The space shuttle Endeavour is once more poised to launch into orbit early Wednesday after a four-day delay caused by a hydrogen gas leak.
Endeavour is slated to lift off at 5:40 a.m. EDT (0940 GMT) from Launch Pad 39A here at NASA's Kennedy Space Center. The shuttle is scheduled for a 16-day mission to the International Space Station (ISS) to deliver a porch-like external research platform for the Japanese Kibo laboratory.
Originally slated to launch June 13, Endeavour's flight was delayed by a leak of gaseous hydrogen from a vent pipe on its external fuel tank. Ground crews worked around the clock to change out a valve and seal between the pipe and the shuttle to eliminate the problem. Mission managers are confident the switch should stop the leak.
"Endeavour is in good shape and the teams are excited to be back working towards a launch," NASA test director Steve Payne said Monday.
A similar gas leak hindered the March launch of the shuttle Discovery. That vehicle underwent the same type of repairs and also managed to lift off four days later than planned.
Launch controllers plan to begin filling Endeavour's external tank with fuel at 8:15 p.m. EDT (0015 GMT) Tuesday. Mission managers should know within a few hours if the leak fix has worked.
NASA delayed the launch of a two new moon missions, the Lunar Reconnaissance Orbiter and a smaller impactor probe, in order to try and launch Endeavour on Wednesday. Those two unmanned probes were also slated to launch on Wednesday from the nearby Cape Canaveral Air Force Station. They are now slated to launch no earlier than Thursday afternoon at 5:12 p.m. EDT (2112 GMT), NASA officials said.
If Endeavour is unable to launch today, NASA plans to stand down until July 11 to allow the moon missions to launch.
Endeavour's seven astronauts, led by commander Mark Polansky, plan a daunting schedule of five spacewalks and complex robotic work for their station construction mission.
"We are the last mission that is taking up Japanese hardware on a space shuttle, you know, really big pieces of equipment that we're going to go ahead and leave behind on the space station for construction," Polanksy said in a preflight interview. "It's, all kidding aside, a very, very busy mission."
Along with the new piece of Kibo, Endeavour will carry up a boatload of spare parts to serve as reserves after the shuttle retires, planned for 2010. The shuttle will also drop off NASA astronaut Tim Kopra to start a six-month stay at the space station as an Expedition 20 flight engineer.
"I'm looking very forward to doing a spacewalk with the STS-127 crew and being able to spend time in space with my crewmates and friends that are part of Expedition 20," Kopra said in a NASA interview. "And that'll be a very exciting time because what we're doing as crew members on that mission is expanding our knowledge of what it means to live in space and the effect of microgravity on the human body."
In Kopra's place, Japanese spaceflyer Koichi Wakata will return home aboard Endeavour to cap off a three-month stint onboard the orbiting outpost. The other Endeavour astronauts include pilot Doug Hurley and mission specialists David Wolf, Chris Cassidy, Thomas Marshburn, all of NASA, and Canadian Space Agency astronaut Julie Payette.
When the shuttle crew arrives at the ISS, they will instantly boost the population there from six to 13 - a record number of people in space at once. While the crowded conditions could create some chaos, the current space station residents say they're looking forward to the visitors.
"When Mark Polansky and his Endeavour crew arrive on the station, things are going to be very busy," said current station resident Robert Thirsk, a Canadian Space Agency astronaut, in a preflight interview. "One of the crew members on board STS-127 is my Canadian astronaut colleague and friend, Julie Payette. This'll be the first time that two Canadian astronauts have been on orbit. It'll be very good to see Julie and her crewmates during the Endeavour mission."
Endeavour's launch will mark NASA's third shuttle flight of the year and the 127th mission for the three-orbiter fleet.
SPACE.com is providing continuous coverage of STS-127 with Staff Writer Clara Moskowitz in Cape Canaveral, Fla., and Senior Editor Tariq Malik in New York. Fuel tanking beings at 8:15 p.m. EDT (0015 GMT) Tuesday with live coverage set for 12:30 a.m. EDT (0430 GMT) Wed.. Click here for mission updates and SPACE.com's live NASA TV video feed.