[meteorite-list] Hayabusa Probe Prepares to Punch an Asteroid

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Nov 3 12:33:11 2005
Message-ID: <200511031731.jA3HVkK08798_at_zagami.jpl.nasa.gov>

http://www.newscientist.com/article.ns?id=dn8259

Hayabusa probe prepares to punch an asteroid
Maggie McKee
New Scientist
03 November 2005

Japan's Hayabusa spacecraft is set to perform the first of three daring
rendezvous with a small asteroid on Friday, as planetary scientists
puzzle over close-up images of the space rock.

The probe has been hovering above the 600-metre-long asteroid Itokawa
since 12 September 2005, mapping most of its surface to a resolution of
less than 1 metre. Now, the spacecraft is preparing to carry out its
dramatic scientific mission, which involves swooping down to the surface
three times in order to return the first asteroid samples to Earth in 2007.

It will make the first approach - to a spot near the asteroid's spin
axis - on Friday at 0500 GMT. Considered a "rehearsal" descent, Hayabusa
will test a laser range finder to judge its distance from the asteroid
and drop a shiny "target marker" to the surface that it will try to
illuminate and photograph.

"The primary rehearsal goal is to see how accurately we can predict the
spacecraft's landing spot," says Don Yeomans, leader of the mission's US
science team at NASA's Jet Propulsion Laboratory in Pasadena,
California. He expects the accuracy to be better than 60 metres.
          
Robot rover

During Friday's run-through, the spacecraft will also release a
10-centimetre-tall robot called MINERVA. The robot carries thermometers
and three cameras to take measurements from the surface, where it will
gently hop around by reacting to a rotating weight within its body.

Hayabusa will then attempt its first sample collection on 12 November
from an expanse of fine dust in the middle of the asteroid called
MUSES-Sea. The second sampling will take place on 25 November from a
broad, flat region on the asteroid's tip dubbed the Woomera Desert.

These flat targets were chosen "as the safest points to attempt a
landing", says Yeomans. In each case, when a fabric cone protruding from
the spacecraft touches the asteroid, it will fire a 5-gram pellet into
the surface and attempt suck up the resulting debris.
          
Failed components

The landings will be controlled autonomously by the spacecraft's
software but will be even more challenging than originally expected
because two of Hayabusa's three stabilising reaction wheels have failed.
But Yeomans says the one remaining wheel, along with onboard hydrazine
thrusters - which will fire with as little thrust as possible to
conserve fuel - can keep the spacecraft correctly oriented. "The mission
is going amazingly well so far," he says.

Asteroid researcher Bill Bottke of the Southwest Research Institute in
Boulder, Colorado, US, agrees. "The pictures are really remarkable - and
very different from any asteroid images taken to date," he told New
Scientist.

While most asteroids appear to be covered largely by regolith - powdery
material created when small meteorites crash into a rocky body -
Itokawa's surface shows only small amounts of the pulverised stuff.
Instead, it appears that "little blocky things" make up most of the
surface, says Bottke.
          
Mysterious surface

This, along with Hayabusa's discovery that the asteroid's density is
less than expected, leads Bottke to believe it may be composed of
smaller rocks that are held together by gravity.

"There's always been a big debate about whether asteroids should be
considered solid pieces of rock or rubble piles," says Bottke. "If some
asteroids are rubble piles and I had to guess what one might look like,
I would think it looks like this."

The dearth of craters on its surface is also puzzling, he adds. "It's
unlikely this body has completely avoided impacts in the past," he says.
He speculates the asteroid may have been gravitationally deformed - and
had its surface rejigged - on previous close passes by Earth.

Alternatively, the asteroid's small size may be to blame. Its surface
gravity - just 0.001% of Earth's - may allow regolith particles that
become charged by the Sun to briefly lift from the surface. "If the Sun
could cause small particles to move a little bit, they might start to
fill in the depressions," Bottke says.

Hayabusa is scheduled to leave Itokawa in December 2005 and should drop
its sample-return capsule down to Australia in July 2007.
 
Received on Thu 03 Nov 2005 12:31:46 PM PST