Last November the Japanese spacecraft Hayabusa made the first ever attempt to land on an asteroid and collect a sample to bring back to Earth. Although it is not yet known whether the sampling was successful, the mission has provided scientists with an unprecedented close-up view of a near-Earth asteroid, detailed in several papers published today in the journal Science.
The asteroid Itokawa is typical of the thousands of asteroids with Earth-crossing orbits. "This mission is so important because it's the first visit we've ever had to an asteroid of this size. It's the smallest, most common type of asteroid. It's the size that we care about. If one of these strikes the earth, it could potentially cause a global catastrophe," says Erik Asphaug, an asteroid expert at the University of California, Santa Cruz, who is not associated with the mission.
Roughly 500 meters long, Itokawa is composed of loosely packed rocky rubble, barely held together under the asteroid's own gravity. Despite predictions by some experts that most asteroids should have such a rubble-pile structure (the result of millions of years of deep space collisions), this is the first time an asteroid of this type has been directly observed. Those studied previously were all found to be solid chunks of rock. Itokawa, however, probably coalesced out of the debris from collisions between these larger objects. Like all asteroids, its mineral composition offers a glimpse of the building blocks of the solar system. Earth and the other inner planets formed from chunks of rock that were similar to Itokawa, made up of the silicates olivine and pyroxene, as well as iron.
Though the Hayabusa mission's data collection was a tremendous success, the scientists at the Japan Aerospace Exploration Agency (JAXA) are less certain about the sampling attempts. A small amount of debris may have been captured as the spacecraft bounced off the asteroid in the first of two touchdowns, so the JAXA team is now attempting to steer it back to Earth. The craft, however, was seriously crippled in a series of setbacks; with no battery power and very little fuel, it will no longer arrive next summer as originally planned. Scientists will have to wait until 2010 to examine what could be the first bits of asteroid ever brought to Earth.