Fischbeck, who has consulted with NASA on the shuttle's delicate heat protection system, wondered why foam had broken off on the launch pad. "It's something you might want to understand before you launch," he said.

The patch of foam fell off an area that covers an expandable bracket holding a liquid oxygen feed 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.

NASA managers decided to go ahead with the launch attempt because of three criteria: They are confident enough foam still is on the bracket to prevent a large piece of ice from forming; that the area of foam where the piece dropped was still intact; and they don't believe the area will be exposed to extreme heat during ascent.

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.

Griffin decided last week that the shuttle should go into orbit as planned, despite the concerns of two top agency managers ¡ª including the top safety officer ¡ª who wanted additional repairs to the foam insulation.

But the two agency officials said the foam loss will not threaten the crew because NASA has a plan for the astronauts to move into the international space station if in-orbit inspections find is serious damage to the spacecraft. The crew would await rescue 81 days later by second space shuttle.

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.