Nasa probe enters orbit around 'potato' asteroid Vesta after four year journey - Telegraph.co.uk

With its iron core and possible lava flows, scientists believe Vesta is more similar to Earth or the moon than most of its other asteroid neighbours.

Ceres, the largest object in the asteroid belt, is relatively close to Vesta, but it formed under vastly different circumstances. The so-called dwarf planet more closely resembles the icy moons of Jupiter and Saturn. Ceres has water-bearing minerals and possibly a weak atmosphere.

The goal of the Dawn mission is to collect enough information about Vesta and Ceres to understand conditions and processes of the early solar system. The spacecraft has three scientific instruments to study surface features and determine chemical composition.

"We are exploring backward in time as far as we can," said lead scientist Christopher Russell, with the University of California at Los Angeles.

Several spacecraft have flown by asteroids before, including Nasa's Galileo probe, which encountered three asteroids on its way to Jupiter.

Dawn, however, is the first probe to go into orbit around an asteroid for a long-term study. It also would be the first spacecraft to rendezvous with more than one object in the solar system.

Dawn travelled 1.7 billion miles to reach Vesta.

It is expected to depart in July 2012 to begin the three-year, 930 million mile trek to Ceres. The mission at Ceres is expected to last six months.