The Mars Observer spacecraft in the clean room.
Click on image for full size
NASA/JPL
Mars Observer
The Mars Observer set out to observe the atmosphere (detailed weather rather than climate), the magnetic field, and the surface of Mars. It carried with it 8 instruments. However, the Mars Observer, called MO for short, 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.