NASA hopes its STEREO mission to explore the gravitational “parking lots” at the L4 and L5 points will reveal secrets about the Moon’s origin. An April 9 article on the official site explains:

L4 and L5 are where an object’s motion can be balanced by the combined gravity of the sun and Earth. “These places may hold small asteroids, which could be leftovers from a Mars-sized planet that formed billions of years ago,” said Michael Kaiser, Project Scientist for STEREO at NASA’s Goddard Space Flight Center in Greenbelt, Md. “According to Edward Belbruno and Richard Gott at Princeton University, about 4.5 billion years ago when the planets were still growing, this hypothetical world, called Theia, may have been nudged out of L4 or L5 by the increasing gravity of the other developing planets like Venus and sent on a collision course with Earth. The resulting impact blasted the outer layers of Theia and Earth into orbit, which eventually coalesced under their own gravity to form the moon.”

[Via Toni Weisskopf.]