[meteorite-list] Stardust's Precious Comet Cargo
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:32:52 2004
Stardust's Precious Comet Cargo
By David Tytell
Sky & Telescope
March 18, 2004
With all the excitement surrounding the Mars Exploration Rovers since
they landed in January, few people realize that another space triumph
happened two days before Spirit touched down on the red planet. On
January 2nd a spacecraft called Stardust successfully flew by Comet
81P/Wild 2, extended a tennis-racket-shaped collector, and, as its
name implies, caught bits of cometary dust to return to Earth. On
Tuesday a standing-room-only crowd of researchers at the Lunar and
Planetary Science Conference in Houston, Texas, eagerly listened as
the Stardust team members shared the early results of the flyby.
The mission's primary goal was to bring back at least 500 particles
bigger than 15 microns in size. According to Stardust's principal
investigator, Donald Brownlee (University of Washington), "We
beat that by a factor of at least a few." The spacecraft also took
72 images of the comet's nucleus from as little as 240 kilometers
(149 miles) away, spotting details with a resolution better than 20
meters per pixel. These shots revealed a body covered with scarps,
vertical cliffs, and a "monument valley" complete with columns,
pyramids, cones, and 100-meter-high spires.
These two views of Comet Wild 2, taken from slightly different
angles, can form a 3-D stereo image. Gaze "through" the screen,
letting your eyes relax and separate as if looking at something in
the far distance. The images of the comet nucleus should drift and
overlap. If you get them to overlap exactly, they will snap into a
"fused" 3-D image. The relief on the comet's surface that you'll
see this way is exaggerated. The actual shape of the nucleus, says
Stardust project manager Thomas Duxbury, "resembles a thick
hamburger patty with a few bites taken out." Courtesy NASA / JPL
Stardust's cameras also confirmed that the comet's 5-km (3-mile)
wide nucleus is quite active. More than a dozen highly collimated jets
of gas and dust were seen spewing from the nucleus, and one of them was
clearly associated with a distinct surface feature. Wild 2 is "unlike
any other body in the solar system," says Brownlee. "There is no
featureless terrain. It's incredibly feature-rich."
Wild 2 came with other surprises. Most notable was the comet's coma,
the hazy cloud of dust and gas that surrounds the nucleus. Astronomers
had suspected that as Stardust passed through the coma, its
dust-flux-monitoring instrument would detect a smooth increase of
particles that would peak at closest approach and recede the same way.
That's what the Giotto spacecraft found when it passed through Comet
1P/Halley's coma in 1986. The predictions couldn't have been more
wrong. Instead Stardust detected the dust in "clumps," recording sudden
bursts of hits followed by lulls. To explain the phenomenon Benton C.
Clark (Lockheed Martin) proposed that Wild 2 actually releases mass in
"chunks and aggregates that later become dust clouds."
Perhaps the reason is the comet's recent history. Wild 2 was originally
in a long-period orbit that kept it far from the Sun. But in 1974 an
encounter with Jupiter changed its orbit, bringing it into the inner
solar system. Thus, in the past three decades Wild 2 likely
experienced fresh stresses and cracks that could have led to the loss
of large chunks of the largely pristine icy body.
Stardust did come with some disappointments. Scientists were unable to
determine the mass and thus the density of the comet's nucleus. The
best they could distinguish from the noisy data was an upper bound of
5 trillion tons. Additionally, the Cometary and Interstellar Dust
Analyzer, a mass spectrometer, only recorded 29 events - only a
hundredth of what was predicted, says Clark. He suspects that as
the craft was being pelted with debris it needed to fire its thrusters
to correct its orientation, which might have caused an "unintended
shadowing" of the instrument. Bad timing is also possible - perhaps
Stardust simply flew past Wild 2 during a period of little outburst
activity. The particles that the instrument did see had an organic-rich
composition and were deficient in rock-forming minerals. But the CIDA
results remain preliminary.
Stardust's sealed sample canister will land in Utah on January 15, 2006.
It will be then whisked away to a special lab being constructed at
NASA's Johnson Space Center in Houston. There scientists will mine
the collected particles looking for embedded gases, ratios of key
atomic isotopes, minerals, and organic compounds.
Received on Fri 19 Mar 2004 08:12:40 PM PST