NASA gave the green light for a shuttle lift-off on Tuesday despite worries about a piece of foam that popped off Discovery's external fuel tank while the spacecraft sat on the launch pad.
The decision was sure to stir more debate about whether the space agency was putting its flight schedule ahead of safety.
The 7.62 centimetre triangular piece of foam that appeared to come from a 12.7 centimetre-long crack on Monday morning is far smaller than the foam chunk that brought down Columbia, killing seven astronauts in 2003. But NASA managers spent most of the time on Monday pondering whether to go ahead with the launch.
Some outside experts said they were uncomfortable with going ahead, although they did not have all the information.
Paul Fischbeck, a Carnegie Mellon University risk and engineering professor who has consulted with NASA on the shuttle's delicate heat protection system noted that NASA said they had never seen foam fall off on the launch pad before.
"The question is why did it happen this time and never before? If it's something you've never seen before, that makes it much more curious. "It's something you might want to understand before you launch."
The patch of foam fell off an area that covers an expandable bracket holding a liquid oxygen fuel line against the huge external tank. NASA engineers believe ice built up in that area from condensation caused by rain on Sunday.