Beyond the Earth, towards the limits of the solar system, there are a large number of asteroids that seem to orbit around an empty point of space. Considering their complete motion, we could say that they revolve around the Sun, although they do not orbit it . But in addition to this, they have an oscillatory movement, a back and forth, around nothing . To put this in context, it could be said that the Moon also revolves around the Sun, since it accompanies the Earth on its path, but it actually orbits around the planet, which is the cause of the lunar movement.
These asteroids are known as Trojan asteroids . The vast majority are located at the distance of Jupiter, although we have also observed some close to Neptune, Uranus, Mars and even Earth . These bodies orbit around what are known as Lagrange points , specifically around the fourth and fifth Lagrange points of the corresponding planet. These points are nothing more than regions of space where the gravitational forces of the Sun and the planet in question seem to compensate , creating a region where a certain balance is possible.
The first three points are located on the line between star and planet. The first between the two bodies , much closer to the planet. The second beyond the planet , a short distance from it. The third beyond the star , at a distance slightly greater than that of the planet’s orbit. These three points however lead to an unstable equilibrium . That is, they lead to a balance in which any slight disturbance causes the body to move away from that balance. As if they were located on top of a hill and any movement would make them fall down the hill.
The fourth and fifth points instead give rise to a stable balance , in which a small deviation brings you back to the balance point. It is around these points that the Trojan asteroids orbit. Two Trojan asteroids are known for Earth , orbiting at its L4 point, that is, preceding it in its orbit around the Sun. Both objects are very small, with less than 400 meters in size for the Trojan discovered in 2010 and just over a kilometer for the one discovered in 2020. Fourteen objects are known from Mars , with sizes similar to the two observed for Earth, while that for Neptune 28 of these objects are known. It is believed that the large Trojan asteroids (more than 100 kilometers in diameter) of Neptune could be more numerous than those of Jupiter , despite the fact that at present about ten thousand Trojan asteroids are known for Jupiter and it is believed that there could be more than a million of these objects that exceeds a kilometer in diameter.
The first Trojan asteroid discovered was what we now call 588 Achilles , which at 130 kilometers in diameter is the fourth largest known Jupiter Trojan today. It was discovered in 1906 by Max Wolf from the Heidelberg observatory in present-day Germany. A few more asteroids were discovered over the next few decades, but these discoveries did not accelerate until the 1970s to the present day and almost 10,000 confirmed asteroids today .
Based on these figures, the number of Jupiter Trojans with a diameter greater than one kilometer has been estimated at more than a million . This amount would be comparable to that of the asteroid belt, with the total mass of the Trojans being about a fifth of the mass of the entire asteroid belt (which, although it may contain a similar number of objects, contains several considerably larger asteroids and even a dwarf planet, Ceres , almost 1,000 kilometers in diameter).
Some of these objects will be visited by NASA’s Lucy mission , which took off in October 2021, with which we hope to obtain a great deal of information about them and the origins of the solar system. However, we already know something about them. For example, we know some details about 624 Hektor , the largest of Jupiter’s Trojan asteroids. Hector has an elongated shape, about twice as long as it is wide, which tells us that it probably resulted from the merger of two smaller, roughly spherical asteroids . It is about 400 kilometers long and almost 200 kilometers wide. This size is apparently enough to retain a satellite, called Skamandrios , which orbits just over 600 kilometers from the asteroid and is just over 10 kilometers in size.
Another asteroid, 617 Patroclus , is also particularly interesting. It is the largest of the known binary Trojan asteroids . That is, this object actually consists of two asteroids orbiting each other at a very close distance. Both parts are very similar in size, one about 115 kilometers in diameter and the other 105 kilometers, and they orbit each other every four days, at a distance of 680 kilometers . Even the smallest and seemingly insignificant bodies in the solar system hide interest.
References:
F. Marchis et al, 2014, The Puzzling Mutual Orbit of the Binary Trojan Asteroid (624) Hektor, The Astrophysical Journal Letters. 783, doi:10.1088/2041-8205/783/2/L37
F. Marchis et al, 2006, A low density of 0.8gcm-3 for the Trojan binary asteroid 617Patroclus, Nature. 439, doi:10.1038/nature04350
