KNOXVILLE — A University of Tennessee geologist is part of NASA’s latest mission, which seeks to study the two largest known asteroids in the solar system.

Dr. Harry Y. “Hap” McSween has been named to NASA’s Dawn mission, which launches in 2006 on a nine-year journey to orbit the asteroids Vesta and Ceres.

“These asteroids are two ‘baby planets’ that are very different from each other yet both contain tantalizing clues about the formation of the solar system,” McSween said.

NASA will use special instruments to observe the two bodies located in the asteroid belt between Mars and Jupiter, McSween said.

Ceres, the largest asteroid, has a diameter of about 600 miles. Scientists are intrigued by its primitive development, including a relatively unscathed surface, water-bearing minerals and possibly a very weak atmosphere and frost, McSween said.

Vesta is the brightest asteroid and the only one visible by the naked eye. Its average diameter is about 320 miles.

Vesta’s surface is dry and has been resurfaced by lava flows, McSween said. Like the Earth’s Moon, it has been hit many times by smaller space rocks, sending out meteorites at least five times in the last 50 million years. McSween is among those scientists who have studied meteorites from these impacts.

During its journey through the asteroid belt, Dawn will rendezvous with Vesta and Ceres, orbiting from 500 miles to about 62 miles above the surface.

The mission will determine these asteroids physical attributes such as shape, size, mass, craters and internal structure, and produce data on more complex properties such as composition, density and magnetism, McSween said.

McSween was a science team member of NASA’s Mars Pathfinder, Mars Global Surveyor and Mars Odyssey missions. He also will be working on the Mars Exploration Rover mission in 2003.

An advisor to NASA and the National Research Council, McSween was president of the Meteoritical Society and chaired the Planetary Division of the Geological Society of America. He won the Leonard medal, the top award in planetary science, in 2001.

Dawn’s principal investigator is Dr. Christopher T. Russell of the University of California, Los Angeles. It is one of two missions selected for funding from 26 NASA proposals in 2001. The second is Kepler, a spaceborne telescope which will look for earth-like planets.

Information about Dawn is available at: