Natural collection spots for asteroids

In an orbiting system of two objects like the Earth and the Moon there are stable positions where another object, like a satellite can remain in a fixed position in relation to the two objects. These are called Lagrange points and there are five of them. Imagine a line which intersects the Earth and the Moon. In our Earth-Moon system L1, L2 and L3 are located on the line. One is just inside the Moon’s orbit between the Earth and the Moon, another is just outside the Moons orbit on the other side of the Moon, and the third is on the other side of the Earth but just inside the orbit of the Moon. These are not very stable and any objects which are not exactly at this point will tend to drift. L4 and L5 are very stable points and they exist on the Moons orbit. One of them is 60 degrees in front of the Moon and the other is 60 degrees behind the moon. There are small asteroids which exist at these points for some of the moons of Saturn. In the Jupiter-Sun system the L4 and L5 points thousands of small asteroids have gathered and remain in Jupiter’s orbit. These two groups of asteroids are called Trojan asteroids and the named asteroids just in front of Jupiter are usually named for Greeks and the named asteroids behind Jupiter are named for Trojans.

(The Great Courses – Black Holes, Tides, and Curved Spacetime: Understanding Gravity)

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