Viking engineers at Jet Propulsion Laboratory today sent command to turn off NASA's Viking Orbiter 1 ending the spacecraft's four-year mission in Mars orbit.

Kermit Watkins, Viking project manager, said the orbiter was shut down because its attitude control gas is expected to be depleted before it completes its next orbit of Mars.

"By sending the turn off command," Watkins said, "instead of allowing it to occur automatically, we will be sure that the spacecraft has been shut down."

Although silent, the spacecraft will continue in orbit well beyond the turn of the century to prevent contamination of the surface for future missions to Mars.

Viking engineers have been carefully monitoring the spacecraft since late July in the expectation that it would soon use the last of its attitude control gas and be unable to maintain the correct orientation. Attitude control gas keeps the spacecraft's radio antenna pointed to Earth and the power producing solar panels aimed at the Sun.

Viking 1 was launched on Aug. 20, 1975, from Cape Canaveral, Fla. It arrived in Mars orbit on June 19, 1976, and has been operating continuously since then.

Viking Lander 1 continues to collect scientific information on the surface of Mars. It is programmed to continue working until December 1994.

The Viking program is managed for NASA's Office of Space Science by Jet Propulsion Laboratory.

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