The seven-member crew of the space shuttle Endeavour boarded the spacecraft Monday, amid uncertainty over whether NASA's fifth launch attempt would again be scuttled by poor weather.
Despite forecasts suggesting only a 40 percent chance of favorable weather conditions, the US space agency went ahead with preparations for the 6:51 pm (2251 GMT) takeoff.
Dressed in bright orange jumpsuits, the crew left their quarters at the Kennedy Space Center and were ferried to launchpad 39A in a silver minibus, accompanied by security vehicles and a helicopter.
Commander Mark Polansky, 53, was the first to board, followed by co-pilot Doug Hurley, 42.
The shuttle's hatch is scheduled to be shut at 4:46 pm (2046 GMT), with the crew conducting multiple tests before takeoff, including on communications with Houston, which controls the mission launch.
Around 10 minutes before liftoff, launch director Pete Nickolenko will conduct a final survey of technical readiness and the weather situation in preparation for launch.
The mission for Endeavour's crew of six Americans and one Canadian is to install a platform on the ISS for astronauts to conduct experiments in the vacuum of space, 350 kilometers (220 miles) above Earth's surface.
So far, unsettled weather poses the greatest threat to Monday's fifth attempt to send the Endeavour on its mission to deliver the final piece of Japan's Kibo laboratory to the International Space Station (ISS).
The astronauts were also scheduled to undertake repair and replacement work, including installing six new batteries in the ISS — weather permitting.
"If a new technical issue crops up or rain gets into the thruster, teams will look to target launch for Tuesday," NASA said on its website, referring to a partially unattached Tyvek cover used to keep rain and debris out of one of Endeavour's thrusters.
The risk of a storm system developing within 20 miles (36 kilometers) of launchpad 39A, where the Endeavour and its seven-astronaut crew are waiting to take off, is the main concern, meterologist Scott McCormick told AFP.
It was just such a storm system that scuppered a launch scheduled for Sunday evening, which was cancelled shortly before takeoff as a thunderstorm developed closeby.
"Looks like the team is ready but the weather is not. At this time we are no-go," the US space agency's launch director Nickolenko said with just minutes to go before liftoff Sunday.
The Endeavour crew left the launch pad at about 8:35 pm (0035 GMT Monday) and returned to crew quarters at the Kennedy Space Center for the night.
Weather problems in the form of lightning strikes were also responsible for the third of four delays to Endeavour's mission on Saturday.
A Friday night storm produced at least 11 lightning strikes around the shuttle's pad, but did not damage the spacecraft.
Takeoff was delayed twice before, when potentially hazardous fuel leaks were discovered, apparently caused by a misaligned plate linking a hydrogen gas vent line with the external fuel tank.
The US space agency said the problem had been fixed, and had filled Endeavour's external fuel tanks with some two million liters (half a million gallons) of low-temperature liquid hydrogen on Sunday before the launch was scrapped.
The crew for the mission includes Canadian Julie Payette, an electrical and information engineer, the only woman on board, who has been in space before, as have two other members of the crew, including shuttle commander Polansky.
The crew's four other members will be on their maiden space voyage.
American aerospace engineer Tim Kopra, 46, will replace Japanese astronaut Koichi Wakata, spending several months aboard the floating space station.
He would be the latest addition to the permanent crew of the ISS, which is a joint collaboration between 16 different countries.
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