[meteorite-list] Hayabusa Yields Earth Defense Clues

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Mar 20 09:34:45 2006
Message-ID: <200603200303.k2K33S603222_at_zagami.jpl.nasa.gov>

http://news.bbc.co.uk/1/hi/sci/tech/4823526.stm

Probe yields Earth defense clues
By Paul Rincon
BBC News science reporter, in Houston, Texas
March 19, 2006

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 shows Itokawa is 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.

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.

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.

Itokawa is covered in a loose, gravel-like regolith which reaches depths
of 2m in some areas.

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

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.
Received on Sun 19 Mar 2006 10:03:27 PM PST