Despite being pelted by rain for four hours before liftoff, being in orbit six days and having to land in the dark on a concrete runway, the space shuttle Challenger ended its mission in better shape than any of the seven preceding flights.

"The spacecraft looks cleaner this time than it ever did before," said Herman K. (Fritz) Widick, shuttle ground operations manager at the Kennedy Space Center. "It's possible we'll be able to get the shuttle out of here as early as Friday, arriving in Florida sometime Saturday."

Depending on how long it takes to drain two small fuel tanks that developed tiny leaks in flight, Widick said, the Challenger will leave the Mojave Desert bolted to the top of a Boeing 747 by Friday or Saturday morning. The 747 is to stop overnight for refueling at Carswell Air Force Base in Fort Worth or Altus Air Force Base in Oklahoma City, depending on the weather.

Either way, it would be the fastest turnaround of the eight shuttle flights. This is what NASA is seeking, using less time to get the shuttle ready for its next flight.

Widick said the Challenger's fuselage appeared to have suffered less damage than on the previous flights. He said only 27 of the hundreds of tiles that protect the fuselage from the heat of reentry will have to be replaced, the fewest needed after a flight. Despite landing on the hard runway, the fuselage underbelly also suffered less damage.

The tires and brakes also came through the first night landing in fine condition, officials said. One of the three brake assemblies broke and its tire blew after landing on the last flight. Widick said the brakes will be removed this time and sent back to the Goodrich factory where they were made to verify that they came through the landing in good shape.

"We're doing this because of the problem we had the last time," Widick said. "This is only for verification."

Today's work involved venting the liquid hydrogen and oxygen from the shuttle's fuel cells. The next job before mating the 100-ton spaceliner to the 747 will be draining the manifolds of two small fuel tanks that leaked during flight. This task could take as long as 16 hours and could delay the shuttle's departure until Saturday.

When the shuttle arrives at the Kennedy Space Center in Florida, its toilet might be removed and sent to the General Electric Co. in King of Prussia, Pa., where it was built. There it would be tested, examined and analyzed to discover the cause of what is now the eighth straight breakdown in flight of the space toilet. The toilet sprung at least three leaks on the eighth flight.

"I'm not at all pleased with the waste management system," Air Force Lt. Gen. James A. Abrahamson, associate administrator of NASA, said here Monday night in saying that the equivalent of a task force would be formed to deal with the lavatory's problems. "I'm very disappointed it's so troublesome at this point."

The next space shuttle flight will take four astronauts and two scientists into Earth orbit, which is at least part of the cause for Abrahamson's concern.

The next flight is scheduled aboard the space shuttle Columbia for Oct. 28 and will carry the $1 billion Spacelab, built by the European Space Agency, and the first European to fly in an American spacecraft.