Sailing around the world every 90 minutes, astronauts John W. Young and Robert L. Crippen today prepared the Columbia for the demanding and potentially hazardous ride home on Tuesday, which will be the first time in 20 years of manned space flight that a spaceship has returned from space and landed like an airplane.

The most critical part of Young and Crippen's 54 1/2-hour flight, the landing at Edwards Air Force Base in California, will begin at 12:30 p.m. EST with a series of complex maneuvers. Network television coverage of the landing will start at noon.

The landing maneuvers begin with an engine firing that slows the space shuttle down over a remote region of the South Indian Ocean, takes the DC9-sized Columbia out of orbit at about 1 p.m. over Guam, brings it north of Hawaii and in over California's Sierra Madre mountains to a dead-stick landing in the Mojave Desert at 1:30 p.m.

The landing will be anything but a "piece of cake," however.

It will involve heat caused by re-entry friction with the atmosphere that will drive temperatures on Columbia's surface as high as 2,800 degrees Fahrenheit and could destroy the craft and the astronauts if any of the insultating tiles are ripped off the bottom.

The winged spaceship will become half-spacecraft, half-airplane, with the astronauts getting just one chance to bring the 75-ton Columbia to a stop on the Edwards runway.

Columbia has no jet engines like an airplane to power it back up if Young and Crippen overshoot the runway. The shuttle is little more than a giant glider: it has a tail, elevons on its wings, and speed brakes to bank left or right, give it some lift and slow it down, but no power.

Columbia lands at more than 200 miles an hour, faster than high performance jet fighters.

The return to earth begins about 3 1/2 hours before the landing when Young and Crippen close the cargo bay doors they have kept open through most of their flight to radiate into space the heat generated by the craft's systems.

If the doors cannot be closed, Young or Crippen will don a spacesuit, grab some tools and move into the bay to work the doors closed. If the doors were open on landing, they would be torn off by a supersonic air stream and would rip the tail off, cut up the fuselage and bring the flight to a catastrophic end.

Near the end of their 36th orbit, Young and Crippen will turn the shuttle upside down and backwards so it is going tail and engines first, then fire their two orbital maneuvering engines to slow the shuttle so it will fall out of orbit. Moving northeast over Indonesia and Iwo Jimo, Columbia then will pitch back over so that it's right-side-up and flying nose first again to begin its rapid descent to earth.

Edging into the atmosphere, the shuttle's computers will fire its small thruster engines to angle the nose up so that the craft rushes through the thickening air at about a 40-degree angle of attack, the term aeronautical engineers use to describe the upward slant of an aircraft's lifting surfaces.

Friction will heat the underside of the shuttle to about 2,300 degrees and drive temperatures to 2,800 degrees on the nose and leading edges of the tail and wings.

Electrified gases formed by the intense heat will temporarily block out radio communication at this time. The astronauts will be on their own with Columbia's computers monitoring its course, rate of descent and other data.

The shuttle's nose and leading edges are made of reinforced carbon-carbon, so called because it is two layers of a graphite bonded together. It is the most heat-resistant substance known to man and can withstand heat of more than 3,000 degrees. Because of its weight, however, most of the underside is covered with much lighter foamed-glass tiles coated black to reradiate the heat to space.

The tiles are critical to re-entry. Even the loss of just a few of the underside tile could destroy the shuttle because there would be no insulation to prevent the scorching heat from burning holes in the aluminum frame and destroying control, electric and other systems.

The period of most intense heat will be at about 200,000 feet. The astronauts' ejection system won't work until they get down to at least 100,000 feet and probably not until 75,000.

At Mission Control in Houston, NASA officials repeated their assertion that there is no damage to the insulating tiles on Columbia's underside that could endanger the craft during re-entry.

Attempts to photograph the belly of Columbia with high-powered cameras provided no usable information, but Don Puddy, one of the flight directors, said NASA is "willing to proceed with re-entry without any additional data on the tiles."

About 16 tiles are damaged, with an 8-inch by 8-inch square on the right engine pod the largest damaged area.

NASA has made several attempts to photograph the craft's underbelly to see if any tiles are missing or damaged, but has been frustrated by poor weather or the angle of the craft as it passed over the high-powered Air Force cameras located in Florida and Hawaii.

Photos taken during the sixth orbit yesterday from a telescope in California were not usuable, and photos taken today on the 17th and 21st orbits were flawed by the craft's angle and clouds.

Gene Kranz, deputy flight director, said NASA and the Department of Defense would try again to take more photographs with other DOD facilities, but said the location of those facilities was classified.

The black tiles that line the bottom of the craft were more thoroughly tested than the thinner white tiles on the upper portion that were ripped off Sunday during ascent, NASA officials said. This was one source of their confidence that the Columbia will be in no danger during re-entry.

The tiles on the bottom will hit peak heat before peak stress during re-entry. By the time they are subjected to the greatest air stress, they will have cooled significantly.

When Columbia falls 200,000 feet, Young and Crippen can begin using the elevons on the wings to control the craft. They won't be able to use the tail at that point because the nose will still be pitched so sharply upward that the tail will be down and out of the main air stream. Still half in space, Young and Crippen will be able to fire thruster engines to roll, pitch and yaw.

Below 150,000 feet, Young and Crippen will pitch the nose forward to reduce the angle of attack to about 14 degrees and begin their final approach. They will fly east over Edwards, bank to the north in a U-turn and land to the west.

The great glider then will make a 22-degree dive for the runway at an air speed of more than 350 miles an hour. At 1,700 feet Young will flatten out the glide, extend the craft's speed brakes and bring the shuttle's wheels down for landing. It is the only time during the entire flight that Young will fly the craft without help from the computers.