Shuttle's cargo is key link for future NASA missions

As NASA Deputy Administrator Dale Myers explains, his agency considers the shuttle's return to service ``the beginning of the future for the space program.'' And, he adds, Discovery's cargo - the TDRS communications satellte - is part of ``the key to that future.''

Once it is fully established, the Tracking and Data Relay Satellite (TDRS) system will enable users to be in almost continuous contact with satellites in low-Earth orbit.

It will let the National Aeronautics and Space Administration work with the planned Hubble Space Telescope and other orbiting observatories as well as with the shuttle and, eventually, a space station. (The Hubble telescope and other observatories are scheduled for launch beginning in 1990.)

Without the TDRS system, these and other major elements of the United States space program's future would be severely handicapped or utterly useless. They would not be able to communicate effectively with the ground.

It is this pivotal role that gives the TDRS satellite the high priority reflected in its status as the first new shuttle payload to fly, notes Richard Truly, NASA's associate administrator.

As of yesterday afternoon, astronauts were scheduled to release the satellite about 6 hours after Discovery's 11:37 a.m. (EDT) takeoff.

Once the new TDRS-3 satellite is on station and working satisfactorily, it will join TDRS-1, now in orbit, to form a complete system. TDRS-2, which was to fill this role, was lost in the Challenger explosion.

The satellites orbit 22,300 miles high, where they travel at the same speed at which Earth rotates. They remain virtually over the same ground location. This is known as geosynchronous orbit.

The satellites also have a far better view than do ground stations of objects in low-Earth orbit (below about 300 miles) where many scientific and manned spacecraft fly.

Ground stations now are in touch with such low-orbiting satellites for only about 15 percent of an orbit. With two TDRS on station, NASA should be in contact with these satellites about 90 percent of the time.

Without this nearly continuous contact and the high-speed communications TDRS provides, users would choke on the data volume the US space program soon will be generating, explains Thomas McGunigal, director of the TDRS program for Contel Federal Systems. Contel owns and operates the TDRS system for NASA.

In its fastest mode, TDRS can transmit in one second the equivalent of the information in a 20-volume encyclopedia with 1,200 pages per book and 2,000 words per page.

NASA needs that kind of capacity. It will be producing a library's worth of data daily when the new scientific satellites, the shuttles, and the space station are all operating at the same time.

Mr. McGunigal says he views TDRS ``as a major step in constructing the infrastructure of space.'' It is, he adds, ``like building a subway'' transit system. Once in place, many people will use it for a long time to come.

NASA's current shuttle schedule calls for Discovery to carry up another TDRS Feb. 18. When this is in operation, the old TDRS-1 will be placed on standby as a spare. It has been working for five years and has suffered some degradation, mainly in backup systems. But it still gives good service, McGunigal says.

NASA will send up TDRS replacements periodically to ensure continuous operation of the system. Plans now call for TDRS launches by shuttle in November 1990 and November 1992, again using Discovery. TDRS launches may eventually move off the shuttle to expendable, unmanned rockets.

HOW THE TRACKING AND DATA RELAY SATELLITE (TDRS) IS DEPLOYED TDRS, a NASA communications satellite, is Discovery's main payload. An initial upper stage (IUS) rocket will boost TDRS from the shuttle's orbit through a transfer orbit to a higher geosynchronous orbit.

1.As TDRS reaches this final orbit, various sections of the satellite will be deployed into operation. This deployment begins about 12 hours and 45 minutes after the space shuttle is launched, and about 6 hours and 30 minutes after the satellite is launched from the shuttle.

2.The solar array panels are released and deployed in operating position.

3.The space-to-ground link antenna is released and deployed.

4.The C-band antenna is released and latched. The satellite then separates from the rocket.

5.Single-access antennas are released and deployed.

6.Total operational deployment is attained. Elapsed time: 37 minutes.

Source: TRW Inc.

An illustration in the Sept. 30 edition portraying deployment of NASA's new communications satellite gave the wrong name of the rocket that boosts the satellite into orbit. The booster is an inertial upper stage rocket.