Pictures Give New Image of Asteroid

Itokawa Now Seen As 'Rubble Pile'

Spectacular images and data obtained by a Japanese spacecraft show that the near-Earth asteroid Itokawa is almost certainly an unusual "rubble pile" composed of boulders, pebbles, and perhaps sand and dust, probably brought together gently -- and mysteriously -- after an ancient collision in space, scientists said yesterday.

"The pictures were just phenomenal, and so different from any other asteroids we've flown by," said planetary geophysicist Olivier Barnouin-Jha of the Johns Hopkins University's Applied Physics Laboratory. "I think most of the people on the team would have argued that as small an object as this should have been a rock -- a solid piece rather than a rubble pile."

Instead, instruments on Japan's Hayabusa spacecraft found that the surface of Itokawa -- described by the research team as a "sea-otter-shaped" asteroid about 1,800 feet long -- was a jumble of boulders and gravel far less dense and far more porous than a solid piece of stone or metal.

"This is the first for-sure rubble-pile asteroid we've ever seen up close," said Michael E. Zolensky of NASA's Johnson Space Center. "The two big chunks -- the head and the body -- are just touching. The pieces are barely hanging on, and the fine-grained stuff fills in the gaps."

Zolensky and Barnouin-Jha are members of a multinational team led by Akira Fujiwara of the Japan Aerospace Exploration Agency's Institute of Space and Astronautical Science. Results are being reported today in seven papers in the journal Science.

Last fall's Hayabusa mission, in which engineers at one point briefly landed the spacecraft on Itokawa in an attempt to collect samples, marked the first touchdown accomplished on a "near-Earth object" -- one of thousands of small asteroids and other bits of celestial material whose trajectories bring them inside Earth's orbit.

The study of these visitors has intensified in recent years because of the potential damage that one of them -- even one as small as Itokawa -- could do in a collision with Earth. NASA has study groups counting them, tracking them and deciding how best to push the dangerous ones out of the way, or perhaps even blow them up.

"This one could make a big mess," said Donald Yeomans, a Hayabusa team member and a senior research scientist at NASA's Jet Propulsion Laboratory. "You could blow it apart, but that would give you more pieces, like a shotgun. It would be much easier to push it or pull it out of the way." Itokawa, however, is in no danger of hitting Earth.

Hayabusa's cornucopia of data and astonishing images, many taken from altitudes of four miles or lower, provided a marked contrast to the soap-opera-like perils and complications that plagued the spacecraft during the mission.

Gyroscope breakdowns forced Hayabusa to use its thrusters to alter its position in space, limiting its instruments' "looks" at the surface. A tiny hopping robot designed to land on the asteroid instead flipped into the void.

Hayabusa landed on the surface of Itokawa on Nov. 27, but its dust collector apparently malfunctioned. Then its thrusters sprang a leak during liftoff and controllers subsequently lost radio contact, causing Hayabusa to miss the return window that would have allowed it to reach Earth in 2007.

Engineers are back in control now and will try to start the journey home next year, aiming for a landing in Australia in 2010. Even though the dust collection didn't work properly, Yeomans said, scientists are hopeful that touchdown on Itokawa stirred up a puff of material that the collector may have sampled.

Zolensky and others said scientists have long thought that many asteroids are rubble piles but have not agreed on how they could have been created. For a low-gravity mush of boulders and stones such as Itokawa to hold together, the mix would have had to be close together when it began to form, and the relative speed of the particles would need to be negligible.

"Otherwise the asteroid would fly apart," Barnouin-Jha said. "Our understanding is that small objects like this should be solid shards that broke off [larger objects] because of collisions, not rubble piles."