Figure: Artist's concept of Mars Global Surveyor.

NASA's Mars Global Surveyor orbiter is the oldest Mars spacecraft currently in operation, and it has been studying the red planet for nearly a decade. Mars Global Surveyor was the first successful U.S. mission launched to Mars since the Viking mission in 1976. After a 20-year absence at the planet, Mars Global Surveyor ushered in a new era of Mars exploration with its five science investigations. Mars Global Surveyor arrived at Mars on September 11, 1997 (September 12, UTC), and has since returned more scientific data about the evolution of the red planet than all other Mars missions combined.

Image Credit: NASA/JPL

http://mars.jpl.nasa.gov/mgs/overview/