On April 23,1980 Viking Orbiter One (VO-1), operating in the blowdown mode, completed a 10-s Mars orbit trim maneuver to position the spacecraft for its final science sequence in May-June 1980. This brings the number of propulsive maneuvers for VO-1 to 23. Total accumulated operating time for the rocket engine is 2896 s, representing a total impulse of 3.93 x 10 6 N-s. The estimated propellant remaining is sufficient to operate the rocket engine for an additional 30 s. VO-1 has completed more than 1700 days in space, 1400 days in orbit around Mars, and more than 2 yr of attitude-control-system operation with helium gas transferred from the propulsion-system pressurant tank. The mass of helium remaining is expected to be sufficient for attitude control through June 1980. This paper presents the flight history of the Viking 75 Orbiter propulsion systems and summarizes the design and test philosophy that have contributed to their success.

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