By Irene Klotz

CAPE CANAVERAL, Fla., May 31 (Reuters) - Space shuttle Discovery is poised for launch on Saturday on a mission to add Japan to the growing number of countries operating full-time space research laboratories in orbit.

In delivering Japan's bus-sized Kibo laboratory to the International Space Station, the United States will fulfill an agreement hashed out two decades ago to partner with other countries in building and operating an orbital base.

"This is a big step for the Japanese community," said Akihiko Hoshide, a 39-year-old astronaut with the Japan Aerospace Exploration Agency who is making his first flight.

Japan is the last of the 16-nation partnership to get its hardware in space and it will be next year before the three-part lab is complete. During its last shuttle mission in March, NASA installed Kibo's storage room, which was launched with spare computers and experiment racks.

The 37-foot-(11-metre-) long, 16-ton main compartment is packed aboard shuttle Discovery, which is set for liftoff at 5:02 p.m. EDT (2102 GMT) from the Kennedy Space Center in Florida.

Meteorologists predicted an 80 percent chance of acceptable weather for launch.

The lab fits so snugly in the shuttle's 50-foot- (15-metre-) long cargo bay that NASA has no room to fly an inspection boom crews routinely use to check their ship for damage after launch. Instead, the last shuttle crew to visit the space station left its boom behind for Discovery to use and return to Earth.

Technicians even had to pull out 35 pounds (16 kg) of spare parts from the shuttle's crew cabin when NASA agreed to fly a pump to fix the space station's balky toilet.

THREE SPACEWALKS

Once in orbit, the Discovery crew plans to conduct three spacewalks to hook up the new lab, work on the station's cooling system and troubleshoot a problem that is hampering a pair of solar wing panels from tracking the sun for power.

For now, there's plenty of electricity for all the station's compartments, which include the U.S. Destiny laboratory, Europe's Columbus module, the Russian-made living quarters and propulsion chambers, two connecting nodes and two airlocks.

After Kibo is attached, the station will be 71 percent complete, with seven construction missions remaining.

One of those flights will be to deliver the final segment of Kibo, a unique porch complete with robot arms for tending to experiments in the open environment of space.

NASA wants to have the space station in perfect working order and fully stocked with spare parts before Sept. 30, 2010, when the shuttle fleet is due to be retired.

In addition to the construction missions and two resupply flights, the U.S. space agency plans a final servicing call to the Hubble Space Telescope, scheduled for October.

The seven-member Discovery crew includes a new flight engineer who will remain behind for a six-month mission aboard the space station. Greg Chamitoff replaces Garrett Reisman who will return home with the rest of the shuttle Discovery crew.

Discovery's flight, which is the 123rd in shuttle program history, is scheduled to last 14 days. (Editing by Jim Loney and Mohammad Zargham)

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