[meteorite-list] Blowing a Hole in a Comet: Take 2

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 27 Sep 2007 17:06:37 -0700 (PDT)
Message-ID: <200709280006.RAA03272_at_zagami.jpl.nasa.gov>

http://science.nasa.gov/headlines/y2007/26sep_next.htm

Blowing a Hole in a Comet: Take 2
NASA Science News
September 26, 2007

Sept. 26, 2007: The flash! The dazzle! The front page of the New York
Times! Two years ago, NASA's Deep Impact spacecraft dropped an 820 lb
copper projectile onto Comet Tempel 1, unleashing an explosion that made
headlines around the world.

Exploding comets tend to have that effect. But how many people know
what happened after the blast? The surprising answer is none--not even
NASA.

Deep Impact's prime mission was to punch a hole in Tempel 1 and look
inside, giving researchers their first glimpse of a comet's internal
structure. But "we were never able to see the crater because the cloud
of debris was so thick," says Michael New of NASA Headquarters.

Why didn't Deep Impact wait until the dust cleared? It couldn't. The
mission was designed from the beginning as a high-speed flyby, giving
extra velocity to the "bullet." Orbiting was not an option. Carried by
its own momentum, Deep Impact sailed away before the cloud had time to
dissipate.

Take 2: NASA is going back for a second look.

"We're sending another spacecraft back to Tempel 1, the Stardust probe,"
says New.

Stardust is famous for its January 2004 flyby of Comet Wild 2. Severely
buffeted by jets of gas and debris flowing from the comet, Stardust
nevertheless managed to snatch thousands of samples of comet dust and
return them to Earth for analysis. "Stardust is one of the great
successes of NASA's Discovery program," says New. (The Discovery program
launches innovative, inexpensive spacecraft every 18-to-24 months on
cutting edge missions. Deep Impact is also part of this program.)

At first, Stardust was simply retired, sailing the void with nothing to
- but now it is being recycled as "Stardust-NExT," short for New
Exploration of Tempel 1. Planetary science professor Joe Veverka of
Cornell University is the mission's principal investigator.

"We're very excited to go back," says Veverka. "Stardust is due to reach
Comet Tempel 1 in 2011. By then the debris cloud will be long gone and
we should get a clear view of the crater."

Peering into the crater, however, "is only half the story," says
Veverka. Before the cloud spoiled the view, Deep Impact's cameras
recorded some surprising things:

For one, the comet is ringed by a strangely-layered "sedimentary"
terrain. There are no rivers on comets, so what causes these features?
"Good question," says Veverka. One possibility: comets might be formed
in layers. "Imagine two small proto-comets smashing into one another,
sticking together and flattening like pieces of playdough," he says. Or
maybe the layers are created via some form of hot erosion when the comet
swings past the sun every 6.5 years. "We just don't know."

Stardust will gather important clues. "We're returning to the comet
almost exactly one orbit--that is, one comet-year--after the first
visit. This gives us a chance to see how solar heating might have
altered Tempel 1's face."

Another surprise was landslides. "Deep Impact saw an enormous flow of
smooth, powdery material" completely covering about a kilometer of
underlying terrain, says Veverka. This feature is as mysterious as the
layers, but it could explain one thing: why Deep Impact's debris cloud
was so troublesome. "We might have hit a patch of deep powder," adds
New. "Fine particles tend to make big clouds that are hard to see through."

"This is why we explore," adds Veverka. "Tempel 1 is an amazing comet."

Veverka notes that recycling a mission like Stardust is cheaper than
sending a whole new spacecraft. "Stardust-NExT costs less than 15% of a
full-up Discovery mission."

"Giving new assignments to veteran spacecraft represents not only
creative thinking and planning, but also a prime example of getting more
from the budget we have," agrees Alan Stern, associate administrator of
NASA's Science Mission Directorate.

Deep Impact is being recycled, too. "We're using Deep Impact for two new
projects," explains New. One is called DIXI (Deep Impact Extended
Investigation): "Deep Impact will fly by Comet Boethin in December 2008
for a close-up investigation of the comet's nucleus." The second is
EPOCh (Extrasolar Planet Observation and Characterization): "Cameras on
Deep Impact will target nearby stars with known giant planets. By
watching these planets transit (pass in front of) their stars, Deep
Impact will be able to determine whether they possess rings and/or
moons." For this work, EPOCh's sensitivity will exceed that of existing
ground and space-based observatories, possibly leading to the discovery
of new Earth-sized planets.

No crater? No problem. Says New: "You can't keep a good Discovery
mission down."
Received on Thu 27 Sep 2007 08:06:37 PM PDT