When is an Asteroid Not an Asteroid?

More space news appears below.  I think I'd use a term like "layered microplanet" to identify bodies by composition characteristics rather than size.  Because that's sort of a crucial thing to know, if a body has a crust and core or is just a lump of undifferentiated stuff.

JPL/NASA News

News release: 2011-100
March 29, 2011

When is an Asteroid Not an Asteroid?

On March 29, 1807, German astronomer Heinrich Wilhelm Olbers spotted
Vesta as a pinprick of light in the sky. Two hundred and four years later, as
NASA's Dawn spacecraft prepares to begin orbiting this intriguing world,
scientists now know how special this world is, even if there has been
some debate on how to classify it.

Vesta is most commonly called an asteroid because it lies in the
orbiting
rubble patch known as the main asteroid belt between Mars and Jupiter.
But
the vast majority of objects in the main belt are lightweights,
100-kilometers-wide (about 60-miles wide) or smaller, compared with
Vesta,
which is about 530 kilometers (330 miles) across on average. In fact,
numerous bits of Vesta ejected by collisions with other objects have
been
identified in the main belt.

"I don't think Vesta should be called an asteroid," said Tom McCord, a
Dawn
co-investigator based at the Bear Fight Institute, Winthrop, Wash. "Not
only
is Vesta so much larger, but it's an evolved object, unlike most things
we
call asteroids."

The layered structure of Vesta (core, mantle and crust) is the key trait
that makes Vesta more like planets such as Earth, Venus and Mars than
the
other asteroids, McCord said. Like the planets, Vesta had sufficient
radioactive material inside when it coalesced, releasing heat that
melted
rock and enabled lighter layers to float to the outside. Scientists call
this process differentiation.

McCord and colleagues were the first to discover that Vesta was likely
differentiated when special detectors on their telescopes in 1972 picked
up
the signature of basalt. That meant that the body had to have melted at
one
time.

Officially, Vesta is a "minor planet" -- a body that orbits the sun but
is
not a proper planet or comet. But there are more than 540,000 minor
planets
in our solar system, so the label doesn't give Vesta much distinction.
Dwarf
planets ? which include Dawn's second destination, Ceres -- are another
category, but Vesta doesn't qualify as one of those. For one thing,
Vesta
isn't quite large enough.

Dawn scientists prefer to think of Vesta as a protoplanet because it is
a
dense, layered body that orbits the sun and began in the same fashion as
Mercury, Venus, Earth and Mars, but somehow never fully developed. In
the
swinging early history of the solar system, objects became planets by
merging with other Vesta-sized objects. But Vesta never found a partner
during the big dance, and the critical time passed. It may have had to
do
with the nearby presence of Jupiter, the neighborhood's gravitational
superpower, disturbing the orbits of objects and hogging the dance
partners.

Other space rocks have collided with Vesta and knocked off bits of it.
Those
became debris in the asteroid belt known as Vestoids, and even hundreds
of
meteorites that have ended up on Earth. But Vesta never collided with
something of sufficient size to disrupt it, and it remained intact. As a
result, Vesta is a time capsule from that earlier era.

"This gritty little protoplanet has survived bombardment in the asteroid
belt for over 4.5 billion years, making its surface possibly the oldest
planetary surface in the solar system," said Christopher Russell, Dawn's
principal investigator, based at UCLA. "Studying Vesta will enable us to
write a much better history of the solar system's turbulent youth."

Dawn's scientists and engineers have designed a master plan to
investigate
these special features of Vesta. When Dawn arrives at Vesta in July, the
south pole will be in full sunlight, giving scientists a clear view of a
huge crater at the south pole. That crater may reveal the layer cake of
materials inside Vesta that will tell us how the body evolved after
formation. The orbit design allows Dawn to map new terrain as the
seasons
progress over its 12-month visit. The spacecraft will make many
measurements, including high-resolution data on surface composition,
topography and texture. The spacecraft will also measure the tug of
Vesta's
gravity to learn more about its internal structure.

"Dawn's ion thrusters are gently carrying us toward Vesta, and the
spacecraft is getting ready for its big year of exploration," said Marc
Rayman, Dawn's chief engineer at NASA's Jet Propulsion Laboratory,
Pasadena,
Calif. "We have designed our mission to get the most out of this
opportunity
to reveal the exciting secrets of this uncharted, exotic world."

The Dawn mission to Vesta and Ceres is managed by the Jet Propulsion
Laboratory, a division of the California Institute of Technology in
Pasadena, for NASA's Science Mission Directorate, Washington. The Dawn
mission is part of the Discovery Program managed by NASA's Marshall
Space
Flight Center in Huntsville, Ala. UCLA is responsible for overall Dawn
mission science. Orbital Sciences Corporation of Dulles, Va., designed
and
built the Dawn spacecraft. The German Aerospace Center, the Max Planck
Society, the Italian Space Agency and the Italian National Astrophysical
Institute are part of the mission team.

For more information about Dawn, visit http://www.nasa.gov/dawn and
http://dawn.jpl.nasa.gov .

Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jccook@jpl.nasa.gov

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