Introduction to special issue

The Falcon Has Landed

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Science  02 Jun 2006:
Vol. 312, Issue 5778, pp. 1327
DOI: 10.1126/science.312.5778.1327

Killer asteroids are the stuff of science fiction movies, but the threat from near-Earth asteroids, at least in the long term, is real. Asteroids are small solid bodies, hundreds of meters to many kilometers in size, with rocky, perhaps icy, surfaces. Understanding of what and how they are built may not only someday help us deflect or destroy one on a collision course with Earth, but tell us how Earth was formed. Asteroids preserve within them detritus left over from the debris disk out of which the planets grew some 4.5 billion years ago. Since then, most asteroids have suffered collisions and impacts and often show the scars. Some can even be linked to collected meteorites that were ejected as a result.

Enough has been learned about asteroids to group and classify them, but basic questions about their formation remain. Overall chemical compositions can be determined from spectroscopy with ground-based telescopes and compared with meteorite samples, but morphological information requires a closer look. The first close-up pictures of lumpy asteroids Ida and Gaspra were snapped by Galileo on its way to Jupiter, and Eros' weathered surface was probed by NASA's Near Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft in 2001.

The next step requires technically challenging missions designed to land on and return samples from a variety of asteroids. The Japanese Hayabusa mission is the trailblazer. Launched in May 2003 and propelled by an ion-drive engine, in September 2005 Hayabusa reached asteroid Itokawa, a tiny rocky asteroid just 500 m across, whose elliptical orbit crosses the paths of both Earth and Mars.

The Hayabusa spacecraft (“hayabusa” means falcon) hovered 20 km above the asteroid and, in November 2005, swooped down to perch for about 30 min on its surface. Whether it managed to grasp any rocks in its robotic talon is not yet known, as the mission scientists had to wrestle hard to pull off the maneuver. Nevertheless, Hayabusa, low on fuel, is now gliding slowly back to Earth, where it may drop its cargo capsule into the Australian desert in 2010.

Whether or not Hayabusa makes it back to Earth, the papers in this issue show that it has already enlarged our view of asteroids. Unlike solid Eros, Itokawa turned out to be a strange rubble world of loose-packed rocks held together by its own tenuous gravity. Its shape suggests that two unequal lumps were squashed together in an earlier collision. Yet few craters are seen on the rocky surface, perhaps because any impacts shake the rubble pile and erase indentations. Fine gravel “seas” also cover several regions where the local gravity is weak. Infrared and x-ray spectrometers carried by Hayabusa show that Itokawa's composition is chondritic and so is typical of many other asteroids.

Hayabusa's hard-won success may yet be crowned by the return of a sample in 2010. But even now, this imaginative mission shows us that rubble-pile asteroids are weird worlds indeed.

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