The crew of the space shuttle Challenger today began shutting down the dozen working scientific experiments aboard Spacelab in preparation for landing the craft on a desert runway at Edwards Air Force Base in California just after noon (EDT) Monday.

Firing Challenger's engines for the first time in six days, commander Robert F. Overmyer maneuvered the 110-ton spaceliner so that its nose was pointed toward the sun. The maneuver put the aft fuel tanks in the shade, thereby cooling them enough to trigger a thermostat and heat the fuel to the temperature needed Monday when he and pilot Frederick D. Gregory fly the craft back to Earth.

"Kind of comforting for old Challenger to start firing those thrusters," Overmyer said. "Good to feel like a moving machine again."

The "moving machine" is to streak back into Earth's atmosphere across the Pacific Ocean at more than 20 times the speed of sound. Crossing the California coast at Long Beach and slowing to four times the speed of sound, the craft is to fly over Los Angeles around 9 a.m. Pacific time, creating a sonic boom likely to be heard throughout Los Angeles County.

The landing is scheduled for 9:10 a.m. Pacific time (12:10 p.m. EDT) on a Mojave Desert lakebed.

"When we arrive at Eddy Edwards AFB , a beer, of course, would be nice," said Gregory, a Washington, D.C., native and the first black astronaut to take the shuttle's controls, "but I think all of us would like an ice cream with chocolate and strawberries."

As crew members began to wrap up their experiments and stow equipment, scientists on the ground gave the three orbiting scientists and two physicians aboard Spacelab an "A" for their efforts.

"We've got enough information to fill 50,000 books," mission manager Joseph Cremin said at the Johnson Space Center here, where the Spacelab flight is directed. "That's a lot more than we expected to get."

The seven-day mission of the European-built Spacelab, its second flight and the first of three scheduled this year, was the first to be flown in what flight directors call a "gravity-gradient" attitude, with the shuttle's tail pointing at Earth and its starboard wing in the direction of its orbit. The spaceliner did not roll or turn in the last six days, minimizing any disturbances engine firings might have caused to Spacelab experiments.

The two squirrel monkeys accompanying the crew apparently adjusted well to space travel.

"We'll be bringing two monkeys back to Earth that are even friendlier than they were to begin with," physician/astronaut William E. Thornton said. "Those primates are part of the crew right now."

Food and waste particles leaking from the cages of the two dozen rats on board have been a recurrent problem, however, and for the third time in the mission had to be cleaned up with vacuum cleaners today by Thornton and Norman E. Thagard.

Thagard recommended that the next time animals accompany humans in space, crew members be issued goggles in addition to the surgical smocks, gloves and masks they have worn while changing the animals' food and waste trays.

On their reentry through the Earth's atmosphere Monday, the monkeys and rats will likely be pressed against the sides of their cages for more than three minutes when Challenger encounters reentry forces equal to almost twice the force of gravity. Veterinarians who trained the animals for their space flight predicted that it would be a "relatively easy G-load" that would not harm or frighten the animals.