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Asteroid Probe Offers New Views of Near-Earth Object

Brian Handwerk
for National Geographic News
June 1, 2006
 
A Japanese spacecraft has delivered an unprecedented look at one of the near-Earth asteroids that frequently fly by our planet.

Hayabusa, Japanese for "falcon," achieved a close encounter with asteroid 25143 Itokawa last November.

At 1,640 feet (500 meters) long, the asteroid is a small, rocky, "S-type" commonly found in the inner regions of the main asteroid belt (virtual solar system).

To some observers Itokawa resembles a lumpy potato. Others see a celestial sea otter with a small "head" and larger "body" (sea otter photo and profile).

Hayabusa hovered over the oddly shaped asteroid taking images and readings before achieving a daring touchdown in an attempt to capture the first ever asteroid-surface samples.

Technical problems intervened, and it's unclear whether Hayabusa was able to secure a sample or whether engineers will be able to guide the spacecraft back to Earth some 180 million miles (290 million kilometers) away.

Despite these setbacks, the researchers say the spacecraft's rendezvous and landing were striking successes.

The mission sent back detailed images of Itokawa's surface, some from such a close vantage point that they include the spacecraft's shadow.

Hayabusa also made important measurements of the asteroid's mass and composition.

"The results obtained for Itokawa make a good benchmark for this type of asteroid," said Akira Fujiwara of the Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA).

"We could say that we have seen real images of the most common type of asteroid in the near-Earth region."

Fujiwara and colleagues published their mission results this week in a special edition of the journal Science.

Orbiting "Rubble Pile"

Past evidence had led most scientists to suspect that very small asteroids were constructed of solid rock. But Itokawa may be best described as a rubble pile in space.

Andrew Cheng is a member of the Hayabusa team and a physicist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.

"It was the first asteroid of that size that we've actually gotten a close look at, and it's very bizarre looking, totally different in many ways," he said.

"We thought that we had a handle on what [a near-Earth asteroid] looked like, but it turns out that it is a totally different kind of object."

The asteroid appears to be a low-density collection barely held together by its own low gravity. Itokawa rotates slowly and would likely break apart if it spun much faster.

Scientists likened the impact of a larger space rock or other item colliding with Itokawa to a rock landing in a bucket of sand.

Such impacts might have created the asteroid's sea otter shape. An alternative theory suggests that the head and body were two separate rubble piles that merged together.

Itokawa doesn't carry any liquid water, but it is home to several "seas"—smooth expanses of fine gravel. One such expanse provided a landing zone for Hayabusa.

The asteroid's rough, boulder-choked surface proved another puzzler for asteroid experts.

"We're trying to come up with scenarios that could explain this," said Beth Ellen Clark, an astronomer at Ithaca College in New York and a member of the Hayabusa team.

"It turns out that the smooth areas are in gravitational lows, so that makes you think that maybe something mobilizes the stuff so that the finer material can roll downhill," she said.

"But that's nonintuitive for a lumpy, potato-shaped asteroid. What's really going on is anybody's guess."

Planetary Building Blocks

Comets and asteroids are thought to contain the largely unchanged building blocks from the formation of the solar system some 4.6 billion years ago.

Near-Earth asteroids are likely the remnants from gathering clusters of rock that formed the inner planets Mercury, Venus, Earth, and Mars (space exploration wallpapers).

"[Research into] the origins of planets gets into the question of our own origins," Clark said.

"Some scenarios for Earth have meteorites and comets impacting the surface and supplying a lot of the volatiles"—chemicals that vaporize at relatively low temperatures—"and organic materials that we now have.

"If those models are correct, the primitive stuff in the solar system is directly related to our origins."

Analysis continues on Itokawa's composition, though initial results suggest its base materials are very primitive.

"A rubble pile like this has not been around since the beginning—almost certainly it's something that was blown to bits and reassembled relatively recently," Johns Hopkins' Cheng said.

"But the material it's made of is probably primordial and last melted, if it ever melted at all, back when the planets formed.

"The material will maintain it's old chemical composition, though as an asteroid Itokawa could be as little as ten or a hundred million years old."

Will Falcon Fly Home?

Asteroids also factor in some models for the destruction of life on Earth (explore an interactive feature on asteroid impacts).

To date, more than 4,100 near-Earth objects have been discovered and nearly 800 of them have been classified as potentially hazardous asteroids that could pose a threat to Earth.

Itokawa, for example, crosses the orbits of both Earth and Mars during its own one-and-a-half-year orbit around the sun.

Learning more about asteroid composition may help scientists defend the planet against a statistically rare but potentially devastating impact like the one often linked to dinosaur extinction.

Meanwhile, Hayabusa's mission continues. The team is hard at work attempting to bring the falcon home to roost.

"They've started the process, but it's going to take a while," Cheng said.

"It's still possible that the craft could get back to Earth, and maybe it has a sample. But we don't know about either [possibility]."

Scientists hope to guide the craft back to Earth and have it drop its cargo capsule into the Australian outback sometime in 2010.

"You have to try," Ithaca's Clark said. "It's hard to give up until you have to, and right now we don't have to."

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