Scientists studying the rounded rocks on the surface of the asteroid 25143 Itokawa, photographed by the Japanese probe Hayabusa, have concluded that sunlight combined with the asteroid's tiny gravity caused them move and thus erode themselves.
As sunlight bounced off the orbiting boulders, photons provided a tiny push. As they radiated back outward as heat, they triggered a recoil effect that added a gentle spin. Over time, these slowly spinning boulders bumped into each other with enough force to wear their edges into smooth surfaces.
Warning! This is only a model, and thus could very well be wrong. It is reasonable however and worth considering as a factor in studying the early formation history of asteroids.
An analysis of the rotation of the peanut-shaped asteroid 25143 Itokawa suggests that its smaller end is actually far denser than its bigger end.
Previously, some researchers have proposed that Itokawa is actually two asteroids in contact with one another, a scenario strongly supported by the new results, the team contends. It's not clear whether the odd configuration results from the merger of a two-asteroid system or merely clumps of material that fell back together after a larger asteroid was blasted apart by an immense collision.
The researchers also found that the asteroid's rotation is actually increasing by 45 milliseconds per year.