Public release date: 1-Jun-2006
Contact: Natasha Pinol
American Association for the Advancement of Science
A sea otter-shaped rubble pile in space
Hayabusa mission offers an intimate portrait of asteroid Itokawa
This news release is also available in Japanese
True to its name, the Japanese spacecraft Hayabusa, which means
in Japanese, hovered over the near-Earth asteroid Itokawa last fall,
taking up-close measurements and photographs. Then it swooped down for
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 2 June 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
blocks of Earth. Some in Earth-crossing orbits may pose significant
future threats," said Brooks Hanson, Science's deputy managing editor,
"The Hayabusa results appearing in Science give us an intimate look at
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
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
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
"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
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
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
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
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
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
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
Okada et al.
"Detailed Images of Asteroid 25143 Itokawa From Hayabusa," by J. Saito
"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.
The American Association for the Advancement of Science (AAAS) is the
world's largest general scientific society, and publisher of the
journal, Science (www.sciencemag.org). AAAS
was founded in 1848, and serves some 262 affiliated societies and
academies of science, serving 10 million individuals. Science has the
largest paid circulation of any peer-reviewed general science journal
the world, with an estimated total readership of one million. The
non-profit AAAS (www.aaas.org) is open to all and
fulfills its mission to "advance science and serve society" through
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