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Sept.
27, 2007: The asteroid belt between Mars and Jupiter
is like the solar system's cluttered old attic. The dusty,
forgotten objects there are relics from a time long ago, each
asteroid with its own story to tell.
These
are stories planetary scientists are eager to hear. Much is
still unknown about our solar system's beginnings. We learn
the basic story in school: A vast disc of gas and dust around
the sun slowly gathered into larger and larger chunks, eventually
forming the planets we know today. But how exactly did this
happen, and why did it produce the kinds of worlds that it
did, including a certain blue planet well-suited for life?
 Right:
NASA's Dawn spacecraft lifts off from Kennedy Space Center
on Sept. 27, 2007. [More]
To
answer these questions, today (Sept. 27th) NASA launched a
robotic probe named Dawn. Its mission: Fly to two giant asteroids,
Ceres and Vesta, and explore them up close for the first time.
Shortly after launch, Dawn signaled ground control to say
that all
is well: the spacecraft is oriented properly and receiving
power from its massive solar arrays. "Dawn has risen,
and the spacecraft is healthy," says the mission's project
manager Keyur Patel of JPL.
Vesta,
for starters
Dawn's
first stop is Vesta—an asteroid that may implicate ancient
supernovas in the solar system's birth.
Telescopic
observations of Vesta and studies of meteorites believed to
have come from Vesta suggest that the asteroid may have been
partially molten early in its history, allowing heavy elements
like iron to sink and form a dense core with a lighter crust
on top.
"That's
interesting--and a bit puzzling," says Dawn's principal
investigator Christopher Russell of UCLA. Melting requires
a source of heat such as gravitational energy released when
material comes together to make an asteroid. But Vesta is
a small world—"too small," he says--only about 530
km across on average. "There would not have been enough
gravitational energy to melt the asteroid when it formed."
 Right:
A Hubble Space Telescope photo of Vesta. [More]
A
supernova may provide the explanation: Some scientists believe
that when Vesta first formed, it was "spiced up"
by aluminum-26 and iron-60 created in possibly two supernovas
that exploded around the time of the solar system's birth.
These forms of iron and aluminum are radioactive isotopes
that could have provided the extra heat needed to melt Vesta.
Once these isotopes decayed, the asteroid would have cooled
and solidified to its present state.
This
idea would explain why Vesta's surface appears to bear the
marks of ancient basaltic lava flows and magma oceans, much
as Earth's moon does. The supernovas would also change the
sequence of events involved in planet formation:
"When
I went to school, the thought was that the Earth got together,
heated up, and the iron went to the center and the silicate
floated on top, producing a core-forming event," Russell
says. This view assumes that smaller planetoids that collided
and merged to form Earth were amorphous masses that hadn't
yet formed their own iron cores. But if chunks of rock the
size of Vesta could melt and form dense cores, "it would
affect the way the planets and their cores grew and evolved."
If
all goes as planned, Dawn would reach Vesta and enter orbit
in the year 2011. Detailed images of Vesta's surface will reveal
traces of its molten past, while spectrometers catalog the minerals
and elements that make up its surface. Vesta's gravitational
field will be mapped out by the motions of Dawn itself as the
probe orbits the asteroid, and that should settle once and for
all whether Vesta indeed has an iron core.
On
to Ceres
After
orbiting Vesta for about 7 months, Dawn will undertake a maneuver
never before attempted: leave the orbit of one distant body,
and fly to and orbit another.
 This
kind of "asteroid hopping" would be practically
impossible if Dawn used conventional rocket fuel. "We
would need one of the largest rockets that the US has to carry
all the propellant," says Marc Rayman, Project System
Engineer for Dawn at JPL. Instead, Dawn uses ion propulsion,
which requires only one-tenth as much propellant.
Right:
A Hubble Space Telescope photo of Ceres. [More]
Dawn's
fuel-efficient ion engines--famously nicknamed "the
Prius of Space"--will propel the craft from Vesta,
arriving at Ceres by 2015.
Measuring
950 km in diameter, Ceres is by far the largest object in
the asteroid belt. Remarkably, it is not a rocky world like
Vesta, but one covered in water ice. "Ceres
is going to be a real surprise to us," says Russell.
Because it appears to harbor a layer of ice 60 to 120 km thick,
the surface of Ceres has probably changed more dramatically
over time than Vesta's, obscuring much of its early history.
But while Ceres may not offer such a clear window onto the
earliest epoch of planet formation, it could teach scientists
about the role that water has played since then. For example,
why can some rocky worlds like Ceres and Earth hold on to
large amounts of water, while others, like Vesta, end up bone
dry?
"Vesta
will tell us about the earliest epoch, and Ceres will tell
us about what happened later," Russell says. Together,
they offer two unique stories from our solar system's past,
and who-knows-how-many lessons about how the planets came
to be.
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Author: Patrick Barry | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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