In March 2011, MESSENGER became the first spacecraft to orbit the planet Mercury. In July of the same year, the Dawn spacecraft became the first to orbit a main-belt asteroid, Vesta. Both MESSENGER and Dawn are missions in the Discovery program, NASA's lowest-cost category of planetary mission. The image above shows Mercury on the left, and Vesta on the right. Both surfaces are marked by impact craters, but the most immediately noticeable difference is that Vesta has a much more irregular shape. This is a consequence of Mercury's far larger gravity, which has squeezed the planet into a sphere. Vesta's weak gravity is less able to overcome the strength of the rocks. Mercury's mass is about 1300 times greater than that of Vesta. The MESSENGER spacecraft is the first ever to orbit the planet Mercury, and the spacecraft's seven scientific instruments and radio science investigation are unraveling the history and evolution of the Solar System's innermost planet. During the one-year primary mission, MDIS is scheduled to acquire more than 75,000 images in support of MESSENGER's science goals.