Asteroid is a big rubble pile

WASHINGTON – 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 Thursday.

“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 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.