Washington  - The space shuttle Endeavour blasted off Wednesday on its sixth attempt to launch since June, headed to the International Space Station to expand Japan's Kibo module.

The seven-member crew is to deliver an outside porch on the module at the station to expose scientific experiments to the extremities of space.

The mission has suffered a slew of delays that kept the craft on the ground for weeks longer than planned.

The Sunday launch was cancelled due to storms and a Saturday launch was also scrubbed due to lightning the night before that struck the launch pad, but spared the shuttle itself.

On Monday, stormy weather again grounded the spacecraft.

In addition, technical glitches forced two postponements last month after technicians detected hydrogen gas leaks during fuelling just hours before scheduled liftoff.

Endeavour, which is expected to dock at the space station on day 3, Saturday, is under the command of US astronaut Mark Polansky, who is Twittering about the journey on Astro_127.

The seven-person crew includes Canadian astronaut Julie Payette, 45.

The launch comes at a busy time for NASA, just weeks after a shuttle flight to repair the Hubble Space Telescope and an unmanned rocket launch to study the moon for the possible return of manned exploration. The moon mission was briefly postponed due to the Endeavour's problems, but got underway June 18.

The US space agency is working to fit in seven further flights of the shuttle fleet before retiring the ageing crafts in late 2010.

The shuttle will bring a new resident to the ISS crew, US astronaut Tim Kopra, to replace Japanese astronaut Koichi Wakata on the recently expanded six-person station crew.

Five spacewalks are planned for the 17-day mission with the installation of new space-station components and the delivery of spare parts, which will help keep the station running after the shuttle programme is retired next year.

The 4,082-kilogramme porch, known as the Japanese Experiment Module - Exposed Facility, will be installed in the first spacewalk. Astronauts Dave Wolf and Kopra will prepare the outside of the Kibo module for the addition. Astronauts Polansky and Canadian Payette will use the shuttle's robotic arm to remove the piece from the shuttle and hand it off to the station's arm, operated by Doug Hurley and Wakata, for installation.

Endeavour will also haul up an unpressurized storage area called the Experiment Logistics Module that will be attached to the station and house the Japanese experiments. The robotic arms on both craft will get a workout during the mission, moving the module from the shuttle to the station. And a Japanese-built arm will be used for its first work.

Later in the mission, the robotic arm will transfer the scientific experiments and a communications device to the porch. The experimental equipment includes an X-ray camera to look at objects in space and a device to measure neutrons, plasma, high-energy light particles, cosmic dust and other particles in space.

During the second spacewalk, astronauts will transfer spare equipment from a cargo carrier to a stowage area on the station's truss, where they can be easily retrieved on future spacewalks. The new equipment includes an antenna, a pump mechanism and a rail mechanism. They will serve as backup parts for the ISS crew after the space shuttles are grounded and no longer able to deliver new parts to the station.

Other spacewalks will focus on changing batteries on one of the station's solar wings and installing cameras on the outside of the craft.

After undocking from the ISS, the Endeavour will deploy two satellites. (dpa)