WASHINGTON - NASA rushed Wednesday to get a special pump on board shuttle Discovery to fix a balky toilet at the international space station, as the launch countdown got under way.

The space station's Russian-built toilet has been acting up for the past week. The three male residents have temporarily bypassed the problem, which involves urine collection, not solid waste.

Russian space officials are providing the pump to launch aboard Discovery on Saturday. The shuttle's seven astronauts arrived at Kennedy Space Center a few hours ahead of the start of countdown Wednesday afternoon.

At the same time, a NASA employee was en route to Florida from Russia with the 1 1/2-foot-long pump and related hardware, which was packed in a diplomatic pouch and carried onto the commercial jetliner as 35 pounds of hand luggage.

To make room for the pump inside Discovery's crammed cabin, NASA was going to pull out some wrenches, a spare part for the space station's oxygen generator, and a microbe-killing device for use in the European space lab.

"Clearly, having a working toilet is a priority for us, so some of these things that we didn't need for the next six months or so could wait," said payload manager Scott Higginbotham.

NASA is also squeezing in a Disney action figure, Buzz Lightyear. The toy popularized by the 1995 movie "Toy Story" will spend several months at the space station as part of an educational program for math and science teachers and their students.

Meanwhile, the main cargo on Discovery is Japan's Kibo lab, a 37-footer that's as big as a school bus. Kibo means "hope" in Japanese.

Launch time on Saturday will be 5:02 p.m. Forecasters put the odds of good launch weather at 80 per cent.

Shuttle commander Mark Kelly took pleasure in seeing Discovery on the launch pad, as he flew in from Houston.

"We hear it's in great shape," Kelly told a small crowd at the runway. "As soon as we get a couple more spare parts that I'm sure some of you guys have heard about ... we're going to be all ready to go."

Astronaut Michael Fossum, who will take part in three spacewalks, noted that he and his crewmates have been training for this mission for a year.

The flight is scheduled to last 14 days.

"It's been a long year, and I think everything's finally in place," Fossum said. "Discovery's perched on the pad, Kibo is ready to go, the weather looks good and we're about as ready as we can possibly be. I think it's time to go fly."