[meteorite-list] Voyage to the Giant Asteroids (Dawn)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 15 Jun 2007 11:13:28 -0700 (PDT)
Message-ID: <200706151813.LAA02484_at_zagami.jpl.nasa.gov>

http://science.nasa.gov/headlines/y2007/15jun_dawn.htm

Voyage to the Giant Asteroids
NASA Science News
June 15, 2007

June 15, 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 about the solar system's beginnings.

These are stories planetary scientists want to hear. Much is still
unknown about our solar system's distant past. 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?

To answer these questions, NASA plans to launch a robotic probe named
Dawn. Its mission: Fly to two giant asteroids, Ceres and Vesta, and
explore them up close for the first time. Liftoff is scheduled for July
2007.

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 Chris Russell, Principal
Investigator for Dawn at the University of California, Los Angeles.
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."

A supernova (or two) 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 radioactive 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
October 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 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 NASA's Jet Propulsion Laboratory. Instead,
Dawn uses ion propulsion, which requires only one-tenth as much
propellant. Dawn's engines proved themselves onboard an earlier,
experimental spacecraft known as Deep Space 1, managed by NASA's New
Millennium Program.

Dawn's fuel-efficient ion engines will propel the craft from Vesta,
arriving at Ceres by February 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 an early window onto planet formation, it could teach
scientists about the role that water has played in planetary evolution
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.
Received on Fri 15 Jun 2007 02:13:28 PM PDT