On September 12, 1997, the Mars Global Surveyor (MGS) entered an elliptical orbit around Mars, beginning a 20-year period of continuous robotic scientific exploration at the Red Planet. After 18 months of aerobraking to circularize its orbit around the Red Planet, MGS began its science and mapping mission, during which it returned more than 240,000 images over 4.8 Martian years, contributing a wealth of new information about Mars, its atmosphere and its satellites. Although MGS stopped transmitting in November 2006, by that time it had been joined at Mars by three other orbiters and on the surface by two rovers. Subsequently, three other orbiters and one more rover have continued the scientific exploration of Mars; one of the orbiters has been active for 16 years. The scientific knowledge obtained by these spacecraft significantly increased our understanding of Mars.