America's Space Transportation System (STS) is on course.
During Columbia's fourth and final test mission, the shuttle's crew is answering questions about the craft's capabilities - and ironing out remaining problems such as a payload door distorted by extreme cold. As on the STS-3 flight, the door latched properly after a period of warming sunshine.
The next flight, STS-5, remains set for November liftoff. It will mark the beginning of the shuttle's operational missions, when it will move from research into a paid, cargo-carrying role.
Columbia's crew started its fifth day in orbit Thursday by checking through the overnight teleprinter messages and eating breakfast. Then on Orbit 60 of this 113-orbit, 7-day mission, both men plunged into a busy round of scientific and military experiments.
When mission control in Houston asked about one surface contamination experiment, Comdr. Thomas K. Mattingly said that it had been left out. ''You might be surprised,'' he added sharply, ''but we haven't had much spare time.'' Later, flight controller Harold Draughon said ''a high priority'' is to give the crew ''a fairly relaxed'' Friday.
The crew spent a good bit of time Thursday wrestling with an auto control ''glitch'' which Navy Captain Mattingly reported ''still has me surprised.''
Dealing with such surprises, whether triggered by too much water in the shuttle's protective tiles, by mechanical problems with equipment, or by computer software problems, is the task of a special trouble-shooting team based in Houston.
The astronauts report any problems directly to the flight controllers seated in the Mission Operations Control Room here in Houston. If the flight controllers don't have an immediate solution or can't call up one on their computer consoles, they pass the problem along to support groups waiting nearby with their own computer banks and specialized equipment.
So far, NASA officials are listing STS-4 as probably their most ''benign'' flight.
But this mission has been a busy one for NASA's payload operations flight controller, Wayne Boncyk, and his industry counterpart, James Moser of Ford Aerospace & Communications Corporation.
Their task is to be thoroughly familiar with every piece of payload equipment shuttled into space.For STS-4, they were ready to deal with any faults in the complex continuous-flow electrophoresis system, a prototype that could lead to setting up chemical manufacturing facilities in space. When the astronauts reported unexpected bubbles in the system, Mr. Boncyk relayed an answer back to Columbia within 90 seconds that the bubbles were normal and wouldn't affect the experiment.
But a serious problem developed with the shuttle's first fare-paying ''Getaway Special,'' a package of scientific experiments packed into an oil-drum-size canister by 10 Utah State University students.
Shortly after the thrill of seeing their experiments launched into space, the students learned that Columbia's crew was unable to switch the package on. This time Mr. Boncyk and Mr. Moser could not come up with a 90-second answer to why the astronauts' calculator-like switch showed no contact with the canister.
NASA's response in such cases is to form a special team to solve the problem. So Boncyk and Moser sat down with diagrams and duplicates of the control equipment involved in the experiment and worked with other experts by telephone.
''Everybody that is potentially useful is drawn into the loop,'' Mr. Boncyk explained, ''because there's no one person with all the necessary knowledge.''
The trouble-shooting team ran simulations and narrowed down the fault to three possible wiring failures. Then it sent 36 lines of correction procedures to the STS crew by teleprinter - only to find that the crew's tight schedule left no time for more than a quick fix-up.
Finally, Mr. Boncyk felt there was a good chance of fixing the system by using an electrical repair kit inherited from Skylab. The repair worked after taking 10 minutes of precious crew time.
For Wayne Boncyk, it was just one more case of ''showing our customers that NASA is capable of delivering on its promises.''