Engineers at the Kennedy Space Center are gearing up to start the shuttle Endeavour's countdown this evening at 8 p.m., setting the stage for launch on a space station assembly mission at 6:36:36 p.m. Wednesday. Forecasters are predicting a 70 percent chance of good weather Wednesday and Thursday, improving to 80 percent "go" on Friday.

"All our systems are in very good shape right now," said NASA Test Director Steve Payne. "Countdown work is on schedule and we have no issues to report. The team is ready, Endeavour's ready and we're looking forward to Wednesday's launch and a safe and successful mission."

Commander Scott Kelly, pilot Charles Hobaugh, Tracy Caldwell, Rick Mastracchio, Dave Williams, educator-astronaut Barbara Morgan and Al Drew flew to the Florida spaceport Friday.

They originally hoped to take off Tuesday evening, but last week NASA managers delayed the flight one day because of time lost replacing a valve in the shuttle's crew module. Located behind the orbiter's toilet, the positive pressure relief valve failed a pressure test and engineers opted to replace it with a valve borrowed from the shuttle Atlantis.

"We found a little tiny piece of debris in the sealing surface, which was causing that slight leak rate," Payne said today. "Once we removed it and retested the valve, it was tight as can be. So there's nothing wrong systemically with any of the valves.

"We did borrow one from OV-104, from Atlantis, and we put it in there and we just finished a leak check, I believe early this morning, and it's very, very tight. So we're good to go as far as valves and we're not concerned about anything system wide."

Shuttle weather officer Kathy Winters said she expects a 70 percent chance of good weather Wednesday, with the only concern being a chance of isolated showers and electrically active anvil clouds moving into the launch area. Two of NASA's three European emergency runways are forecast "go," as is Edwards Air Force Base in California's Mojave Desert.

The outlook is much the same Thursday, still 70 percent go with less chance of anvil clouds, and by Friday, Winters said drier air is expected, prompting an 80 percent go forecast.

"The weather looks good overall," Winters said.

Endeavour has enough on-board liquid hydrogen and oxygen for it's three electricity producing fuel cells to make four launch attempts in five days. If the shuttle isn't off the ground by then, the team would stand down for 48 hours to top off fuel cell supplies before making additional attempts.

A detailed countdown timeline, along with NASA's television schedule (revision A), a launch windows chart and the latest version of the crew's flight plan are available here.

• TV Sked (rev. A)
• Countdown Timeline
• Launch Windows
• Flight Plan

Mission planners are prepared for two options - an 11-day three-spacewalk mission and a 14-day four-EVA flight - depending on how a new station-to-shuttle power transfer system works. The SSPTS was designed to let the shuttle plug into the station's solar power grid, reducing the load on the orbiter's fuel cells and extending the docked phase of the mission.

Until the SSPTS is plugged in and verified operational, however, Endeavour's mission is officially considered an 11-day flight. Because of last-minute planning and the one-day launch delay, the flight plan posted on the CBS News Quick-Look page is slightly out of synch. Revision A of the NASA TV schedule is up to date through the official 11-day scenario. Updates will be posted as warranted.

Additional coverage for subscribers:
VIDEO: SUNDAY COUNTDOWN AND WEATHER UPDATE PLAY
VIDEO: CREW ARRIVES AT KENNEDY SPACE CENTER PLAY
VIDEO: COMMENTS FROM EACH OF THE ASTRONAUTS PLAY
MORE: STS-118 VIDEO COVERAGE
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