# Can you see something active in the sky apart from satellites? Can there be amateur time-domain astronomy?

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What I mean is an event unfolding that is viewable by naked eye or telescopes, and doesn't take comparing days of footage to see moving pixels. Apart from satellites, is there something that moves or happens so quickly that humans could sense it within a reasonable time? The whole sky is like a still painting, I'd love to see parts of it being "animated". Things on my mind are:

• Crescent moon occulting a star (revealing or hiding it)
• Solar and Lunar eclipses

Or more exotic things like:

• Stars exploding (catching that as it happens, is it even possible?)
• Other planets' moons moving a reasonabl/detectable amount in 30 minutes perhaps
• Slow pulsar blinking (they are still super fast afaik)
• Star changing its brightness and not blinking due to haze/turbulence

If it moves or flashes it isn't astronomy, it is meteorology or technology.

There are only a few exceptions to this: Meteors are an atmospheric phenomenon, and a meteor will appear to move rapidly across the sky. But because they "come from space" and occur well above the clouds they are often considered to be part of astronomy.

As you note, eclipses and occultations happen quickly enough for the changes to be visible. The moons of Jupiter do move notably during the night, and when one moves into or out of shadow, it can appear or disappear over the course of a couple of minutes: easily noticeable.

A supernova could brighten quickly enough for the variation to be visible over a night's observation: You wouldn't see it suddenly appear, but over the course of a night it could appear brighter at the end of the night than at the beginning.

Similarly, Algol will fade from magnitude 2 to 3.5 over a few hours. It isn't quick enough to notice the change in the moment, but it is quite clear if you are observing over a few hours.

Pulsars are too dim to be seen with normal equipment even the bright nearby crab pulsar has a magnitude of 16.5 (and at one flash in 33 milliseconds, it is too quick to see by eye)

GRB 080319B was an exceptional object. It was a gamma-ray burst. It gave a flash of gamma rays that lasted a little over a minute. If you had happened to know exactly where to look you could, (marginally with the naked eye, but easily with binoculars) have seen the optical counterpart to the gamma-ray burst. This was created by the formation as a massive early star collapsed to a black hole producing a jet of energy that happened to point our way.

The sun is changing and on a small scale it does change on the scale of minutes, but it is hard to see any movement at that scale with basic equipment. If you can get your hands on a Hydrogen-alpha solar telescope you will, be able to see prominances that change over a period of hours.

Rarely an asteroid will pass so close so as to be visible. Apopsis will have a close approach in 2029 and will appear as a slowly moving star.

Jupiter is active at radio frequencies. You can tune in to Jupiter with the right equipment and hear it changing at rates from a few fractions of a second to a few seconds, see e.g. radiosky.com > Jupiter Central

These are exceptions. In general, it is rare for anything so big and powerful that it can be seen over a distance of many light-years to be able to change fast enough that those changes can be noticed by our eyes.

If you have even a modest pair of binoculars and can hold them steady or support them against something so you can watch the four bright Galilean moons of Jupiter than you can watch them blink off and on again as the eclipse each other, i.e. pass through each others shadows!

This happens about twice every 12 years (Jupiter's orbital period) as the plane that contains Jupiter's satellites passes through Earth.

### And you are in luck for this!

These are happening RIGHT NOW! See answers to When will the next series of mutual eclipses of Jupiter's moons begin?

Actually the Moon occults bright planets at irregular but fairly frequent intervals. You can consult websites that predict occultations to find out more, but for fun see answers to Does a lunar occultation of Mars happen twice a year?

As @JamesK's answer points out you need a telescope to see pulsars blinking, but you also need to find a pulsar that blinks slowly enough that your eye can detect it. The Crab Nebula pulses at about 30 Hz and some people can notice it, but it would be better to choose one with a somewhat lower frequency. For more on that see answers to

### What about near Earth asteroid passes?

Answer(s) to Has a near earth object in heliocentric orbit ever been bright enough to be visible to the unaided eye? suggest we will be able to see some near Earth asteroids (NEOs) as they pass near Earth. In particular, from here:

99942 Apophis (2004 MN4) has an apparent magnitude between 1.7 and 2.7 based on these estimates. Its close approach date is April 13 2029.

### But out of luck for transits of Mercury and Venus any time soon:

• The next transits of Mercury are in 2032, 2039, 2049 and 2052…
• The next transits of Venus are in 2125, 2247, 2255 and 2360…

Many stars are double, and some orbit each other fast enough and far enough for amateurs to be able to detect them and measure them, and see the change in positions over the course of a few years.

On the scale of years to months to weeks to sometimes days, are also variable stars. James K did mention Algol in his answer, but there are literally thousands of variable stars. I suggest visiting the website of the American Association of Variable Star Observers-despite the name, it has members from around the world. https://aavso.org

On September 20th, 2016, Victor Buso was testing his camera mounted on his 40-cm Newtonian telescope when he captured the first moments of a supernova. He was observing NGC 613, a spiral galaxy at a distance of 26.4 Mpc, because at that time it was located near the zenith. The exposure time was 20 s. An analysis of his images show remarkably fast rise rate in brightness of 43 ± 6 mag d$$^{−1}$$. This paper in Nature gives all the details. Bersten, M., Folatelli, G., García, F. et al. A surge of light at the birth of a supernova. Nature 554, 497-499 (2018). https://doi.org/10.1038/nature25151

https://doi.org/10.1038/d41586-018-02331-4

## Can you see something active in the sky apart from satellites? Can there be amateur time-domain astronomy? - Astronomy

What does all this sighting information mean?

Time is when the sighting opportunity will begin in your local time zone. All sightings will occur within a few hours before or after sunrise or sunset. This is the optimum viewing period as the sun reflects off the space station and contrasts against the darker sky.

Visible is the maximum time period the space station is visible before crossing back below the horizon.

Max Height is measured in degrees (also known as elevation). It represents the height of the space station from the horizon in the night sky. The horizon is at zero degrees, and directly overhead is ninety degrees. If you hold your fist at arm's length and place your fist resting on the horizon, the top will be about 10 degrees.

Appears is the location in the sky where the station will be visible first. This value, like maximum height, also is measured in degrees from the horizon. The letters represent compass directions -- N is north, WNW is west by northwest, and so on.

Disappears represents where in the night sky the International Space Station will leave your field of view.

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## Smallsats and Pizza Boxes Lasers and Krypton Gas Thrusters

For as small as the Starlink satellites are — in the “smallsat” class and weighing in at about 250 kg each — they’re packed with all sorts of fun stuff. As pointed out in the Real Engineering video below, each Starlink satellite is essentially a flying, solar-powered wireless router. Phased-array antennas on the Earth-facing side of the satellite will link to “user terminals,” the oft-described “pizza box” ground stations that will provide Internet services to groups on the ground. The satellite also has onboard Hall-effect krypton gas thrusters for station keeping and for the eventual de-orbit burn when the satellite passes its best-by date.

Perhaps the most interesting bit of tech onboard each satellite is a set of lasers. While none of the 180 or so Starlink satellites launched so far have been equipped with lasers, the intention is to use them for the all-important job of “backhaul” communications: the ability to link nearby satellites together optically to find a path between any two ground stations. This has significant throughput benefits over traditional terrestrial fiber-optic links, since the speed of light in glass is about half of that in a vacuum. In theory, Starlink connections have the potential to greatly reduce the latency that exists in terrestrial links. But of course the satellites need those lasers first, and they need to work.

The improved latency of Starlink is probably the key to understanding what Mr. Musk is trying to accomplish here. The ability to provide low-latency transcontinental connections could be incredibly lucrative, especially to the financial markets, where time is literally money. Given the lengths that high-frequency traders will go to shave a few milliseconds off a link, SpaceX could name their price for a reliable link that saves 30 milliseconds or more. Any of the other stated benefits of Starlink, like providing Internet access to underserved locations, will ride on the back of the waves of profit the service will unleash.

## Contents

### Petit's moon Edit

The first major claim of another moon of Earth was made by French astronomer Frédéric Petit, director of the Toulouse Observatory, who in 1846 announced that he had discovered a second moon in an elliptical orbit around Earth.

It was claimed to have also been reported by Lebon and Dassier at Toulouse, and by Larivière at Artenac Observatory, during the early evening of March 21, 1846. [7]

Petit proposed that this second moon had an elliptical orbit, a period of 2 hours 44 minutes, with 3,570 km (2,220 mi) apogee and 11.4 km (7.1 mi) perigee. [7] This claim was soon dismissed by his peers. [8] The 11.4 km (37,000 ft) perigee is similar to the cruising altitude of most modern airliners, and within Earth's atmosphere. Petit published another paper on his 1846 observations in 1861, basing the second moon's existence on perturbations in movements of the actual Moon. [7] This second moon hypothesis was not confirmed either.

Petit's proposed moon became a plot point in Jules Verne's 1870 science fiction novel Around the Moon. [9]

### Waltemath's moons Edit

In 1898 Hamburg scientist Dr. Georg Waltemath announced that he had located a system of tiny moons orbiting Earth. [10] [11] He had begun his search for secondary moons based on the hypothesis that something was gravitationally affecting the Moon's orbit. [12]

Waltemath described one of the proposed moons as being 1,030,000 km (640,000 mi) from Earth, with a diameter of 700 km (430 mi), a 119-day orbital period, and a 177-day synodic period. [7] He also said it did not reflect enough sunlight to be observed without a telescope, unless viewed at certain times, and made several predictions of its next appearances. [12] "Sometimes, it shines at night like the sun but only for an hour or so." [12] [13]

E. Stone Wiggins, a Canadian weather expert, ascribed the cold spring of 1907 to the effect of a second moon, which he said he had first seen in 1882 and had publicized the find in 1884 in the New-York Tribune when he put it forward as probable cause of an anomalous solar eclipse of May of that year. He said it was also probably the "green crescent moon" seen in New Zealand and later in North America in 1886, for periods of less than a half-hour each time. He said this was the "second moon" seen by Waltemath in 1898. Wiggins hypothesized that the second moon had a high carbon atmosphere but could be seen occasionally by its reflected light. [14]

The existence of these objects put forward by Waltemath (and Wiggins) was discredited after the absence of corroborating observation by other members of the scientific community. Especially problematic was a failed prediction that they would be seen in February 1898. [7]

The August 1898 issue of Science mentioned that Waltemath had sent the journal "an announcement of a third moon", which he termed a wahrhafter Wetter und Magnet Mond ("real weather and magnet moon"). [15] It was supposedly 746 km (464 mi) in diameter, and closer than the "second moon" that he had seen previously. [16]

### Other claims Edit

In 1918, astrologer Walter Gornold, also known as Sepharial, claimed to have confirmed the existence of Waltemath's moon. He named it Lilith. Sepharial claimed that Lilith was a 'dark' moon invisible for most of the time, but he claimed to be the first person in history to view it as it crossed the Sun. [17]

In 1926 the science journal Die Sterne published the findings of amateur German astronomer W. Spill, who claimed to have successfully viewed a second moon orbiting Earth. [13]

In the late 1960s John Bagby claimed to have observed over ten small natural satellites of Earth, but this was not confirmed. [7] [18]

### General surveys Edit

William Henry Pickering (1858–1938) studied the possibility of a second moon and made a general search ruling out the possibility of many types of objects by 1903. [19] His 1922 article "A Meteoritic Satellite" in Popular Astronomy resulted in increased searches for small natural satellites by amateur astronomers. [7] Pickering had also proposed the Moon itself had broken off from Earth. [20]

In early 1954 the United States Army's Office of Ordnance Research commissioned Clyde Tombaugh, discoverer of Pluto, to search for near-Earth asteroids. The Army issued a public statement to explain the rationale for this survey. [21] Donald Keyhoe, who was later director of the National Investigations Committee on Aerial Phenomena (NICAP), a UFO research group, said that his Pentagon source had told him that the actual reason for the quickly initiated search was that two near-Earth objects had been picked up on new long-range radar in mid-1953. In May 1954, Keyhoe asserted that the search had been successful, and either one or two objects had been found. [22] At The Pentagon, a general who heard the news reportedly asked whether the satellites were natural or artificial. Tombaugh denied the alleged discovery in a letter to Willy Ley, [9] and the October 1955 issue of Popular Mechanics magazine reported:

Professor Tombaugh is closemouthed about his results. He won't say whether or not any small natural satellites have been discovered. He does say, however, that newspaper reports of 18 months ago announcing the discovery of natural satellites at 400 and 600 miles out are not correct. He adds that there is no connection between the search program and the reports of so-called flying saucers. [23]

At a meteor conference in Los Angeles in 1957, Tombaugh reiterated that his four-year search for natural satellites had been unsuccessful. [24] In 1959, he issued a final report stating that nothing had been found in his search.

### Modern status Edit

It was discovered that small bodies can be temporarily captured, as shown by 2006 RH 120 , which was in Earth orbit in 2006–2007. [1]

In 2010, the first known Earth trojan was discovered in data from Wide-field Infrared Survey Explorer (WISE), and is currently called 2010 TK 7 .

In 2011, planetary scientists Erik Asphaug and Martin Jutzi proposed a model in which a second moon would have existed 4.5 billion years ago, and later impacted the Moon, as a part of the accretion process in the formation of the Moon. [25]

In 2018, it was confirmed two dust clouds orbited Earth at the Moon's L4 and L5 points, [26] known as the Kordylewski clouds. These were nicknamed "Earth's hidden moons". [27]

The interpretation of some bodies has led to sometimes bold statements in the astronomy press, though often allowing for other interpretations: [3]

Earth has a second moon, of sorts, and could have many others, according to three astronomers who did calculations to describe orbital motions at gravitational balance points in space that temporarily pull asteroids into bizarre orbits near our planet.

Although no other moons of Earth have been found to date, there are various types of near-Earth objects in 1:1 resonance with it, which are known as quasi-satellites. Quasi-satellites orbit the Sun from the same distance as a planet, rather than the planet itself. Their orbits are unstable, and will fall into other resonances or be kicked into other orbits over thousands of years. [3] Quasi-satellites of Earth include 2010 SO 16 , (164207) 2004 GU 9 , [28] (277810) 2006 FV 35 , [29] 2002 AA 29 , [30] 2014 OL 339 , 2013 LX 28 , 469219 Kamoʻoalewa and 3753 Cruithne. Cruithne, discovered in 1986, orbits the Sun in an elliptical orbit but appears to have a horseshoe orbit when viewed from Earth. [3] [31] Some went as far to nickname Cruithne "Earth's second moon". [31]

The key difference between a satellite and a quasi-satellite is that the orbit of a satellite of Earth fundamentally depends on the gravity of the Earth–Moon system, whereas the orbit of a quasi-satellite would negligibly change if Earth and the Moon were suddenly removed because a quasi-satellite is orbiting the Sun on an Earth-like orbit in the vicinity of Earth. [32]

Earth possesses one known trojan, a small Solar System body caught in the planet's gravitationally stable L4 Lagrangian point. This object, 2010 TK 7 , is roughly 300 metres across. Like quasi-satellites, it orbits the Sun in a 1:1 resonance with Earth, rather than Earth itself.

Computer models by astrophysicists Mikael Granvik, Jeremie Vaubaillon, and Robert Jedicke suggest that these "temporary satellites" should be quite common and that "At any given time, there should be at least one natural Earth satellite of 1 meter diameter orbiting the Earth." [33] Such objects would remain in orbit for ten months on average, before returning to solar orbit once more, and so would make relatively easy targets for manned space exploration. [33] "Mini-moons" were further examined in a study published in the journal Icarus. [34]

It has been proposed that NASA search for temporary natural satellites, and use them for a sample return mission. [1] [35]

### 1913 Edit

The earliest known mention in the scientific literature of a temporarily captured orbiter is by Clarence Chant about the Meteor procession of 9 February 1913: [32]

It would seem that the bodies had been traveling through space, probably in an orbit about the sun, and that on coming near the earth they were promptly captured by it and caused to move about it as a satellite. [36]

Later, in 1916, William Frederick Denning surmised that:

The large meteors which passed over Northern America on 9 February 1913, presented some unique features. The length of their observed flight was about 2,600 miles [4,200 km], and they must have been moving in paths concentric, or nearly concentric, with the earth's surface, so that they temporarily formed new terrestrial satellites. [37]

### 2006 Edit

On 14 September 2006, an object estimated at 5 meters in diameter was discovered in near-polar orbit around Earth. Originally thought to be a third-stage Saturn S-IVB booster from Apollo 12, it was later determined to be an asteroid and designated as 2006 RH 120 . The asteroid re-entered solar orbit after 13 months and is expected to return to Earth orbit after 21 years. [38]

### 2015 Edit

In April 2015, an object was discovered orbiting the Earth, and initially designated 2015 HP116 , but more detailed investigation quickly showed the object to be the Gaia spacecraft, and the object's discovery soon was retracted. [39]

On 3 October 2015, a small object, temporarily designated WT1190F, was found to be orbiting the Earth every

23 days, and had been orbiting since at least late 2009. It impacted the Earth on 13 November 2015 at 06:18:34.3 UTC (±1.3 seconds).

### 2016 Edit

On 8 February 2016 an object

0.5 meter in diameter was discovered orbiting the Earth with a period of 5 days and given the temporary designation XC83E0D, and most likely lost. The object was later identified as the lost artificial satellite SR-11A, or possibly its companion SR-11B, which were launched in 1976 and lost in 1979. [ citation needed ]

On 8 April 2016, an object, given the temporary designation S509356, was discovered with an orbital period of 3.58 days. Although it has the typical area-to-mass ratio (m 2 /kg) of satellites, it has a color typical of S-type asteroids. It was later identified as the Yuanzheng-1 stage from the launch of Chinese navigation satellites. [40]

### 2017 Edit

On 8 December 2017, the object YX205B9 was discovered with an orbital period of 21 days, on an eccentric orbit taking it from slightly beyond the geocentric satellite ring to almost twice the distance of the Moon. It was later identified as the booster stage from the Chang'e 2 mission. [41] [42]

### 2018-2020 Edit

2020 CD 3 was discovered in 2020, and orbited around Earth from 2018 to May 2020. [43] [6]

2020 SO, discovered in September 2020, will enter orbit of Earth between October 2020 and late May 2021.

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## War in Space May Be Closer Than Ever

The world&rsquos most worrisome military flashpoint is arguably not in the Strait of Taiwan, the Korean Peninsula, Iran, Israel, Kashmir or Ukraine. In fact, it cannot be located on any map of Earth, even though it is very easy to find. To see it, just look up into a clear sky, to the no-man&rsquos-land of Earth orbit, where a conflict is unfolding that is an arms race in all but name.

The emptiness of outer space might be the last place you&rsquod expect militaries to vie over contested territory, except that outer space isn&rsquot so empty anymore. About 1,300 active satellites wreathe the globe in a crowded nest of orbits, providing worldwide communications, GPS navigation, weather forecasting and planetary surveillance. For militaries that rely on some of those satellites for modern warfare, space has become the ultimate high ground, with the U.S. as the undisputed king of the hill. Now, as China and Russia aggressively seek to challenge U.S. superiority in space with ambitious military space programs of their own, the power struggle risks sparking a conflict that could cripple the entire planet&rsquos space-based infrastructure. And though it might begin in space, such a conflict could easily ignite full-blown war on Earth.

The long-simmering tensions are now approaching a boiling point due to several events, including recent and ongoing tests of possible anti-satellite weapons by China and Russia, as well as last month&rsquos failure of tension-easing talks at the United Nations.

Testifying before Congress earlier this year, Director of National Intelligence James Clapper echoed the concerns held by many senior government officials about the growing threat to U.S. satellites, saying that China and Russia are both &ldquodeveloping capabilities to deny access in a conflict,&rdquo such as those that might erupt over China&rsquos military activities in the South China Sea or Russia&rsquos in Ukraine. China in particular, Clapper said, has demonstrated &ldquothe need to interfere with, damage and destroy&rdquo U.S. satellites, referring to a series of Chinese anti-satellite missile tests that began in 2007.

There are many ways to disable or destroy satellites beyond provocatively blowing them up with missiles. A spacecraft could simply approach a satellite and spray paint over its optics, or manually snap off its communications antennas, or destabilize its orbit. Lasers can be used to temporarily disable or permanently damage a satellite&rsquos components, particularly its delicate sensors, and radio or microwaves can jam or hijack transmissions to or from ground controllers.

In response to these possible threats, the Obama administration has budgeted at least \$5 billion to be spent over the next five years to enhance both the defensive and offensive capabilities of the U.S. military space program. The U.S. is also attempting to tackle the problem through diplomacy, although with minimal success in late July at the United Nations, long-awaited discussions stalled on a European Union-drafted code of conduct for spacefaring nations due to opposition from Russia, China and several other countries including Brazil, India, South Africa and Iran. The failure has placed diplomatic solutions for the growing threat in limbo, likely leading to years of further debate within the UN&rsquos General Assembly.

&ldquoThe bottom line is the United States does not want conflict in outer space,&rdquo says Frank Rose, assistant secretary of state for arms control, verification and compliance, who has led American diplomatic efforts to prevent a space arms race. The U.S., he says, is willing to work with Russia and China to keep space secure. &ldquoBut let me make it very clear: we will defend our space assets if attacked.&rdquo

Offensive space weapons tested
The prospect of war in space is not new. Fearing Soviet nuclear weapons launched from orbit, the U.S. began testing anti-satellite weaponry in the late 1950s. It even tested nuclear bombs in space before orbital weapons of mass destruction were banned through the United Nations&rsquo Outer Space Treaty of 1967. After the ban, space-based surveillance became a crucial component of the Cold War, with satellites serving as one part of elaborate early-warning systems on alert for the deployment or launch of ground-based nuclear weapons. Throughout most of the Cold War, the U.S.S.R. developed and tested &ldquospace mines,&rdquo self-detonating spacecraft that could seek and destroy U.S. spy satellites by peppering them with shrapnel. In the 1980s, the militarization of space peaked with the Reagan administration&rsquos multibillion-dollar Strategic Defense Initiative, dubbed Star Wars, to develop orbital countermeasures against Soviet intercontinental ballistic missiles. And in 1985, the U.S. Air Force staged a clear demonstration of its formidable capabilities, when an F-15 fighter jet launched a missile that took out a failing U.S. satellite in low-Earth orbit.

Through it all, no full-blown arms race or direct conflicts erupted. According to Michael Krepon, an arms-control expert and co-founder of the Stimson Center think tank in Washington, D.C., that was because both the U.S. and U.S.S.R. realized how vulnerable their satellites were&mdashparticularly the ones in &ldquogeosynchronous&rdquo orbits of about 35,000 kilometers or more. Such satellites effectively hover over one spot on the planet, making them sitting ducks. But because any hostile action against those satellites could easily escalate to a full nuclear exchange on Earth, both superpowers backed down. &ldquoNeither one of us signed a treaty about this,&rdquo Krepon says. &ldquoWe just independently came to the conclusion that our security would be worse off if we went after those satellites, because if one of us did it, then the other guy would, too.&rdquo

Today, the situation is much more complicated. Low- and high-Earth orbits have become hotbeds of scientific and commercial activity, filled with hundreds upon hundreds of satellites from about 60 different nations. Despite their largely peaceful purposes, each and every satellite is at risk, in part because not all members of the growing club of military space powers are willing to play by the same rules&mdashand they don&rsquot have to, because the rules remain as yet unwritten.

Space junk is the greatest threat. Satellites race through space at very high velocities, so the quickest, dirtiest way to kill one is to simply launch something into space to get in its way. Even the impact of an object as small and low-tech as a marble can disable or entirely destroy a billion-dollar satellite. And if a nation uses such a &ldquokinetic&rdquo method to destroy an adversary&rsquos satellite, it can easily create even more dangerous debris, potentially cascading into a chain reaction that transforms Earth orbit into a demolition derby.

In 2007 the risks from debris skyrocketed when China launched a missile that destroyed one of its own weather satellites in low-Earth orbit. That test generated a swarm of long-lived shrapnel that constitutes nearly one-sixth of all the radar-trackable debris in orbit. The U.S. responded in kind in 2008, repurposing a ship-launched anti-ballistic missile to shoot down a malfunctioning U.S. military satellite shortly before it tumbled into the atmosphere. That test produced dangerous junk too, though in smaller amounts, and the debris was shorter-lived because it was generated at a much lower altitude.

More recently, China has launched what many experts say are additional tests of ground-based anti-satellite kinetic weapons. None of these subsequent launches have destroyed satellites, but Krepon and other experts say this is because the Chinese are now merely testing to miss, rather than to hit, with the same hostile capability as an end result. The latest test occurred on July 23 of last year. Chinese officials insist the tests&rsquo only purpose is peaceful missile defense and scientific experimentation. But one test in May 2013 sent a missile soaring as high as 30,000 kilometers above Earth, approaching the safe haven of strategic geosynchronous satellites.

That was a wake-up call, says Brian Weeden, a security analyst and former Air Force officer who studied and helped publicize the Chinese test. &ldquoThe U.S. came to grips decades ago with the fact that its lower orbit satellites could easily be shot down,&rdquo Weeden says. &ldquoGoing nearly to geosynchronous made people realize that, holy cow, somebody might actually try to go after the stuff we have up there.&rdquo

It was no coincidence that shortly after the May 2013 test, the US declassified details of its secret Geosynchronous Space Situational Awareness Program (GSSAP), a planned set of four satellites capable of monitoring the Earth&rsquos high orbits and even rendezvousing with other satellites to inspect them up-close. The first two GSSAP spacecraft launched into orbit in July 2014.

&ldquoThis used to be a black program&mdashsomething that didn&rsquot even officially exist,&rdquo Weeden says. &ldquoIt was declassified to basically send a message saying, &lsquoHey, if you&rsquore doing something funky in and around the geosynchronous belt, we&rsquore going to see.&rsquo&rdquo An interloper into geosynchronous orbit need not be an explosives-tipped missile to be a security risk&mdasheven sidling up to an adversary&rsquos strategic satellites is considered a threat. Which is one reason that potential U.S. adversaries might be alarmed by the rendezvous capabilities of GSSAP and of the U.S. Air Force&rsquos highly maneuverable X-37B robotic space planes.

Russia is also developing its own ability to approach, inspect and potentially sabotage or destroy satellites in orbit. Over the past two years, it has included three mysterious payloads in otherwise routine commercial satellite launches, with the latest occurring in March of this year. Radar observations by the U.S. Air Force and by amateur hobbyists revealed that after each commercial satellite was deployed, an additional small object flew far away from the jettisoned rocket booster, only to later turn around and fly back. The objects, dubbed Kosmos-2491, -2499 and -2504, might just be part of an innocuous program developing techniques to service and refuel old satellites, Weeden says, though they could also be meant for more sinister intentions.

Treaties offer little assurance
Chinese officials maintain that their military activities in space are simply peaceful science experiments, while Russian officials have stayed mostly mum. Both nations could be seen as simply responding to what they see as the U.S.&rsquos clandestine development of potential space weapons. Indeed, the U.S.&rsquos ballistic missile defense systems, its X-37B space planes and even its GSSAP spacecraft, though all ostensibly devoted to maintaining peace, could be easily repurposed into weapons of space war. For years Russia and China have pushed for the ratification of a legally binding United Nations treaty banning space weapons&mdasha treaty that U.S. officials and outside experts have repeatedly rejected as a disingenuous nonstarter.

&ldquoThe draft treaty from Russia and China seeks to ban the very things that they are so actively pursuing,&rdquo Krepon says. &ldquoIt serves their interests perfectly. They want freedom of action, and they&rsquore covering that with this proposal to ban space weapons.&rdquo Even if the treaty was being offered in good faith, Krepon says, &ldquoit would be dead on arrival&rdquo in Congress and would stand no chance of being ratified. After all, the U.S. wants freedom of action in space, too, and in space no other country has more capability&mdashand thus more to lose.

According to Rose, there are three key problems with the treaty. &ldquoOne, it&rsquos not effectively verifiable, which the Russians and Chinese admit,&rdquo he says. &ldquoYou can&rsquot detect cheating. Two, it is totally silent on the issue of terrestrial anti-satellite weapons, like the ones that China tested in 2007 and again in July 2014. And third, it does not define what a weapon in outer space is.&rdquo

As an alternative, the U.S. supports a European-led initiative to establish &ldquonorms&rdquo for proper behavior through the creation of a voluntary International Code of Conduct for Outer Space. This would be a first step, to be followed by a binding agreement. A draft of the code&mdashwhich Russia and China prevented from being adopted in last month&rsquos UN discussions&mdashcalls for more transparency and &ldquoconfidence-building&rdquo between spacefaring nations as a way of promoting the &ldquopeaceful exploration and use of outer space.&rdquo This, it is hoped, can prevent the generation of more debris and the further development of space weapons. However, like the Russian-Chinese treaty, the code does not exactly define what constitutes a &ldquospace weapon.&rdquo

That haziness poses problems for senior defense officials such as General John Hyten, the head of the U.S. Air Force Space Command. &ldquoIs our space-based surveillance system that looks out at the heavens and tracks everything in geosynchronous a weapons system?&rdquo he asks. &ldquoI think everybody in the world would look at that and say no. But it&rsquos maneuverable, it&rsquos going 17,000 miles per hour, and it has a sensor on board. It&rsquos not a weapon, okay? But would [a treaty&rsquos] language ban our ability to do space-based surveillance? I would hope not!&rdquo

Is war in space inevitable?
Meanwhile, shifts in U.S. policy are giving China and Russia more reasons for further suspicion. Congress has been pressing the U.S. national security community to turn its attentions to the role of offensive rather than defensive capabilities, even dictating that most of the fiscal year 2015 funding for the Pentagon&rsquos Space Security and Defense Program go toward &ldquodevelopment of offensive space control and active defense strategies and capabilities.&rdquo

&ldquoOffensive space control&rdquo is a clear reference to weapons. &ldquoActive defense&rdquo is much more nebulous, and refers to undefined offensive countermeasures that could be taken against an attacker, further widening the routes by which space might soon become weaponized. If an imminent threat is perceived, a satellite or its operators might preemptively attack via dazzling lasers, jamming microwaves, kinetic bombardment or any other number of possible methods.

&ldquoI hope to never fight a war in space,&rdquo Hyten says. &ldquoIt&rsquos bad for the world. Kinetic [anti-satellite weaponry] is horrible for the world,&rdquo because of the existential risks debris poses for all satellites. &ldquoBut if war does extend into space,&rdquo he says, &ldquowe have to have offensive and defensive capabilities to respond with, and Congress has asked us to explore what those capabilities would be. And to me, the one limiting factor is no debris. Whatever you do, don&rsquot create debris.&rdquo

Technology to jam transmissions, for example, appears to underpin the Air Force&rsquos Counter Communications System, the U.S.&rsquos sole acknowledged offensive capability against satellites in space. &ldquoIt's basically a big antenna on a trailer, and how it actually works, what it actually does, nobody knows,&rdquo Weeden says, noting that, like most space security work, the details of the system are top secret. &ldquoAll we basically know is that they could use it to somehow jam or maybe even spoof or hack into an adversary&rsquos satellites.&rdquo

For Krepon, the debate over the definitions of space weapons and the saber-rattling between Russia, China and the U.S. is unhelpfully eclipsing the more pressing issue of debris. &ldquoEveryone is talking about purposeful, man-made objects dedicated to warfighting in space, and it&rsquos like we are back in the Cold War,&rdquo Krepon says. &ldquoMeanwhile, there are about 20,000 weapons already up there in the form of debris. They&rsquore not purposeful&mdashthey&rsquore unguided. They&rsquore not seeking out enemy satellites. They&rsquore just whizzing around, doing what they do.&rdquo

The space environment, he says, must be protected as a global commons, similar to the Earth&rsquos oceans and atmosphere. Space junk is very easy to make and very hard to clean up, so international efforts should focus on preventing its creation. Beyond the threat of deliberate destruction, the risk of accidental collisions and debris strikes will continue to grow as more nations launch and operate more satellites without rigorous international accountability and oversight. And as the chance of accidents increases, so too does the possibility of their being misinterpreted as deliberate, hostile actions in the high-tension cloak-and-dagger military struggle in space.

&ldquoWe are in the process of messing up space, and most people don&rsquot realize it because we can&rsquot see it the way we can see fish kills, algal blooms, or acid rain,&rdquo he says. &ldquoTo avoid trashing Earth orbit, we need a sense of urgency that currently no one has. Maybe we&rsquoll get it when we can&rsquot get our satellite television and our telecommunications, our global weather reports and hurricane predictions. Maybe when we get knocked back to the 1950s, we&rsquoll get it. But by then it will be too late.&rdquo

## 24.5 Black Holes

Let’s now apply what we have learned about gravity and spacetime curvature to the issue we started with: the collapsing core in a very massive star. We saw that if the core’s mass is greater than about 3 MSun, theory says that nothing can stop the core from collapsing forever. We will examine this situation from two perspectives: first from a pre-Einstein point of view, and then with the aid of general relativity.

### Classical Collapse

Let’s begin with a thought experiment. We want to know what speeds are required to escape from the gravitational pull of different objects. A rocket must be launched from the surface of Earth at a very high speed if it is to escape the pull of Earth’s gravity. In fact, any object—rocket, ball, astronomy book—that is thrown into the air with a velocity less than 11 kilometers per second will soon fall back to Earth’s surface. Only those objects launched with a speed greater than this escape velocity can get away from Earth.

The escape velocity from the surface of the Sun is higher yet—618 kilometers per second. Now imagine that we begin to compress the Sun, forcing it to shrink in diameter. Recall that the pull of gravity depends on both the mass that is pulling you and your distance from the center of gravity of that mass. If the Sun is compressed, its mass will remain the same, but the distance between a point on the Sun’s surface and the center will get smaller and smaller. Thus, as we compress the star, the pull of gravity for an object on the shrinking surface will get stronger and stronger (Figure 24.12).

When the shrinking Sun reaches the diameter of a neutron star (about 20 kilometers), the velocity required to escape its gravitational pull will be about half the speed of light. Suppose we continue to compress the Sun to a smaller and smaller diameter. (We saw this can’t happen to a star like our Sun in the real world because of electron degeneracy, i.e., the mutual repulsion between tightly packed electrons this is just a quick “thought experiment” to get our bearings).

Ultimately, as the Sun shrinks, the escape velocity near the surface would exceed the speed of light. If the speed you need to get away is faster than the fastest possible speed in the universe, then nothing, not even light, is able to escape. An object with such large escape velocity emits no light, and anything that falls into it can never return.

In modern terminology, we call an object from which light cannot escape a black hole , a name popularized by the America scientist John Wheeler starting in the late 1960s (Figure 24.13). The idea that such objects might exist is, however, not a new one. Cambridge professor and amateur astronomer John Michell wrote a paper in 1783 about the possibility that stars with escape velocities exceeding that of light might exist. And in 1796, the French mathematician Pierre-Simon, marquis de Laplace, made similar calculations using Newton’s theory of gravity he called the resulting objects “dark bodies.”

While these early calculations provided strong hints that something strange should be expected if very massive objects collapse under their own gravity, we really need general relativity theory to give an adequate description of what happens in such a situation.

### Collapse with Relativity

General relativity tells us that gravity is really a curvature of spacetime. As gravity increases (as in the collapsing Sun of our thought experiment), the curvature gets larger and larger. Eventually, if the Sun could shrink down to a diameter of about 6 kilometers, only light beams sent out perpendicular to the surface would escape. All others would fall back onto the star (Figure 24.14). If the Sun could then shrink just a little more, even that one remaining light beam would no longer be able to escape.

Keep in mind that gravity is not pulling on the light. The concentration of matter has curved spacetime, and light (like the trained ant of our earlier example) is “doing its best” to go in a straight line, yet is now confronted with a world in which straight lines that used to go outward have become curved paths that lead back in. The collapsing star is a black hole in this view, because the very concept of “out” has no geometrical meaning. The star has become trapped in its own little pocket of spacetime, from which there is no escape.

The star’s geometry cuts off communication with the rest of the universe at precisely the moment when, in our earlier picture, the escape velocity becomes equal to the speed of light. The size of the star at this moment defines a surface that we call the event horizon . It’s a wonderfully descriptive name: just as objects that sink below our horizon cannot be seen on Earth, so anything happening inside the event horizon can no longer interact with the rest of the universe.

Imagine a future spacecraft foolish enough to land on the surface of a massive star just as it begins to collapse in the way we have been describing. Perhaps the captain is asleep at the gravity meter, and before the crew can say “Albert Einstein,” they have collapsed with the star inside the event horizon. Frantically, they send an escape pod straight outward. But paths outward twist around to become paths inward, and the pod turns around and falls toward the center of the black hole. They send a radio message to their loved ones, bidding good-bye. But radio waves, like light, must travel through spacetime, and curved spacetime allows nothing to get out. Their final message remains unheard. Events inside the event horizon can never again affect events outside it.

The characteristics of an event horizon were first worked out by astronomer and mathematician Karl Schwarzschild (Figure 24.15). A member of the German army in World War I, he died in 1916 of an illness he contracted while doing artillery shell calculations on the Russian front. His paper on the theory of event horizons was among the last things he finished as he was dying it was the first exact solution to Einstein’s equations of general relativity. The radius of the event horizon is called the Schwarzschild radius in his memory.

The event horizon is the boundary of the black hole calculations show that it does not get smaller once the whole star has collapsed inside it. It is the region that separates the things trapped inside it from the rest of the universe. Anything coming from the outside is also trapped once it comes inside the event horizon. The horizon’s size turns out to depend only on the mass inside it. If the Sun, with its mass of 1 MSun, were to become a black hole (fortunately, it can’t—this is just a thought experiment), the Schwarzschild radius would be about 3 kilometers thus, the entire black hole would be about one-third the size of a neutron star of that same mass. Feed the black hole some mass, and the horizon will grow—but not very much. Doubling the mass will make the black hole 6 kilometers in radius, still very tiny on the cosmic scale.

The event horizons of more massive black holes have larger radii. For example, if a globular cluster of 100,000 stars (solar masses) could collapse to a black hole, it would be 300,000 kilometers in radius, a little less than half the radius of the Sun. If the entire Galaxy could collapse to a black hole, it would be only about 10 12 kilometers in radius—about a tenth of a light year. Smaller masses have correspondingly smaller horizons: for Earth to become a black hole, it would have to be compressed to a radius of only 1 centimeter—less than the size of a grape. A typical asteroid, if crushed to a small enough size to be a black hole, would have the dimensions of an atomic nucleus.

### Example 24.1

#### The Milky Way’s Black Hole

where c is the speed of light, G is the gravitational constant, and M is the mass of the black hole. Note that in this formula, 2, G, and c are all constant only the mass changes from black hole to black hole.

As we will see in the chapter on The Milky Way Galaxy, astronomers have traced the paths of several stars near the center of our Galaxy and found that they seem to be orbiting an unseen object—dubbed Sgr A* (pronounced “Sagittarius A-star”)—with a mass of about 4 million solar masses. What is the size of its Schwarzschild radius?

#### Solution

This distance is about one-fifth of the radius of Mercury’s orbit around the Sun, yet the object contains 4 million solar masses and cannot be seen with our largest telescopes. You can see why astronomers are convinced this object is a black hole.

### A Black Hole Myth

Much of the modern folklore about black holes is misleading. One idea you may have heard is that black holes go about sucking things up with their gravity. Actually, it is only very close to a black hole that the strange effects we have been discussing come into play. The gravitational attraction far away from a black hole is the same as that of the star that collapsed to form it.

Remember that the gravity of any star some distance away acts as if all its mass were concentrated at a point in the center, which we call the center of gravity. For real stars, we merely imagine that all mass is concentrated there for black holes, all the mass really is concentrated at a point in the center.

So, if you are a star or distant planet orbiting around a star that becomes a black hole, your orbit may not be significantly affected by the collapse of the star (although it may be affected by any mass loss that precedes the collapse). If, on the other hand, you venture close to the event horizon, it would be very hard for you to resist the “pull” of the warped spacetime near the black hole. You have to get really close to the black hole to experience any significant effect.

If another star or a spaceship were to pass one or two solar radii from a black hole, Newton’s laws would be adequate to describe what would happen to it. Only very near the event horizon of a black hole is the gravitation so strong that Newton’s laws break down. The black hole remnant of a massive star coming into our neighborhood would be far, far safer to us than its earlier incarnation as a brilliant, hot star.

### Making Connections

#### Gravity and Time Machines

Time machines are one of the favorite devices of science fiction. Such a device would allow you to move through time at a different pace or in a different direction from everyone else. General relativity suggests that it is possible, in theory, to construct a time machine using gravity that could take you into the future.

Let’s imagine a place where gravity is terribly strong, such as near a black hole. General relativity predicts that the stronger the gravity, the slower the pace of time (as seen by a distant observer). So, imagine a future astronaut, with a fast and strongly built spaceship, who volunteers to go on a mission to such a high-gravity environment. The astronaut leaves in the year 2222, just after graduating from college at age 22. She takes, let’s say, exactly 10 years to get to the black hole. Once there, she orbits some distance from it, taking care not to get pulled in.

She is now in a high-gravity realm where time passes much more slowly than it does on Earth. This isn’t just an effect on the mechanism of her clocks—time itself is running slowly. That means that every way she has of measuring time will give the same slowed-down reading when compared to time passing on Earth. Her heart will beat more slowly, her hair will grow more slowly, her antique wristwatch will tick more slowly, and so on. She is not aware of this slowing down because all her readings of time, whether made by her own bodily functions or with mechanical equipment, are measuring the same—slower—time. Meanwhile, back on Earth, time passes as it always does.

Our astronaut now emerges from the region of the black hole, her mission of exploration finished, and returns to Earth. Before leaving, she carefully notes that (according to her timepieces) she spent about 2 weeks around the black hole. She then takes exactly 10 years to return to Earth. Her calculations tell her that since she was 22 when she left the Earth, she will be 42 plus 2 weeks when she returns. So, the year on Earth, she figures, should be 2242, and her classmates should now be approaching their midlife crises.

But our astronaut should have paid more attention in her astronomy class! Because time slowed down near the black hole, much less time passed for her than for the people on Earth. While her clocks measured 2 weeks spent near the black hole, more than 2000 weeks (depending on how close she got) could well have passed on Earth. That’s equal to 40 years, meaning her classmates will be senior citizens in their 80s when she (a mere 42-year-old) returns. On Earth it will be not 2242, but 2282—and she will say that she has arrived in the future.

Is this scenario real? Well, it has a few practical challenges: we don’t think any black holes are close enough for us to reach in 10 years, and we don’t think any spaceship or human can survive near a black hole. But the key point about the slowing down of time is a natural consequence of Einstein’s general theory of relativity, and we saw that its predictions have been confirmed by experiment after experiment.

Such developments in the understanding of science also become inspiration for science fiction writers. Recently, the film Interstellar featured the protagonist traveling close to a massive black hole the resulting delay in his aging relative to his earthbound family is a key part of the plot.

Science fiction novels, such as Gateway by Frederik Pohl and A World out of Time by Larry Niven, also make use of the slowing down of time near black holes as major turning points in the story. For a list of science fiction stories based on good astronomy, you can go to http://bit.ly/astroscifi.

### A Trip into a Black Hole

The fact that scientists cannot see inside black holes has not kept them from trying to calculate what they are like. One of the first things these calculations showed was that the formation of a black hole obliterates nearly all information about the star that collapsed to form it. Physicists like to say “black holes have no hair,” meaning that nothing sticks out of a black hole to give us clues about what kind of star produced it or what material has fallen inside. The only information a black hole can reveal about itself is its mass, its spin (rotation), and whether it has any electrical charge.

What happens to the collapsing star-core that made the black hole? Our best calculations predict that the material will continue to collapse under its own weight, forming an infinitely squozen point—a place of zero volume and infinite density—to which we give the name singularity . At the singularity, spacetime ceases to exist. The laws of physics as we know them break down. We do not yet have the physical understanding or the mathematical tools to describe the singularity itself, or even if singularities actually occur. From the outside, however, the entire structure of a basic black hole (one that is not rotating) can be described as a singularity surrounded by an event horizon. Compared to humans, black holes are really very simple objects.

Scientists have also calculated what would happen if an astronaut were to fall into a black hole. Let’s take up an observing position a long, safe distance away from the event horizon and watch this astronaut fall toward it. At first he falls away from us, moving ever faster, just as though he were approaching any massive star. However, as he nears the event horizon of the black hole, things change. The strong gravitational field around the black hole will make his clocks run more slowly, when seen from our outside perspective.

If, as he approaches the event horizon, he sends out a signal once per second according to his clock, we will see the spacing between his signals grow longer and longer until it becomes infinitely long when he reaches the event horizon. (Recalling our discussion of gravitational redshift, we could say that if the infalling astronaut uses a blue light to send his signals every second, we will see the light get redder and redder until its wavelength is nearly infinite.) As the spacing between clock ticks approaches infinity, it will appear to us that the astronaut is slowly coming to a stop, frozen in time at the event horizon.

In the same way, all matter falling into a black hole will also appear to an outside observer to stop at the event horizon, frozen in place and taking an infinite time to fall through it. But don’t think that matter falling into a black hole will therefore be easily visible at the event horizon. The tremendous redshift will make it very difficult to observe any radiation from the “frozen” victims of the black hole.

This, however, is only how we, located far away from the black hole, see things. To the astronaut, his time goes at its normal rate and he falls right on through the event horizon into the black hole. (Remember, this horizon is not a physical barrier, but only a region in space where the curvature of spacetime makes escape impossible.)

You may have trouble with the idea that you (watching from far away) and the astronaut (falling in) have such different ideas about what has happened. This is the reason Einstein’s ideas about space and time are called theories of relativity. What each observer measures about the world depends on (is relative to) his or her frame of reference. The observer in strong gravity measures time and space differently from the one sitting in weaker gravity. When Einstein proposed these ideas, many scientists also had difficulty with the idea that two such different views of the same event could be correct, each in its own “world,” and they tried to find a mistake in the calculations. There were no mistakes: we and the astronaut really would see him fall into a black hole very differently.

For the astronaut, there is no turning back. Once inside the event horizon, the astronaut, along with any signals from his radio transmitter, will remain hidden forever from the universe outside. He will, however, not have a long time (from his perspective) to feel sorry for himself as he approaches the black hole. Suppose he is falling feet first. The force of gravity that the singularity exerts on his feet is greater than on his head, so he will be stretched slightly. Because the singularity is a point, the left side of his body will be pulled slightly toward the right, and the right slightly toward the left, bringing each side closer to the singularity. The astronaut will therefore be slightly squeezed in one direction and stretched in the other. Some scientists like to call this process of stretching and narrowing spaghettification. The point at which the astronaut becomes so stretched that he perishes depends on the size of the black hole. For black holes with masses billions of times the mass of the Sun, such as those found at the centers of galaxies, the spaghettification becomes significant only after the astronaut passes through the event horizon. For black holes with masses of a few solar masses, the astronaut will be stretched and ripped apart even before he reaches the event horizon.

Earth exerts similar tidal forces on an astronaut performing a spacewalk. In the case of Earth, the tidal forces are so small that they pose no threat to the health and safety of the astronaut. Not so in the case of a black hole. Sooner or later, as the astronaut approaches the black hole, the tidal forces will become so great that the astronaut will be ripped apart, eventually reduced to a collection of individual atoms that will continue their inexorable fall into the singularity.

From the previous discussion, you will probably agree that jumping into a black hole is definitely a once-in-a-lifetime experience! You can see an engaging explanation of death by black hole by Neil deGrasse Tyson, where he explains the effect of tidal forces on the human body until it dies by spaghettification.

An overview of black holes is given in this Discovery Channel video excerpt.

## Software Tutorial

The NOAA satellites only pass overhead at certain times of the day, broadcasting a signal. These signals appear at around

137 MHz, and only when a satellite is passing overhead. Each satellite has a different frequency. Currently only NOAA satellites 15, 18 and 19 are operational, their frequencies are shown below.

An example of a NOAA APT weather satellite signal is shown zoomed in and out on the frequency spectrum directly below and an example audio file of the signal is shown further below.

APT Signal Zoomed in APT Signal Zoomed Out https://www.rtl-sdr.com/wp-content/uploads/2013/05/NOAAAPT.mp3

### WXtoImg Tutorial

WXtoImg is a free weather satellite decoding program which can decode the APT signal, and also tell you the times and frequencies of the NOAA satellites passing overhead. There is also a paid version of WXtoImg which can unlock more features, however it is not required for use with RTL-SDR. To use WXtoImg and SDRSharp together follow the instructions below.

1. Next open WXtoImg, and then set your Ground Station Location, (which is the coordinates of your antenna) by going to Options -> Ground Station Location. The city you are in should suffice, but you can be more accurate by entering in an exact latitude and longitude if you want.

1. In WXtoImg set your audio piping method which you have chosen. To do this go to Options -> Recording Options, and ensure the correct device is selected under the soundcard option.Also, here you can adjust the "Record only when active APT satellites are overhead" "with maximum elevation above (degrees)" and "record only when satellite is above (degrees)" settings. You may want to reduce the default values if you have an antenna with a good view of the sky and find that WXtoImg stops recording or doesn't start fast enough even though the APT signal is present in SDRSharp.

1. Now you will need to update your Kepler files. These files contain the information about satellite locations. They need to be periodically updated, because satellites drift in their orbit over time. Go to File -> Update Keplers to do this. Make sure you have an internet connection for the update.
1. Now you can go to File -> Satellite Pass List, and find a time when a satellite will be passing overhead. Take note of the frequency as well.

1. When the time comes for the satellite to appear, open WXtoImg, and then go to File->Record, and click on Auto Record. The recording and decoding will begin when the satellite appears on your horizon, and stop when it goes out of view according to the times in the satellite pass list.

1. Open SDRSharp select the audio piping method you are using under the Audio Output drop down box and then tune to the frequency that the satellite will be broadcasting at. Adjust the gain settings in SDRSharp under the Configure button so that you get good reception of the signal. Set the receive mode to WFM, filter bandwidth to 34 kHz and Filter Audio set to OFF. It may also be useful to ensure Snap to Grid is unchecked.
1. As the RTL-SDR is not frequency accurate, and also due to the Doppler effect, the signal may not be at the exact frequency it should be at. Just adjust the frequency in SDRSharp until it is centered on the satellite signal. You may also increase the filter bandwidth beyond 34 kHz if there are no nearby interfering signals to cover the entire travel of the signal.
1. Adjust the volume in SDRSharp and/or Windows volume settings so that the volume bar in the bottom right hand corner of WXtoImg shows a green color.

WXtoImg should now be decoding and showing the weather satellite image as it is received. You may need to periodically adjust the frequency to center the signal as the Doppler effect will cause it to move. But, with the RTL-SDR adjusting for the Doppler shift is not critical as the filter bandwidth can be simply set larger than 34 kHz (try 36 -40 kHz) so that it is large enough to receive the entire signal even as it as it shifts.

Once the image has been fully received, you can play with the options under the Enhancements and Projection menu in order to add false color and enhance the received image.

### Orbitron Tutorial

It is not entirely necessary for these NOAA satellites, but if you want the Doppler effect to be automatically adjusted for in SDRSharp or you want to automatically record all satellite passes then you can use free a program called Orbitron, which with the aid of a plugin, will interface with SDRSharp.

1. Download the SDR# Orbitron DDE tracking and scheduler plugin from here.
1. Extract all the plugin files to the SDR# directory. With notepad or another text editor open Plugins.xml. Within the <sharpPlugins> </sharpPlugins> tags add the line <add key="DDE Tracking Client" value="SDRSharp.DDETracker.DdeTrackingPlugin,SDRSharp.DDETracker" /> at the end.
1. Open Orbitron in Administrator Mode (if in Windows Vista/7/8), by right clicking it, and selecting Run as Administrator. Orbitron may open in full screen mode. Press Alt+Enter to exit full screen if you wish.You will probably be initially presented with a TLE file update screen. You can leave all the boxes as default. Click on the update button, which is the icon with a globe and lightning bolt. Orbitron will download the new TLE files. The TLE files contain the satellite orbit information, and will need to be periodically updated every few days. Running Orbitron in Administrator mode is important, as otherwise the updated TLE files will not be saved.

1. In order to have Orbitron accurately track the satellites it is important that your Windows PC time is accurate. Orbitron comes with a method to synchronize your PC time to the NTP servers, which provide accurate time. In the setup screen click on the Time Synch tab, and click on the Synchronize PC clock button (looks like a lightning bolt) to automatically synchronize the time. You may also wish to select the Synchronize PC clock when Orbitron starts checkbox if your PC is always connected to the internet.
1. Close Orbiton. Now open Notepad in Administrator mode, by right clicking its shortcut in the Start Menu, and clicking on Run as Administrator.
1. In Notepad, go to File->Open, and browse to your OrbitronConfig folder. The full path is probably probably installed in “Program Files (x86)OrbitronConfig”. Open Setup.cfg.
1. At the bottom of the Setup.cfg text file, add these two lines, making note that you should specify the path to your own local install of SDR#. Here we assume you’ve installed SDR# to C:sdrsharp.

1. Now open Orbitron and in the main tab set the refresh interval to 1 second. This is the drop down box in the lower right of the panel.
1. Set your home location by clicking the location tab on the bottom. You can select your city on the right side if you don’t know your exact longitude and latitude.

1. Next click on Load TLE and load the noaa.txt file, or the file for whatever other satellite you are interested in tracking.

1. For NOAA weather satellite images we are interested in NOAA satellites 15, 18 and 19, as they are the only satellites working, so place a check next to those. Double clicking on a satellite name will select it and show it in the map window.
1. For each satellite enter the correct downlink frequency under Dnlink/MHz, e.g. for NOAA 15 enter 137.62 MHz. The corrected doppler frequency will be automatically calculated.
1. Go to the Main Tab in Orbitron and click on the Setup button (looks like a crossed hammer and spanner).
1. Go to the Miscellaneous tab and ensure that AOS Notification: Show Notice is selected, with the elevation set to 0. (Increase the elevation if you only want to start tuning to the frequency when the satellite is higher in the sky and thus gives you better reception).
1. Go to the Extra tab and ensure that “AOS Notification: Make satellite” active is checked.
1. In SDR# ensure you have set the PPM offset correction properly and then press Play and go to the Tracking DDE Client plugin. Click on the Config button.
1. Here we need to set up the scheduler instructions. First enter the name of the satellite you want to track in the Satellite name text box, making sure to replace any spaces in the name with underscores. For example if you wanted to track NOAA 15, then you’d set the Satellite name to NOAA_15.
1. When a satellite comes into view the scheduler will automatically run the commands written in the AOS text box in the scheduler. When it leaves view it will run the commands in the LOS box. Under AOS using the available commands and the two left arrow buttons (<<) add the commands “radio_modulation_type<WFM>”, “radio_bandwidth_Hz<38000>” and “radio_tracking_frequency_On” to the top AOS box with each command on a seperate line. Also add the command “radio_tracking_frequency_Off” to the bottom LOS box. This will ensure that the correct modulation and bandwidth is automatically set as well as tuning the frequency to the Receiver/doppler frequency specified by Orbitron. Close the Scheduler configuration box.

1. Next ensure that the scheduler is enabled by checking the Scheduler box in the plugin.
1. Now back in Orbitron go to the Rotor/Radio tab, and set the “Dnlink” mode to FM-W and the Driver to SDRSharp. Click the icon with two windows next to the Driver drop down box and make sure it is pressed in. If a box pops up saying it could not find the driver then you may have specified the path in step 8 incorrectly. Click Yes and then select the SDRSharpDriverDDE.exe file in the SDR# folder.
1. In SDR# the Tracking DDE Client plugin should now show that Orbitron is connected and information about the currently selected satellite in Orbitron will show in the plugin. When a satellite appears overhead the frequency will immediately snap to the doppler frequency specified by Orbitron.

1. Finally, if desired WXtoImg can be made to automatically output a live webpage of the latest weather satellite images. This option can be found in WXtoImg under Options->Auto Processing Options->Web Page Settings.
• An LNA such as this or this may improve signal reception, especially if you run a long coax feed line from the antenna to the dongle.
• Ensure that your antenna has a good unobstructed view of the sky.
• You probably won't get very good results without a proper satellite antenna such as a QFH or turnstile.

## Part 1: How Many Alien Probes Could Have Come From Stars Passing By Earth?

1. Searching for Extraterrestrial Artifacts

Alien astronomy at our present technical level may have detected our biosphere many millions of years ago. The Great Oxidation Event occurred around 2.4 billion years ago it was a rise in oxygen as a waste product due to organisms in the ocean carrying out photosynthesis. Long-lived robotic probes could have been sent to observe Earth long ago. I will call such a probe a “Lurker,” a hidden, unknown and unnoticed observing probe, likely robotic. They could be sent here by civilizations on planets as their stars pass nearby.

Long-lived alien societies may do this to gather science for the larger communicating societies in our Galaxy. The great virtue of searching for Lurkers is their lingering endurance in space, long after they go dead.

Here, in part 1, I estimate how many such probes could have come here. This is explained in detail in [1].

In Part 2, titled ‘A Drake Equation for Alien Artifacts’, I propose a version of the Drake Equation to include searching for alien artifacts that may be located on Moon, Earth Trojans and co-orbital objects [1]. I compare a Search for Extraterrestrial Artifacts (SETA) strategy of exploring near Earth for artifacts to the conventional listening-to-stars SETI strategy.

1.1 Observing Earth

From Figure 1, the time over which our biosphere has been observable from great distances, perhaps thousands of light years, due to oxygen in the atmosphere, is a very long time, measured in the billions of years [7,8]. The first oxidation event occurred about 2 .5 billion years ago and the second, largest oxidation event about 0.65 billion years ago, so 0.65 10 9 < TL <2.5 10 9 years.

An ET civilization that passes nearby can see there’s an ecosystem here, due to the out-of-equilibrium atmosphere. They could send interstellar probes to investigate.

Figure 1. History of Oxygen content of Earth’s atmosphere is observable from great distances. Dashed line is present value. Horizontal axis is in millions of years before present. (Wikipedia Commons)

2. How Often Do Stars Pass By Our Sun?

It is not widely known that stars pass close to our solar system. The most recent encounter was Scholz’s Star, which came 0.82 light-years from the Sun about 70,000 years ago [3]. A star is expected to pass through the Oort Cloud every 100,000 years or so, as Scholz’s Star did, shown in Figure 2.

Bailer-Jones et al. showed that the number of stars passing within a given distance R, NS (R), scales as the square of that distance [4]. This comes about because Earth is in a flow of stars circling the galactic center, so the cross-sectional area is what matters, which gives an R 2 scaling, rather than the volume,

R 3 . Figure 3 shows that several stars have approached or will approach our solar system over 10 5 years.

Figure 2. Our most recent visitor: Scholz’s Star came within 0.82 light-years from the Sun about 70,000 years ago (NASA).

Bailer-Jones et al., using accurate 3D spatial and 3D velocity data for millions of stars from the Second Gaia Data Release has shown that a new passing star comes within one light year of our Sun every half million years, 100 within 10 light years [4].

With the number of stars passing within a given distance, NS (R), and R the distance of the star from the Sun in light years, the rate of passing stars is:

So a new star comes within 10 ly every 5,000 years [3]: during our 10,000-year agricultural civilization, two new stars have come within 10 ly.

Figure 3. Stars come very close to Earth frequently. About 2 stars come within a light year every million years. An ET civilization that passes nearby can see there’s an ecosystem here, due to the out-of-equilibrium atmosphere. They could send interstellar probes to investigate. (stackexchange.com)

3. How Many Lurkers May Have Come Here?

To calculate the number of Lurkers that could be located at various sites nearby to Earth, such as the Moon, Earth Trojan zone or the co-orbitals, I make the following estimates. The quantities to use in calculating this concept are shown in Table 1.

There are two factors to evaluate: 1) How often do stars get within a given range of Earth? 2) How long would a Lurker reside in a given location near Earth?

Of course, a key factor we do not know is what fraction of the stars have spacefaring civilizations.

Table 1 Passing Stars Parameters

The number of Lurkers that could arrive and now be found, NL, would be fip times TL, the orbital lifetime of the object upon which the Lurker is resident, times the passing star rate, [dNS(R)/dt] from Eq. 1:

We don’t know fip, but we can calculate the ratio

Now we make estimates of NL/fip. Details of these estimates below can be found in [1].

4.0 Locations for Lurkers Near Earth

The time that Lurkers would be in the solar system, TL, will be limited by the lifetime of the orbits they are in. That is determined by the stability of the orbit of the near-Earth object it lands on. This provides an upper bound to how long they could be around. The Moon, Earth Trojans and co-orbitals of Earth lifetimes are:

4.1 The Moon

Searching on the Moon has recently been advocated [5, 6]. Our Moon is thought to have formed about 4.5 billion years ago, long before life appeared. Then the Earth ecosystem would not attract attention. Later, life became evident in our atmosphere.

We have had the Lunar Reconnaissance Orbiter in low orbit around the Moon since 2009. It has photographed about 2 million sites at sub-meter resolutions. We can see where Neil Armstrong walked! The vast majority of these photos have not been inspected by the human eye. Davies and Wagner have proposed searching these millions of photographs for alien artifacts, which would require an AI for initial surveys [5]. Development of such an AI is a low-cost initial activity for finding alien artifacts on the Moon, as well as Earth Trojans and the Earth co-orbitals. A recent AI analysis of 2 million images from LRO revealed rockfalls over many regions of the Moon [9]. So we have proof a search for artifacts of

1-meter scale could be done by AI.

Figure 4 The Apollo 17 site as seen by the Lunar Reconnaissance Orbiter. Note that Moonbuggy tracks can be clearly seen. A study of the >2 million such photos could detect possible artifacts on the Moon (NASA).

4.2 Earth Trojans

Figure 5 shows the many Jupiter Trojans, located at stable Lagrange Points near that planet. There may be many such objects in the Earth Trojan region [11],

60 degrees ahead of and following Earth. Their lifetime is likely to be on the order of billions of years, and some objects there may be primordial, meaning that they are as old as the Solar System, because of their very stable Lagrange Point orbits [11-14].

Figure 6 shows a portion of the orbit of the only Earth Trojan found so far, 2010 TK7. It oscillates about the Sun–Earth L4 Lagrange Point,

60 degrees ahead of Earth [15]. Its closest approach to Earth is about 70 times the Earth-Moon distance. It is not a primordial Earth Trojan and is estimated to have an orbital lifetime of 250,000 years, when it will go into a horseshoe orbit about the sun. It is clear why there are no other Trojans of the Earth yet found: they are hard to observe from Earth.

There are large stable regions at Lagrange Points, so Trojans may exist for long time scales. It is possible that primordial Earth Trojans exist in the very stable regions around the Lagrange Points. Orbital calculations show that the most stable orbits reside at inclinations <10° to the ecliptic there they may survive the age of the solar system, so again we use the oxygen time,

2.5 Gyr. So Trojans’ orbital lifetimes can vary from 2 10 5 years to 2.5 10 9 years.

Figure 5. The many Jupiter Trojans, which lead and follow the planet at

Figure 6. Portion of the orbit of the one Earth Trojan found so far, 2010 TK7. (NASA)

4.3 Earth Co-orbitals

See [16] for a discussion of the co-orbitals of Earth. A large number of tadpole, horseshoe and quasi-satellites that approach near to Earth appear to be long-term stable. Figure 7 shows to orbit of the nearest one, 2016 HO3. Morais and Morbidelli, using models of main asteroid belt sources providing the co-orbitals and their subsequent motions, estimate lifetimes to run between 1 thousand and 1 million years. They conclude that the mean lifetime for them to maintain resonance with Earth is 0.33 million years (17).

Figure 7. Orbits around the Sun of Earth and the nearby quasi-satellite 2016 HO3. It comes within 5 million km of Earth (NASA).

5. Conclusions

In [1] the above remarks are quantified. Here I summarize the calculations in the Table, for probes traveling from 10 ly and 100 ly. (Note that, since co-orbitals have a finite lifetime on their orbits near Earth, Table 2 refers to this is the number of probes that may have landed on what was at the time a co-orbital but will now have wandered off somewhere.)

Table 2: NL/fip: The number of Lurkers, from stars that pass by our Solar System that could have arrived and now could be found, for several nearby astronomical bodies, divided by fip, the fraction of stars that have civilizations that develop interstellar probe technology and launch them.

• Clearly, the Moon and the Earth Trojans have a greater probability of success than the co-orbitals.
• Of course, fip is the factor we don’t know: how many civilizations develop interstellar probe technology and launch them.
• The great virtue of searching for Lurkers is their lingering endurance in space, long after they go dead.
• Close inspection of bodies in these regions, which may hold primordial remnants of our early solar system, yields concrete astronomical research. It will yield new astronomy and astrophysics, quite apart from finding Lurkers.
• A suggestion for SETI observers: Look at the specific stars that have passed our way in the last 10 million years and ask how many of them are ‘sunlike’ and/or are known to have habitable planets. Observe those stars closely for possible emissions to Earth [16].

For discussion of approaches to study these objects, starting with passive observations, and going on to missions to them, see Reference 14, section 4, “SETI Searches of Co-orbitals”. The actions and observations are:

1. Launch robotic probes and manned missions to conduct inspections, take samples.

2. Conduct passive SETI observations.

3. Use active planetary radar to investigate the properties of these objects

4. Conduct active simultaneous planetary radar ‘painting’ and SETI listening of these objects.

5. Launch robotic probes and manned missions to conduct inspections, take samples.

This argues for a Search for Extraterrestrial Artifacts (SETA) strategy of exploring near Earth for alien artifacts [2].

1. J. Benford, “How Many Alien Probes Could Have Come From Stars Passing By Earth?”, JBIS 74 76-80, 2021.

2. J. Benford, “A Drake Equation for Alien Artifacts“, Astrobiology 21, 2021.

3. E. Mamajek et al, “The Closest Known Flyby Of A Star To The Solar System” ApJ Lett., 8003 L17, 2015.

4. C. A. L. Bailer-Jones et al, “New Stellar Encounters Discovered in the Second Gaia Data Release”, Astronomy & Astrophysics 616 A37, 2018.

5. P.C.W. Davies, R.V. Wagner, “Searching for Alien Artifacts on the Moon”, Acta Astronautica, doi:10.1016/j.actaastro.2011.10.022, 2011.

6. A. Lesnikowski, L. Bickel and D. Angerhausen, “Unsupervised Distribution Learning for Lunar Surface Anomaly Detection”, arXiv:2001.04634. 2020.

7. X. L. Kaltenegger, Z. Lin and J. Madden, ““High-resolution Transmission Spectra of Earth Through Geological Time”, Astroph. Lett., 2041, 2020.

8. Y. V. S. Meadows et al., “Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment“, Astrobiology 18, 620, 2018.

9. V. Bickel V. et al., 2020 Impacts drive lunar rockfalls over billions of years, Nature Communications, 11:2862 | https://doi.org/10.1038/s41467-020-16653-3

10. R. Malhotra, “Case for a Deep Search for Earth’s Trojan Asteroids”, Nature Astronomy 3, 193, 2019.

11. M, Ćuk, D. Hamilton and M. Holman, “Long-term stability of horseshoe orbits”, Monthly Notices Royal Astronomical Society, 426, 3051, 2012.

12. F. Marzari, H. Scholl, “Long term stability of Earth Trojans”, Celestial Mechanics and Dynamical Astronomy, 117, 91, 2013.

13. Zhou, Lei Xu, Yang-Bo Zhou, Li-Yong Dvorak, Rudolf Li, Jian, “Orbital Stability of Earth Trojans”, Astronomy & Astrophysics, 622, 14, 2019.

14. R. Dvorak, C. Lhotka, L. Zhou, “The orbit of 2010 TK7. Possible regions of stability for other Earth Trojan asteroids”, Astronomy & Astrophysics, 541, 2012.

15. P. Wiegert, K. A. Innanen and S. Mikkola, “An Asteroidal Companion to the Earth”, Nature, 387, 685, 1997.

16. J. Benford, “Looking for Lurkers: Objects Co-orbital with Earth as SETI Observables”, AsJ, 158:150, 2019.

17. M. Morais and A. Morbidelli, ‘The Population-of Near-Earth Asteroids in Co-orbital Motion with the Earth”, Icarus 160, 1, 2002.

Hi
I am a common layman.
Just curious, can we develop some technology so that we can use the gravitational waves or other cosmics waves or neutrinos to drive our space crafts.

While the search for lurkers seems attractive, I find the idea of passing stars a red herring. The frequency of “close” passes and the unknown density of technological civilizations means that if sparse, the passing stars are likely uninhabited. As we are already considering interstellar craft, whether beamed sails or more exotic propulsion technologies, it seems to me that any technological species will simply send out their probes as fast as possible and live with the speed of light limitations if no FTL communication or flight is possible once telescopic observations have indicated likely target worlds. Waiting thousands of years to pass by a star, which may or may not have life, seems like an unlikely scenario, especially if probes can travel at decent fractions of c.

I am sure that some simple math would indicate at what density ETI must exist to make close passes a better strategy than just sending out probes for the distances required. My intuition is that this density is high. So high that we might just detect them telescopically as soon as we look with instruments of sufficient resolution and capabilities.

If lurkers are deliberately camouflaged, like terrestrial hides, I suspect they would be hard to detect. They might look like rocks, or be buried in the lunar regolith. They might look like asteroids if lurking in a gravitational stable/metastable position. Or they might be stealthed in other ways. For example, we already have radar stealthed aircraft that if left at the L5 point would be extraordinarily hard to detect either by radar or optical means. If the lurker is dead and inert, it certainly cannot respond to signals, and therefore requires a painstaking search.

As Douglas Adams said: “Space is big, really big…” The solar system, even from the asteroid belt inwards would take a very long time to search. It would make those police lines to search for evidence across miles of the countryside for evidence seem trivial by comparison.

Let’s hope that any probes want to be found and make themselves conspicuous even as inert objects. A large magnetic field seems an obvious choice…

I don’t think the premise here is that implausible. It is possible that technological civilizations manage to destroy themselves before they have launched probes to more than a dozen systems – probes that seem ambitious enough merely in reaching an orbit in a nearby star system, without many finer considerations of camouflage.

A bit of an aside here but we have been discussing in a previous post whether or not the Oort Cloud actually exists. Could Scholz’s star have removed most of the Oort cloud objects as it passed through?

What is the best compilation you could find of stars that have passed near Earth? Wikipedia writers use a chart based on “The Close Approach of Stars in the Solar Neighborhood”, Quarterly Journal of the Royal Astronomical Society, volume 35, 1994 … which might as well be the Paleolithic compared to modern astronomical resources.

Besides probe-builders are checking up on their work, a long history of passing stars might help us understand why light pollution does such little damage to the ecology. Not that it isn’t bad enough already, but I would think that if we hadn’t spent a few millennia with some overly bright star in the sky every now and then, many organisms would be much more hard-wired in their expectations for night.

I should thank James Benford for coming back and responding below! It was Ref. 4 and sources citing it. I see why he used the older illustration though – Ref. 4 does a fair amount of ‘fearing to tread’, presenting a table with daggers to indicate which entries are particularly likely to be bogus. Sometimes it is nice to have a canon you can shoot something with, even if you have doubts about its accuracy. In any case, the paper ( https://arxiv.org/abs/1805.07581 ) develops a model with perihelia fairly randomly arranged in the plane, so within a certain distance the odds are proportional to the inverse radius squared: encounter rates of 78.6 +- 8.7 per Myr within 2 pc, and 19.7 +- 2.2 per Myr within 1 pc.

The authors didn’t say this, but I suppose if you wait a million years and something should come within half a light year – wait four, and cut that in half again. (As it happens, we are nearly into the lucky leap megayear – see https://en.wikipedia.org/wiki/Gliese_710 ) The above is true if you can assume the encounter rate is constant, but they are at 1.5 sigma for a change. Ignoring that, I get that sometime in Earth’s history a star should have passed within 500 AU. Must have been a show!

A 2019 paper working from this base ( https://arxiv.org/pdf/1911.01735.pdf ) points to C/2002 A3 as a comet perturbed by the passage of HD 7977, a G0 dwarf (1.1 solar masses), passing 1.3 light years away, 2.8 million years ago. A comet that nominally had an eccentricity of 0.25, before the star passed within 740 AU of it!

“The great virtue of searching for Lurkers is their lingering endurance in space, long after they go dead.”

I disagree. If these probes are indeed Lurkers there will be steps taken to prevent detection. Like our own geosync satellites that approach their normal end of life, the last of their fuel is used to eject them from their orbit to place them safely out of the way.

Of course this assumes they don’t want the probes to be found. Or perhaps they don’t care because they see no risk to themselves from accidental discovery of the probes.

As is usual with all such questions there is an overabundance of assumptions, and that is likely to persist for quite some time. The questions are nevertheless interesting even when they are not immediately addressable.

Titan has fuel-and I still half expect front-end loaders on Miranda

Sans O₂, hydrocarbons ain’t fuel – but in an atmosphere of hydrocarbons, a tank of O₂ is fuel.

(1) If the “lurker” is sent to do scientific investigation of Earth, it will need an orbiter in low polar orbit for global mapping and one or more landers to gather ground truth, including the most pressing science for any investigator interested in life, which will be analysing the DNA of living organisms. Why then do you want to examine the Moon and the Earth-Sun trojan orbits?

(2) If an alien civilisation was interested enough in the Solar System to do scientific investigation here, what was there to prevent it coming here and colonising the system? Note that natural selection will preferentially populate passing stars with colonising cultures rather than sedentary ones.

Satellites in orbit may have limited lifetimes. Probes placed on the surface of the Earth might corrode away unless made of noble metals such as Au or Pt.

A thought experiment – if there was a star system passing by about 1 light year from us, would we be talking about sending something crewed there? If so, if there is such a thing as an advanced civilization out there, maybe they think the same way? If they have, there might be a few star systems around that might not harbour habitable planets in the usual sense, but there might still be ‘civilized worlds’ present in the system, one way or the other. And they might not be so much in a hurry to reach really distant star systems from theirs, as settling just one might take some time. So they simply take their time and use the opportunities as they present themselves. Anyway – if so, we could be looking at star systems that might not be so habitable, and they might have advanced cicilization still.

With stone age technology a species evolved for tropical rainforests and perhaps temperate grasslands – Homo sapiens – adapted to every climate but did not inhabit the antarctic, perhaps due to remoteness and a lack of megafauna.

With different molecular machinery, different evolution and different technology, the uninhabitable might be habitable.

As far as lurkers are concerned, I think they are not worth spending resources on. With all due respect if advanced ET’s want us to find an artifact they will make it abundantly easy to do so and if they don’t we won’t find anything. A crash site of some type or accidental loss of an alien probe that we then find is entirely different and probably doesn’t qualify as a lurker, although we could end up splitting hairs about that. If there are such things and we end up finding one it will be extremely serendipitous and not to be planned on.

The Moon survey seems inexpensive to me. Software to scan the images of the surface and flagging possible anomalous objects. At some point, we will have similar large archives of high-resolution images of asteroids and other celestial objects that we can scan and analyze automatically.

Surely the point is that if we don’t even look, we cannot find anything. I don’t think we need to initiate an expensive search program, but we should piggyback artificial object detection on existing missions, much as SETI piggybacked on radiotelescope observations to look for ETI signals.

Actually that seems right Alex. Here I am participating in Planet Hunters TESS and not thinking about how such a resource could be used to cheaply do a Moon survey looking for lurkers. My bad. :)

And if we actually found a lurker on the moon it would justify NASA’s current “make work” project to land a woman on the moon in the next few years.

At the risk of biting my own tail:
Assume that NL is greater than or equal to 1, because if it isn’t we may find lots of neat stuff, but we won’t find a lurker.
This implies that Fip is greater than or equal to 1/500,000 to 1/130,00 i.e. 2e-6 to 7.7e-6.
Given the geologic time scales involved these don’t seem unreasonable, but the question remains: How many technological space fairing civilizations would do this, that is, what is F(prime directive)?

Lurkers placed on the moon might be located to give an unrestricted view of the earth and a communications line-of-sight to their home base or relay station away from earth, on a plane perpendicular to the moon’s orbital plane, and tangential to the moon at the earth-moon axis. Such an area on the moon at a fair elevation could be of particular interest.

SETA is wonderful direction of research. IMO, finding artifacts is by far the most probable way to break the state of knowledge about ETI. And we don’t need to consider “they are hiding”. While it seems reasonable that any exploratory mission would obey some codes (like non-contamination), it is very unlikely that all explorers intend to camouflage their missions and that indeed all of them _succeeded_ in that. An opposite assumption just doesn’t pass Occam’s Razor. We won’t find Lurkers, by their design, but the vast majority of ETAs would be long-dead and inert space debris. So the most straightforward way to estimate the probability of finding ETAs is just to estimate the total accumulated mass of artificial material in an “interesting” stellar system. It depends on many unknowns but impressive numbers can result even from assumption that civilizations use means that are surely feasible, and are constrained by them. Like, they use gravitational lensing to study systems and identify interesting ones, but they can study xenobiology only in-situ and use thermonuclear propulsion to send probes. If any habitable system receives an expedition in ten million years, corresponding to appearance of a civilization with explorative lifespan of 10000 years within 50 light-years every 10 MYr during the last 4 Gyr, then I guess the total mass of ETAs in the Solar system is in the range of thousands to billions of tons. Of course, Archaean Earth was nothing to look at, but it was not alone. There was early warm and wet Mars and still-not-boiled-dry Venus – three worlds that looked habitable from afar instead of the current single one. That does not count possible extinct or abandoned colonies.

Of course, it is the needle-in-haystack task, but at least the needles and the haystack are not many parsecs away! They are within reach of observation and exploration ranges of our current and near-future instruments. On decameter-scale, LSST or another survey mission could look for objects in the asteroid belt with unusual light curves, suggesting non-natural shape. On cm-scale, as it was said in the article, an automated lunar orbiter can continously take images of surface and look for something weird by an on-board neural (calibrated and/or validated by Apollo and other landing sites), sending images and coordinates to Earth for futher inspection if something shows above threshold. (just occured to me, any object that emits it’s own light in the lunar night would be quite interesting and very easy to detect, though living probes are much rarer thing and a whole another cause). A fleet of small solar-sail or electric powered satellites can disperse in the asteroid belt, wandering from one object to another, scrutinizing them for scientific and mining purposes and looking for something unusual on their surfaces or in their shapes.

And all of this is near-term even compared to Breakthrough Starshot, and profoundly more promising than search for distant technosignatures. Even if we find CFCs in transit spectrum of earthlike world, or identify solid-state laser emission from a cloud around a distant star, we won’t be able to do anything about it in centuries, but even a single 2 GYr-old piece of alien junk from asteroid belt is a treasure trove in our hands.

To be able to develop a space travel, an alien civilization
should develop one feature called “creativity”. Otherwise
it would be just an animal kingdom. How long they
could stay in this state nobody knows. ..might be billions
of years. We cannot say what sparks creativity either.

“Far out in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy lies a small unregarded yellow sun. Orbiting this at a distance of roughly ninety-two million miles is an utterly insignificant little blue green planet whose ape-descended life forms are so amazingly primitive that they still think digital watches are a pretty neat idea.”

― Douglas Adams, The Hitchhiker’s Guide to the Galaxy

After a few hundred years of industrial civilization, we seem to think manufactured space craft with semi-smart software are a pretty neat idea. Which makes us think other civilizations will send Bracewell probes (“lurkers”) to our system. We think up ingenious ideas to reduce the effort needed, as well as what assumptions are needed to detect live or dead ones.

However, the longest living “artifact” is staring us in the face – life iteself. It has a near 4 billion year history of replication that has withstood at least 5 major catastrophes, but each time created new phenotypes that have increased the richness of the terrestrial bisophere. It has even managed to evolve a species that can consider sending machines and life to other star systems.

As far as we know, all terrestrial life has a single common ancestor, there is no “shadow life” detected. This implies that life either originated on Earth, or if seeded from elsewhere, was done once only, and never repeated. The only counterargument I can see that would allow other seedings is that all life in the galaxy is based on the same fundamental biology, or that other civilizations know of our type of biology and send only compatible seeds, and that subsequent seedings just integrate with the existing biology.

If we become a stable civilization that can live sustainably for millions of years, it might make sense to seed other, sterile worlds, with terrestrial life that can be sent as dormant spores, eggs, and seeds, and let evolution do its work to evolve new forms and build rich biospheres. We could start in the relatively near future, adapting whatever advanced propulsion technologies we have to deliver payloads of dormant life to worlds ready to be “greened”. Such a program won’t be a simple as sprinkling some bacteria to start a culture, as we may want to add multicellular life too, and that may require some effort where species need to be co-existing to survive.

Lastely, let me return to the idea of multiple seeds of Earth but with compatible biology. Imagine a galactic library with catalogs of worlds and their basic biology, wrested by long ages of exploration by probes. A civilization could select the biology needed for Earth and send some “firmware” updates to enhance the existing life. How might this be done? Using retroviruses that can insert their new instruction sets into the DNA of host species. Hoyle may have been wrong that comets deliver viral epidemics, but just maybe deliberate introduction of viruses into the terrestrial biosphere is possible, to push evolution by “punctuated equilibrium” and shorten the time needed to “uplift” a species to join the galactic club.

The transition from ape brain and intelligence (chimpanzee IQ

60, subsaharan human IQ 80+) is a phenomenon that has not been adequately explained. Perhaps a firmware upgrading from afar as suggested by Alex Tolley did the trick.

“The Runaway Brain” by Christopher Wells does a pretty good job of explaining the prehistoric growth in human intelligence without the need for aliens.

We don’t need to go to the stars. We could just seed Venus with only slight advances in our technology.

The other side of the coin is that such objects are common and would destroy themselves so as not to give information to immature cultures such as ours. Perhaps all those iron/nickel objects that we keep finding falling from the sky are the smelted remains of ET’s lurkers.

I don’t think any advanced technological civilization would bother to send physical artifacts. What for? A space-based telescope with an aperture of 150,000 km or so could resolve the Solar System down to 10cm pixels at a distance of 1 ly. That’s certainly a BIG telescope, but (1) building one is probably not a lot harder than building a bunch of interstellar probes (and the scopes to receive their transmissions), and (2) you only need to build ONE, and then you can use it for all kinds of things, and all kinds of observations. Plus you keep your technology right next to you and it’s easy to repair or upgrade.

Why do we build probes to inspect other worlds in our system. There are so many things that require physical contact to understand. For example, you cannot find out much about biology just by looking at a planet through a telescope. You need samples to dissect and analyze.

Carl, that is an excellent point!

Building a crazy big telescope has gotta be orders of magnitude easier than pursuing interstellar propulsion (….with travel times in a reasonable time frame…. lets just say 100 years to Proxima).

>The vast majority of these photos have not been inspected
>by the human eye. …develop AI algorithm… …low cost

What about making a Zooniverse project (https://www.zooniverse.org/) out of them? Then we would not only be talking “low cost”, but virtually “no cost”. I’d be happy to look through a couple of photos per day, and I’m guessing others would be too. The Zooniverse user base is no laughing matter, and many “comparable” photo-based project are available right now.

Having those Moon photos collecting dust seems like the wrong thing to do.

Is there anyone looking for or planning to look for signs of Lurkers phoning home? My assumption is that Lurkers would lie dormant and either periodically phone home or phone home if they encounter technology like a radar scan of the trojan they are sitting on or nuclear weapons test or use. Is there any way we could look back and see if there was any evidence of such activity coincident with North Korea underground testing? A one time signal would be ignored. But if a signal was picked up twice – each time following a underground detonation?

Searching the lunar surface imagery database is a fairly cheap thing to do, but what do we tell the computer to look for? Training the deep learning system with images of our own hardware on the surface induces a bias “I don’t know what that thing is, but it doesn’t look like a LEM descent stage or a Lunakhod, so I’ll keep looking”…

Lurkers may not even be hidden. Non-interference/staying out of sight are OUR cultural values, they may not necessarily apply to visiting aliens. Especially if, at the time of the visit/observation, there’s no apparent threat from we Earthlings.

To the first part of your post Steve, we could use volunteer citizen scientists to manually look at millions of images of the Moon just as we do when hunting for exoplanets. I don’t have all the answers as to how it would be set up but I would imagine looking for any object with geometric shapes such as right angles, perfect circles or spheres, antenna-like structures etc. etc. I would gladly participate. :)

What about the dyson swarm out in the Kuiper belt? Can we see that, if it existed?

The answer: 0 to essentially infinity. As our understanding of the evolution of life on Earth increases, and assuming that this is the prime template for the evolution of complex organism, the likelihood d of finding intelligent life on other planets markedly decreases.

More practically speaking: if there is an advanced civilization that can find a way to traverse the universe in real time, we had better hope they are friendly.

As with so many other issues aired on this forum, the question we should be discussing here is not whether or not this scenario is possible,
(it certainly is), but whether or not its existence or resolution is probable.

The Solar System is a very big place, and on astronomical time scales it is constantly changing and evolving, usually in locally destructive circumstances. Even with fast and cheap technology, it would take eons to explore it thoroughly. Unless a Lurker has been deliberately designed to be long-lived and easily found, its ruins or derelict is not likely to be discovered by our level of technology. It it has survived, it is probably deep inside a planetary crust, at the bottom of an ocean, encased under kilometers of ice, or drifting in orbit billions of kilometers away in the cold and dark, indistinguishable at a distance from any of an infinity of other lumps of ice, metal or rock.

Any search for these guys should be based on these realities, and hopefully piggy-backed onto some other research program with a higher probability of success.

We need a space program because one day we will likely stumble onto just such a discovery of immense practical, scientific, or philosophical value. But we should not be planning our explorations with the idea of turning any one of them up specifically.

It would be counter-productive and a squandering of resources. And worse, to our impatient species, it would provide us within a very short time an excuse to cease all our explorations. The Fermi Paradox comes to mind. Is it a valid point? Certainly. Is it a good reason, after less than a century of cursory and superficial searches, to cease searching altogether? Certainly not.

This is needles in haystacks where there are many large haystacks and there may be no needles. If there are needles there is no reliable reason to believe they are hanging out in haystacks. But, all we can see is haystacks and so…

“piggy-backed onto some other research program”

…that’s about the best to be hoped for under the circumstances.

Clearly looking for alien artifacts in the region of the solar system near Earth is a credible alternative approach, a strategy of ETI archeology. The formulation that will be given here on Tuesday is a way of discussing the SETA strategy and comparing it to SETI. I maintain that SETA is a credible strategy.

Many suggestions here make a false assumption: assuming that all aliens act the same way and that you know what that way is. Alex and others should have a look in the Drake equation paper, reference 2, a version of which will appear on this site on Tuesday 20th April. You’ll see a number of scenarios worked out using varying assumptions about alien technology and intentions. Then perhaps you can reconsider what you’ve written here.

As for camouflage, as several others as suggested, think of it this way: how many of our probes have had camouflage? Answer: none. Why? Because we don’t care if someone finds it and we don’t want to waste mass on unnecessary items. Bringing camouflage across interstellar distances is unlikely to happen.

Alex appears to be assuming I’m suggesting searching the entire inner solar system. Far from it. I’m proposing specific locations, mostly small objects, which can be inspected with existing technology.

Others say Lurkers would reside close to Earth as is practical, low earth orbit or Geo. They are not attractive possibilities. Low Earth orbit is stable for a time of less than a century, so would be gone soon. Geosynchronous is stable 1000-10,000 years. So both are short-term orbit on the long times we could have been visited for. Moreover, we have a complete map of everything in geo orbit. If there’s something alien there, we already know it. That would’ve leaked out by now so it’s not credible. And landers will be destroyed quickly by Earth’s environment.

Mike Serfas: Consult reference 4 and later citations of it.

Torque xtr: I completely agree!

Steve Muise: Software inspections of LRO images are already being done for rockfalls (reference 8) and for artifacts (reference 5).

Henry Cordova: Studies have shown that the artifacts we have on the Moon will last millions of years.

As for camouflage, as several others as suggested, think of it this way: how many of our probes have had camouflage? Answer: none. Why? Because we don’t care if someone finds it and we don’t want to waste mass on unnecessary items.

As a physicist, you don’t consider that what you study will respond to your experiments with agency. AFAIK, not one of our space probes was expecting to meet with any organism or artifact with agency. Under those circumstances, camouflage is a waste of mass.

However, this changes if you want to study anything with agency. If you have ever fished by a river or lake, you know that you wear drab, brown/green clothing, and stay very still. If you want to observe wildlife up close, you sit in a hide, make no noise, and position the hide downwind. Any probe that is intent on closer study of a living Earth will want to camouflage itself in some way to prevent its presence from being detected by the organisms it wishes to study. If course, if you want to fish with explosives, or in an analogous case the probe can paralyze or kill anything it wants to sample and ignore other study types, then it probably doesn’t need camouflage. [C.f. fictional UFO abductions.] If the camouflage is active, like cephalopod or flatfish skin, then a dead probe will possibly no longer be camouflaged.

Whether a probe is camouflaged or not will depend on its purpose and whether it is still active or not. Given our current engineering technology and your assumption about probes arriving with close approaching stars, the logic is that any visiting probe is long dead and will be an inert artifact. Change those assumptions and the camouflage issue is not moot.

I am very much looking forward to part 2.

I’d like to know how many ort clouds our sun has passed through and if the two systems trade comets and other ort cloud bodies.
The earth is visible as a transiting body for stars lying along the ecliptic. How many nearby stars and for how long? is this a more likely source for contact than passing stars.

Reference 6 is an interesting start. An unsupervised ML algorithm was trained on Apollo 17 images from the high-res LRO images (0.5-1.5 m/pixel). The algorithm was able to detect the Apollo 15 LM descent vehicle from images around that site. Their metric suggests that they can reduce the search requirements for human inspection 50x. Clearly, this is a promising start to do the proposed lunar analysis for lurkers.

Reference 5 is a review of possible technosignature features to look for. Davies suggests both ML to prune the image set, and crowdsourcing human volunteers – albeit that needs work to define what to look for.

There is a website Moon Zoo that crowdsources volunteers to look for specific natural features. Adding features to look for might be bolted onto this site’s search.

As they are searching for any alien artifacts, I would be happy if someone could find Surveyor 4 to determine if it landed intact or exploded in 1967, as contact was lost with it just seconds before it touched down on the lunar surface.

Then we need to find Luna 9 and 13, which will be tough because they may be hard to distinguish from lunar boulders. Not sure if any of the failed members of the early Soviet Luna lander series survived their encounters with the Moon.

I have also wondered if the seismometer encased in a sphere of balsa wood survived the impact of Ranger 4 on the lunar farside in 1962? I mean, that is what it was designed to do. Ranger 3 and 5 both missed the Moon, so they are undoubtedly still intact and circling the Sun.

This paper is a report on a NASA workshop on technosignatures. Section 4.3 pp31-32) deals with alien probes to our system and includes “lurkers”.

Yes, ET could be in “our” backyard. The probability of a 100% explored Milky Way is higher than a 100% colonized. The energy required to explore the galaxy is less then that required to colonize. Colonization would require some amount of prior exploration. If time is less of a concern, the cost to produce 1 pixel to 10 cm resolutions at 1 light year with a probe is less than a telescope. Telescopes could never deliver a probe’s broad spectrum of data. One of the greatest potential threats to a people will be other people. It is rational game play to observe other players. Survive its evolution and mature biotech will allow for self-replicating probes, mass production of non-replicating probes or space ship people

Stealth can not be considered a strictly anthropological motivation. Too many non humans employ stealth. It is used so frequently, it can’t be considered a niche strategy either. Remaining unseen offers many practical benefits. The behavior of the observed isn’t polluted. Once the observation tools are discovered, the observed will potentially end the experiment by interfering with the tools. If you are a space ship person, do you think humans would respect your person hood?

I wouldn’t simplify modeling by assuming all probes sent by a people would be similar. The probe sent to a system with a high likelihood for life or intelligent life could have different capabilities, like stealth. I would even make the case that our existence increases the odds a probe is in the Solar system.

Local probes or space ship people are a safer target for METI. The pro METI arguments that are actually gaslighting, ring true for local observers. Keep the message quiet and there is no risk of revealing our presence or of influencing another people.

Lastly, the possibility a probe could be a person can not be discounted. Any action plan for searching and interacting with probes must deal with the potential.

The novel “Existence” by David Brin offers an interesting take on the variety of probes and motivation possible. It is difficult to discuss the treatment of probes without spoiling. Definitely recommend it.

I’m reminded that the iconic view of interstellar travel and contact in the US is Star Trek. [Semi] military vessels, crewed like warships, with weapons. Crew members on away teams carry hand weapons (phasers) that are used liberally. The starships have shields against weapons but make no attempt to shield themselves from sensors.

In the UK, the iconic series is Doctor Who. The TARDIs is small (on the outside), has few passengers, and the Doctor has a rule about no weapons. A working TARDIS has a chameleon circuit to make the ship blend in with its surroundings. It seems to have broken after selecting a 1950s/1960s police box. The Doctor’s nemesis, the Master, usually travels in a TARDIS that has a working chameleon circuit.

I just wonder if the difference in consideration of probe stealth might not be cultural?

I suspect production budgets are also a reason why one show got a believable starship and one got a telephone booth.

Breakthrough Listen Searched for Signals From Intelligent Civilizations Near the Center of the Milky Way

MAY 2, 2021 BY BRIAN KOBERLEIN

The Breakthrough Listen project has made several attempts to find evidence of alien civilizations through radio astronomy. Its latest effort focuses attention on the center of our galaxy.

The idea behind Breakthrough Listen is that if alien civilizations are out there, they probably emit radio signals either intentionally or unintentionally. Most of their work has focused on observing stars with potentially habitable planets, the idea being that just as we emit radio signals, so do they. But by looking at the center of our galaxy, they’ve begun to search for more ambitious aliens.

The central region of our galaxy is a great place to point your telescope if you want to listen for signals across thousands of stars. It’s the region of the Milky Way where stars are most densely clustered. The downside is that the center of the Milky Way falls outside the galactic habitable zone.

Breakthrough Listen Searches The Crowded Center of the Milky Way for Possible Signals From Intelligent Beings

## Astronomy and Astrophysics in the New Millennium (2001)

### INTRODUCTION

Astronomical discoveries of the past decade&mdashimages of the hot universe at an epoch before the first galaxies and stars emerged, of other solar systems beginning to take form, of planetary systems beyond our own&mdashhave captured the imagination of scientists and citizens alike. These startling advances are the result not only of the collective creative efforts of scientists and engineers throughout the United States and around the world, but also of the generous investments in astronomy over much of the past 50 years by federal and state governments, foundations, and individuals.

In the decades ahead, the pace of discovery&mdashremarkable as it has been over the past&mdashwill accelerate. Astronomers stand poised to examine the epoch when galaxies similar to our Milky Way first took form, to image Earth-like planets beyond our solar system, and to learn whether some show evidence of life. To take these next steps will require significant investments of both imagination and public resources.

Because the magnitude of these investments will be large, it is fair to ask why astronomical research should merit such support. Perhaps the most persuasive, but least quantifiable, justifications lie in the importance American society has always attached to exploring new frontiers, and in the deep human desire to understand how we came to be, the kind of universe we live in, whether we are alone, and what our ultimate fate will be. Exploring frontiers of unimaginable mystery and beauty, astronomy speaks compellingly to these fundamental questions.

As researchers, astronomers experience the excitement of discovery most vividly and are the first to glimpse new answers to ancient questions. As a community of citizens fortunate to live in a society that supports them generously, astronomers believe strongly that &ldquofrom those to whom much is given, much is asked.&rdquo It is in that spirit that the committee offers below an accounting of astronomy&rsquos more tangible contributions to broader societal goals.

### THE ROLE OF ASTRONOMY IN PUBLIC SCIENCE EDUCATION

Astronomers&rsquo most significant contribution to society lies in the area of science education, broadly conceived to include (1) raising public awareness of science, (2) conveying scientific concepts to students at all

levels and to their teachers, and (3) contributing to educating a technically capable and aware citizenry. Astronomy is relevant to each of these goals, and it can act as a pathfinder in stimulating people&rsquos interest in all of science.

#### THE RELEVANCE OF ASTRONOMY

Astronomy excites the imagination. The beauty of the night sky and its rhythms are at once stunning and compelling. The boldness of our collective efforts to comprehend the universe inspires us, while the dimensions of space and time humble us. Astronomy encompasses the full range of natural phenomena&mdashfrom the physics of invisible elementary particles, to the nature of space and time, to biology&mdashthus providing a powerful framework for illustrating the unity of natural phenomena and the evolution of scientific paradigms to explain them. In combination, these qualities make astronomy a valuable tool for raising pubic awareness of science, and for introducing scientific concepts and the process of scientific thinking to students at all levels. A few reminders serve to illustrate the potential of astronomy to advance public science education goals.

Astronomy is all around us. Just look up! Who has not looked at the night sky and wondered at the panoply of stars there? We are all aware of the motion of the Sun through the sky during the day and the changing phases of the Moon at night. The motions of astronomical objects determine the day-night cycle, the seasons of the year, the tides, the timing of eclipses, and the visibility of comets and meteor showers. Easily observed astronomical events have formed the basis for time keeping, navigation, and myths or sagas in cultures around the world.

Much of astronomy is visual and can be appreciated for its aesthetic appeal as well as its illustrative power. Images of deep-sky objects convey the beauty of the universe, even to those who are too young to understand their context or implications.

Astronomy is a participatory science. Many nonscientists have astronomy as a lifelong avocation. Astronomy is one of the few sciences in which amateurs by the tens of thousands have formed active organizations (e.g., the Planetary Society, with membership exceeding 130,000), and many amateurs make significant scientific contributions to such fields as the monitoring of variable stars and measuring positions of moving objects. Telescope and magazine sales suggest that nearly 300,000 citizens take some active interest in amateur astronomy. The American

Astronomical Society has formed a working group to foster partnering between professional and amateur astronomers. Many amateurs freely share their excitement about science with local teachers and students through such programs as Project ASTRO, which links astronomers with 4th through 9th grade teachers and classes in 10 sites around the country.

Astronomy offers the possibility of discovery. The chance to find a never-before-seen supernova, nova, comet, or asteroid is very exciting, especially to nonprofessionals. Both the distribution of astronomical data and software via the Internet and the ready availability of sophisticated imaging devices on moderate-cost small telescopes enable amateur astronomers to play an active and growing role in discovering new objects, searching for transient and variable objects, and monitoring them.

Astronomy inspires work in the arts. From poetry and music to science fiction books and films, the ideas and discoveries of modern astronomy serve as inspiration for artists, for youngsters, and for the public at large. In the process, the works inspired by astronomy can serve as goodwill ambassadors for the value and excitement of physical science to many in society who do not otherwise come into contact with the sciences.

#### CONVEYING ASTRONOMY TO THE PUBLIC

Statistics confirm the widespread interest in astronomy.

Planetariums and observatories are popular visitor destinations. There are approximately 1,100 planetariums in North America. About 30 percent of these serve school groups only, while about 70 percent do both school and public shows. Approximately 28 million visits are made to the planetariums in the United States each year. For many school children from urban areas, such a visit may be their only introduction to a dark night sky and to the wonders of the universe.

Observatory visitor centers are similarly popular. They provide a place where families learn about science together. For example, the seven observatories that belong to the Southwestern Consortium of Observatories for Public Education (McDonald, the National Solar Observatory at Sacramento Peak, Kitt Peak National Observatory, the Very Large Array (VLA), Lowell Observatory, Whipple Observatory, and Apache Point), collectively host more than 500,000 visitors annually and reach more than 4,000 teachers through workshops. The new Visitor Center at Arecibo in Puerto Rico hosts an average of 120,000 visitors each year. Most science

museums have sections on astronomy and hold weekend, evening, and summer programs on astronomical sciences.

Astronomy serves as an introduction to science for nearly 10 percent of all college students&mdashmore than 200,000 each year, nationwide. For many, astronomy will be the only science course they will ever take. To examine and improve the effectiveness of teaching science via introductory astronomy courses&mdashmany of which are offered at community colleges and small colleges without extensive research programs&mdashthe Astronomical Society of the Pacific and the American Astronomical Society are jointly sponsoring a series of symposia and discussions at their meetings. The first such symposium was held in Albuquerque in 1998, and another one entitled &ldquoThe Cosmos in the Classroom&rdquo was held in Pasadena in July 2000.

Discoveries in astronomy are well covered by the media. For example, staff of the New York Times and the Dallas Morning News, two of the leading papers in terms of science coverage, each develop on average more than one astronomy story per week. News conferences of the American Astronomical Society are heavily attended, covered by many news media and often held up as a model by other sciences and scientific organizations. Dozens of astronomy columns now run in newspapers and magazines. Many focus on sky phenomena, while others report on recent developments. Perhaps the best known of these is the regular series of science articles published in Parade, the national Sunday supplement&mdasha series begun by the late Carl Sagan and now continued by David Levy.

Magazines devoted exclusively to astronomy enjoy wide circulation&mdashnearly 300,000 combined for Sky and Telescope and Astronomy. Many other national magazines, such as Popular Science, National Geographic, Discover, and Scientific American, cover astronomy regularly and report that their astronomical stories or issues are among the most popular. It is no coincidence that when Scientific American began a new quarterly magazine devoted to single-topic issues, the first was entitled &ldquoThe Magnificent Cosmos.&rdquo

Astronomy reaches an extraordinary audience of radio listeners. The program &ldquoEarth and Sky&rdquo is carried by about 900 radio stations in the United States, and the program is heard about 280 million times each year. &ldquoStarDate/Universo&rdquo reaches an audience of about 8.7 million listeners weekly. Surveys in Michigan and Florida showed that 51 percent and 36 percent, respectively, of the listeners discussed what they

had heard on the &ldquoEarth and Sky&rdquo program with other adults or children. Eighty percent of the listeners felt the program &ldquoexpanded their knowledge of science.&rdquo Gender, ethnicity, and occupational status did not correlate with whether or not a person listened to the series. These statistics show that well-presented astronomy stories have an extremely large and diverse audience.

Astronomical sites are among the most popular science destinations on the Web. The American Astronomical Society has found that news stories carried on Web sites often stimulate stories on affiliated television networks. Web sites offer the additional advantage of coverage in depth since they are not limited in terms of space in the same way as newspapers and television broadcasts. Web sites of the Jet Propulsion Laboratory (JPL) and the Space Telescope Science Institute (STScI) are enormously popular and provide the public with a sense of shared participation in the startling discoveries of planetary probes and the Hubble Space Telescope. For example, the Web provided real-time access for millions to view spectacular events such as the impact of Comet Shoemaker-Levy on Jupiter and the adventures of Pathfinder and Sojourner on Mars. The JPL and the U.S. Geological Survey have developed a planetary photojournal Web site that is accessed by 100,000 users who download 700,000 files every month. These Web sites, as well as those run by the Astronomical Society of the Pacific and the American Astronomical Society, provide resources used by thousands of teachers throughout the nation&mdashand bring the excitement of science from the frontiers of research directly into the classroom.

Public interest in astronomy has fueled a number of successful small businesses. Several hundred million dollars are spent each year by hobbyists, small telescopes users, and travelers journeying to witness astronomical events. The catalog of educational materials in astronomy from the nonprofit Astronomical Society of the Pacific reaches about 300,000 people each year.

#### ASTRONOMY IN PRECOLLEGE SCIENCE EDUCATION

The national science education standards developed by the National Research Council (NRC, 1996) specify age-appropriate content goals for the teaching of science in grades K-12. However, content goals alone are not enough. Although students may be able to give the correct answers to traditional problems and questions, these correct answers often mask fundamental misconceptions. Largely to address this prob-

lem, the national science education standards suggest an emphasis on the teaching of science as inquiry. Engaging students in the active process of inquiry can help them to develop a deeper understanding of both scientific concepts and the nature of science. Through inquiry, students can gain an appreciation of how we know what we know about science.

Astronomy lends itself extraordinarily well to inquiry-based teaching and allows teachers to take advantage of the natural fascination students have with the field. Many astronomical phenomena can be observed by students directly with no special equipment, and astronomy-based investigations (focusing on topics like light and color, for example see Figure 4.1) can naturally lead students to explore concepts that inform other scientific fields.

Consequently, astronomers and astronomy educators have invested significantly in developing hands-on activities to support science curricula at all levels. The best of these are collected in The Universe at Your Fingertips: An Astronomy Activity and Resource Notebook (edited by A. Fraknoi et al., Astronomical Society of the Pacific, San Francisco, 1995), a resource and activity notebook that is now in use in almost 15,000 schools around the country.

Over the past decade, astronomers also began to work closely with educators to bring data from spacecraft and observatories directly into the classroom and museums (an example is shown in Chapter 5 in Figure 5.2). Programs such as Hands-on Universe (sponsored by Lawrence Berkeley National Laboratory), Hands-on Astrophysics (sponsored by the American Association of Variable Star Observers), Telescopes in Education (sponsored by NASA), and Research-Based Science Education (sponsored by NSF/NOAO) allow students to explore and use newly acquired astronomical data. Simple image analysis tools are now widely available and, when used in connection with images from planetary exploration and telescopic observations, can be powerful tools in engaging the imaginations of students. Programs like these have already led to well-publicized examples of students discovering a supernova and a new Kuiper Belt object. An increasing number of schools are able to connect to the Internet, thereby making access to astronomical data and images widely available.

A number of astronomical organizations and groups have also been working directly with K-12 teachers, providing training, materials, and classroom visits by teams comprising both professional and amateur astronomers (see Figure 5.1 in Chapter 5). By the end of 1999, for

FIGURE 4.1 School children visiting the exhibit Light! Spectra! Action! at the Adler Planetarium and Museum (in Chicago) learn how astronomers use light and spectra to tell the properties of stars. Photographs provided by D. Duncan.

example, Project ASTRO (developed initially by the Astronomical Society of the Pacific) had established about 700 astronomer-teacher partnerships and had reached more than 50,000 students around the country. Through such projects as the AASTRA program sponsored by the American Astronomical Society, the SPICA and ARIES programs at the Harvard-Smithsonian Center for Astrophysics (see Figure 4.2), and the Astronomical Society of the Pacific&rsquos Universe in the Classroom workshops, several thousand teachers have learned how to be more effective in conveying astronomy and science to their students. The astronomical community has recognized the value of such efforts and is seeking ways to expand their reach to a larger number of teachers throughout the United States.

FIGURE 4.2 Elementary school student using a gnomon to follow the motion of the Sun&rsquos shadow. This program is a part of the Earth in Motion module of Project ARIES at the Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts. The project is funded by the National Science Foundation, the Smithsonian Astrophysical Observatory, and Harvard University. Photograph courtesy of the Harvard-Smithsonian Center for Astrophysics.

The variety of organized science education outreach efforts built on astronomical themes has been growing rapidly and promises to increase throughout the decade as NASA and the National Science Foundation encourage investigators and teams to add education components to their funded research. A good, frequently updated summary of current national astronomy education projects can be found on the Web site of the Astronomical Society of the Pacific (<www.aspsky.org/education/naep.html>).

Because of the importance of linking the public investment in research to advancing public science education goals, the astronomical community has worked hard to identify areas where successes have been achieved, efforts that are highly leveraged, and ways that those gains can be propagated. Recommendations aimed at better coordinating these efforts in the new decade are described in Chapter 5.

### THE PRACTICAL CONTRIBUTIONS OF ASTRONOMY TO SOCIETY

Federal support of curiosity-driven scientific research has historically led to a broad range of contributions to technological advances with long-term benefits to society. Indeed, national investment in curiositydriven scientific research is widely viewed as an essential element of U.S. economic strength and competitiveness. Despite its focus on the extraterrestrial, astronomy has made important contributions on Earth as well. In large measure, these contributions derive from the need to measure precise positions, luminosities, and structural details in faint and distant cosmic sources, to measure time with exquisite precision, and to analyze large statistical samples of objects spanning a wide range of physical, chemical, and evolutionary conditions. All these activities have led to numerous benefits to society that are discussed in more detail below. In some areas, astronomers have pioneered the technology, while in others they have worked symbiotically with industry and the defense sector in developing and perfecting the appropriate technologies.

#### ANTENNAS, MIRRORS, AND TELESCOPES

Large mirrors or antennas that focus and image light, infrared radiation, or radio waves are used not only by astronomers but also by, for example, the communications industry, the military (e.g., in surveil-

lance), and the scientists who use telescopes that look down from space to study Earth&rsquos ecosystem and resources. In order to produce a sharp image, either large-diameter mirrors or antennas are required, or the radiation must be collected on widely spaced individual mirrors or antennas and then combined&mdasha technique called interferometry.

Besides size, another key to a high-quality image is producing a very accurately shaped mirror or antenna. Astronomers have made major contributions to mirror and antenna technology. Examples include developing mirror materials (lightweight materials in particular), mirror designs, precision shaping and metrology (shape testing), procedures for correcting the effects of bending under the force of gravity, technologies to correct for the blurring effect of the atmosphere (e.g., a technology called adaptive optics), interferometry, and the technology for steering the beams and efficiently collecting the radiation in large radio telescopes. Besides the obvious applications noted above, there are additional spinoffs. One notable example is in the area of adaptive optics. Techniques developed by astronomers for adaptive optics are being refined to produce ophthalmic instruments that can image the retina of an eye and measure an individual&rsquos eye aberrations in unprecedented detail. The potential exists for low-cost diagnosis of eye disease, as well as for specification of parameters for either contact lenses that will provide &ldquosupernormal vision&rdquo or corrective eye surgery.

Adaptive optics techniques and techniques to manufacture and figure ultralightweight, ultrahigh-precision mirrors are examples of synergy between investments in defense-related technology and in astronomy. The rapid growth of adaptive optics over the past decade owes much to the declassification of techniques developed in the service of national security interests. Mirrors for the Hubble Space Telescope are a direct descendent of efforts in service of surveillance during the 1970s and 1980s, while today, NASA and the National Reconnaissance Office are partners in efforts to develop next-generation, large space-based mirrors.

#### SENSORS, DETECTORS, AND AMPLIFIERS

Perhaps the biggest technology spinoff contributed by astronomy has been the development or improvement of devices that convert light and other forms of radiation into images. Historically, astronomy pushed the development of photographic film to greater sensitivities and resolution. However, film has now been largely replaced by electronic sensors, detectors, and amplifiers&mdashdevices that enable accurate digitized mea-

surements of brightness over a wide range of wavelengths. In this section, astronomy&rsquos contributions to signal detection are discussed by frequency band, starting with the high-frequency x-ray band and moving to ever lower frequencies: ultraviolet/optical, infrared, and radio.

X rays partially penetrate opaque objects and can thus be used to image their &ldquoinsides.&rdquo One prominent example is provided by the luggage scanners used as security devices in airports. The most common version of this device is a spinoff from space x-ray astronomy, where the requirement to observe weak cosmic signals resulted in the development of high-sensitivity x-ray detectors. Application of these detectors to luggage scanners enabled the use of low x-ray dosages to obtain good images, thus enhancing their safety for operators and passengers alike. X-ray astronomy detectors, with their sensitivity to single photons and to low-energy x rays, are also ideally suited for fundamental biomedical research, for cancer and AIDS research, and for drug and vaccine development. These sensitive detectors have led to a plethora of x-ray medical imaging devices, including those used to search for breast cancer, osteoporosis, heart disease (the thallium stress test), and dental problems. The last is a new development that uses x-ray charge-coupled devices (CCDs miniature electronic detectors) to replace dental x-ray film, a change that will reduce exposure to x rays. Another exciting development is the x-ray microscope. A microscope is, in effect, a miniature telescope. X-ray astronomy has led to the development of the Lixiscope, a portable x-ray microscope to be used to image small objects and fine detail, with applications in energy research and biomedical research. It is widely used in neonatology, out-patient surgery, diagnosis of sports injuries, and Third World clinics. The Lixiscope is NASA&rsquos second largest source of royalties. In a somewhat different technique called x-ray diffraction, a &ldquosuper-microscope&rdquo is achieved that is capable of studying tiny molecular structures. This technique utilizes the interference of the x rays with each other after they scatter off a sample surface. X rays are preferred because they resolve molecular structure. Astronomical advances in detector sensitivity and focused beam optics have allowed the development of systems with much shorter exposure times, and have allowed researchers to use smaller samples, avoid damage to samples, and speed up their data runs. Biomedical and pharmaceutical researchers have used these systems for basic research on viruses, proteins, vaccines, and drugs, as well as for cancer, AIDS, and immunology research.

At ultraviolet (UV) and optical frequencies astronomers have pushed the development of more sensitive CCDs and of large arrays of CCDs.

Cooled silicon CCD arrays developed for optical astronomy now dominate in a multitude of industrial imaging applications. The basic performance of these detectors has been improved by a thinning process developed by astronomers. CCD manufacturers have adopted this technique for use on Earth satellites (e.g., to watch for lightning strikes in the atmosphere) and in surveillance applications. In the UV, CCD development undertaken for a Hubble Space Telescope instrument was later incorporated in a stereotactic breast biopsy machine, which detects tumor positions accurately enough to steer the biopsy probe, thereby reducing the need for surgery and cutting costs by 75 percent (see the Scientific Imaging Technologies Web site at <www.site-inc.com/newsbreastcancer.htm>). In addition, UV detectors developed for the Hubble Space Telescope are being considered as a key element in a helicopter-based system aimed at rapid detection of power-line failures in remote areas.

Objects on Earth radiate most of their energy at infrared (IR) frequencies. In addition, infrared radiation can in some cases be more penetrating than visible light, thus rendering it useful for looking &ldquoinside&rdquo objects, in analogy to x rays. For both of these reasons, the development and/or improvement of sensitive IR detectors, large-format arrays, and IR techniques by infrared astronomers has had significant benefit to society. In this area, there has been a symbiotic relationship with the Department of Defense, which has invested large amounts of money in IR detector development for defense applications. Improvements made by astronomers have contributed to the final versions of the detectors used in the Strategic Defense Initiative and for night-vision devices. In the industrial sector, IR detector arrays developed by astronomers are being used in the semiconductor industry in IR microscopes that examine computer chips for flaws. In the medical sector, IR detectors and spectroscopes are being used to diagnose cervical cancer and genetic diseases and to image malignant tumors and vascular anomalies.

Not only radio and television, but also all satellite and much telephone communication is accomplished with radio waves. Radio astronomers have provided the impetus to many technical advances that have improved the stability, widened the bandwidth, and reduced the noise and interference of radio communications: low-noise maser, parametric, and other transistor amplifiers that have had wide application in the communications industry. Astronomers have perfected highradio-frequency systems that have found application in devices to detect concealed weapons, to see through fog and adverse weather for aircraft landing systems, and to image human tissue (e.g., in mammograms).

#### SPECTROMETERS AND DEVICES TO FOCUS RADIATION

Astronomers have driven the development of ever more precise instruments, called spectrometers, that separate and analyze the different frequencies present in a beam of radiation. In addition, they have perfected precision techniques to focus radiation into spots too small to be visible. These developments have been highly beneficial to the industrial, defense, and medical sectors of the economy.

NASA supported the development of a novel x-ray spectrometer, the microcalorimeter, for x-ray astronomy, but this new device can also be used to analyze the chemical elements in a small sample. Applications include materials science research, rapid trace-element analysis for the semiconductor industry (semiconductor wafer testing), and biomedical research, which requires low doses for biological samples. X-ray spectrometers developed in part in response to the needs of astronomy are also used in x-ray laser materials science and in fusion energy research, as well as in the nuclear nonproliferation program. UV spectrometers are used in laboratory analysis equipment. IR spectrometers remotely analyze the composition of the atmosphere. Spaceborne and ground-based radio spectrometers remotely monitor temperature, winds, humidity, and chemical composition in the atmosphere with applications to weather prediction, global warming, and pollution monitoring. The depletion of ozone has been monitored with astronomical radio telescopes equipped with radio spectrometers. Spaceborne radio spectrometers also sense ground-level quantities such as soil moisture, vegetation cover, ocean height and sensitivity, oil spills, snow cover, and iceberg hazards. Essential components of all these spectrometers have been invented or perfected by the astronomical community.

Efforts in UV and x-ray astronomy pioneered the development of technologies crucial for UV and x-ray lithography, a process by which fine beams of radiation etch lines in a material. Very fine line widths are needed by the semiconductor and microchip manufacturing sector to make advanced computer chips, transistors, and other microelectronic devices. In the medical sector, astronomical technology invented to focus x rays is being put to use in precision deposition of x-ray radiation to destroy cancerous tumors.

#### IMAGE RECONSTRUCTION

Astronomers are bedeviled by faint and blurred images that are often swamped by large amounts of noise or static. An analogous problem would be faint TV reception, superimposed on the static produced by a hair dryer operating nearby. Consequently, astronomers have been at the forefront of efforts to improve and sharpen images, to reduce extraneous noise, and to extract the maximum information from the radiation received. One example of this effort is a system of image analysis tools and computer applications programs developed by astronomers at the National Optical Astronomy Observatories: IRAF, the Image Reduction and Analysis Facility. IRAF has been used not only by thousands of astronomers worldwide, but also by researchers outside astronomy engaged in underwater imaging, mapping of the aerosols in the atmosphere, medical imaging for detection of breast cancer, decoding of human genetic material (in connection with the Human Genome Project), numerous defense-related applications, visualization of the images from electron microscopes, and many other applications. AIPS, the Astronomical Image Processing System developed at the National Radio Astronomy Observatory, is another software package for manipulation of multidimensional images that is used routinely in nonastronomical image analysis applications. Astronomers have also contributed to the advancement of tomography, which enables construction of three-dimensional images out of a series of two-dimensional pictures. Tomographic imaging is used widely in both medical x-ray imaging and industrial applications. The image reconstruction work of R. Bracewell, a pioneering radio astronomer, is widely cited by the medical imaging community. Techniques pioneered by astronomers, such as &ldquowavelet smoothing&rdquo and &ldquomaximum entropy,&rdquo have been used for pattern recognition in areas like mammography and to sharpen images for police work.

#### PRECISION TIMING AND POSITION MEASUREMENTS

Interferometry is the main technique used by astronomers to measure with ultrahigh precision the position in the sky of astronomical objects. Interferometers employ two or more telescopes located some distance apart that precisely measure the time difference in the arrival of radiation from a source. To do this properly requires extremely accurate clocks, since the time differences are extremely short. Astronomers played a significant role in refining the hydrogen maser clock, which is

now widely used for space communications and in the defense sector. The interferometric timing technique to locate radiation sources has had widespread application, including finding noise sources (such as faulty transmitters that interfere with communications satellites), locating cellular phones to track locations of 911 calls, measuring the tiny shifts of Earth&rsquos crust before and after earthquakes, and precisely locating people and vehicles using the Global Positioning System precision surveying network.

#### DATA ANALYSIS AND NUMERICAL COMPUTATION

Astrophysics has been a major driver of supercomputer architecture and computational science for nearly 50 years. Computations of stellar evolution by the pioneering astronomer Martin Schwarzschild occupied nearly half of the time of one of the first computers (MANIAC). Computers are severely challenged by the gigabytes of data streaming in daily from modern astronomical sensors and large sky surveys, and by the large computational speeds required for both simulations and database searches. These requirements are stimulating the development of large computers and innovative hardware components. Beowulf computers, which provide simple commodity supercomputing, were developed by astronomers to enable sophisticated numerical simulations. The idea of designing special-purpose hardware for a specific task has also flourished in astronomy. Two examples of such hardware are the GRAPE computer chips for doing large-scale gravitational N-body simulations (details are available at <grape.c.u-tokyo.ac.jp/grape/>), and the Digital Orrery for calculating the motions of the bodies in our solar system (now retired at the Smithsonian Institution in Washington, D.C.). The Gordon Bell Prize&mdasha prestigious award for significant achievement in the application of supercomputers to scientific and engineering problems&mdashwas won by astronomers in 1992, 1995, 1996, 1997, and 1998. FORTH, a high-performance computer programming language and operating system, was developed at the National Radio Astronomy Observatory and has been used in hand-held computers carried by Federal Express delivery agents and by automotive engine analyzers in service stations, in environmental control systems in airports, and by Eastman Kodak in quality control for film manufacturing.

Many software developments were also either created by astronomers or received much of their impetus for improvement from them. Fast Fourier transforms and other image-processing techniques were

greatly improved by radio astronomers and later by optical astronomers. Some of the more popular grid-based computational fluid dynamics techniques that are used in applications such as weather prediction were either created or improved by astronomers. Another particle-based hydrodynamic technique, smoothed particle hydrodynamics, was both invented and improved by astronomers and has found uses outside astronomy, for example in modeling ballistic impacts. Magnetohydrodynamic codes and numerical simulations of plasmas developed by astronomers contribute to design efforts aimed at harnessing fusion power. Digital correlation techniques for spectral analysis of broadband signals have been adapted for use in remote sensing, oceanography, and oil exploration. IDL, a commonly used graphical package, originated as visualization software for the Mariner Mars 7 and 9 space probes. &ldquoNumerical Recipes,&rdquo 1 a collection of numerical algorithms that is now widely used throughout science, started as an astronomy course on scientific computing. To handle the large databases being produced by astronomical surveys, several groups are collaborating with computer scientists to push forward the frontiers of database mining. Inexpensive and errorfree methods of archival mass data storage have been invented by astronomers. Such developments will obviously have far-reaching applications. Finally, astronomy serves as a prolific and productive training ground for many computational scientists.

#### EARTH&rsquoS ENVIRONMENT AND PLANETARY SURVIVAL

Astronomical studies are essential to understanding the evolution of Earth&rsquos atmosphere and the factors that drive climate changes. Geological evidence suggests that in past millennia, Earth&rsquos climate&mdashas well as the atmosphere and oceans that control it&mdashwas remarkably different. It is now certain that the astronomical environment, including changes in the Sun&rsquos brightness, the influx of cosmic rays, variations in Earth&rsquos orbit, and the influx of zodiacal dust, is an important driver of major long-term climatic changes, such as the ice ages, as well as some smaller and more rapid changes. Together, astronomical and geological observations provide the framework for understanding the response of the biosphere to external change, which is an essential precursor to comprehending and predicting the relative importance of changes that may be wrought by modern industrial activity.

Deepening our understanding of the factors that control climatic conditions on Earth will depend critically on continued careful observa-

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