The astronaut crew patch explains the purpose of the third flight of the space shuttle Columbia, which is scheduled to rocket into orbit Monday morning on the first leg of a seven-day flight in space.

The patch created by artists for astronauts Jack R. Lousma, 46, and C. Gordon Fullerton, 45, shows Columbia's towering tail silhouetted against the gold and orange spikes of the sun's rays, a symbol of the priorities given this third in a planned series of four test flights due to end this summer.

Lousma and Fullerton will fly Columbia around the earth in a way that will expose the tail, nose and cargo bay to heat hotter than boiling water and cold so extreme that many gases turn into liquids.

The cargo bay will be exposed to the sun's heat for 26 hours, the tail for 30 hours and the nose for 80 hours, a flight plan that will raise temperatures of some parts of the spacecraft to as much as 250 degrees Fahrenheit.

While the sufaces that are exposed to the sun are being heated up, those in the shade will be chilled to as low as 215 degrees below zero Fahrenheit.

There is a method to all this madness, primarily to discover how Columbia, its rocket engines, fuel tanks, myriad electronics and thousands of tiles protecting its fuselage bear up under the extremes of hot and cold in space where the temperature difference between the day and night side of the earth is as much as 500 degrees because of the absence of atmosphere.

"We're ready and we know Columbia's ready," shuttle commander Lousma said today after landing at Patrick Air Force Base just ahead of Fullerton in a T38 jet trainer. "We've come a long way and we've got a long way to go."

By 6 p.m. today, the countdown on launchpad 39A was proceeding with clockwork precision toward the scheduled launch at 10 a.m. Monday. Launch crews had encountered so few problems in the countdown they were running two hours ahead of schedule late today.

Even the changeable Cape Canaveral weather was cooperating. The clear and sunny skies prevailing over the Kennedy Space Center on Friday and today are forecast again for Sunday and Monday, meaning that weather should not delay the launch.

The shuttle still will land a week from Monday at the White Sands Missile Range in southern New Mexico instead of California's Edwards Air Force Base where the dry lake beds that serve as runways were turned to mud last week by heavy spring rains. The desert floor at White Sands long has been designated as a backup shuttle landing site and its gypsum surface is so rock hard it takes an air hammer to drive holes in it.

The first priority of the third shuttle flight is to expose the spacecraft's tail to sunlight to see how its fuel tanks pump fuel and its rear engines restart under high temperatures. The second priority then is to point the tail away from the sun to see how the engines restart after a long exposure to the cold.

The third priority is to heat-soak the 65-foot-long cargo bay, which this time will be carrying more than 21,000 pounds of instruments and experiments. The aim is to investigate how the extreme heat in space will affect the scientific cargo the shuttle will carry into orbit for the next 10 years.

While the cargo bay doors are open to the sun, an instrument in the cargo bay called an X-ray polarimeter will scan the sun's surface for signs of solar flares, which still puzzle scientists although they occur as often as 20 times in a year. Scientists believe that one key to learning the mechanisms that trigger a solar flare is the X-rays it emits when it ignites.

This experiment is so important that if a flare occurs while the shuttle is in flight, the astronauts will be ordered to interrupt any other activity to turn the craft so the cargo bay is aimed at the sun if it isn't already. Two months ago, a flare rippled across the sun's surface while the crew was simulating a mission and they were ordered to make this maneuver in their simulator.

Another instrument in the cargo bay is a device called the plasma diagnostics package, which will be moved out of the cargo bay by the 50-foot-long robot arm bolted to the floor of the cargo bay near its cockpit. The arm will hold the instrument about 40 feet out from the cargo bay where it will be left for hours at a time to detect the "plasma wake" left in the upper reaches of the earth's ionosphere by the shuttle's rapid passage.

On board also will be a clear plastic container the size of a household medicine cabinet, containing 72 live insects, 24 adult moths, 12 adult drone honeybees, 24 moths and 12 houseflies who will be hatched while the shuttle is in space. The purpose of this experiment, which was designed by 18-year-old high school senior Todd E. Nelson of Rose Creek, Minn., is to determine how adult insects fly and how newborn insects react to their first few minutes of life in weightlessness.

Countless jokes are being told among the press corps about this insect farm, but if anybody can handle this "house fly horseplay," it is Lousma, who is commanding his first space flight.

A onetime University of Michigan football player who is easily one of the most robust of the astronauts, Lousma also is one of the few people to have gone through Marine boot camp twice although he didn't have to. Lousma chose to do so when he became an officer candidate because he said he enjoyed it so much the first time.