Challenger's crew headed the 100-ton spaceliner today for the first shuttle landing by night.

If everything through the night went as well as it had gone the six days in orbit, Richard H. Truly, Daniel C. Brandenstein, Dale A. Gardner, Guion Stewart Bluford II and Dr. William E. Thronton will have touched Challenger down on concrete runway with simulated sunlight at Edwards Air Force Base in California's Mojave Desert at 3:40 a.m. EDT on Labor Day.

"The forecast is for excellent weather in California," flight director Harold Draughon said at the Johnson Space Center, from which Challenger has been directed. "There is absolutely nothing to indicate we won't land on time at Edwards, which is where we want to go."

If the astronauts were not able to land on time at the end of their 97th orbit, they would be ordered down at Edwards at 2:10 a.m. Tuesday EDT instead of heading for alternate landing sites at White Sands, N.M., or Florida's Kennedy Space Center.

Flight directors chose Edwards for the first night landing because of miles of flat lake beds at either end of its 15,000-foot-long runway. The shuttle would have had leeway if the pilots brought it down short or overshot the runway.

"We just don't have the extra room at the other landing sites," Air Force Lt. Gen. James A. Abrahamson, associate NASA administrator, has said. "This is a conservative agency, and we want to make the first night landing at Edwards where we have lots of room for error."

One reason the agency scheduled this eighth shuttle liftoff and landing at night is that night landings may be scheduled in Florida, where the weather at some times of year is better at night than during the day.

It is also no secret that the Pentagon, the shuttle's largest future customer, wants the space agency to demonstrate a night landing because it might want to return one of its shuttle missions to Earth in a hurry, which might mean at night.

"We don't want to be restricted to just daytime operations," Abrahamson said in a recent interview.

For the first time, the public has not been invited to the landing site to watch. The flight directors fear that automobile lights moving about might confuse the astronauts as they head in.

The first landing of the shuttle in darkness uses one of the world's brightest floodlight systems. High-intensity xenon floodlamps were installed at runway 22 at Edwards to serve as "portable sunlight" for Challenger's first night landing.

Designed to be visible to the astronauts from 50 miles, the lamps create the closest thing to sunlight. Each of the six xenon searchlights produces an 800 million-candlepower beam.

"The fundamental idea here is to make sure that the landing area itself is basically a daytime operation from the pilot's point of view," said Garry Layton, shuttle project manager at Dryden Flight Research Facility at Edwards. "And where the rubber meets the road, it will be essentially daylight."

The astronauts were to do nothing different on their final approach than the seven previous crews did for daylight landings. Commander Truly and pilot Brandenstein were to take over the controls from the landing computers when the shuttle was to bank for the final time at an altitude of 14,500 feet, heading toward the runway.

A flawless first night landing for Challenger would complement a near-perfect flight. Only minor things, what flight directors describe as "nuisances," have gone wrong.

A pump that circulates warm fluid through the underside of the shuttle when it is blocked from the sun failed on the mission's first day but was fixed. At times, the astronauts had trouble communicating through a new $100 million communications satellite, the Tracking and Data Relay Satellite.

A television camera in the elbow of their 50-foot-long robot arm came apart Friday, and one of the shuttle's four in-flight computers apparently was hit today by a stray cosmic ray, causing it to fall "out of sync" with three other computers controlling the spacecraft's flight.

Even after it fell behind the other computers, the stricken computer still was working and processing information. Flight directors bumped it from first in command to fourth.

"The computer really is okay," Draughon said. "But we moved it out of string because it has this one black mark against it."

Despite all these little "nuisances," Draughon described the eighth shuttle flight as "the cleanest spacecraft so far. It's had the fewest failures and accomplished everything we set out to do."