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Last Updated: Sunday, 19 March 2006, 20:59 GMT
Probe yields Earth defence clues
By Paul Rincon
BBC News science reporter, in Houston , Texas

Asteroid Itokawa, Jaxa
Itokawa orbits more than 250 million km from Earth
Japan's Hayabusa spacecraft is providing an unprecedented insight into one of the many asteroids that cross into Earth's neighbourhood.

Data from the mission suggests Itokawa could be a relatively young body formed out of debris from the collision of two larger objects.

Scientists presented their results at a major science US conference held in Houston, Texas.

The mission could also provide clues to preventing asteroid strikes on Earth.

"If you're going to mitigate against an object on an Earth-threatening trajectory, you're going to first want to know its composition and secondly its structure," said Dr Don Yeomans, the scientist in charge of monitoring Near-Earth Objects (NEOs) at the US space agency (Nasa).

"We'd have the technology to deal with them, but you have to know the enemy," he told the BBC News website.

Best defence

Because there are many more small NEOs like Itokawa than large ones, this asteroid was a good target, the Jet Propulsion Laboratory (JPL) scientist added.

There is considerable debate among scientists about the best way to deal with an asteroid headed for our planet. Proposed methods include firing nuclear weapons at an incoming object to blow it up, and detonating a nuclear bomb just close enough to deflect its course - a so-called stand-off explosion.

Itokawa's age is probably in the order of several million years; so it is relatively young
Jun'ichiro Kawaguchi, Hayabusa mission manager
Knowing an asteroid's mineral make-up and whether it is fairly solid or more porous would help scientists determine which would work best.

A so-called "rubble pile" asteroid with a porous structure might absorb enough energy from a stand-off explosion to continue unimpeded, it is claimed.

"For this particular type of asteroid - an S-type object - we know its composition well. And this one seems to be a rubble pile," Dr Yeomans explained.

'Potato' mystery

Itokawa's "rubble pile" structure (it is estimated to be 40% porous) holds the key to how it was created.

Scientists think two large objects may have hit each other, shattering into debris. Some portion of this primordial material then merged to form a smaller body - Itokawa.

Hayabusa (Jaxa)
The Hayabusa mission has been a troubled one and communication has only just been re-established
Its characteristic potato-like shape may even suggest Itokawa is a so-called "contact binary" - an object created when two smaller chunks of debris strike each other very slowly and stick. How this happens is still a puzzle.

The low number of craters on the asteroid provides a clue to its age, say Hayabusa scientists. Heavily cratered objects are considered to be ancient due to the heavy bombardment that is believed to have taken place in the early Solar System.

"Itokawa's age is probably in the order of several million years; so it is relatively young," Hayabusa mission manager Jun'ichiro Kawaguchi told the BBC News website.

But not everyone in the audience here at the Lunar and Planetary Science Conference was convinced the craters identified by Hayabusa's scientists really are craters.

Long return

Asteroids previously visited by spacecraft, such as 433 Eros and 253 Mathilde, are covered in impact depressions.

And, according to Don Yeomans, there could be other reasons why the surface is so smooth.

"I think it suggests that somehow the materials have been seismically shaken when objects have hit Itokawa. The whole asteroid shakes and the loose regolith or soil rolls into what were once craters and areas where the gravity is highest," he explained.

The flat areas of Itokawa are covered in a loose, gravel-like regolith. But much is covered in large, jagged boulders.

Like other S-type asteroids, the composition of Itokawa resembles the most common type of meteorite found on Earth - the chondrites.

The asteroid is named after Hideo Itokawa, a father of Japan's space programme

Hayabusa reached near-Earth asteroid (25143) Itokawa late last year. It was designed to collect the first dust and rock samples from an asteroid and return them to Earth; but the mission has been plagued by problems.

Two reaction wheels, which maintain the spacecraft's orientation, failed on approach to the asteroid. Then, a robot designed to hop about on the surface of Itokawa drifted into space after being released from its mothership.

The two scheduled landing attempts to collect samples were also failures, though scientists think a container may have passively collected some soil.

Controllers re-established communications with the spacecraft this month after a fuel thruster leak in December prevented the craft from pointing its antenna towards Earth. They hope to send it on the three-year journey back to Earth in 2007.

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26 Jul 04 |  Science/Nature

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