- Shuttle's 39th and final flight lifts off from Kennedy Space Center in Florida at 4.50pm local time (9.50pm GMT)
After 143million miles and nearly a year in orbit, space shuttle Discovery has blasted off for the last time.
Tens of thousands witnessed history in the making from the Florida launch site. Countless more watched from surrounding towns. Roads leading into Kennedy Space Center were jammed.
Discovery should reach the space station Saturday. It will deliver a compartment full of supplies as well as the first humanoid robot to fly in space.
Making history: Discovery blasts off with a roar at Cape Canaveral today
Into the night: The shuttle can be seen roaring towards the edges of space after lift-off today
One last time: Thousands gathered at Cape Canaveral to witness Discovery's last journey
The launch took place just after the scheduled 4.50pm local time (9.50pm GMT), Fuelling began shortly after sunrise.
This time, Nasa is confident that no hydrogen gas will leak and no cracks will develop in the fuel tank.
Both those problems cropped up during the initial countdown in November and the repairs took almost four months.
Discovery will head to the International Space Station (ISS) with a crew of six, as well as a load of supplies and a humanoid robot.
This will be the 39th flight for Nasa's oldest surviving shuttle, which first blasted off in 1984.
It promises to be a sentimental journey for the six astronauts assigned to the mission, as well as the supporting cast of thousands who have painstakingly prepared the world's most travelled rocketship.
Lift off: The space shuttle Discovery blasts off from the Kennedy Space Center in Cape Canaveral, Florida,this afternoon for the very last time
Emotional last journey: Tens of thousands of people gathered near Cape Canaveral to watch the shuttle's final launch
Once more, Nasa's fleet leader is paving a new road, one that leads to shuttle retirement and an uncertain future for the U.S. space program.
When Discovery returns from the ISS, it will be the first of the three surviving shuttles to be decommissioned this year and shipped off to a museum, in this case the Smithsonian Institution.
But the end of the 30-year shuttle program is still months down the road.
For now, Nasa prefers to focus on Discovery's last hurrah, an 11-day mission to deliver a bundle of space station supplies and an experimental humanoid robot that will become the first of its kind in space.
Nasa spokesman Charles Bolden said: 'Discovery is the most flown spacecraft in history.
'People don't understand. They say it matter-of-factly. There is no other multi-flown spacecraft than the shuttle.'
It's been an uncharacteristically bumpy exit for Discovery.
Fuel tank cracks - one of the most challenging problems to strike the shuttles - cropped up during the initial countdown in early November.
It took until January for Nasa to understand the cracking in the centre portion of the tank that holds instruments, and to be assured the repairs would work.
Then last month, the lead spacewalker was injured in a bicycle crash and had to be replaced on the crew.
Launch director Mike Leinbach said Discovery has been 'a great ship... an amazing machine'.
'This is her 39th mission,' he said yesterday. 'We'd have quite a few left in her had the program been extended, but it wasn't.'
'Landing day is going to be tough,' he noted, as will landing day be for shuttle Endeavour in the spring and especially for shuttle Atlantis in the summer.
On our way: The Discovery's astronauts leave crew quarters for the launch pad ahead of its 39th and final flight
Team effort: Commander Steve Lindsey (front row, right) leads the crew followed by pilot Eric Boe (front, left) and back row, left to right, mission specialists Alvin Drew, Nicole Stott, Steve Bowen and Mike Barratt
Revealed: Discovery awaits lift-off at Kennedy Space Center in Florida last night after its rotating service structure (left) is moved away
Mr Leinbach expects there to be a lot of choked up people on the runway 'because it's the end of a 30-year program that not only have we worked in... but we've grown to love and appreciate and feel like we're doing something special for the country and really the world.'
Discovery has long been a favourite at Nasa, at least unofficially.
The oldest of the surviving shuttles that first flew in 1984, Discovery carried the Hubble Space Telescope into orbit in 1990, returned Mercury astronaut John Glenn to space at age 77, and got Nasa flying again after the Challenger and Columbia shuttle disasters.
To mark the historic nature of the flight, the six astronauts are taking up a medallion from the Royal Society that was struck in honour of 18th century British explorer James Cook. His ships included the Discovery, one of the exploring vessels after which the shuttle was named.
The future, as outlined by the White House, involves turning orbital rides over to private companies in order to free Nasa up for grander outer space travel. Asteroids and Mars are on the must-see list.
Launch manager Mike Moses said yesterday: 'We're still working on the what's next part.'
The new approach is neither good nor bad, he stressed: 'We can't keep doing what we're doing with the budgets we have, so this is the new future.'
As long as the space shuttles keep flying, the U.S. government can't invest enough money in a successor spaceship, said White House science adviser John Holdren.
Nasa's finest: The Discovery crew is (from left) Steve Lindsey, Eric Boe, Steve Bowen, Alvin Drew, Michael Barratt and Nicole Stott
Long history: Discovery's robot arm holds the Hubble Space Telescope some 380miles above Earth in 1990
Debut: Discovery blasts off from Kennedy Space Center on its first flight, STS-41-D, on August 30, 1984
He said last week: 'We need to move toward a more modern, more efficient and safer way to get our astronauts in low Earth orbit.'
Mr Bolden, a former astronaut, said his number one priority, for now, is getting the last three shuttle missions flown safely.
He expects to be 'crying like a baby' when Atlantis makes the final landing of the final flight.
MEET DISCOVERY'S SEVENTH CREW MEMBER... A HUMANOID ROBOT
With his gold, metallic head and powerful white torso, he looks either like a creation from Star Wars or a member of an American Football team.
But the time has finally arrived for Robonaut 2, or R2 as he is known, to become the seventh crew member on board this week's Discovery launch.
Confident: Seventh crew member Robonaut 2 will become the first humanoid robot in space
Nasa will send R2 to become a permanent resident of the ISS where he will become the first humanoid robot in space.
R2 was developed jointly by Nasa and General Motors in a joint bid to develop a robotic assistant that can work alongside human astronaut in space.
The 21-stone robot consists of a head and a torso with two, strangely muscly, arms.
These powerful arms are 2ft 8in long and are powerful enough to carry 20 pounds each in Earth’s gravity.
But the robot’s fingers are also extremely sensitive and allow R2 to operate machinery with almost the same skill as a human astronaut.
