Mars Global Surveyor entered a 258 x 54021 km x 93.3 deg polar orbit around Mars on 12 September 1997 after a 22 minute burn of its main engine. After a long aerobraking phase to a lower circular orbit, the spacecraft began its primary mission of photographing and observing changes on the Martian surface in March 1999. After nearly ten years of service, the last signals from MGS were received on 3 November 2006. The spacecraft went silent after an incorrect software upload caused its solar arrays to lose power.

The Mars Climate Orbiter was the second flight of the Mars Surveyor Program. The probe was to enter a 160 km x 38600 km polar orbit around Mars on September 23,1999, and use aerobraking to reach a 373 km x 437 km x 92.9 degree sun-synchronous mapping orbit by November 23 1999. While the Mars Orbit Insertion burn began as planned on September 23, 1999 at 08:50 GMT, no signal was received after the spacecraft went behind the planet. Subsequent investigation showed that the spacecraft had plunged deep into the Martian atmosphere, with its closest approach to Mars being 57 km. It was concluded that the spacecraft burnt up in the atmosphere. It was later found that cutbacks in tracking, combined with incorrect values in a look-up table imbedded deep in the spacecraft software (use of pounds force instead of newtons) were to blame. This failure led to a shake-up of NASA's 'faster, better, cheaper' approach to unmanned spaceflight. Additional Details: here....