A string of recent rocket failures in the US space programme had led to some nervousness at Nasa, but the countdown progressed normally to blast off and Discovery rose high into the Florida morning sunshine shortly after 0649 EDT (1149 BST).

Watch Discovery blast off into the Florida sky

"We're going to have a great flight," Mission Commander Kent Rominger said just before launch.

On board are a crew of five Americans, one Russian and one Canadian. They are taking almost two tonnes of supplies to the ISS. This includes spare parts, tools, laptop computers and clothes for the ISS's first permanent residents, due to move in early next year.

Space walk

Discovery will also deliver components for two cranes that are needed for future construction work on the embryonic station.

Cranes are required to build the ISS

Two of Discovery's astronauts will start to erect the cranes on the hull of the ISS during a six-hour space walk. They will change 18 worn-out battery chargers that feed energy to the Zarya module, the first Russian built ISS unit put up last November.

The astronauts will also attempt to repair the malfunctioning communications system linking the American-built Unity module to the control centre in Houston.

BBC's Richard Hollingham: The ISS has been held back by the troubled Russian economy

The construction work on the $60bn space station is falling behind schedule.

The Russians failed to meet their deadlines for completion of the service module which will act as the living quarters for the first ISS crew.

Disco ball

Although more than 40 missions will be required to complete the building work, it is possible Discovery's may be the only one to launch in 1999 if the service module does not go up as planned in November.

Gil Moore explains how Starshine works

The Space Shuttle Atlantis is supposed to follow the unmanned Russian mission in December with more equipment.

Discovery has other work to complete during its 10-day mission. It will deploy the Project Starshine satellite. This scientific project will help school children worldwide to measure the way the Earth's atmosphere responds to explosions on the Sun.

Starshine looks just like a disco ball

The satellite looks just like a disco ball with its 800 tiny mirrors.

"By taking a simple little mirror and putting it up in space and having it be visible to kids on the ground by reflecting flashes of sunlight after sunset or before sunrise, we can actually measure the density of the upper atmosphere," Gil Moore, Director of Nasa's Project Starshine, said.