Like E.T.'s alien comrades, US scientists studying Mars are anxiously awaiting a call from an errant planetary explorer - the Viking 1 lander. This long-playing spacecraft - originally designed to operate for only 90 days - had regularly supplied weather information and other data since it landed on Mars July 20, 1976. But last Nov. 20, it mysteriously fell silent. Now the Mars team at the National Aeronautics and Space Administration (NASA) Jet Propulsion Laboratory (JPL) in Pasadena, Calif., which has tried a variety of measures to awaken the spacecraft, waits hopefully for a call from Viking on May 5.
The lander's computer is programmed to turn on the transmitters automatically at that time. A JPL spokesman says it's virtually the last chance to make contact with Viking.
The Viking mission was carried out by twin spacecraft, each with a ''mother ship'' that orbited Mars and dispatched a robot landing vehicle to the surface.
Scientists considered the mission a completed success several years ago. Nevertheless, the flow of data from Viking 1 was a welcome bonus. The NASA budget has provided $200,000 a year for the Viking 1 program.
The unexpected break in Viking 1's communications was an ironic tragedy. JPL experts say they suspect that in the team's well-meaning attempt to prolong the life of the craft's batteries, the Viking computer was inadvertently reprogrammed to turn the communications antenna away from Earth.
Viking's atomic generators, which change the heat of radioactive material into electricity, can supply power through 1994. However, communications depend on a secondary set of rechargeable batteries. These have to be put through a regular ''reconditioning'' cycle of discharge and recharge to prolong their life. Without such exercise, the batteries would have given out some time ago. It was in the course of changing the reconditioning cycle that the trouble arose last fall.
JPL controllers sent up correct instructions. But they unwittingly loaded them into a section of the Viking computer's memory that contained the antenna pointing instructions. This appears to have reprogrammed the computer so that the antenna was turned away from Earth. The JPL team has tried to correct the programming, sending instructions with high radio power in hope that the spacecraft would receive them. But so far there has been no sign of success.
Thus, the JPL team can only wait until May 5, when the spacecraft is expected to respond on its own. If nothing is heard then, says Geoffrey Briggs, deputy director of NASA's Earth and Planetary Exploration Division, the effort to recontact Viking 1 will wind down. A JPL spokesman says one more attempt would be made to contact Viking 1 the following week.