The Mars Observer (MO)set out to observe the atmosphere, the magnetic field, and the surface of Mars. It carried with it 8 instruments. However, the Mars Observer failed on August 21, 1993, just prior to entering Martian orbit. Over the long cruise from Earth to Mars, some of the fuel leaked into the engines. When the engines were ignited for orbit entry, the fuel tanks blew up.
Out of the failure of Mars Observer came the Mars Surveyor Program. The instruments carried by MO will now be carried by the Mars Global Surveyor, currently gathering data, Mars '98, as well as Mars 2001.