Discovery had to make a 6.7 kilometre journey back from the launch pad to its hangar for extra safety modifications on 26 May. Credit: © NASA

It has taken more than two years to get the space shuttle Discovery ready for launch, after the Columbia disaster of February 2003. NASA staff have faced a rigorous list of safety requirements to make their shuttles shipshape, and there have been a number of mishaps along the road to lift-off. Even now Discovery isn't perfect.

After the Columbia shuttle burned up on re-entry to Earth, an accident investigation board was called in to identify risks on the shuttles. They came back with a list of 15 key recommendations.

The independent Return to Flight Task Group, which was set up to check on NASA's progress, announced on 28 June that NASA had met 12 of these 15 (see 'Shuttle safety recommendations').

But not getting a perfect scorecard is hardly a defeat for the shuttle. As the task group noted, "Some of these recommendations were relatively easy, most were straightforward, a few bordered on the impossible." On 30 June, NASA administrator Michael Griffin said that the remaining risks were acceptable, and that Discovery was ready to launch.

Repairs and delays

Many of the changes have required NASA to go back to the drawing board and completely redesign components. The use of insulating foam on the craft, some of which broke off and caused the Columbia problem, has been reassessed. And Discovery now bristles with sensors to check for damage, which took considerable time to fit (see 'Discovery: the improved shuttle' interactive graphic). The other two shuttles, Atlantis and Endeavour, are being fitted up the same way.

With these modifications complete, Discovery was initially slated for a May launch, and rolled out to its launch pad at the Kennedy Space Center, Florida, on 7 April. But while filling the external fuel tank with liquid oxygen, engineers found that ice was accumulating on the pipe that carries the ultracold fluid from the tank into the shuttle's engines.

If the ice fell off, it could have damaged part of the shuttle, so the whole craft was rolled back into its hangar to have its external tank swapped for one with an extra heater fitted to the pipe. This will hopefully prevent icing. After the switch, the shuttle was rolled back to the launch pad on 15 June.

Nobody's perfect

The shuttle still isn't perfect, although NASA's team is taking steps to meet the board's last three recommendations.

Perhaps unsurprisingly, it has proven impossible to stop small chunks of foam from falling off; given the vibrations and extreme conditions of lift off, the team expects that to be inevitable. And the shuttle's exterior has been strengthened, but it isn't completely impregnable to debris strikes.

NASA has also been unable to prove that they are capable of repairing holes in the protective tiles and carbon plates that protect the shuttle once it has reached orbit. A crucial part of Discovery's mission will be to test some of these repair techniques on some spare tiles (see 'The mission' ). But the only opportunity to prove it really works will come when a shuttle needs to be repaired in space.

Of course it is impossible to make the shuttles completely safe. "There's no question they have improved aspects of the shuttle system," says Ali Mosleh, an expert in reliability engineering at the University of Maryland, College Park, who has worked with NASA on a probabilistic risk analysis for the shuttle. The risk is now "better, probably by some percentage", he says. "But not by an order of magnitude."

NASA is ready to take a chance with Discovery. The next nail-biting mission will come in September, when Atlantis, now being fitted with the same safety modifications, takes off. But for any of the flights of the three shuttles between now and the end of their lifetime in 2010, there is no guarantee. Travelling into space is still, after all, a risky business.