Number 271 (Story #3), May 16, 1996 by Phillip F. Schewe and Ben Stein BINARY ASTEROIDS. Doublet craters account for 10% of all impact structures on Earth and Venus. A prevalent theory holds that such impacts come about when a single asteroid is fragmented either when it rips through the planet's atmosphere or when it is pulled apart by gravitational tidal forces just before impact. A new study by William Bottke (Caltech) and Jay Melosh (Arizona) shows that the relatively wide separation of craters in doublet events can best be explained by supposing that tidal fragmentation into parts had occurred at some earlier stage, as with Comet Shoemaker-Levy. The researchers suggest that such weakly-bound "rubble-pile" asteroids and asteroid satellites (such as the Ida-Dactyl system imaged by the Galileo spacecraft) might be more common than we thought. (Nature, 2 May 1996.)