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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.
Right:
Deep Impact strikes Comet Tempel 1. [More]
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 do—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:
Above: Surprising terrain on Comet Tempel
1. Credit: Deep Impact.
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."
 Right:
Comet Tempel 1--the view from Stardust in 2011. [Larger
image] [animation]
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."
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Author: Dr.
Tony Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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