[meteorite-list] New Data Reveals Mysteries of Asteroid Itokawa

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Mar 20 23:13:22 2006
Message-ID: <200603201910.k2KJArn29460_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn8868-new-data-reveals-mysteries-of-asteroid-itokawa-.html

New data reveals mysteries of asteroid Itokawa
Maggie McKee
20 March 2006

Close-up views of asteroid Itokawa taken by Japan's Hayabusa spacecraft
at the end of 2005 continue to puzzle scientists. They debated nearly
every aspect of the asteroid - including its formation, age and
composition - during the first major release of mission data on Friday.

Hayabusa was designed to bring the first asteroid samples back to Earth
by firing pellets into the space rock and scooping up the resulting
debris. But it probably failed to fire any pellets
during two landings on Itokawa in November 2005, making it unlikely to
have captured much - if any - rocky debris.

However, the spacecraft took high-resolution images, spectra and density
measurements of the 550-metre-long space rock, and mission scientists
presented the observations at the Lunar and Planetary Science Conference
in Houston, Texas, US.

"We'd learn a lot more if we got samples back," says Jay Melosh, a
planetary scientist at the University of Arizona in Tucson, US, who was
not involved with the mission. "But it's given us a glimpse of a kind of
object we've never seen before."

Shaken up

Most asteroids appear to be covered largely by fine regolith - the rock
dust created by the impact of small meteorites. But Itokawa contains
only small amounts - its finest-grain material appears to be
gravel-sized particles. "This suggests there is some process removing
fine dust or burying it under the surface," Melosh told New Scientist.

Furthermore, the gravel-like regolith is not distributed evenly but is
concentrated in flat expanses that cover about one-fifth of Itokawa's
surface. The rest of the surface is "rough" terrain littered with
metre-sized boulders, which suggests that some process is moving the
gravel into the flat zones.

One possible mechanism could be impacts by other space rocks. Melosh and
his colleagues have previously shown that impacts on small asteroids
could cause them to shake for several hours, moving regolith around
their surfaces.
          
Age range

This shaking might also explain another of Itokawa's mysteries, says
Hayabusa team member Naru Hirata of Kobe University in Japan, by
covering up craters. Researchers see far fewer craters than they expect
- just 60 candidates larger than a few metres across. Alternatively, he
says, small craters may never form in the first place - a small
meteoroid hitting Itokawa may simply crush one of the many boulders on
its surface and not produce a crater.

Such effects imply Itokawa has been pummelled by more space rocks than
its youthful appearance suggests. Assuming that this is the case,
Chikatoshi Honda of the Japan Aerospace Exploration Agency (JAXA) in
Kanagawa, Japan, estimates it is between 10 million and 100 million
years old.

But other team members argue that the dearth of craters should be taken
at face value - and that Itokawa is genuinely young. Itokawa is thought
to have formed just outside the orbit of Mars in the main asteroid belt
between Mars and Jupiter. Then, close encounters with Mars probably
pulled it closer to the Sun into its current orbit, which crosses that
of the Earth, says team member Makoto Yoshikawa of JAXA.
          
Rubble fusion

But its current orbit is very sensitive to the gravity of objects around
it, making long-term estimates of its past or future motion difficult.
Given this uncertainty, Itokawa "could have formed only one or two
million years ago", says team member Andy Cheng of Johns Hopkins
University in Maryland, US.

He says the asteroid may have quickly left the asteroid belt, where it
would have had the highest chance of being struck by other space rocks,
and spent most of its lifetime within the orbit of Mars.

Researchers are also divided over how Itokawa formed. Estimates of its
density reveal it is 39% empty space. That suggests it coalesced from
the debris of an ancient asteroid impact. But whether it formed as a
single rubble pile or as two that later collided and stuck together
remains unclear.

Hayabusa team members favour the latter scenario, pointing out that its
shape resembles a sea otter, with a small head curving towards a larger
body. They say these two components were once separate and at some point
fused together at the "neck".
          
Hot and cold

But Melosh says typical collisions in the asteroid belt occur at speeds
of 2 kilometres per second - too fast for objects to attach to one
another. If Itokawa is a so-called "contact binary", he says, the
collision would have to have been "a very special event at very, very
low velocity". Instead, he says Itokawa may have been stretched into an
elongated shape by an external gravity field as it passed by a massive
object, such as the Earth.

The rock's composition is also unclear. Spectral observations of
minerals taken from Hayabusa suggest the rock was not altered by heat in
its past. But those from ground-based telescopes in Hawaii suggest it
partially melted after being heated to more than 1000?C.

Pinning this history down may reveal when and where in the solar system
its component rocks formed. And that would inform how to interpret the
histories of meteorites that fall to Earth.

Ion engines

But the issue can only be settled definitively by studying samples
returned from the asteroid, says Melosh. Mission members hope the
spacecraft accidentally collected some dust from the asteroid during its
two landings and are hoping to send the probe back towards Earth in early
2007.

But it is far from certain that the spacecraft can make the journey. In
December, the spacecraft spun out of control after a fuel leak and was
out of contact with mission managers for more than a month, with its
antenna pointing in the wrong direction. During this time, its circuits
may have been damaged by the cold of space, mission manager Jun'ichiro
Kawaguchi told New Scientist.

Now, mission managers are turning on all of the 200 or so heaters on the
spacecraft and will test the first of the spacecraft's three ion engines
in about a month, with the other two engines turned on successively over
the next six months. The ion engines will be used to propel the craft
back to Earth, so it is crucial that they function, says Kawaguchi. "As
of today, we have not been able to confirm if the ion engines operate,"
he says.
Received on Mon 20 Mar 2006 02:10:52 PM PST