Astronauts Joe Henry Engle and Richard H. Truly were told today to bring their spaceliner Columbia back to Earth on Saturday, a move that shrinks their planned five days in orbit to a little more than two days.

"We think it's the prudent thing to do at this point in our test flight program," Johnson Space Center director Christopher C. Kraft said this afternoon at a news conference in Houston that was piped in here. "We have thought this out very carefully and we thought it was the best thing to do."

Putting the best face on it that he could, Kraft explained that one reason Engle and Truly were being

Coverage of the Columbia's landing will begin at 3 p.m. on CBS and NBC and at 4:10 p.m. on ABC. called home early in the second flight of the reusable space shuttle is that they accomplished most of their major objectives on the first and second days in orbit.

Kraft said that the loss of one of the shuttle's three fuel cells and potential bad weather at Edwards Air Force Base, Calif., where the crew will land, influenced the decision to call Engle and Truly back.

"The best weather situation for landing is tomorrow afternoon but that was not the decision swinger," Kraft said. "We're still in a position that if the weather got worse or something happened where we're better off in orbit, we would do so. We have many options open to us and we think we chose the best one."

But NASA officials and Air Force weathermen kept a wary eye on the normally blue desert skies, which were full of clouds today.

Lt. Col. Winston Crandall of Edwards' weather detatchment said the landing weather probably will be "marginal. We're saying partly cloudy with a chance of mostly cloudy at high levels."

Crandall said a cloud system was off the California coast, and "we're watching it hour by hour. We're optimistic that the major mass is going to be through before tomorrow."

If weather conditions preclude a landing at the Rogers Dry Lake, the shuttle would be routed to the White Sands Missile Range in New Mexico.

Though acutely aware that the failed fuel cell made them candidates for a shortened mission, Engle and Truly took the news hard when they heard it. The rookie astronauts clearly were enjoying themselves in space and wanted to wring five full days out of their mission.

"Okay, now you get to hear the bad news," astronaut Sally Ride told Engle and Truly from Houston's Mission Control Center at 11:38 a.m. EST. "We're running a minimum mission and we're bringing you home early."

"Oh, okay," Engle said, sounding disappointed. "That's not so good."

"You guys did too good a job," Ride quickly replied. "That's why we're going to bring you home early."

After a long silence, Truly broke in: "We're both feeling pretty good. We've got plenty of energy left. If you want to extend the day, go right ahead." Now, it was Ride's turn to return the silence. "Roger," she said after pausing.

While Engle and Truly were making history with the second flight of Columbia, Ride made a bit of history also. She became the first woman astronaut to speak from the Mission Control Center to astronauts in space.

The conversation between Ride and the Columbia astronauts was not without an aside. At one point, Truly explained how he was about to exercise the shuttle's mechanical arm. Said Ride: "Sounds good to me." Answered Truly: "You sound pretty good, too."

Instead of landing at mid-day on Tuesday, Engle and Truly are scheduled to touch down in California's Mojave Desert at 4:22 p.m. EST Saturday. Weather forecasters said that layers of broken clouds are expected at 14,000 feet and at 22,000 at Edwards on Saturday. Visibility between 30 and 60 miles and southwesterly winds gusting up to 15 knots are also forecast, which are adequate for landing.

The abbreviated shuttle mission is the third manned American space flight to be shortened by mechanical trouble. The flight of Gemini 8 in 1966 was cut short when the spacecraft went into a spin and the voyage of Apollo 13 in 1970 was aborted when one of its oxygen tanks exploded.

The failed fuel cell that shortened the mission of Engle and Truly came as a surprise to shuttle planners. The cells had been used without failing on 25 manned American spacecraft since 1965. They provide astronauts with electricity and drinking water and the cells on Columbia were considered to be improved models of earlier ones on Gemini and Apollo.

"If we made a list beforehand of what things we'd expect to bite us on this flight, I think we'd all put the fuel cells at the bottom of the list," shuttle program manager Glynn S. Lunney said. "Their whole history showed an excellent performance."

Apparently, the fuel cell failed when water collected in the membrane that lets hydrogen and oxygen combine in precisely measured amounts to generate electricity. The water corroded the membrane and began to break it up, creating an uncontrolled and dangerous situation where oxygen and hydrogen could come together in large enough amounts to explode.

If they were disheartened by the shortened mission, shuttle program officials never showed it today. Deputy director of flight operations Eugene Kranz pointed out that Engle and Truly had already accomplished most of their objectives, including an exercise of the shuttle's mechanical arm and a long run of the instruments in the shuttle cargo bay to get a radar map of the Earth's more rugged terrain and read color differences in the ocean to follow the movements of large schools of fish.

"This is a test flight program and we have yet to use this system 100 percent," Kranz said. "We have not reached the point where we have such defense in depth that we can absorb damage and press on with our mission." CAPTION: Picture 1, Joe Henry Engle and Richard H. Truly conduct tests during second day of the shuttle mission.; Picture 2, Photograph beamed back shows exercise extending craft's mechanical arm. Earth is at the top and shuttle's cargo bay at bottom. Photos by UPI: Picture 3, Johnson Space Center director Christopher C. Kraft, deputy flight director Eugene Kranz announce changes in mission. "We think it's the prudent thing to do," Kraft said. AP