NASA fueled a space shuttle for the first time in more than two years Thursday, conducting a critical prelaunch test of Discovery's redesigned external tank.

The 154-foot, rust-colored tank underwent major design changes after the Columbia disaster, which was caused by a chunk of insulating foam that fell off the tank during liftoff and gashed the shuttle's wing.

None of the astronauts was aboard Shuttle Discovery (search) for the daylong tanking test, a rare event in the 24-year history of the shuttle program. But the entire launch team was on hand in the firing room, and eight inspectors were at the pad to watch for any ice or frost on the tank.

Ice could be even more dangerous than foam if it broke off during liftoff and smacked into the shuttle. But the ice inspectors reported that both the shuttle and its fuel tank appeared to be in excellent condition.

Discovery is scheduled to blast off no earlier than May 15 on a flight to the international space station (search). It will be the first shuttle mission since the Columbia tragedy in February 2003.

Midway through the tanking test, NASA's new administrator, Michael Griffin (search), addressed employees for the first time and promised that the resumption of shuttle flights will be his highest priority.

"All other commitments will flow around what I have to do in order to support the return-to-flight decision," he said in a televised speech from NASA (search) headquarters in Washington.

NASA began filling Discovery's tank with more than 500,000 gallons of super-cold liquid hydrogen and liquid oxygen in the morning. The practice countdown ended at the 31-second mark in late afternoon, followed by the slow draining of the fuel for use later.

The last time NASA conducted such a test was in 1998, also on Discovery. The test does not include any firing of the engines.

After a 1.67-pound piece of foam brought down Columbia and killed all seven astronauts, NASA removed foam from some places on the tank and applied the insulation differently, to prevent big chunks from breaking off. Heaters were also installed to prevent the formation of ice at spots that no longer have insulation.

NASA plans to reassess the launch date soon, in light of all the delays in getting Discovery to the pad.

The latest snag was a 11/2-inch hairline crack found in the tank's foam right before the shuttle was moved to the launch pad last week. Engineers believe there is no need to repair the crack, saying that even if a piece of foam did come off there, it would not strike the ship.