CAPE CANAVERAL, Fla. – NASA gave the green light Monday night for a Fourth of July shuttle liftoff despite worries about a piece of foam that popped off Discovery’s external fuel tank while the spacecraft sat on the launch pad.

“It’s going to be a go for tomorrow,” NASA spokeswoman Lisa Malone said Monday, giving a thumbs up.

The 3-inch triangular piece of foam that appeared to come from a 5-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.

An evening meeting of Discovery’s mission management team gave the go-ahead “without dissent,” said William Gerstenmaier, NASA’s associate administrator for space operations.

Some outside experts said they were uncomfortable with going ahead, although they didn’t have all the information.

Paul Fischbeck, a Carnegie Mellon University risk and engineering professor who has consulted with NASA on the shuttle’s heat protection system, noted that NASA said it 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,” he said. “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 then “pinched” some of the foam, forming the crack and causing the piece of foam to drop off, officials said.

The piece of 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 bad weather.