When the asteroid gets close to the Earth, it starts moving away again. Why doesn the Earths gravity pull it right in?
Non-technical answer: Any orbit is a balance between two competing forces: gravity, which pulls things together, and the speed of the bodies, which (if it is in the right direction) tends to make them move away from each other. Think of a satellite in orbit: if it was not moving, it would fall down onto the planet, but its speed keeps it in orbit. However, gravity and speed are not completely independent of each other: gravity makes things go faster as they fall, and a moving object traveling in the wrong direction will not stay in orbit. It is an interplay between these two which allows Cruithne to avoid the Earth. As Cruithne gets closer to our planet, the Earth’s gravity does pull it towards it; but because of the trajectory of the asteroid, the Earth’s gravity just makes its speed change in such a way that its orbit starts to move away from the Earth. Gravity never repels the asteroid in the strict sense but the result is the same: the asteroid reverses direction and begins to move
