Several potential causes that may have been responsible for the loss of the Mars Observer spacecraft last August have been identified by a special review board at the Jet Propulsion Laboratory.

The panel, chaired by JPL Deputy Assistant Laboratory Director Dr. R. Rhoads Stephenson, was appointed by JPL Deputy Director Larry N. Dumas as required by JPL management procedures after contact was lost with Mars Observer on August 21 three days before it was to enter orbit around the red planet.

According to Stephenson, the board's findings are generally consistent with those of an independent mission failure review board appointed by NASA Administrator Daniel Goldin and chaired by Dr. Timothy Coffey of the Naval Research Laboratory.

NASA is formulating a corrective action plan based on the independent review board's recommendations.

"Each of the review teams weighted the various hypotheses slightly differently, but we came to the same general conclusions about the loss," said Stephenson.

The JPL board's report says one of several potential causes was most likely to have caused the loss:

-- A breach of the spacecraft's propulsion system, due to one of three possible scenarios;

-- Electrical power loss due to a massive short in the power subsystem;

-- Loss of function that prevented both the spacecraft's main and backup computers from controlling the spacecraft;

-- Loss of both the main and backup transmitters due to failure of an electronic part.

Stephenson added that determining the cause of the loss was especially difficult because the spacecraft was purposely not transmitting data to Earth at the time of the failure.

Mars Observer had turned off its transmitter as a precautionary measure to protect the transmitter tubes from shock just before it pressurized its onboard propellant tanks on August 21. Three days later the spacecraft was due to fire its main engines to place it in orbit around Mars.

At the end of the tank pressurization, Mars Observer was supposed to turn its transmitter back on. Ground controllers, however, never received a signal.

The possibility of a propulsion subsystem breach actually includes three different possible scenarios, the JPL board said:

-- Liquid oxidizer (nitrogen tetroxide) may have migrated past a check valve in the pressurization lines; during the tank pressurization, the oxidizer could have been forced into lines containing the fuel, liquid monomethylhydrazine, causing the line to burst;

-- The pressure regulator could have failed, causing the oxidizer tank to over pressurize and burst;

-- A small pyrotechnic device, or squib, that was fired to open a valve in one of the pressurization system's lines could have been ejected from the pyro valve like a bullet and damaged the fuel tank.

Among the other main categories of failure hypotheses, a massive power subsystem failure could have been caused by a short at one of the main bus power diodes.

Loss of function in the spacecraft's computers could have occurred at the time the pyrotechnic devices, or squibs, were fired in the propulsion subsystem. Under this hypothesis, the squib firing could have generated an electromagnetic pulse that caused the spacecraft's main command processor to "hang" in a state in which neither the main or backup computer was able to control the spacecraft.

Loss of both the spacecraft's transmitters could have resulted if a component failed in a control unit which prevented either of the transmitters from being powered on.

In addition to its findings on direct causes of the Mars Observer failure, the JPL board's report also made general observations and recommendations to improve spacecraft design and implementation in the future.

JPL managed the Mars Observer mission for NASA's Office of Space Science.

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