The launch is scheduled for Saturday at 1949 GMT. The official countdown begins Wednesday at 2100 GMT.

With the Columbia disaster in February 2003 still fresh in everyone's mind, another catastrophe would immediately end the 25-year-old shuttle program.

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.

During the STS-121 mission, the astronauts will carefully inspect the shuttle for any damage and test new equipment and procedures to increase safety.

The crew will also deliver critical cargo for the ISS, while Fossum and Sellers will conduct two or three spacewalks to do maintenance work on the space station. Reiter will then remain in the ISS, joining American astronaut Jeffrey Williams and Russian cosmonaut Pavel Vinogradov.

NASA gave the green light for the launch despite calls from its top safety and engineering officials for a delay to ensure that potentially damaging foam insulation does not peel off the orbiter's external fuel tank yet again.

Columbia's wing was struck by a piece of foam that fell off the tank during takeoff, dooming the spacecraft as it returned to earth with seven astronauts aboard.

Despite spending hundreds of millions of dollars to fix the problem, foam came off Discovery's fuel tank in July 2005 in the first mission since the Columbia disaster, prompting NASA to ground the shuttle fleet again.

After a flight readiness review, NASA administrator Michael Griffin announced that this launch would go ahead despite dissenting views expressed by chief safety officer Bryan O'Connor and chief engineer Chris Scolese. The launch has a July 1-19 window.

O'Connor and Scolese had asked for a six month delay to make more modifications to insulating foam on the external fuel tank.

But they accepted Griffin's decision because the seven astronauts could take refuge in the ISS and wait for a rescue spacecraft if Discovery suffered irreparable damage.

"We both feel that there remain issues with the orbiter -- there is the potential that foam may come off at time of launch," O'Connor and Scolese said in a joint statement last week. "We do not feel, however, that these issues are a threat to safe return of the crew."

NASA will have several cameras in place for the launch to detect any falling debris during liftoff. The ISS also has equipment to visually inspect the shuttle.

Griffin warned that another disaster would likely mean the end of the shuttle program.

"If we have a major incident in launching the space shuttle, I would not wish to continue with the program," he said.

The success of the shuttle, with its ability to carry large payloads, is also critical to the completion of the ISS. NASA plans 16 more flights to finish the space station by 2010, the year the shuttle fleet will be retired.

Without a shuttle, the United States would have to rely on Russia's Soyuz spacecraft to access the ISS.

NASA is already looking beyong the shuttle program with plans to build a Crew Exploration Vehicle (CEV) by 2014.

The CEV is essential to President George W. Bush's space exploration ambitions announced in 2004, which include a return to the Moon in 2018 and eventually the first manned mission to Mars.

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STS-121 Mission