NASA pulls off historic rescue to save crucial data relay satellite

In August, when Guion S. Bluford becomes the first black US astronaut to orbit Earth, he will have a $100 million, 5,000-pound satellite to exercise. Nearly lost after its April 4 launch, this Tracking and Data Relay Satellite (TDRS) - the key to future orbital communications - has now been placed accurately on station.

It is in geostationary orbit (moving at the same rate at which Earth turns) 22,367 miles above the equator. From its position, it can transfer messages and data between other satellites and the ground even when those satellites are out of range of ground antennas. This can help eliminate recurring radio blackouts during shuttle missions, among other things.

If extensive tests now planned show that the TDRS has not been damaged, the Spacelab shuttle mission in September should have much of the communications needed to handle its massive orbit-to-ground data flow. Perhaps 60 percent of the experiments could be adequately supported.

Also, operators of the Landsat 4 Earth resources satellite should be able to restore communication with the high-resolution Landsat instrument package. Orbit-to-ground radio contact was lost several months ago when a Landsat transmitter malfunctioned. This instrument package, however, called the Thematic Mapper, was designed to communicate primarily through the TDRS. According to a Goddard Space Flight Center spokesman, tests with Landsat 4 should begin this month.

Maneuvering the TDRS into its proper orbit has been one of the most demanding rescue operations in National Aeronautics and Space Administration (NASA) history. Astronauts on the space shuttle Challenger's first flight placed TDRS-A - the first of three such satellites to be launched - in a low orbit. An Inertial Upper Stage (IUS), a powerful booster supplied by the United States Air Force, was to carry TDRS-A to geostationary orbit. A guidance failure, however, caused the IUS-TDRS combination to tumble. The satellite was quickly separated from the IUS, sustaining what so far appears to have been minor damage. It ended up in an orbit with a high point (apogee) of 21,970 miles and a low point (perigee) of 13,574. This was far from the circular, 22,300-mile-high geostationary orbit.

Engineers working for NASA, for TRW Inc. (TDRS builders), and for Space Communications Inc. (TDRS operators) lofted the satellite into its proper orbit using its attitude control thrusters. Small, sustained burns of some of these thrusters - whose nozzles are no bigger than thimbles - could do the job by increasing the satellite's velocity at critical points. A boost at apogee raises the height of perigee, and vice versa.

Happily, TDRS-A had abundant fuel to do this. Originally, the satellite was to have carried a commercial radio unit, as well as the NASA equipment. So extra fuel was added for maneuvers connected with the unit. Then the commercial unit was canceled. But NASA left the extra fuel on board. After 39 maneuvers involving just over 44 hours and 5 minutes of thruster burning time, nearly 500 pounds of this fuel is left. NASA officials say this is more than twice the amount needed for the expected 10-year operating life of TDRS-A.

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