When is an Asteroid Not an Asteroid?

MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109 TELEPHONE 818-354-5011
http://www.jpl.nasa.gov

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

News release: 2011-100 March 29, 2011

When is an Asteroid Not an Asteroid?

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2011-100&cid=release_2011-100

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 .

-end-

To remove yourself from this mailing, please go to http://www.kintera.org/TR.asp?a=kmKVJiM1IgJYJsL&s=iiLYI1PFJeKKI0NLLsH&m=iwKYI9NUJnL6ItI

To remove yourself from all mailings from NASA Jet Propulsion Laboratory, please go to http://www.kintera.org/TR.asp?a=dfIHKXNzG9ILL8K&s=iiLYI1PFJeKKI0NLLsH&m=iwKYI9NUJnL6ItI