Nasa will today to make a third attempt to launch space shuttle Discoveryas it seeks to ease concerns over a small crack in the ship's fuel tank.
The mission, set for liftoff at 7.38pm Irish time from the Kennedy Space Center in Florida, is critical if Nasa is to finish the half-built International Space Station before the fleet is retired in 2010.
Another accident or serious problem likely would ground the shuttles permanently.
Nasa managers decided the small foam crack was not a launch hazard, but the agency is painfully aware of the potential consequences of foam falling off the shuttle's fuel tank.
Shuttle Columbiaand seven astronauts were lost in 2003 after debris striking the orbiter caused it to disintegrate on its return to Earth. Large chunks of foam also fell off the tank during the first post- Columbiamission last year.
Discovery's launch will be only the second since the accident. Despite two major redesigns, more work on the tanks is pending.
Nasa administrator Michael Griffin decided to proceed with Discovery's launch over the objections of his chief engineer and head of safety.
There was no dissent among mission managers and engineers on the risk posed by the loss of a .09-ounce slice of foam from an oxygen fuel line.
Inspectors discovered a five-inch long, half-inch wide crack in the fuel tank foam and a wedge of foam that broke away following Discovery's second launch attempt on Sunday.
The crack is believed to have been caused by condensation trailing down the shuttle's liquid oxygen fuel line and solidifying into ice due to the extreme cold of the cryogenic fuel flowing through the pipe.
When the fuel tank was emptied after Sunday's weather-related delay, the ice melted and the foam cracked as the line expanded in the warm air.