The Monday decision was sure to stir more debate about whether the space agency was putting its flight schedule ahead of safety.
The 3-inch triangular piece of foam that appeared to come from a 3-inch-long crack late Sunday or early Monday is far smaller than the foam chunk that brought down Columbia, killing seven astronauts in 2003. But NASA managers spent most of Monday pondering whether to go ahead with the launch.
NASA has the best weather forecast yet for today's launch. Word of the good forecast came as technicians started pumping super-cold rocket fuel into shuttle Discovery's rust-colored fuel tank, a process that takes about three hours.
NASA scrubbed two previous launch attempts because of approaching thunderstorms.
CBS News correspondent Bob Orr reports that the piece of foam that cracked off weighs only a tenth of an ounce — about the same as a penny. Engineers say it would take a piece of foam more than twice that heavy to critically damage the shuttle's heat shield, Orr reports — so had the foam, come off in flight, it would not have been a threat to Discovery or its seven astronauts. It is far smaller than the foam chunk that brought down Columbia, killing seven astronauts in 2003.
Still, he says, NASA officials are concerned there may be other small cracks in the foam that they can't see. They're also they're worried that ice could form on the exposed part of the bracket presenting a potential threat on 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 Sunday.
The tank expanded when the super-cold fuel was drained after Sunday's launch was canceled because of the weather. The ice that formed "pinched" some of that foam, causing the quarter-inch-wide crack and the piece of foam to drop off, officials said.
The size of the fallen foam was less than half the size of one that could cause damage, NASA officials said.
Inspectors spotted the crack in the foam insulation during an overnight check of the shuttle. NASA had scrubbed launch plans Saturday and Sunday because of weather problems.
The forecast for a Tuesday liftoff was better than previous days, with just a 40 percent chance that storm clouds would prevent liftoff.
NASA Administrator Michael Griffin decided last week that the shuttle should go into orbit as planned, despite the concerns of two top agency managers — including the top safety officer — who wanted additional repairs to the foam insulation.
The mission for Discovery's crew this time is to test shuttle-inspection techniques, deliver supplies to the international space station and drop off European Space Agency astronaut Thomas Reiter for a six-month stay.