Vesta--Is it Really an Asteroid?   
       
   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.   
       
   Many astronomers call Vesta an asteroid because it lies in the main asteroid   
   belt between Mars and Jupiter. But Vesta is not a typical member of that   
   orbiting rubble patch. The vast majority of objects in the main belt are   
   lightweights, 100 kilometers wide or smaller, compared with Vesta, which is a   
   530 kilometer-wide behemoth.   
       
   "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."   
   [...]   
   A model of the protoplanet Vesta, using scientists' best guess to date of what   
   the surface of the protoplanet might look like. It was created as part of an   
   exercise for NASA's Dawn mission. [more]   
       
   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."   
       
   Production Editor: Dr. Tony Phillips | Credit: Science@NASA   
       
   More Information   
   Dawn -- mission home page   
       
   Credits: 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.   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.59   
    * Origin: NCS BBS (1:3828/7)