After weeks of repair work to plug a hydrogen leak on the space shuttle Endeavour, NASA is now watching the weather for the spacecraft's planned Saturday launch.
Endeavour has a 40 percent chance of good weather for its weekend launch toward the International Space Station, though NASA is hopeful that the potential for nearby thunderstorms and thick clouds will not hinder the much-delayed construction flight.
"That is always the remaining issue that we have," NASA test director Steve Payne said of the weather in a Thursday briefing. "If there's a hole in the clouds, we'll go for it. If not, then we'll go for it the next day. But we would rather get it done on Saturday."
Endeavour and a seven-astronaut crew are currently slated to blast off Saturday evening at 7:39 p.m. EDT (2339 GMT) from a seaside launch pad at NASA's Kennedy Space Center in Cape Canaveral, Fla. The countdown clock began ticking down toward liftoff late Wednesday night.
Payne said NASA has launched space shuttles with a 40 percent chance of good weather in the past. The forecast will improve slightly on Sunday, though a recent trend of afternoon thunderstorms is expected to continue.
"On launch day we are expecting thunderstorms to affect the countdown, especially in the afternoon," shuttle weather officer Todd McNamara said.
NASA has four days to send up Endeavour. If the shuttle is not flying by Tuesday, it will have to wait for Russia to launch an unmanned craft with much-needed supplies. That would push the shuttle liftoff to July 27.
Endeavour's mission has been delayed since mid-June due a hydrogen gas leak that prevented two consecutive launch attempts. Engineers replaced a misaligned connector plate and a vent line seal to plug the leak.
Commanded by veteran spaceflyer Mark Polansky, Endeavour's six-man, one-woman crew is slated to fly a marathon 16-day mission to the International Space Station. The astronauts will ferry a new member of the station's six-man crew to the outpost and deliver a porch-like experiment platform — the last piece of Japan's massive Kibo lab — as well as other spare parts. Five spacewalks are planned for the mission.
Also launching on Endeavour are new solar array batteries, as well as a spare antenna, a cooling system pump module and a drive unit for the station's railcar, said Scott Higginbotham, NASA's payload manager. They will be attached to storage platforms outside the station during the mission's spacewalks, he added.
Aside from the vexing weather forecast, Endeavour appears to be in ship shape for Saturday's launch attempt. Payne said the shuttle has no technical issues at this time.
"Good things come to those who wait," Higginbotham said. "It's time to go fly and complete Kibo."
Space.com and The Associated Press contributed to this report.