[meteorite-list] When is an Asteroid Not an Asteroid?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 29 Mar 2011 15:08:10 -0700 (PDT)
Message-ID: <201103292208.p2TM8A1r015345_at_zagami.jpl.nasa.gov>

http://www.jpl.nasa.gov/news/news.cfm?release=2011-100

When is an Asteroid Not an Asteroid?
Jet Propulsion Laboratory
March 29, 2011

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 at jpl.nasa.gov

2011-100
Received on Tue 29 Mar 2011 06:08:10 PM PDT