Columbia's astronauts were back star-gazing Friday after scientists on the ground fired up the $150 million Astro observatory, which has been crippled by repeated malfunctions.

Troubles have persisted throughout the mission and have forced the astronauts to skip more than 100 planned observations. About 250 celestial targets were to have been studied.The latest problem occurred Thursday when a backup computer terminal aboard the spacecraft that had been running the observatory's three ultraviolet telescopes overheated and shut down. The other computer terminal failed Sunday.

Lint, which can accumulate from such items as astronauts' clothing, paper products and hand towels on the shuttle, was found clogging the air vents of the first failed terminal, flight director Gary Coen said. He said the second terminal hadn't been fully inspected yet.

Coen said the cause of the computer failures has not been determined and he didn't know if it was connected to the lint buildup.

Despite the setbacks, the shuttle's seven astronauts planned to go ahead Friday with a science lesson from space to middle school students at two NASA centers. The youngsters were to ask questions of the crew members after the lesson.

Meanwhile, the Astro observatory is being run by commands sent from the ground that guide the telescopes toward their desired targets. The astronauts, using a joystick and aided by information from the ground, can then zero in on an object.

The process succeeded several times Thursday night and early Friday, giving scientists hope that many more observations can be made before Columbia's 10-day flight ends Tuesday.

"Our attitude here is that we've got a mode we feel is going to work and we're going to go get the science," mission manager Jack Jones said. But he added he didn't know yet how efficient the operation would be.

Spacelab Mission Operations Control's John David Bartoe congratulated the astronauts Friday morning.