Vesta is an asteroid that is thought to be a remnant protoplanet with a differentiated interior and a mean diameter of about 530 km. Comprising an estimated 9% of the mass of the entire asteroid belt which lies between Mars and Jupiter. It is the second-most-massive object in the belt after the dwarf planet Ceres. NASA's Dawn spacecraft is on track to begin the first extended visit to a large asteroid. The mission expects to go into orbit around Vesta on July 16 and begin gathering science data in early August. Vesta resides in the main asteroid belt and is thought to be the source of a large number of meteorites that fall to Earth.
Vesta's shape is relatively close to a gravitationally relaxed oblate spheroid, but the large concavity and protrusion at the pole combined with a mass less than 5×1020 kg precluded Vesta from automatically being considered a dwarf planet under International Astronomical Union rules. Vesta may be listed as a dwarf planet in the future, if it is convincingly determined that its shape, other than the large impact basin at the southern pole, is due to hydrostatic equilibrium, as currently believed. Initially when first discovered Ceres, Pallas, Juno and Vesta were classified as planets.
After traveling nearly four years and 1.7 billion miles, Dawn is approximately 96,000 miles away from Vesta. When Vesta captures Dawn into its orbit on July 16, there will be approximately 9,900 miles (16,000 kilometers) between them. When orbit is achieved, they will be approximately 117 million miles away from Earth.
After Dawn enters Vesta's orbit, engineers will need a few days to determine the exact time of capture. Unlike other missions where a dramatic, nail-biting propulsive burn results in orbit insertion around a planet, Dawn has been using its placid ion propulsion system to subtly shape its path for years to match Vesta's orbit around the sun.
"Navigation images from Dawn's framing camera have given us intriguing hints of Vesta, but we're looking forward to the heart of Vesta operations, when we begin officially collecting science data," said Christopher Russell, Dawn principal investigator, at UCLA.
Dawn's three instruments are all functioning and appear to be properly calibrated. The visible and infrared mapping spectrometer, for example, has started to obtain images of Vesta that are larger than a few pixels in size. During the initial reconnaissance orbit, at approximately 1,700 miles the spacecraft will get a broad overview of Vesta with color pictures and data in different wavelengths of reflected light. The spacecraft will move into a high-altitude mapping orbit, about 420 miles above the surface to systematically map the parts of Vesta's surface illuminated by the sun; collect stereo images to see topographic highs and lows; acquire higher-resolution data to map rock types at the surface; and learn more about Vesta's thermal properties.
Dawn then will move even closer, to a low-altitude mapping orbit approximately 120 miles (200 kilometers) above the surface. The primary science goals of this orbit are to detect the byproducts of cosmic rays hitting the surface and help scientists determine the many kinds of atoms there, and probe the protoplanet's internal structure.
"We've packed our year at Vesta chock-full of science observations to help us unravel the mysteries of Vesta," said Carol Raymond, Dawn's deputy principal investigator at JPL. Vesta is considered a protoplanet, or body that never quite became a full-fledged planet.
Dawn launched in September 2007. Following a year at Vesta, the spacecraft will depart for its second destination, the dwarf planet Ceres, in July 2012.
For further information: http://www.jpl.nasa.gov/news/news.cfm?release=2011-192&rn=news.xml&rst=3044#1