The small asteroid Itokawa is just a loosely packed pile of rubble that collected after a collision between asteroids, according to a slew of new studies based on data from Japan's Hayabusa spacecraft. The asteroid appears to be plagued by recurring impacts and tremors today, making its continued survival a mystery.Hayabusa made two attempts to collect samples from the 535-metre-long space rock in November 2005. The attempts appear to have failed, but that will not be clear unless the spacecraft can be returned to Earth, which scientists are hoping to do in 2010. But during its approach, the spacecraft did take images and other data on Itokawa's topology, composition and gravity field.What they found was completely unexpected. "Five years ago, we thought that we would see a big chunk of monolithic rock, that something so small doesn't have the ability to hold onto any pieces," says Erik Asphaug, a planetary scientist at the University of California in Santa Cruz, US, who is not involved with the mission. "Everything we suspected about it turned out to be wrong."

The spacecraft showed a surface littered with boulders and gravel, suggesting it was made of the debris from a larger asteroid that was shattered in a past collision. The latest observations from Hayabusa put an approximate size limit on that parent body. Onboard gamma-ray and infrared spectrometers reveal the asteroid is composed of the "raw materials" of planets, such as olivine, pyroxene and metallic iron, says Asphaug. But these materials do not appear to have melted and separated, as would be expected if the parent body was larger than about 200 kilometres across, he says.

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