Discovery; TDRS and other plans.

Discovery: TDRS and other plans

The major mission activity in the upcoming flight of Discovery, representing the shuttle program's return to life 32 months after the Challenger disaster, continues a satellite project that never did get off on the right foot.

It was in the early 1970s that NASA first began planning the use of a network of satellites to replace the ground stations with which it tracks spacecraft and relays their data. "The hope was to greatly increase the amount of time for which satellites -- including the then-untried shuttle itself -- could communicate with the ground. The first Tracking and Data-Relay Satellite (TDRS-1) was launched on April 4, 1983 (Challenger's maiden flight). But trouble with its booster rocket left it in too low an orbit, which required nearly three months of gradual nudges to correct. The second TDRS was destroyed in 1986 with Challenger itself.

The latest TDRS is the heaviest item in Discovery's payload, weighing more than 2 tons, plus 16 tons for its booster. TDRS-1 is now relaying data for the Solar Maximum Mission satellite, the Solar Mesosphere Explorer, the Earth Radiation Budget Satellite and Landsats 4 and 5. NASA plans to phase out many of its ground stations, but the TDRS system must first consist of two operational satellites plus a third in orbit as a spare. The third is tentatively scheduled for launch early next year, to be followed later by one to replace the now-aging TDRS-1.

Besides deploying the new TDRS, Discovery's astronauts are to conduct microgravity experiments in materials processing and life sciences, as well as observing lightning and other phenomena below. They also plan to test a system of secure on-board communications using infrared devices like television remote-control units, which produce no radio transmissions that can be picked up outside the shuttle.