Moon story waxes fuller.

The story of how the moon was born may finally be complete. According to the most widely accepted scenario, a Mars-size rock slammed into the Earth 4.5 billion years ago, spewing material that coalesced into the moon. But where this gigantic impactor came from has remained unclear.

The moon's chemical composition suggests that the impactor formed as close to the sun as Earth did, but another factor made that seem impossible: As Earth was forming, its gravity acted like a vacuum cleaner, gobbling up nearby debris. How the impactor could have grown to the size of Mars has been an enigma.

In the March Astronomical Journal, Edward Belbruno and J. Richard Gott of Princeton University propose that the giant impactor could have gradually formed at one of two gravitational sweet spots called L4 and L5, which are situated as far from the sun as is the Earth. At these two points, any object stays put relative to the sun and Earth.

In computer simulations, the researchers found that debris could have collected at either L4 or L5 into a Mars-size protoplanet and then been nudged away from its sweet spot by other gravitational pulls. Were that to happen, the protoplanet would then orbit the sun in a yo-yoing pathway smack in the middle of Earth's trajectory, with a chance of hitting Earth.--E.K.