Recent data from the Japanese Hayabusa spacecraft suggests that the asteroid it visited at the end of last year, called Itokawa, is a loosely held pile of rubble. According to the craft�s survey the asteroid is still plagued by impacts and tremors, making scientists question how the pieces have managed to continue to cling together.

Before the mission, the Japanese Aerospace Exploration Agency (JAXA) had assumed that Itokawa must be one large piece of rock, as they thought such a small asteroid would have insufficient mass for its gravity to hold lots of smaller pieces together. The recent data has come as a surprise to the agency.

According to New Scientist Space, measurements of Itokowa�s gravity field suggest that 40 per cent of the asteroid is empty space. In comparison, a handful of sand is only around 20 per cent porous.

"That is astonishing," says Erik Asphaug, a planetary scientist at the University of California , USA. "It's very hard to get porosities greater than that. You've got to start balancing things delicately, like you were building a house of cards," he says. "The only way to do it is to gently pack the stuff together."

Images taken by Hayabusa showed the asteroid�s surface was littered with small pieces of rock. This suggests it was originally a larger, more solid asteroid that was broken up in a collision. The pieces were later pulled together by their own gravity, making the asteroid we see today. Usually such collisions pack down the rock pieces, making them less porous, so this finding has surprised scientists.

The 535 metre-long asteroid appears to have few impact craters. Scientists believe this is because each impact disturbs the gravel which then flows into the craters, hiding them from view.

Eros, a 33 kilometre-long asteroid, is the only other to have been studied in such detail. Eros was once one solid piece of rock that was then involved in a collision, shattering it into chucks. Although the rock was shattered, the pieces were held together in their previous positions under their own gravity.

JAXA have also announced that Hayabusa does have enough fuel left in its ion engines to make it back to Earth. Scientists were worried that the engines may have used up too much fuel for the return trip, after being used to control the craft�s orientation following a chemical fuel leak and the malfunction of one of the craft�s stabilisers. The craft was designed to collect samples from Itokawa and return them to Earth, but there is only a slight chance some material may have been knocked into the sample chamber during the sample collection attempts.

Related News
Hayabusa probe approaches asteroid Itokawa
Hayabusa lander misses target
UK names to land on asteroid
First Hayabusa sample collection fails, but UK names are delivered to asteroid
Japanese announce that Hayabusa did land on Itokawa
Hayabusa collects asteroid sample
Hayabusa's return to Earth postponed until 2010