NASA hopes to launch Endeavour to the international space station on Wednesday to deliver a fresh resident crew and a replacement joint for the robot arm. The shuttle will bring back the three men who have been living on the orbiting outpost for six months.
Forecasters on Monday put the odds of good launch weather at 60 percent. Storm clouds remain a concern, but thunderstorms should be far enough inland to allow for a safe shuttle flight.
Endeavour's crew will be able to make three launch attempts in four days, reports CBS News Space Consultant Bill Harwood.
Over the weekend, mission managers bumped the launch to Tuesday, then Wednesday to replace a bad valve in Endeavour's left orbital-maneuvering system. The valve failed late in the countdown Thursday night, but engineers managed to solve the problem only to have thunderstorms force a delay, the first of four.
The valve failed again late Friday during testing, and the subsequent replacement and leak checks took longer than expected.
The bottle-size valve regulates the pressure of nitrogen gas used to operate the fuel valves for the orbital-maneuvering engine.
The space station's U.S. residents, Daniel Bursch and Carl Walz, will set a NASA space endurance record by the time they return to Earth.
They have been living on the outpost, along with their Russian commander, Yuri Onufrienko, since early December.
CBS News Space Consultant William Harwood has covered America's space program full time for more than 15 years, focusing on space shuttle operations, planetary exploration and astronomy. Based at the Kennedy Space Center in Florida, Harwood provides up-to-the-minute space reports for CBS News and regularly contributes to Spaceflight Now and The Washington Post.