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If the asteroids penetrate beyond the orbit of Jupiter, wouldn't there be others that penetrate beyond the orbit of Mars, closer to the Sun? The first of such cases was discovered on August 13, 1898 by a German astronomer, Gustav Witt. He detected asteroid 433 and saw that his period of revolution was only 1.76 years, that is, 44 days less than that of Mars. Therefore, its average distance from the Sun must be less than that of Mars. The new asteroid was called Eros.
Eros proved to have rather high orbital eccentricity. In the aphelion, it is within the asteroid belt, but in the perihelion, it is only 170 million kilometers from the Sun, not much more from the distance from the Earth to the Sun. Since its orbit is inclined with respect to that of the Earth, it does not approach it as much as it would if both orbits were in the same plane.
Anyway, if Eros and the Earth are in the appropriate points of their orbits, the distance between both will be only of 23 million kilometers. This is a little more than half of the minimum distance between Venus and Earth, and it means that, if we did not count our own Moon, Eros was, at the time of its discovery, our closest neighbor.
It is not a very large body. Judging by the changes in its brightness, it is brick-shaped, and its average diameter is about five kilometers. Anyway, it is not a despicable thing. If it collided with Earth, a catastrophe would occur.
In 1931, Eros approached a distant point just 26 million kilometers from Earth, and a vast astronomical project was established to accurately determine its parallax, so the distances of the Solar System could be determined more accurately than ever. The project was successful, and the results were not improved until radar rays were reflected from Venus.
An asteroid that approaches Earth more than Venus, is called (with some exaggeration) Earth grater. Between 1898 and 1932, only three more earth rozers were discovered, and each of them approached our planet less than Eros.
However, this mark was broken on March 12, 1932, when a Belgian astronomer, Eugéne Delporte, discovered the asteroid 1,221, and saw that, although its orbit was regular with respect to that of Eros, it managed to approximate 16 million kilometers of Earth's orbit He called the new asteroid Amor (the Latin equivalent of Eros).
On April 24, 1932, exactly six weeks later, the German astronomer Karl Reinmuth discovered an asteroid he called Apollo, because he was another Earth grater. It was an amazing asteroid since, in its perihelion, it is only 95 million kilometers from the Sun. It moves not only inside the orbit of Mars, but also inside the Earth, and even that of Venus . However, its eccentricity is so great that in the aphelion it is 353,000,000 kilometers from the Sun, further from what happens to Eros. The period of revolution of Apollo is, therefore, 18 days longer than that of Eros. On May 15, 1932, Apollo approached within 10,725,000 kilometers of the Earth, less than 30 times the distance of the Moon. Apollo is less than two kilometers wide, but it is large enough so that it is not welcome as a "grater." Since then, any object that approaches the Sun more than Venus does, has been called an Apollo object.
In February 1936, Delporte, who had already detected Love four years earlier, spotted another earth slasher whom he called Adonis. Exactly a few days before its discovery, Adonis had passed only 2,475,000 kilometers from Earth, or only a little more than 6, 3 times the distance from the Moon to us. And what is more, the new Earth grater has a perihelion of 65 million kilometers, and at that distance it is very close to the orbit of Mercury. It was the second Apollo object discovered.
In November 1937, Reinmuth (the discoverer of Apollo), spotted a third, which he called Hermes. It had passed 850,000 kilometers from Earth, just a little more than twice the distance from the Moon. Reinmuth, with the available data, calculated a gross orbit, according to which Hermes could pass only 313,000 kilometers from Earth (a smaller distance from the one that separates us from the Moon), as long as Hermes and Earth were found at the appropriate points of their orbit. However, since then Hermes has not been detected again.
On June 26, 1949, Baade discovered the most unusual of Apollo objects. Its period of revolution was only 1, 12 years, and its orbital eccentricity was the best known in asteroids: 0.827. In its aphelion, it is safe in the asteroid belt between Mars and Jupiter, but, in its perihelion, it approaches 28,000,000 kilometers from the Sun, closer than any planet, including Mercury. Baade called this asteroid Icarus, according to the young Greek mythology who, flying through the air with the wings that his father had created, Daedalus, got too close to the Sun, which melted the wax that secured the feathers of the wings on his back, and he fell to death.
Since 1949, other Apollo objects have been discovered, but none has come as close to the Sun as Icarus. However, some have an orbital period of less than one year and, at least, one is closer, at each point of its orbit, to the Sun than the Earth.
Some astronomers estimate that there are about 750 Apollo objects in space, with diameters of one kilometer and more. It is believed that, in the course of a million years, four respectable Apollo objects have reached Earth, three to Venus, and one to both Mercury, Mars or the Moon, and seven have seen their orbits altered in such a way that Everyone has left the Solar System. The number of Apollo objects, however, does not decrease over time, so others are likely to be added from time to time due to gravitational disturbances of objects in the asteroid belt.
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