NASA faced the prospect of more stormy weather Monday as it tried for the fifth time to launch shuttle Endeavour to the international space station.
The seven astronauts assigned to the space station construction mission waved and shook their hands in the air as they headed out to the launch pad at midafternoon.
Weather conditions were green, or go, as the crew climbed aboard one at a time, but areas of rain popped up to the west, a cause for concern.
"While we all hope that today's the day, we have a saying that you never know for sure until the solid rockets light off," commander Mark Polansky said earlier in a Twitter update.
Thunderstorms prevented Endeavour and seven astronauts from blasting off Sunday evening. Forecasters said there was a 60 percent chance the weather would force yet another delay Monday evening for the space station construction mission.
NASA has until Tuesday, possibly Wednesday, to launch Endeavour with the final piece of Japan's space station lab. Otherwise, it must wait until the end of July because of a Russian supply ship that's awaiting liftoff.
Sunday's countdown made it all the way to the nine-minute mark before the liftoff was scrubbed.
Saturday's launch attempt was foiled several hours in advance by a series of lightning strikes around the pad that required extra checks of the many critical shuttle systems. Back in June, hydrogen gas leaks held everything up. Repairs to a misaligned plate on the fuel tank took care of the leaks.
The only technical concern Monday was a loose cover on a shuttle thruster. Rain at the pad almost certainly would force a launch delay; any water that gets into the thruster would freeze in space, and the thruster would become inoperable.
Endeavour holds the third and final segment of Japan's enormous $1 billion space station lab, named Kibo, or Hope. It's a porch for experiments that need to be exposed to the vacuum of space. The shuttle also is loaded with large spare parts for the space station and hundreds of pounds of food for the six station residents.
When the shuttle astronauts arrive at the space station, they will make up the biggest crowd ever in a single place in orbit: 13 people.
Endeavour will spend nearly two weeks at the space station. In all, the flight will last 16 days. Five spacewalks are planned to hook up the Japanese lab's new porch, replace space station batteries and perform other maintenance.