NASA's space shuttle Discovery sits on launch pad 39A at the Kennedy Space Center on Thursday in Cape Canaveral, Fla. (Red Huber/The Orlando Sentinel/Associated Press) NASA technicians are putting the finishing touches on the space shuttle Discovery on Friday, one day before its scheduled liftoff from the Kennedy Space Center in Florida.

Discovery and its eight-member flight crew are expected to take off Saturday at 5:02 p.m. ET if the weather and other conditions stay favourable. The forecast calls for an 80 per cent chance of acceptable conditions at launch time, NASA said Friday in a news release.

Isolated coastal showers may be in the area Saturday morning, but a sea breeze will develop in the afternoon, clearing the coast and causing any showers to move inland, NASA said.

"After months of hard work and preparation, Discovery and its crew are ready to fly," NASA test director Jeff Spaulding said during a morning briefing on the shuttle's countdown status.

"All of our systems are in great shape. We're tracking no issues and we're right on schedule for tomorrow's launch."

The STS-124 mission, led by commander Mark Kelly, is the second of three flights that will launch components to complete the Japanese Aerospace Exploration Agency's Kibo laboratory at the International Space Station. The crew will make three spacewalks and install Kibo's large Japanese pressurized module and robotic arm system.

The 14-day mission will also deliver new station crew member Greg Chamitoff and return flight engineer Garrett Reisman who will end a three-month stay aboard the space station.

On Thursday, NASA made a last-minute change to its cargo aboard the space shuttle Discovery, swapping 16 kilograms of equipment for something far more pressing: a toilet pump.

The pump was needed to fix a broken toilet aboard the space station. The toilet can still handle solid but not liquid waste, NASA said.

The breakdown has forced the three-person crew at the space station to periodically manually flush the urinal component, a process that takes 10 minutes and two people.

"Insert that into your daily life, and you can see it would be quite inconvenient," Kirk Shireman, NASA's deputy space station program manager, said at a news conference.