Countdown for Shuttle Begins After Crews Clean Fluid System

The countdown started Saturday for Challenger’s once-aborted science mission after the launch team cleaned troublesome air from the space shuttle’s hydraulic fluid system.

“Hopefully, we’ll depart more expeditiously than last time,” said astronaut Gordon Fullerton, commander of the seven-man crew that came within three seconds of liftoff July 12.

The astronauts spoke with reporters after flying here Saturday from their training base in Houston. The countdown began at 9 a.m., aiming for liftoff at 12:23 p.m. PDT Monday.

A problem with excess air in the ship’s hydraulic fluid had threatened to delay the flight for 24 hours, but technicians worked through the night to bleed out the air by circulating the fluid through a machine on the launch pad. The fluid is used to activate moving parts on the shuttle, including valves and wing flaps.

Shuttle managers had decided Friday to start the countdown on time, hoping to resolve the problem while the count continued. The fluid-cleansing operation was completed several hours sooner than expected.

Challenger’s first launch attempt on the mission was aborted when a computer sensed that a coolant valve in engine No. 2 had not closed properly and commanded all three engines to shut down.

During seven days in orbit, the astronauts, including five scientists, are to operate a $78-million set of instruments, most of them in Challenger’s cargo bay. Most, including several telescopes, are designed to study the stars, sun and galaxies.