Hoping for a break in the weather, the five Challenger astronauts stowed gear tonight and prepared to land their 100-ton spaceliner Friday morning on the world's flattest and most expensive runway.

It would be the first space shuttle to return directly to the Kennedy Space Center, from which all seven shuttle flights have been launched.

"There are still a lot of clouds and a lot of moisture in the area but they should start moving out tonight," Air Force Maj. Donald J. Greene said today.

Just to be on the safe side, flight directors gave the astronauts three different landing times at two landing sites. The first two attempts would be at 6:53 a.m. and 8:29 a.m. EDT at the Kennedy Space Center here. The other would be 9:56 a.m. EDT at Edwards Air Force Base in California's Mojave Desert where five of the six previous shuttle crews have landed.

"What we want is good visibility so they can pick up landmarks as they return, and we don't want to go through any rain showers because you're looking a tile damage if you do," said landing recovery director Hugh Whitaker.

Falling at 11,000 feeet a minute and landing at supersonic speeds, the spaceliner would enter a rain shower as if it were flying into buckshot. The thousands of tiles that cover its fuselage and protect it from the heat of reentry could suffer severe damage, a mishap that would not be life threatening but would cost time and money to repair.

Sounding like four men and a woman who were quite ready to come home, astronauts Robert L. Crippen, Frederick H. Hauck, John M. Fabian, Sally K. Ride and Norman E. Thagard spent part of their day "stowing the cabin" and part being silly.

One held up a sign saying "The Doctor Is In" as Thagard, a physician, held "office hours" and showed televiewers the experiments he was running in a search for clues to the causes of space sickness. So far, this has been the first flight since the initial shuttle mission to have no space-sick astronauts.

The astronauts were read the news today instead of getting it on their teleprinters. One story concerned doctors who were opening up offices in odd places. Thagard suggested they add spaceships to the list.

Just before the crew went to sleep this afternoon, astronauts John Blaha and Guy Gardner on Earth sang a duet that started out, "Goodnight, Lady . . ." to the tune of "Goodnight, Ladies."

The Challenger astronauts replied with a recording of "The Entrance of the Gladiators," the familiar circus march by Hungarian composer Julius Fucik.

If they land at the Kennedy Space Center as expected, the astronauts will touch down on a 15,000-foot runway so precise that the difference in elevation from one end to the other is .12 of an inch. No other surface on earth is believed to be so uniform.

So much swamp had to be drained, so much land had to be dug up and filled and so much concrete had to be poured in such a precise way that the runway cost $25 million when it was completed in 1976. It is estimated that cost would be $70 million today. It is the most expensive runway ever built, including those dug out of volcanic rock on islands and those cut out of any mountains on Earth.

Over 1.5 million bags of cement were laid down to pave the runway, enough to cover a two-lane highway 80 miles long. Canals 12 feet deep run along both sides of the strip to drain water from countless summer rainstorms.

Strong wire fences also line either side of the runway to keep out wild pigs, bobcats and panthers roaming the Merritt Island Wildlife Refuge near the runway.

Inside the fence and in the two canals are an estimated 140 alligators that turn out to be more pesky than the animals the "pig fence" keeps out. In the last month alone, alligators had to be carried off the runway three times, and, if and when, the shuttle lands here Friday two agents from the U.S. Fish and Wildlife Service will be on hand to carry away any other wandering alligators.

The biggest pests of all are birds from the refuge, the nesting ground for hundreds of species. Training flights by astronauts are scheduled only for the early morning and early evening hours, when the refuge's thousands of buzzards and other soaring birds are at rest.