杏吧原创

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鈥檚 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鈥檚 topology, composition and gravity field.

What they found was completely unexpected. 鈥淔ive years ago, we thought that we would see a big chunk of monolithic rock, that something so small doesn鈥檛 have the ability to hold onto any pieces,鈥� says Erik Asphaug, a planetary scientist at the University of California in Santa Cruz, US, who is not involved with the mission. 鈥淓verything we suspected about it turned out to be wrong.鈥�

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.

Hot heart

Onboard gamma-ray and infrared spectrometers reveal the asteroid is composed of the 鈥渞aw materials鈥� 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.

Nonetheless, Hayabusa鈥檚 cameras reveal that some large boulders appear layered, 鈥渓ike you鈥檇 broken off a rock from the side of a river bed,鈥� he says. That suggests Itokawa鈥檚 parent body was large enough to heat up at its centre and develop some internal structure, even if it wasn鈥檛 large enough to melt. 鈥淭here could have been hydrothermal processes conducting water around, similar to on Earth, where steam passes through rocks and alters their compositions,鈥� he told New 杏吧原创.

Measurements of the asteroid鈥檚 gravity field also suggest it coalesced from the debris of a previous collision. Hayabusa scientists used the data 鈥� combined with measurements of the space rock鈥檚 size 鈥� to estimate its density. It appears to be 40% porous, or filled with empty space.

鈥淭hat is astonishing,鈥� says Asphaug, adding that a handful of sand has a porosity of 20%. 鈥淚t鈥檚 very hard to get porosities greater than that. You鈥檝e got to start balancing things delicately, like you were building a house of cards,鈥� he says. 鈥淭he only way to do it is to gently pack the stuff together.鈥�

Tamping down

But that raises another mystery, he says, since repeated impacts with other space rocks over millions of years should have made Itokawa denser. 鈥淓very time you have an impact, you鈥檙e going to tamp it down,鈥� he says.

And Itokawa certainly appears to have had its share of cosmic run-ins, even though it does not show many craters. New craters are thought to be buried by gravel that flows into them when Itokawa shudders after being struck by a space rock. This shaking is also thought to have buried the powdery dust created in such impacts, leaving only larger boulders and gravel-sized rocks visible.

Only one other asteroid has been studied so extensively, a 33-kilometre-long rock called Eros. Rather than being a pile of rubble that reassembled after an impact, that asteroid appears to be a single rock that was shattered, with its pieces kept in place. Asphaug argues that more asteroids should be visited by spacecraft, in part to determine how to nudge one out of the way if it threatened to collide with Earth.

Itokawa does cross the Earth鈥檚 orbit during its 1.5-year-long path around the Sun, but calculations show it will probably never hit the planet. But Asphaug says an asteroid the size of Itokawa is expected to strike the Earth once every 50,000 years, making robotic 鈥� or even human 鈥� missions to asteroids of compelling interest. 鈥淵ou want to be ahead of the game鈥� in the event that an asteroid is found on an impact course, he says.

Journal reference: Science (vol 312, p 1328 to 1353)