On the 16th of July 2011 the NASA space probe Dawn moved into orbit around the asteroid Vesta, 188 million km from the Earth, where it will remain for the next year.
An artist's impression of the Dawn space probe.
Vesta was discovered by German astronomer Heinrich Wilhelm Olbers in 1807, the forth object to be discovered in what we now call the Asteroid Belt. It has a diameter of about 530 km and is the second most massive object in the belt, potentially containing 9% of the total mass of the Asteroid Belt. Vesta orbits at a distance of 2.5 AU (i.e. 2.5 times as far from the sun as the Earth is). It is roughly spherical in shape, but is slightly to small to be considered a dwarf planet. Scientists believe that its shape indicates that it underwent at least partial melting early in its history, due to the decay of radioactive elements in its core - the same process that keeps the interior of the Earth molten. Vesta has a number of prominent craters, most notably the 460 km diameter (80% of that of the whole asteroid) at it's south pole.
Vesta.
The Dawn probe is designed to shed light on the early development of the solar system by examining Vesta and Ceres, the two largest objects in the asteroid belt, both of which are thought to be relatively unchanged since their formation, early in the history of the system. The Dawn probe is powered by an ion drive, a new technology which has the potential to vastly improve our ability to explore the solar system. The drive uses two large solar panels to ionize Xenon gas, which is then fired through an electric field creating thrust. This generates less thrust than a conventional rocket engine, but is much smaller and can maintain thrust for much longer, making it a far more efficient system as long as you do not wish to get anywhere in a hurry.
A diagrammatic representation of the ion drive which powers the Dawn probe.
See also Asteroid 2011MD.