NASA engineers move ahead with a rescheduled 2:38 p.m. Fourth of July launch for the space shuttle Discovery. Technicians were concerned by a small piece of insulating foam that had fallen off the shuttle's fuel tank.

Originally scheduled for Saturday, weather concerns have led to two delays. Mission engineers have a launch window that extends to July 19.

The shuttle's fuel tank, shaped like a big orange bullet, is about the size of a grain silo. It holds hundreds of thousands of gallons of liquid oxygen and hydrogen, which is kept supercold.

Whenever the tank is drained or filled, a group known as the Ice Team uses cameras and telescopes to look for buildups of ice, as well as any cracks in the tank's thin layer of insulating foam.

The technicians spotted a crack about three inches long, in foam that covers a metal bracket. The team found a small triangular chunk of foam that had fallen from the shuttle to the launch pad.

Falling foam has been blamed for causing the destruction of space shuttle Columbia three years ago, when a piece of foam fell off during launch and fatally damaged the heat shield on the left wing.

But John Shannon, head of NASA's mission management team, says the chunk of foam that fell off this time would have been too small to do the same kind of damage during launch.

If the shuttle doesn't launch Tuesday, NASA will have another chance on Wednesday. If they cannot get the shuttle into space by July 19, the launch window will close until the end of August.

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