Wings and prayers

Asked what went through his mind while waiting for countdown, John Glenn, the pioneer astronaut and the first American to orbit the Earth, replied: "I felt exactly how you would feel if you were getting ready to launch and knew you were sitting on top of two million parts - all built by the lowest bidder on a government contract." It would be no surprise if similar thoughts occurred to the crew of the space shuttle Discovery as it was prepared for take-off yesterday. The first shuttle to launch since the ill-fated Columbia mission in February 2003, Discovery's journey had already been delayed two weeks ago thanks to unreliable fuel sensors, introduced as a fail-safe after the spectacular explosion of the space shuttle Challenger in 1986. Engineers had not managed to fix the problem, but the launch went ahead anyway.

Given the fate of Columbia and Challenger, the crew of Discovery would be right to be worried. So far Nasa has made 113 shuttle launches, with two ending in tragedy. Yet Nasa has calculated that the "probabilistic risk assessment" of a fatal accident by Discovery is one in 100 - less risky than climbing Mount Everest (15 in 100), but reckless compared with the one in 2 million chance of dying in an airline crash. Discovery is the oldest of the remaining shuttles and a veteran of 30 missions. The words that may have run through the minds of Discovery's crew could easily have been those of the brilliant physicist Richard Feynman, who commented after investigating the fate of Challenger: "Nasa exaggerates the reliability of its product to the point of fantasy."

Unfortunately, the risks of space flight are always going to be high, especially with the 1970s technology in use on the shuttle. But there is a pressing need to get aid to the International Space Station, itself little more than a caravan floating on a cosmic building site. The ISS needs to be gently nudged back into its proper position, and only the shuttle can do it and bring vital supplies and equipment for the pair of astronauts on board. For the next 12 days commander Eileen Collins and her crew are going to be flying on a wing and many prayers.