True to its name, the Japanese spacecraft Hayabusa, which means "falcon" in Japanese, hovered over the near-Earth asteroid Itokawa last fall, taking up-close measurements and photographs. Then it swooped down for a brief landing and the first-ever sample attempt on an asteroid.
The first peer-reviewed set of scientific results from this mission appears in the June 2, 2006 issue of the journal Science, published by AAAS, the nonprofit science society. These findings will help researchers understand the structure and composition of the "near-Earth asteroids" that periodically whiz past our planet.
"Asteroids are relics from our early solar system and provide critical information on its early evolution and some represent the early building blocks of Earth. Some in Earth-crossing orbits may pose significant future threats," said Brooks Hanson, Science's deputy managing editor, physical sciences.
"The Hayabusa results appearing in Science give us an intimate look at a near-Earth asteroid and provide information on its makeup and relation to collected meteorites. The Hayabusa misson also opens up a new frontier of space research in which we investigate other asteroids," he said.
Asteroid Itokawa, named after Japanese rocket scientist Hideo Itokawa, was chosen as Hayabusa's "prey" in part because it is one of the most common types of rocky near-Earth asteroids, the so-called "S-type" asteroids. The relatively tiny asteroid is only 500 meters long.
"The results obtained for Itokawa make a good benchmark for this type of asteroid. We could say that we have seen real images of the most common type of asteroid in the near-Earth region," said lead Science author Akira Fujiwara of the Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA).
Some of the photographs taken by Hayabusa's camera even show the spacecraft's shadow, revealing how close the spacecraft came to the asteroid during its hover.
In a special issue of journal devoted to the Hayabusa mission, Fujiwara and his colleagues report that asteroid Itokawa has two parts, a smaller "head" and larger "body," giving it the shape of a sea otter, and appears to consist of rubble.
Asteroid structure has been a puzzle for scientists thus far. In theory, because of the pummeling that asteroids receive by other objects, they should be clusters of fragments that have re-accreted. Previously studied asteroids, however, generally appear to be lumps of solid rock.
"Although it is still unknown why these other asteroids do not show a rubble pile structure, at least now we have the first example of a rubble pile asteroid," said Fujiwara.
The rubble is very loosely packed and porous, just barely held together by the asteroid's own gravity. If an object collided with Itokawa, it would probably be like a rock landing in a bucket of sand. The signs of impacts with small space rocks get erased as the rubble shifts after the impact, so there are very few craters on Itokawa.
The asteroid's head-and-body structure might have arisen as the rubble shifted in response to impacts. Or, the head and body may once have been separate rubble piles that gradually merged.
Fujiwara and his colleagues report that, unlike previously explored asteroids, Itokawa's surface has patches of both rough, boulder-strewn terrain and "seas" of uniformly sized, finer gravel particles, which appear velvety-smooth in the photographs from the mission. Hayabusa's touchdown took place on one such smooth patch called "Muses Sea." (This is a play on words; Hayabusa was originally called MUSES-C).
Because it was designed to take samples from its destination and return them to Earth, the Hayabusa mission has also offered important engineering lessons. The spacecraft used an electronic ion propulsion system, whose efficiency should be critical to future missions in deep space, and autonomous navigation technology for its delicate approach and landing. Hayabusa also attempted a "touch and go" sampling effort intended to bring the first asteroid material back to Earth, but it may not have been successful.
The spacecraft, now low on fuel, is slowly gliding back to Earth, where it may drop its cargo capsule into the Australian desert in 2010.
The mission encountered several major technical difficulties, which Erik Asphaug of the University of California, Santa Cruz notes in a "Perspective" article that accompanies the research.
"Yet despite these heartbreaking setbacks, Hayabusa has been a stunning success for asteroid science and deep space concept testing, as reported in an exciting set of mission reports in this issue. These are the culmination of heroic efforts to make things go right in the face of multiple setbacks. Failures are not uncommon in deep space, and in this case ingenuity and perseverance paid off in remarkable ways," Asphaug writes.
The papers in the special Science issue are:
"The Rubble-Pile Asteroid Itokawa as Observed by Hayabusa," by A. Fujiwara et al.
"Near-Infrared Spectral Results of Asteroid Itokawa From the Hayabusa Spacecraft," by M. Abe et al.
"X-Ray Fluorescence Spectrometry of Asteroid Itokawa by Hayabusa," by T. Okada et al.
"Detailed Images of Asteroid 25143 Itokawa From Hayabusa," by J. Saito et al.
"Mass and Local Topography Measurements of Itokawa by Hayabusa," by S. Abe at al.
"Pole and Global Shape of 25143 Itokawa," by H. Demura et al.
"Touchdown of the Hayabusa Spacecraft at the Muses Sea on Itokawa," by H. Yano et al.
"Adventures in Near-Earth Object Exploration," by E. Asphaug.
This research was funded by JAXA, NASA, Kobe University and the University of Aizu.
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