Discovery was set to blast off from Kennedy Space Centre at 3:49 pm EDT, only the second shuttle launch since the Columbia disaster in 2003. Vice President Dick Cheney was among the dignitaries expected to attend.

Storm clouds expected in the afternoon remained the chief obstacle to launch, although the forecast improved Saturday from a 40 percent to a 60 percent chance that the weather would be good enough for a launch.

A last-minute technical problem popped up Saturday morning, with the failure of a heater used to keep propellant from freezing in a firing thruster. The thruster isn't used during launch, but it can control the shuttle's position in space and during a rendezvous with the international space station.

"It's not a show-stopper, in terms of rendezvous," said Nasa spokesman Kyle Herring. "You can manage it."

The countdown proceeded as normal, and Nasa was unsure what effect it would have on the launch. Space agency managers have said they want all four thrusters working properly.

Fueling, completed under three hours, showed that four new fuel tank sensors worked properly. The four sensors designed to prevent the main engines from running too long or not long enough during the climb to space had been replaced after one of them gave an electrical reading that was slightly off. The swap-out pushed back Discovery's launch in May.

The launch Saturday will test Nasa Administrator Michael Griffin's decision to go ahead with the mission despite the concerns of two top agency managers who fear foam flying off the fuel tank might harm the space shuttle.

Bryan O'Connor, the space agency's chief safety officer, and chief engineer Christopher Scolese recommended at a flight readiness review meeting two weeks ago that the shuttle not fly until further design changes are made to 34 areas on the fuel tank known as ice-frost ramps. These wedge-shaped brackets run up and down the tank holding in place pressurization lines. Foam insulation is used to prevent ice from building up on the tank when it is filled with supercold fuel. Small pieces of foam have snapped off during previous launches.

"We now have a NASA in which senior officials feel free to discuss and debate openly complex, difficult and subtle technical topics that affect the flight," Griffin said Saturday. "No matter what decision I made, I would have been disappointing somebody.

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O'Connor and Scolese agreed with Griffin's rationale that the risk was only to the shuttle and not the crew since the astronauts could take refuge in the international space station until a rescue vehicle is sent up, so they didn't appeal Griffin's decision.

"First of all, it's not a democracy. We don't take a vote. We don't need 100 percent of the people to say it's OK," astronaut Scott Kelly, whose identical twin, Mark, is Discovery's pilot, said of Griffin's decision. "He made the decision and I think it's the right decision to proceed with the launch."

Astronaut Stephen Robinson, who was part of last year's crew that made the first return to flight since Columbia disintegrated in 2003, said he was encouraged that the biggest technical debate was focused on the ice-frost ramps.

"If we're down to worrying about something like an ice-frost ramp, we must be doing a lot of things right," Robinson said. "There are much bigger things to worry about."

Nasa engineers redesigned the external fuel tank after the Columbia accident, and again after a piece of foam insulation came off the tank during Robinson's mission last year. In the most recent change, more than 35 pounds of foam have been removed in what Nasa describes as the biggest aerodynamic change ever made to the shuttle's launch system. Nasa tried other design changes to the ice-frost ramps, such as removing foam, but they didn't hold up well in wind tunnel tests.

The 1-pound slab of foam insulation broke off Discovery's external fuel tank two minutes after liftoff last year and, unlike in the case of Columbia, missed hitting the shuttle. Nasa responded by grounding shuttle flights for almost a year.

Discovery's seven-member crew will test shuttle inspection and repair techniques, bring supplies and equipment to the international space station and deliver the European Space Agency's Thomas Reiter for a six-month stay aboard the orbiting outpost.

Astronauts Piers Sellers and Mike Fossum will make two spacewalks and possibly a third, which would add a day to what is planned to be a 12-day mission. The crew also includes commander Steve Lindsey and mission specialists Lisa Nowak and Stephanie Wilson.