Nasa's space shuttle programme is finally back on track after being grounded for more than two years. Discovery, one of three shuttles remaining from the original five-strong fleet, is scheduled for launch on July 13 - the first manned US spaceflight since the Columbia disaster in February 2003. Nasa managers admit that despite 21/2 years of modifications and redesigns, risks remain and the agency has failed to fulfil three of the safety goals laid down after the Columbia tragedy. But with space exploration still so relatively young, they say, certain risks can only be minimised - not eliminated. 'Somebody referred to this flight as a test flight, which it is,' said Nasa administrator Michael Griffin. 'They are all test flights.' 'We are being as smart about this as we know how to be, but we are up against the limits of our human knowledge. If someone wants more, they are going to have to find smarter humans.' Seven astronauts died when Columbia broke up over the US state of Texas. Investigations showed that the vehicle's heat shield had been damaged 13 days earlier by a briefcase-sized chunk of foam cladding that fell from the external fuel tank during launch. As Columbia re-entered Earth's atmosphere, super-hot gases seeped through the hole, burning the shuttle and its occupants to cinders. Among the 15 recommendations made by the Columbia Accident Investigation Board, an independent panel set up after the disaster, was that Nasa should not return to flight until the problem of falling foam had been fixed. It also urged the agency to devise ways in which heat-shield damage can be spotted and repaired by astronauts while in space. But Mr Griffin said a 100 per cent fix was impossible and that while safety precautions were better than ever, he had been working to 'recalibrate expectations a little bit'. 'As the years and the generations unroll, we will learn how to make it [space flight] routine as we have done with airline travel after 100 years, so we have a way to go.' Discovery had been scheduled for lift-off last month, but the mission was postponed when hairline cracks were found in the fuel tank after it was rolled out to the launch pad. Weather permitting, Mission STS-114 will lift off from Cape Canaveral, Florida, on the afternoon of July 13, having been cleared for launch during a two-day meeting of Nasa officials that concluded yesterday. The seven-strong crew, led by veteran pilot Eileen Collins, will do test manoeuvres in space.