Mars Observer was a NASA mission to study the surface, atmosphere, interior and magnetic field of Mars from Martian orbit.
The mission was designed to operate for one full Martian year (687 Earth days) to permit observations of the planet through its four seasons.
Contact with Mars Observer was lost on August 21, 1993, three days before scheduled orbit insertion, for unknown reasons and has not been re-established. It is not known whether the spacecraft was able to follow its automatic programming and go into Mars orbit or if it flew by Mars and is now in a heliocentric orbit. Later investigation concluded the most probable cause of the mishap was a fuel line rupture during fuel tank pressurization which would have caused the spacecraft to spin uncontrollably. Although none of the primary objectives of the mission were achieved, cruise mode data were collected up to loss of contact. The total cost of the Mars Observer mission including development, construction, launch, and ground support is estimated at $813 million.
Later investigation indicated this was due to a propulsion system explosion caused by propellants leaking past faulty valves.