Almost as if it were planned, a solar flare took place on the surface of the sun today--a day after astronauts Jack R. Lousma and C. Gordon Fullerton pointed a solar telescope directly at the sun and less than a day before they are scheduled to return to earth in the space shuttle Columbia.

The successful observation of the solar flare on their next-to-last day in space was typical of the way Lousma and Fullerton have flown the third in a series of four planned shuttle test flights.

The astronauts spent most of today getting Columbia ready for its Monday landing, scheduled to take place at 2:27 p.m. EST on the Northrup Strip, a desert runway, at the White Sands Missile Range in southern New Mexico.

Lousma and Fullerton have accomplished everything they set out to do since they left the earth last week, including observing an event on the surface of the sun that they never could have predicted would happen.

"We were lucky, we got a good flare that gave us 100,000 X-ray counts a minute for as long as 10 minutes today," said Dr. Robert Novick of Columbia University, where the X-ray polarimeter that recorded the flare was built. "This is the first time an instrument in space with the sensitivity of this instrument has witnessed a solar flare."

In New Mexico, the weather report for the desert landing site forecast broken clouds at 25,000 feet, and scattered clouds at 13,000 and 6,000 feet. Light winds in the morning will increase to 10 to 18 knots by noon, with gusts up to 28 knots, which could blow enough dust into the air to reduce visibility to less than seven miles in some parts of the missile range.

Mission rules require a seven-mile visibility for landing.

The White Sands landing will be the first and probably only time the space shuttle is brought down in New Mexico. Lousma and Fullerton normally would have landed Columbia at Edwards Air Force Base in California's Mojave Desert, but heavy spring rains flooded the runways there, covering them with mud a week before the astronauts took off from Cape Canaveral. The next landing of the space shuttle is scheduled again for Edwards; all subsequent ones are expected to be made on a three-mile-long concrete runway at the Kennedy Space Center in Florida.

The astronauts went to sleep at 9 p.m. EST, two hours earlier than most nights, so they will be fresh Monday morning for their return. About an hour before landing time, Lousma and Fullerton will slow down their spacecraft by doing a braking burn of their engines over Australia, then descend on a path that takes them north of Hawaii and then southeast over Los Angeles and into New Mexico.

Flying at 16 times the speed of sound across the coast of California, Columbia will fly over Phoenix at a low enough altitude that its sonic boom should be heard by almost half the people who live in Arizona. The spacecraft will probably be too high over California for people there to hear the boom.

The astronauts will be flying almost the same path they would take if they were landing at Edwards in California, but they will brake their engines four minutes later.

Lousma and Fullerton will also do a banking maneuver as they fly across the eastern end of the Pacific Ocean to put them on a course for New Mexico.

The astronauts will try to land at the missile range in a crosswind, provided the crosswinds do not exceed 20 knots. One of the goals of this mission is to see how the 100-ton spaceliner behaves in a crosswind, a condition sure to come up when the shuttle begins landing routinely at Kennedy Space Center.

Columbia will be returning to earth without 37 of its heat protective tiles, almost all of which it lost when it took off from Cape Canaveral. Most of these tiles were located in the nose of the spacecraft and apparently came loose when aerodynamic forces ripped them off at liftoff.

As many as 12 of the missing tiles were located in the aft section of the shuttle near the body flap that slows down the craft's speed when it lands.

"The missing tiles were all tiles that were not densified to make them stronger before launch," shuttle program director Glyn S. Lunney said in an interview. "One of the things we're going to do before the next flight is to densify those tiles to give them a stronger bond to the spacecraft."

None of the missing tiles is crucial to the safety of the shuttle during the heat of reentry.