NASA has spent millions of dollars trying to prevent foam from breaking off at liftoff. Engineers were startled when it broke off Discovery during last year's mission, but it didn't harm the shuttle.

The external 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.

Inspectors spotted the crack in the foam insulation during a check of the shuttle.

The forecast for a Tuesday liftoff was better than previous days, with just a 40 percent chance that bad weather would prevent liftoff.

Griffin decided two weeks ago that the shuttle should go into orbit as planned, despite the concerns of two top agency managers - the top safety officer and chief engineer - 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 serious damage to the spacecraft. The crew would await rescue 81 days later by a 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.