Discovery’s crew members will be showing up with a last-minute addition to their cargo: replacement parts for a broken toilet aboard the station. The toilet has separate systems for dealing with solid and liquid waste. The unit that stopped working last week was supposed to direct urine flow and separate the liquid from air for storage. Two replacement units that were on board the station have also failed.

Julie Payette, a Canadian astronaut, said that despite the many toilet jokes that had been made in the news media over the past week, “We actually take this extremely seriously. In our book, the hygiene cabinet  the toilet  is perhaps one of the most important systems on any spacecraft.” It should go without saying, Ms. Payette said, that “we’re humans.”

“We generate waste,” she continued. “We need a way to dispose of it.”

But she expressed confidence the Russians would be able to repair the system, because Russian engineering tends to be robust and repairable. “They have really good engineers,” she said.

The mission includes three spacewalks to help install Kibo, perform station maintenance and to test techniques for cleaning a malfunctioning rotary joint that is a critical part of the station’s power supply.

That joint, 10 feet in diameter, rotates one of the station’s enormous solar arrays so that it faces the Sun during each orbit. The National Aeronautics and Space Administration idled the joint last year, when it was found to have been damaged by metal shavings that fouled its inner workings and were being ground in by the operation of the joint.

Mr. Gerstenmaier said at the news conference that the station would probably be able to operate into next year before the problems limited the use of the growing station and the joint would have to be repaired.

At the news conference, Michael D. Griffin, NASA’s administrator, beamed as he talked about a week in which NASA not only launched a shuttle crew but also landed a robotic craft, Phoenix, on Mars. He joked, “It’s so great that not even having to do a press conference  two press conferences in a week  can ruin it.” But, he added: “It is not easy. We could talk until 6 a.m. tomorrow and I wouldn’t touch all the details that would demonstrate how hard it is. And yet these teams make it look easy.”