CAPE CANAVERAL, Fla. — NASA fueled Discovery for liftoff today on a backbreakingly difficult space station construction mission, despite a gloomy forecast calling for rain right around launch time.

The space shuttle was set to blast off at 11:38 a.m.

Just before dawn, scattered showers drenched the area. Meteorologists expected more rain clouds to move into the area later in the morning and quite possibly force a delay.

Hoping to beat the poor weather odds, NASA pumped more than 500,000 gallons of super-cold liquid hydrogen and oxygen into Discovery’s external tank, modified since the last mission to prevent dangerous ice buildup and reduce the potential for launch debris.

The wings, however, were not altered in any way, even though a safety engineering group pressed for a delay because of concern over three panels with possible flaws.

Commander Pamela Melroy, only the second woman to lead a shuttle mission, expressed her confidence late last week about flying Discovery, as have many of the senior managers who decided to skip wing repairs. A possible cracking problem with the protective coating on three of the wing panels was deemed an acceptably low risk.

A hole in the wing brought down Columbia in 2003.

Discovery and its crew of seven will embark on a two-week mission that is considered the most challenging and complex in the nine years of orbital assembly of the international space station.

The shuttle will carry up an Italian-built live-in compartment, about the size of a small bus, that the astronauts will attach to the space station. It’s named Harmony, the choice of schoolchildren who took part in a national competition. About 130 of those youngsters traveled to Cape Canaveral to witness the launch.