The Mars Observer is a satellite launched to orbit Mars. It was launched on September 25, 1992. At the time of its launch, it was the first U.S. mission to Mars in 17 years. The Mars Observer cost $813,000,000 to create.
The Mars Observer was designed to study the geology, geophysics, and the climate of Mars. The Observer was meant to take high-resolution photographs of the surface of Mars, and to send those pictures back to Earth.
Unfortunately, the Mars Observer did not complete its mission. On August 22, 1993, NASA lost contact with the Observer, right as it was about to begin its orbit of Mars. However, some future missions to Mars have used some of the instruments designed for the Observer, such as the Mars Global Surveyor and the 2001 Mars Odyssey.