The shuttle Discovery blasted off on Sunday on a mission to outfit the International Space Station with a final pair of solar wings ahead of the arrival in a few weeks of an expanded space crew.

The spacecraft launched at 7.43pm (11.43pm GMT) from the Kennedy Space Centre in Florida. Just over eight minutes later, the shuttle entered orbit and was soaring at 28 000 km/h.

The journey was expected to take two days to reach the ISS, where the seven-member crew was to deliver and install the fourth and final pair of solar wings on the orbiting ISS.

Mike Leinbach, launch director for the mission, said the lift-off was picture perfect.

"I have seen a lot of launches ... and this was the most visually beautiful," he told reporters in a briefing.

"It was just spectacular. When the orbiter and the tank, booster got up in the sun light ....it was just gorgeous."

The mission, one of the last major tasks of the more than decade-long effort to construct the station, has been shortened by one day after a hydrogen leak last week led to a scrub of an earlier launch date.

But NASA officials said that the problem had been cleared up and that there has been no recurrence of the malfunction.

The leak was discovered on Wednesday, when the external tank was 98 percent full of liquid hydrogen prompting it to be emptied for the checks. In all, the shuttle mission was delayed five times since February.

Once the Discovery mission installs the solar truss — the last major segment to be attached to the ISS which itself was begun in 1998 — the space station will become fully operational and capable of housing six astronauts, NASA said.

The mission will also allow space officials to make a swap of personnel, exchanging Japanese astronaut Koichi Wakata for US astronaut Sandra Magnus, who will be returning to Earth after four months in space.

Wakata, of the Japan Aerospace Exploration Agency, is to become the first Japanese crew member on the station.

Shortened but on track

Last week's scrubbed launch forced space officials to shorten what had been planned as a 14-day mission to 13 days, and to scrap one of four planned spacewalks.

However, officials said the scheduling adjustments should not affect the mission to deliver and install a fourth pair of solar panels to the ISS.

Installing the solar panels on the $100-billion station was to take a two-astronaut team four space walks of more than six hours each to complete, according to NASA's original plans.

The pairs of solar panels, containing 32 800 solar cells, are each 35 metres long. The final array, once in place, should boost available energy to the ISS to 120 kilowatts — equivalent to that used in about 50 houses — from the current 90.

The extra power will help run the expanded array of scientific experiments to be conducted in the ISS, which saw the addition over the past year of NASA workspace and a pair of international laboratories — Europe's Columbus and Japan's Kibo.

The main purpose of the space station is to provide a zero-gravity environment for scientific experiments.

However, at present, there is insufficient staff to simultaneously conduct research and maintain the space station.

Additional energy from the soon-to-be-installed solar panels will supply power for onboard laboratories and for the station's crew, which will double from three to six in May.

AFP