Discovery's seven astronauts yesterday carried out final preparations for their attempt to land their stricken space shuttle.
In a critical manoeuvre, pilot Jim Kelly freed the shuttle from the International Space Station and inched the craft away from its docking port before firing the spaceship's jets to reposition it for its voyage home tomorrow. Space station commander Sergei Krikalev wished Discovery's crew a 'safe trip and soft landing'.
That landing promises to be a nail-biting affair, however, for it will be the first attempt by a space shuttle to return to Earth since Columbia blew apart at the end of its flight two years ago.
Nasa had hoped for a flawless return to orbit, but the latest mission - supposed to resurrect America's manned spaceflight programme - has gone badly awry. Serious damage to the craft has been discovered during the mission.
Last week astronaut Steve Robinson had to remove protruding gap fillers from Discovery's hull during a spacewalk, while a ruptured thermal blanket near a cabin window was also found.
'We will just have to get through it and see how it turns out,' said Nasa chief Mike Griffin at a briefing about tomorrow's landing. 'This manned spaceflight stuff is really hard.'
The agency is particularly jittery because it had to ground its entire shuttle fleet last week after a large chunk of insulating foam was spotted falling from an external fuel tank during Discovery's launch, a problem that doomed Columbia in 2002.
Managers reckoned they had fixed the foam insu-lation problems during the 30-month grounding of the fleet that followed that tragedy. More than £1 billion was spent on refits. Now Nasa is back to square one.
'We are seeing the death throes of the space shuttle,' said Dr John Logsdon, director of the space policy institute at George Washington University. 'The problem is not that the machinery is old but the whole concept was a mistake. The shuttle is an evolutionary dead-end.'
For a start, the shuttle is overly complex; it has 300,000 moving parts. By contrast, SpaceShipOne, the world's first private spaceplane - flown for the first time last year - has 30. As one space engineer put it: 'Complexity means fuck-ups. Simplicity means safety.'
The decision to place the manned orbiter beside, rather than on top of, the shuttle's fuel tanks has also had dire consequences: Columbia was wrecked by falling insulation from the top of a fuel tank, which damaged its heat protection tiles so it disintegrated during re-entry.
In addition, using a single craft to carry both humans and cargo is now seen as a major blunder. Hence Nasa's decision - revealed last week - to replace the shuttle with two very different spacecraft: a manned capsule to be launched on top of a medium-sized rocket and a huge cargo launcher to put Moon and Mars vehicles in orbit.
'Separating crew and cargo functions and putting manned capsules on top of rockets will improve safety enormously,' added Logsdon. 'Astronauts have less than a one in 100 chance of dying on the shuttle at present. That is simply not good enough.'
The problem is that the new rockets will not be ready until 2011, requiring Nasa to fly at least another 10-15 shuttle flights to complete the space station. If there is no finished space station, there can be no missions from it to the Moon and Mars.
Thus the space agency's anxiety over Discovery, which is set to land at 9.46am BST. However, that tension is certainly not reflected on board the shuttle, whose crew has been remarkable calm and buoyant.
Yesterday, deputy shuttle programme manager Wayne Hale praised them for their 'consummate professionalism and coolness beyond belief' during one of the most testing missions undertaken by US astronauts.
The greatest risk to Discovery - which is carrying several tons of rubbish and debris that has accumulated over the past two-and-a-half years at the space station - will come tomorrow from ultra-hot plasma gases battering the leading edge of the wings and the nose cone.
If any damage to these has occurred but has not been spotted, another Columbia disaster could occur. The remaining two shuttle craft would be grounded permanently and America's space plans wrecked.
'It looks like a clean bird,' said Griffin. 'Nevertheless I'll celebrate when the wheels come to a halt.'