By Paul Rincon
BBC News science reporter, at the Kennedy Space Center
For Nasa, returning the shuttle to flight has been one of the biggest challenges it has faced since it was set up in 1958.
A safe mission is crucial for the future of Nasa
A safe mission for Discovery and its crew is crucial for demonstrating the agency can recover from the Columbia accident in February 2003, which killed seven astronauts.
And it is critical for demonstrating that Nasa has restored its ability to put humans in orbit, an important step in pushing forward the vision for space exploration.
The vision, laid out in January 2004 by President George W Bush, aims to return humans to the Moon, to use it as a staging point for a manned mission to Mars. Nasa's overriding priority is to now transform that vision into a reality.
But the return to flight also signals the beginning of the end for the space shuttle, which will be retired by 2010 to make way for its replacement, the Crew Exploration Vehicle (CEV) which will carry out many of the goals in the vision.
"[Nasa] has to demonstrate its ability to launch, but at the same time show that it is washing its hands of the programme," says Dr Eligar Sadeh, assistant professor in space studies at the University of North Dakota.
"There is very serious talk of phasing out one of the shuttles in the next two years, a second one in 2009 and the last remaining orbiter by 2010."
Since Columbia broke up trying to re-enter the atmosphere on 1 February 2003, International Space Station (ISS) crews have depended on Russia both to carry them to the station and to send up supplies.
The ISS partners have to agree a final form for the platform
The station remains half-finished, and much of the hardware that still has to be added is configured to be carried only by the shuttle.
"Nasa's working very hard to figure out with its international partners what an acceptably finished ISS might look like and the fewest number of shuttle flights you need to get to that point. Balancing risk with commitment is a tricky business," said John Logsdon, director of the Space Policy Institute at George Washington University's Elliott School of International Affairs.
It was originally thought that the shuttle would need to make 28 journeys to and from the station in order to complete it, but the number of flights could now be scaled down to as few as 15.
Even so, the possibility remains that another failure could occur to one of the three remaining shuttles as they carry out the final construction missions to the ISS. Such a scenario would be a tremendous setback after the two-and-a-half years spent making the shuttle as safe as possible to fly.
"It's not going to be the end of Nasa by any stretch of the imagination. What [a failure] would mean is the immediate end of the shuttle programme," said Dr Sadeh.
Cancellation of the shuttle programme need not result from a catastrophe along the lines of Columbia or Challenger. Even a failure that forced a shuttle crew to use the International Space Station as a safe haven while awaiting a rescue mission could force the vehicle into immediate retirement.
"But I think the resolve, the determination to move on to the vision for space exploration and develop the Crew Exploration Vehicle would probably be stronger than what was before. In this country, there has never been the political will to terminate human spaceflight," Dr Sadeh explains.
Robotic missions such as Cassini have enjoyed great success recently
For many Americans, the ability to send humans into space is a question of national prestige and a symbol of technological pre-eminence.
A CNN/USA Today/Gallup poll released earlier this month found nearly three-quarters of Americans supported a continuing shuttle programme. The poll drew a mixed response when subjects were asked about their confidence in Nasa as an agency.
But other programmes at Nasa will inevitably suffer as it is transformed to realise the vision for space exploration. These include the robotic exploration missions that have provided some of the agency's biggest recent successes.
"Savings have to be made elsewhere, so one saving has been the Jupiter Icy Moons Orbiter and other science programmes are beginning to feel the squeeze," said Dr Andrew Coates, of the Mullard Space Science Laboratory at University College London, UK, and a scientist on the robotic Cassini mission to Saturn.
In terms of internal politics, it could escalate long-standing rivalries between engineering and science sectors within Nasa. John Logsdon said he thought a "partnership" between human and robotic space exploration was the best way to proceed.
The International Space Station could potentially be a source of conflict outside the agency despite the resumption of shuttle flights.
Some think US export rules designed to protect sensitive military technology falling into the wrong hands could prevent astronauts from using some space station equipment.
Robots should be part of a "partnership"
"Right now, we're seeing a lot of concern over US export control regulations and a lot of low opinions of US foreign policy. We're seeing the Europeans, the Chinese and the Russians co-operating in many spheres," said Dr Sadeh.
This and other issues could serve to isolate the US, rekindling old rivalries - and creating new ones - in space, say analysts.
The return to flight may bring some of the spark back to Nasa, but it is clear that testing times lie ahead for the agency. The organisation has a new direction, but with it many new obstacles to negotiate. And the long, arduous return to flight is simply the beginning.