The space shuttle Challenger returned to Earth last week with a cracked windshield, apparently caused by a collision in orbit with a tiny fragment of space rock.

The crack--a quarter-inch across and an eighth of an inch deep--caused no concern to the five astronauts aboard and did not impair their vision. But it was the first time in 20 years that a window of a manned American spacecraft was struck by a foreign object in space.

"On our third day in space, we noticed a small impact crater on window number five, near where I was sitting," Pilot Frederick H. Hauck said yesterday at a post-flight news conference at the Johnson Space Center in Houston.

"We don't believe we saw it prior to Day Three so we don't believe it was caused on ascent. The speculation is an impact with some kind of foreign object, and a micrometeorite might be a good guess."

Shuttle Commander Robert L. Crippen said scientists are examining the windshield to see if any foreign material is embedded in it.

Added Crippen: "We weren't nervous about it at all because it didn't damage the window."

Like motoring tourists on a cross-country expedition, the astronauts also suffered a broken wheel brake assembly just after they landed eight days ago at Edwards Air Force Base in California. Crippen said it must have happened after the crew left Challenger and while the 100-ton spaceliner was being towed off the lake-bed runway.

"It didn't happen during landing because we used the brakes on landing and had no trouble," Crippen said. "Of course, I think we need to understand what went on with the brake, and I'm sure they'll go ahead and find it."

On the fifth day of their six-day flight the onboard toilet broke. It was at least the fourth toilet failure of the seven shuttle flights. What has failed each time is the "slinger," a device that rotates waste by centrifugal force in the shuttle toilet to dispose of it in the absence of gravity.

"The slinger just quit," Crippen explained. "We ended up using it as a one-holer, like people living in the country used to do, and it worked just fine."

Hauck added: "We had plenty of privacy, even though we were a crew of five. We even put curtains up over and in front of the head bathroom so nobody would be inconvenienced."

Much of the talk at the news conference was about Sally K. Ride, the first American woman in space, and how she felt to be the only woman in a crowded cabin with four men 180 miles from the Earth.

Ride took it in stride. She said it was the "most fun I've ever had and ever expect to have."

Ride said she was "pleasantly surprised" that five people could move around in the cabin at the same time. "We never got in each other's way," she said.

Added Hauck: "You can move around in three dimensions up there, you know. Down here, you always have to have your feet on the ground."

Commander Crippen was asked what it was like to fly with a woman crew member. He answered: "I think the women we've got in our office can do the same job men can do in space. I'd be happy riding up there with Sally again."

For the first time in six flights, none of the shuttle astronauts became spacesick. Astronaut Norman E. Thagard, a medical doctor who went along to do experiments that might provide clues to spacesickness, said he now has no idea of what causes it and what its cure might be.