FAJARDO, Puerto Rico–A fascinating idea came up in an informal chat I had yesterday with asteroid expert Erik Asphaug of the University of California, Santa Cruz. The early solar system was a veritable shooting gallery. Our moon is thought to have formed when a Mars-size body hit Earth and threw out a cloud of debris that coalesced in orbit around our young planet. In his talk Monday at the annual Division for Planetary Sciences meeting here, Asphaug reported that the incoming body had to hit at a fairly low velocity. Any faster, and the debris would have scattered into interplanetary space. In that case, whatever body coalesced would not have been a moon, but a planet in its own right. Riffing on his talk, Asphaug has a provocative answer for one of my favorite questions in planetary science: Why doesn’t Venus have a moon ? How did it manage to dodge all the bullets flying around the early solar system? Asphaug suggests that maybe it didn’t. Maybe Venus got hit worse than we did, so that a planet rather than a moon was the outcome. [More]