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.
MESSENGER image of planet Mercury (left)
Date acquired: September 29, 2009
Image Mission Elapsed Time (MET): 162741055
Instrument: Wide Angle Camera (WAC) of the Mercury Dual Imaging System (MDIS)
WAC filter: 7 (748 nanometers)
Scale: Mercury's diameter is 4880 km (3030 mi.)
Dawn image of asteroid Vesta (right)
Date acquired: July 18, 2011
Instrument: Dawn Framing Camera, clear filter
Scale: Vesta's diameter is about 530 km (329 mi.)
These images are from MESSENGER, a NASA Discovery mission to conduct the first orbital study of the innermost planet, Mercury. For information regarding the use of images, see the MESSENGER image use policy.