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Sunlight Puts Spin On Binary Asteroids

July 9, 2008

Asteroids with moons, which scientists call binary asteroids, are common in the solar system. A longstanding question has been how the majority of such moons are formed. In this week’s issue of the journal Nature, a trio of astronomers from Maryland and France say the surprising answer is sunlight, which can increase or decrease the spin rate of an asteroid.

Derek Richardson, of the University of Maryland, his former student Kevin Walsh, now Poincar© Fellow in the Planetology Group in the Cassiop©e Laboratory of CNRS at the  Côte d’Azur Observatory, France, and that group’s leader, co-author Patrick Michel outline a model showing that when solar energy “spins up” a “rubble pile” asteroid to a sufficiently fast rate, material is slung off from around the asteroid’s equator. This process also exposes fresh material at the poles of the asteroid.

If the spun off bits of asteroid rubble shed sufficient excess motion through collisions with each other, then the material coalesces into a satellite that continues to orbit its parent. Because the team’s model closely matches observations from binary asteroids, it neatly fills in missing pieces to a solar system puzzle. And, it could have much more down-to-earth implications as well. The model gives information on the shapes and structure of near-Earth binary asteroids that could be vital should such a pair need to be deflected away from a collision course with Earth.

Finally, the authors say, these findings suggest that a sample return mission to such a binary asteroid could bring back exposed pristine material from the poles of the parent asteroid, providing a chance to probe the internal composition of an asteroid without having to dig into it.

Solar Spin Power

It’s estimated that about 15 per cent of near-Earth and main-belt asteroids with diameters less than 10 kilometers have satellite Scientists have determined that these small binary asteroid pairs were not formed at the beginning of the solar system, indicating that some process still at work must have created them. “It was at first thought the moons in these asteroid pairs probably formed through collisions and/or close encounters with planets,” said Richardson, an associate professor of astronomy at the University of Maryland. “However, it was found that these mechanisms could not account for the large number of binary asteroids present among near-Earth and inner main belt asteroids.”

Recent studies have outlined a thermal process – known as the YORP effect after the scientists (Yarkovsky, O’Keefe, Radzievskii, Paddack) who identified it – by which sunlight can speed up or slow down an asteroid’s spin. Widespread evidence of this mechanism can be seen in the




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