NASA began its official countdown to the space shuttle Discovery鈥檚 launch at 1700 EDT (2100 GMT) on Wednesday. But weather forecasts suggest the critical mission could be delayed from its scheduled blast-off on Saturday.
Discovery鈥檚 engines are due to start on Saturday at 1549 EDT (1949 GMT) from Florida鈥檚 Kennedy Space Center. The mission would carry a seven-person crew to the International Space Station (ISS).
NASA says initial preparations are going ahead smoothly despite earlier concerns from engineers about the safety of foam insulation that had come off the shuttle鈥檚 fuel tank during its previous launch in July 2005.
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Space officials went ahead with the countdown, but Kathy Winters of NASA鈥檚 meteorological service said gathering storm clouds meant the flight had a 60% chance of being postponed. The launch window extends from 1 July to 19 July.
The mission, known as STS-121, 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鈥檚 Thomas Reiter of Germany.
Foam risk
During the 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 on the ISS, joining American astronaut Jeffrey Williams and Russian cosmonaut Pavel Vinogradov (see ).
NASA gave the green light for the launch despite calls from its top safety and engineering officials for a six-month delay to ensure that potentially damaging foam insulation does not peel off the orbiter鈥檚 external fuel tank yet again (see ). The two officials accepted NASA鈥檚 decision because the crew could take refuge on the ISS and wait for a rescue shuttle if Discovery suffered irreparable damage.
Surveillance cameras
This flight will be the second since the February 2003 Columbia disaster. Columbia鈥檚 wing was struck by a piece of foam that fell off the tank during takeoff, dooming the orbiter as it returned to Earth with seven astronauts aboard.
For this mission, NASA will have a number of cameras in place to detect any falling debris during liftoff. The ISS also has equipment to visually inspect the shuttle. Griffin has warned that another disaster would likely mean the end of the shuttle programme: 鈥淚f we have a major incident in launching the space shuttle, I would not wish to continue with the programme.鈥�
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 US would have to rely on Russia鈥檚 Soyuz spacecraft to access the ISS.
