On its five billion kilometre journey, the $267m spacecraft will take data using three instruments: a visible-light camera and two spectrometers for mapping neutrons and visible and infrared light. These will be used to detect surface minerals and catalogue the elements present on Ceres and Vesta.

Dawn

After being flung around Mars in a “gravitational slingshot”, Dawn should reach Vesta in August 2011. Roughly 500 km in diameter, Vesta is the largest asteroid in the asteroid belt and comprises a rocky core with a surface of solidified lava. Scientists will use data from Dawn to try to understand what caused the giant crater at Vesta’s south pole — an event that reduced the asteroid's mass by 1%, and could have ejected enough material to form many of the other asteroids in the belt and meteorites found on Earth.

The second rendezvous for Dawn will be in 2015 when it reaches the dwarf planet Ceres. Astronomers believe Ceres has a different structure to Vesta, and could contain ice or even liquid water buried underneath its crust.

The main question for scientists is how these two very different types of bodies could have formed in the same region. When the planets began to emerge some 4.5 billion years ago, rocky planets formed close to the Sun where it was hotter, leaving icy planets to form farther away. Had it not been for Jupiter’s huge gravitational influence, Vesta and Ceres could have gathered enough material for them to become planets in their own right. The Dawn mission hopes to compare the evolutionary path they took, and in doing so throw light on the early Solar System.

It was not an easy journey to the launch pad for Dawn, however. The mission was first cancelled in 2003, but reinstated a year later. Then, in October 2005, NASA was forced to “stand down” Dawn because of cost overruns and a critical fault with its ion engines. In addition, the mission team admitted that there had been a power struggle between scientists working on small-scale missions and the NASA management. It was only after a new management system was introduced that Dawn was put back on track at the end of March this year.