What Happened to the Mars Observer?
An independent NASA review board concluded that the most likely cause of the loss of communication with the Mars Observer was a rupture in a line in the propulsion system during the start of fuel tank pressurization. The board cautioned that "There was no specific evidence about what actually transpired during the pressurization sequence".
On August 21, 1993 the spacecraft transmitters were turned off during the final approach to Mars to protect the components against shock from the pressurization sequence. After the transmitter was turned off the tanks were supposed to be pressurized and then the transmitters turned back on and communications with Earth resumed, but no further signals were ever received on Earth. The hypothesis is that a small amount of nitrogen tetroxide may have leaked through the check valves during the 11 month voyage to Mars and condensed in the pressurization lines. During pressurization, the oxidant would have mixed with the monomethylhydrazine fuel, causing combustion and rupture of the fuel lines. The resultant high-pressure expulsion of gasses through the rupture would have started the spacecraft spinning uncontrollably and making communication with Earth impossible. The reason that this was considered to be the most likely scenario is that the pressure check valves were not designed for a mission in which tank pressurization took place after 11 months, but rather for a pressurization soon after launch. The decision to pressurize just before entering Mars orbit was made after the check valves were already built. It is therefore likely that some oxidant could have leaked past the valves over the 11 month period.
Other scenarios that were considered likely were: 1) A failure of the pressure regulator, causing the oxidizer tank to burst, 2) A massive short in the electrical system, or 3) Damage to the fuel tank caused by an impact from one of the pyrotechnic devices fired to open valves in the pressurization lines.
Press Release 1
Reports on the loss of Mars
Observer:
MARS OBSERVER SPACECRAFT LOSES COMMUNICATIONS WITH EARTH
MARS OBSERVER MISSION STATUS
MARS OBSERVER ORBITAL ELEMENTS
PRELIMINARY OBSERVATIONS
Conclusions: