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After more than four years exploring Mars, NASA's Viking Orbiter 1 has almost reached the end of its mission.

The orbiter has used almost all its attitude-control gas, that keeps its solar panels pointed to the Sun and the antenna aimed at Earth. When the gas is exhausted -- probably about July 23 -- controllers at Jet Propulsion Laboratory will send commands to turn off Viking Orbiter 1 to end its long and productive mission.

Meanwhile, Viking Lander 1 is programmed to operate unattended on Mars into 1990, perhaps to be extended into 1994.

During the most recent phase of Viking's mission the orbiter has taken about 30 pictures day of region southwest of Olympus Mons and the three volcanoes on the Tharsis Ridge, an area of particular interest for its large, river-like channels. That sequence will continue through July 12.

Project officials plan full program for Orbiter 1's final days. How much of the program can be completed will depend on the amount of attitude-control gas remaining. Here is timeline of events that are proposed to be carried out if the orbiter continues to be cooperative:

July 13, 14: The spacecraft will take series of highresolution photos of the summit caldera of Olympus Mons. These should be the highest-resolution ever obtained of the caldera of the solar system's largest known volcano.

July 15 through 18: The spacecraft will perform three controlled burns of its rocket engines. Those burns are part of series of engineering tests to provide data that will benefit future space missions. One effect of the burns will be to change the spacecraft's orbit from its present 370 kilometers (230 miles) periapsis and 34,000 kilometers (21,127 miles) apoapsis to 350 kilometers (220 miles) periapsis and 56,000 kilometers (34,800 miles) apoapsis. The new orbit will satisfy the condition, in accordance with planetary-quarantine provisions, of not impacting, and thereby not contaminating, the planet before the year 2019.

July 20 to 23: Additional engineering tests will be conducted, principally on the radio system, and the final orbit will be determined.

Current extrapolation of the supply and usage rate of attitude-control gas indicates July 23 is the approximate date of depletion, but flight controllers say there is probably one-week uncertainty in that date, so the orbiter could run out of gas as early as July 16 or as late as July 30.

If it should turn out that there is still gas remaining, then about July 27 Orbiter 1 will make final high-altitude global survey of the visible portion of the Mars disk -- 25 pictures through each of three filters, for total of 75 frames.

When the gas is exhausted, the orbiter's radio transmitter will be commanded off for the last time, and the spacecraft will continue silently orbiting Mars for many decades.

Viking 1 was launched to Mars Aug. 20, 1975, and arrived June 19, 1976. Viking Lander 1 touched down on the Martian surface

July 20, l976, with 90-day mission expectation. That mission completed, it has now observed the planet for more than two full Mars years -- four Earth years on July 20.

As long as it survives, Viking Lander 1 will continue to collect photos and weather data from the Martian surface and, on command from Earth, transmit them on approximately weekly basis.

Viking Orbiter 2 ran out of attitude-control gas and was commanded off July 24, 1978. Viking Lander 2 was turned off after its last relay transmission April 11, 1980.

The Viking Project -- today with fewer than 30 people engaged in flight operations and science data processing -- is managed for NASA's Office of Space Science by Jet Propulsion Laboratory.

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