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.

While two astronomers aboard the shuttle helped ground teams guide the telescopes, two other crew members conducted a live science lesson Friday morning for middle school students at two NASA centers."You may have heard of some of the problems we've been having," astronaut Jeffrey Hoffman told the youngsters. "It's been quite an interesting and dynamic mission."

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 inspected.

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 crew proceeded with Friday's lesson, the first in what NASA hopes will become a regular science series for youngsters.

Hoffman and Sam Durrance, before starting 12 hours of observation work, talked to the students for more than a half-hour about star formation and the many different kinds of radiation emitted from celestial objects.

To mark his debut as a teacher, Hoffman wore a dress shirt and tie - the first tie ever worn in space "as far as I know," he said.

"I can tell you it works quite well. You have to be a little careful in zero gravity," Hoffman said, pushing down his floating tie.