The Dawn spacecraft was the first to ever orbit an object in the asteroid belt between Mars and Jupiter, and has been studying the asteroid Vesta since July 2011, revealing unprecedented detail on its distant past and providing astronomers with a better picture of the early history of our solar system. Now, however, it's time to say goodbye – in only a few days' time, Dawn will make its escape from Vesta's gravitational grasp and start a two and a half-year journey toward the dwarf planet Ceres.
In 2007, Dawn began an epic 3-billion mile (5-billion km) journey to study the two most massive celestial bodies in the asteroid belt. It concluded its first stint in July 2011, when it entered the orbit of the asteroid Vesta and started collecting thousands of pictures and spectral analysis data that has helped determine the asteroid's elemental composition. We now know that geological events in its distant past caused it to melt completely, forming a layered body with an iron core that make Vesta much closer to a small planet than a typical asteroid.
Now, the second leg of its journey is approaching. The probe was initially scheduled to leave on August 15, but problems on the reaction wheel – part of a system that helps the spacecraft point precisely – caused that date to slip by a few weeks. The issues have now been fully resolved and Dawn is scheduled to leave orbit on September 4 to reach Ceres, the largest among the asteroids in the belt, in early 2015.
With a mass four orders of magnitude smaller than Earth's, Vesta exerts a limited gravitational pull on the spacecraft, but even so, the escape from orbit will be far from instantaneous. Dawn has a highly efficient ion propulsion system that uses electricity to ionize xenon – which is found in abundance in the area – to generate thrust. The 12 inch (30 cm)-wide ion thrusters aren't especially powerful, but they can maintain constant thrust for months at a time, slowly increasing Dawn's elevation until the pull of gravity is no longer felt.
The exploration of the dwarf planet Ceres will be the last phase of Dawn's mission. Just as it did with Vesta, it will take comprehensive spectral data and thousands of highly detailed pictures of the dwarf planet – now appearing to us just as a blurry dot in the sky – to uncover more details on the early history of our solar system.