By Jean-Louis Santini
Florida - Nasa began its official countdown on Wednesday to the Discovery shuttle's launch with a downbeat weather report showing that the critical mission could be delayed from Saturday.
Discovery's engines had been due to start on Saturday at 3.49pm from the the Kennedy Space Centre, with a five-man and two-woman crew headed to the International Space Station (ISS) at 10 times the speed of a flying bullet.
Nasa said initial preparations were going ahead smoothly despite earlier concerns from engineers about the safety of foam insulation that had come off the shuttle's fuel tank during its previous launch last year.
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Space officials went ahead with the countdown but Kathy Winters of Nasa's meteorological service said gathering storm clouds meant a 60-percent chance the flight - only the second since the February 2003 Columbia disaster - would be postponed.
With the Columbia's memory still fresh at Nasa, another catastrophe could put a permanent end to 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.
Reiter will remain in the ISS, joining American astronaut Jeffrey Williams and Russian cosmonaut Pavel Vinogradov.
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.
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.
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 at launch.
Nasa will have several cameras in place 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 beyond the shuttle program with plans to build a Crew Exploration Vehicle by 2014. - Sapa-AFP
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