Washington  - The crew of space shuttle Endeavour is preparing to install a new porch outside the International Space Station (ISS), but don't expect any astronauts to kick up their heels and relax.

The porch outside the Japanese Kibo module will expose scientific experiments to the extremities of space and is the main objective of the shuttle flight, which will bring the final pieces of Kibo to ISS when it launches Saturday.

Endeavour is set to take off at 7:39 pm (2339 GMT) Saturday from the Kennedy Space Centre in Florida after a slew of delays kept the craft on the ground for weeks longer than planned.

Last month, NASA was forced to postponed the launch twice in just four days, after technicians detected hydrogen gas leaks during fuelling, just hours before scheduled liftoff. The problem was similar to a malfunction in March that delayed the launch of the Discovery shuttle.

Officials said Wednesday that they were ready to fuel the shuttle, but that the threat of thunderstorms could yet end up keeping Endeavour on the ground. Forecasts showed a 40-per-cent chance of acceptable weather for blastoff.

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 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 Mark Polansky and Canadian Julie 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)