prelaunch test of Discovery's external tank, redesigned to remove excess foam following the Columbia disaster.

None of the astronauts was aboard Discovery 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 along with inspectors on the lookout for any ice or frost forming on the tank.

Ice could be even more dangerous than foam if it broke off during Discovery's planned May liftoff and smacked into the shuttle.

NASA began filling Discovery's 154-foot-tall (46-meter-tall), rust-colored tank with more than 500,000 gallons (1.9 million liters) of super-cold liquid hydrogen and liquid oxygen in the morning. Managers waited three hours before giving the go-ahead because of the possible threat of approaching thunderstorms.

The fuel will remain in the tank until the practice launch countdown ends at the 31-second mark, in late afternoon. Then it will be drained for future use; about half the fuel dissipates into the air.

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

NASA wants to see how Discovery's newly redesigned tank reacts to the cryogenic fuel, and where any ice or frost forms on the exterior. Engineers are especially interested in the areas of the tank where foam insulation was removed or applied differently, to avoid large chunks from breaking off during launch.

A 1.67-pound (750-gram) piece of foam brought down Columbia in 2003, striking the ship's left wing at more than 500 mph (800 kph) at liftoff and letting in the searing atmospheric gases of re-entry two weeks later. The wing melted from the inside out, and the spacecraft shattered over Texas, killing all seven aboard.

Discovery is scheduled to blast off no earlier than May 15, on a delivery and repair mission to the international space station that is considered a test flight. Shuttle managers plan to reassess the launch date soon, in light of all the delays in getting Discovery to the pad.

The latest snag was a 1 1/2-inch-long (3-centimeter-long) 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, which is located on the side of the tank opposite the shuttle, and say that even if a piece of foam did come off there, it would not strike the ship.

         Fixing NASA: Continuing Coverage of the Space Shuttle Return to Flight