Astronaut Nicole Stott gives the "OK" sign as she prepares to board the Space Shuttle Discovery before launch at the Kennedy Space Center in Florida in this image from NASA TV August 28, 2009. [Agencies]

After two thwarted launch attempts this week, the shuttle lifted off its seaside launch pad at 11:59 pm (0359 GMT on Saturday), shattering the dead-of-night calm with a thundering roar and a brilliant burst of flame that temporarily turned the Florida sky as bright as day.

Bad weather and a suspect fuel valve sensor had kept Discovery grounded since Tuesday. The weather was threatening throughout the final hours of Friday's launch countdown, but skies cleared long enough for NASA to send Discovery on its way.

Aboard the shuttle were four veteran astronauts and three rookie fliers, including 46-year-old Nicole Stott, who will be left behind on the space station to serve as one of its flight engineers. The shuttle is expected to reach the station on Sunday night.

During nine days at the orbital outpost, the shuttle crew plans to conduct three spacewalks to install a new ammonia coolant tank and prepare the $100 billion outpost for its next connecting hub.

The station, a project of 16 nations, is scheduled to be finished next year after more than a decade of construction 220 miles (355 km) above Earth.

The shuttle is carrying more than 7 tonnes of food, laboratory equipment, supplies and experiments, including six mice, three of which have been genetically altered for a bone-loss experiment that might lead to new treatments for osteoporosis.