Lagrange L4/L5 points and the origin of our Moon and Saturn's moons and rings.

The current standard theory of the origin of the Moon is that the Earth was hit by a giant impactor the size of Mars causing ejection of debris from its mantle that coalesced to form the moon; but where did this Mars-sized impactor come from? Isotopic evidence suggests that it came from 1 AU radius in the solar nebula, and computer simulations are consistent with its approaching Earth on a zero-energy parabolic trajectory. How could such a large object form at 1 AU in a quiescent disk of planetesimals without having already collided with the Earth at an earlier epoch before having the chance to grow large? Belbruno and Gott propose that the giant impactor could have formed in a stable orbit from debris at the Earth's Lagrange point L(5) (or L(4)). It would grow quietly by accretion at L(5) (or L(4)), but eventually gravitational perturbations by other growing planetesimals would kick it out into a horseshoe orbit and finally into a chaotic creeping orbit, which Belbruno and Gott show would, with high probability, hit the Earth on a near zero-energy parabolic trajectory.