The delay of the sixth space shuttle mission has put a critical squeeze on the remaining shuttle schedule. This is because Mission 6 is to deploy one of two communication satellites that are designed to give astronauts almost continuous contact with Mission Control in Houston. These satellites are crucial to the success of several subsequent missions, which can't tolerate the communications blackouts currently experienced when astronauts are out of sight of a ground tracking station.
The second of the two communications satellites is to be deployed by Mission 8.
The long delay of Mission 6 means - first scheduled for launch Jan. 20 but now set for March 19 or 20 - there is very little lead time now available to have the complete communications satellite system in orbit when it will first be needed next September.
Called TDRSS (Tracking and Data Relay Satellite System), the new service will use two satellites located 130 degrees apart in longitude and orbiting 36,000 kilometers (22,300 miles) above the equator. This is the so-called geosynchronous orbit along which a satellite travels at the same rate at which the Earth spins. Thus the satellite remains over a given spot on the planet's surface.
From this vantage point, the TDRSS ''repeaters'' - which simultaneously receive and rebroadcast radio signals between points that can't communicate directly - will be able to ''see'' satellites orbiting at lower levels 85 percent of the time or better. With TDRSS in place, the National Aeronautics and Space Administration (NASA) plans to close six ground stations next year. These include facilities on Ascension Island, Guam, and Hawaii, at Santiago in Chile, and at Botswana and Dakar in Africa. The deep space network, which tracks interplanetary spacecraft, will remain.
TDRSS will use six satellites in all. Three will be in orbit at any one time. One of these will be a spare that can be placed in service quickly if needed. Three others will be held in reserve on the ground, ready for rapid launching. With this kind of backup, NASA officials expect TDRSS to be highly reliable.
However, until two TDRSS satellites actually are in place and working, NASA can't switch over to the new service. Yet Spacelab - a shuttle-carried laboratory in which scientists will work - must have the virtually continuous communications with Earth that TDRSS will provide.
The first Spacelab mission now is scheduled for late September. It will be STS-9 (Space Transportation System mission 9) in shuttle jargon. Hence the concern over slippage of STS-6's launch date.
This is the first mission of the shuttle Challenger. Problems (mainly leaks) with the main engines have caused repeated delays. NASA officials hope to launch sometime in mid to late March. This is very late indeed if the TDRSS system is to be ready by early fall.
The second TDRSS is to be launched by STS-8, since STS-7 has other duties. NASA anticipates a roughly 90-day turnaround between shuttle launches under the best of circumstances. Everything will have to go smoothly and in timely fashion from here on out if Spacelab is to make it into orbit by late September.
Robert O. Aller, director of NASA's TDRSS division, had considered this ''a tight and . . . very optimistic schedule,'' even without the STS-6 delay. Launching a TDRSS satellite is a tricky operation that involves three different sets of ground controllers.
To begin with, NASA doesn't own TDRSS. It is owned and operated by Spacecom (Space Communications Company), a private company that leases the system to NASA. Spacecom, in turn, is owned by Continental Telecom Inc. and Fairchild Industries. The Spacecom ground station is at White Sands, N.M.
The US Air Force also is involved. TDRSS will be released from the shuttle at an altitude of 283 km (176 miles). From there, it will be boosted to the 36,000 km (22,300 miles) geosynchronous orbit by a rocket system called the Inertial Upper Stage (IUS). This is owned by the Air Force and controlled from a ground station at Sunnyvale, Calif.
To orbit a TDRSS satellite, shuttle controllers at Houston (and the astronauts) have to coordinate with controllers at these other centers in a complex ballet whose movements must be synchronized to the second.
There are several opportunities to launch the TDRSS package during STS-6. But it is by no means assured that the complicated procedure can be successfully carried out at any of these times.