Discovery blasted off yesterday for the first US shuttle mission since the Columbia disaster in February 2003, which forced the US to completely rethink its space program.
Thirteen days after calling off a previous attempt to get the shuttle back in orbit because of a fuel gauge problem, Discovery lifted off into clear blue skies at the Kennedy Space Center at 2:39pm GMT.
"It is time for you to return to flight. Godspeed and we will see you in a couple of weeks," a NASA mission control manager said to the crew just before the engines were ignited.
It was an emotional moment for the NASA experts and managers who watched horrified as Columbia broke up in flames as it re-entered the Earth's atmosphere on Feb. 29, 2003, killing all seven crew.
Discovery jettisoned its ascent motors two minutes after liftoff as planned and then headed into orbit at 7,200km an hour. NASA called it a perfect launch.
The shuttle and its seven crew, including a Japanese astronaut Soichi Noguchi, will spend 12 days in space to link up with the International Space Station (ISS).
The Columbia disaster forced a major rethink of the whole US space program. The Discovery mission will test many new procedures introduced because of the disaster.
The new crew members appeared upbeat as they entered Discovery. They smiled and waved as they boarded the shuttle, clad in their distinctive orange spacesuits.
Noguchi grinned as he held up a handwritten sign saying: "out to launch."
The National Aeronautics and Space Administration went ahead with the launch even though there was a risk that a fuel sensor which forced the postponement of the earlier launch could play up again.
As the shuttle's huge external tank was being filled during the night, engineers kept a close eye on the fuel sensor.
Tests had indicated the sensor was working normally, but NASA managers have said that even if it fails they would still go ahead with the launch, provided they understand the cause of the glitch.
The sensor is one of four that monitor hydrogen levels in the tank to prevent a premature shut-off of the shuttle's three engines.
During the 12-day mission, the crew will deliver supplies and equipment to the ISS.
Noguchi and other members of the crew will also carry out space walks to test some of the new measures introduced after the Columbia tragedy which was blamed on a small piece of insulation foam that broke off after liftoff and struck the orbiter's left wing. It caused a crack that allowed superheated gases to penetrate the structure just minutes before the planned landing.
Once Discovery reaches orbit eight minutes after liftoff, Commander Eileen Collins was scheduled to tip the orbiter slightly so the crew can see the external tank as it is jettisoned, to determine whether any insulating material fell off.
Cameras on the outside of the shuttle allowed experts on the ground to monitor for debris.
Collins will also flip the orbiter over as it nears the ISS Thursday, so the astronauts aboard the space station can inspect the shuttle's thermal protection system.
During one of three planned spacewalks on the Discovery, Noguchi and his US counterpart Stephen Robinson will test repair techniques adopted after the Columbia disaster.
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