On 13 December 2011, NASA's Dawn spacecraft reached its lowest orbit around giant asteroid Vesta (530 km across), the most massive asteroid, and relayed its most detailed images of Vesta’s surface. The images show numerous small craters and other terrains.
A gallery of images can be found online at:
http://www.nasa.gov/mission_pages/dawn/multimedia/gallery-index.html
The asteroids are small rocky objects, orbiting the Sun mainly between the orbits of Mars and Jupiter, in a zone known as the asteroid belt. It is believed that the asteroids are primitive material, leftovers from the primordial cosmic cloud from which the Solar System formed. By studying asteroids, scientists hope to achieve a better understanding of the origin and evolution of the Solar System.
The Dawn spacecraft was launched into space in September 2007. It reached Vesta, and entered orbit around it, on 16 July 2011. Dawn applies sophisticated instruments to map the surface of Vesta, probe its internal structure and study its surface chemical composition. Dawn’s initial Vesta orbit has been gradually modified, to achieve the ultimate mapping orbit, only 210 km above Vesta’s surface.
In July 2012, Dawn will depart Vesta, and cruise in interplanetary space, toward its next target, dwarf planet Ceres (about 950 km across), the largest object in the asteroid belt. It is expected that Dawn will reach Ceres in February 2015.
Vesta orbits the Sun every 3.6 years, at an average distance of approximately 353 million km. Ceres orbits the Sun every 4.6 years, at an average distance of approximately 414 million km. For comparison, Earth’s average distance from the Sun is approximately 150 million km.
References
NASA
www.nasa.gov/
Wikipedia
Further Reading
http://www.nasa.gov/dawn
http://dawn.jpl.nasa.gov