With the Columbia's memory still fresh at NASA, another catastrophe could put an immediate end to the 25-year-old shuttle program, which is scheduled to retire in four years.

Discovery is currently scheduled to launch on Saturday at 3:49 pm (1949 GMT), taking a five-man and two-woman crew to the International Space Station (ISS) for a crucial mission that will determine whether NASA has improved the shuttle's safety. The official countdown started late Wednesday.

"We are in good shape, we are just waiting for the weather to clear for us," Mike Leinbach, the shuttle launch director, told reporters at the Kennedy Space Center in Cape Canaveral, Florida, where thunder could be heard Thursday afternoon.

John Shannon, chairman of the mission management team, added that there were "no constraints" to launch other than bad weather.

"We are ready to go for Saturday," Shannon said.

There is a 60 percent chance that a storm will scuttle the first launch attempt Saturday, weather officials said, adding that a similar probability was predicted for Sunday.

Vice President Dick Cheney will attend the launch, his office said.

The shuttle has a July 1-19 launch window for the mission during which the shuttle will also deliver critical equipment and supplies for the ISS.

It will be just the second mission since Columbia disintegrated into a ball of fire as it returned to Earth in February 2003.

A piece of foam insulation had fallen off Columbia's external fuel tank during liftoff and pierced its heat protective skin.

NASA spent more than a billion dollars to fix the problem, but foam peeled off Discovery's fuel tank last July in the first launch since the disaster, prompting the space agency to ground the shuttle fleet again.

NASA officials decided to go ahead with the second mission despite objections from the agency's top safety and engineering officials, who called for a delay to redesign foam on the ice/frost ramp on the fuel tank.

But officials reiterated Thursday that only small pieces of the ice/frost ramps that prevent ice-buildup on the fuel tank risked peeling off during liftoff.

"We expect that we will lose some foam, but it is not of the size or the mass that is capable of damaging the vehicle catastrophically," Shannon said.

The agency also has a contingency plan in which the seven astronauts would abandon ship if the shuttle suffered irreparable damage and take refuge in the ISS while waiting for a rescue mission, officials said.

NASA has several cameras in place to see if debris falls off during liftoff and hits the ship. The ISS will also take pictures of Discovery during its approach.

A new concern for the launch is the presence of birds. Last year, a bird struck Discovery's external fuel tank shortly after liftoff.

NASA officials said traps were set to keep vultures away from the launch's air space.

Shannon said NASA will be able to determine in day six of the mission whether the shuttle suffered any damage.

The 115th shuttle flight will be headed by commander Steven Lindsey. His crew includes co-pilot Mark Kelly, and mission specialists Michael Fossum, Lisa Nowak, Stephanie Wilson, Piers Sellers and the European Space Agency's Thomas Reiter of Germany.

Reiter will remain in the ISS, joining American astronaut Jeffrey Williams and Russian cosmonaut Pavel Vinogradov.

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