Nasa has set 13 July as a launch date for the first shuttle flight since the Columbia disaster over two years ago, saying that the agency's best minds have made the spacecraft as safe as possible.
"We are being as smart about this as we know how to be, but we are up against the limits of our human knowledge," Nasa Administrator Michael Griffin said on Thursday in pronouncing the space shuttle Discovery ready for lift-off.
"If someone wants more, they're going to have to find smarter humans."
The space agency set the date after more than two years of frustrating delays, setbacks and modifications to the shuttle, and a two-day, high-level review of whether Discovery is ready.
Go for launch
"Based on a very thorough and very successful flight-readiness review, we're currently 'go' for launch of Discovery on 13 July," Griffin said.
Launch director Mike Leinbach said his team was celebrating with hearty backslaps. "It's a great, great feeling to be less than two weeks from launch," he said.
Leinbach said his only worry, at least for now, was the seemingly non-stop stormy weather. As he addressed the late-afternoon news conference, thunder rumbled and rain poured down.
"Based on a very thorough review, we're currently 'go' for launch of Discovery on July 13"
Michael Griffin, Nasa Administrator
Earlier in the week, an advisory panel concluded that Nasa failed to meet three of the 15 safety recommendations issued by the Columbia accident investigators in 2003.
Despite many improvements, the shuttle is still vulnerable to pieces of foam or ice falling off the external fuel tank at lift-off, and the astronauts still have no reliable way of fixing damage to their ship's thermal shielding once in orbit, the group said.
But Griffin and others at Nasa said they believe those risks have been reduced to an acceptable level. He said Nasa did everything possible to make the fuel tank safer and to develop rudimentary patches for Discovery's crew, in case of small holes in the shuttle's thermal skin.
"The proximate causes of the loss of Columbia have been addressed. Many other things which could have been of concern or would have been of concern have also been addressed," Griffin said.
"We honestly believe this is the cleanest flight we have ever done. The only other flight that will ever be cleaner is the next one."
Griffin said spaceflight is always risky. "We've done what we can do to minimise that based on the state of our knowledge today," he said. Discovery will carry seven astronauts to the international space station, along with sorely needed supplies and replacement parts.
If Discovery suffers irreparable damage en route, the astronauts will move into the station and await a rescue by Atlantis -a situation Nasa considers an extreme last resort.
All seven of Columbia's crew were
killed when the shuttle broke apart
Griffin said he met on Thursday Discovery's commander, Eileen Collins, and her crew, who told him they did not want Nasa rushing but assured him they were "go for launch".
A large chunk of fuel-tank insulating foam smashed a hole in Columbia's left wing during lift-off in January 2003 and caused the shuttle to break apart during re-entry two weeks later. All seven astronauts were killed.
Nasa's main focus following the accident was on keeping big pieces of foam from falling off the fuel tank. It was not until this year that engineers fully realised the dangers posed by ice, which can form on the tank once the super-chilled fuel is loaded.
That prompted Nasa to delay the mission from May to July and install another heater on the redesigned tank.