- Scientists at the Jet Propulsion Laboratory may get some
clues to a mystery Friday when NASA's Cassini space probe comes within
1,250 miles of Phoebe, one of Saturn's most mysterious moons.
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- Phoebe has boggled astronomers for over 100 years because
of its dark surface and retrograde orbit, which sends it spinning around
Saturn in the opposite direction from the planet's 30 other known moons.
Many scientists now believe the 137-mile-wide moon may be a captured asteroid
that has remained unchanged since it was first formed in the outer solar
system. If Friday's flyby proves that theory to be true, data gathered
from Cassini's instruments could shed new light on how planets are formed.
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- "The key thing here is that Phoebe is such a unique
object in the Saturn system (that) we wanted to make sure we got a chance
to observe it," said Amanda Hendrix, a planetary scientist with the
JPL in Pasadena, California. "We're basically getting to sample an
object that came from farther out in the solar system, and we're getting
to do that for free."
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- Of course, the Cassini mission itself is far from free
-- the orbiter and its attached Huygens probe will cost NASA, the European
Space Agency and the Italian Space Agency over $3.26 billion by the time
the mission is over. But the timing and trajectory of Cassini's main mission
to Saturn puts it on a path that swings by Phoebe, allowing researchers
to study the quirky moon as a bonus.
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- Cassini will spend approximately 30 hours photographing
Phoebe up close and taking measurements with other instruments. The climax
of the event will come at 1:56 p.m. PDT on Friday, when the orbiter will
pass within 1,250 miles of the moon.
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- In contrast, the Voyager 2 spacecraft that photographed
Phoebe in 1981 only came within 1.4 million miles of Phoebe. Though the
images taken during that mission helped scientists get a better understanding
of Phoebe, the photos this time around will be 1,000 times more detailed
and are expected to reveal features of the moon that have never before
been seen.
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- The photos will help JPL scientists get "a really
good idea of the size and exact volume of Phoebe," said Hendrix. "This
can be coupled with tracking data to know the mass better than is known
now, and this will tell us about density and internal structure -- whether
it's made up of ice, or rock and ice."
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- All this information will be compared with what is known
about asteroids in the Kuiper Belt, an area of icy debris 7.5 billion to
9.3 billion miles from the sun. If the data match, scientists will be able
to say with a good deal of certainty that Phoebe is indeed a "Centaur"
-- an asteroid that has migrated from the Kuiper Belt into the inner solar
system.
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- The data from the Phoebe flyby will also be compared
with data collected from a later flyby of Iapetus, one of Saturn's larger
moons. Scientists hope the comparison will reveal whether the dark side
of Iapetus is composed of dust from Phoebe.
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- Cassini is scheduled to rendezvous with Saturn on June
30, where it will become the first spacecraft ever to orbit the planet.
The orbiter will spend the next four years studying Saturn and its moons.
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- NASA officials are crossing their fingers in hopes of
some big surprises along the way: most notably, the discovery of more satellites,
or small moons, orbiting Saturn. "Cassini will almost certainly discover
new moons around Saturn," said Hendrix, noting that the prior Galileo
space probe found more around Jupiter.
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- Another big moon event will take place on Dec. 24, when
Cassini releases the European Space Agency's Huygens probe above Titan,
Saturn's largest moon. The probe will drift through Titan's atmosphere,
taking as many measurements as possible before crashing into Titan's surface.
Titan is especially intriguing to astronomers and astrophysicists because
it contains its own atmosphere and has a moon of its own, which essentially
makes it a planet that has been caught by Saturn's gravitational pull.
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- NASA officials are still unclear how the Cassini mission
will end once its scheduled observations are completed. If the spacecraft's
instruments and power systems show little sign of wear in 2008, the space
agency could decide to extend the mission, just as they did with the Mars
Exploration Rovers. Or, if time and the stresses of space have taken their
toll, Cassini could get sucked in by Saturn's gravitational pull and burn
up in its atmosphere, much as the Galileo spacecraft did in Jupiter's atmosphere.
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