NASA Hopes to Launch Shuttle Endeavour on Wednesday

This story was updated at 8:00 p.m. EDT.

NASA will try to squeeze in a launch attempt for the space shuttle Endeavour on Wednesday, just days ahead of a different rocket also set to send two unmanned probes to the moon this week, mission managers said Sunday.

The NASA moon probes were previously scheduled to fly on June 17, but the agency would delay their mission in order to get one more chance to launch Endeavour after a potentially dangerous hydrogen gas leak prevented a Saturday liftoff. A final decision on which mission will launch first will be made Monday afternoon.

?From a priority standpoint, what we are trying to do in the agency is maximize our launch opportunities,? said LeRoy Cain, NASA?s deputy shuttle program manager, in a Sunday briefing at NASA?s Kennedy Space Center in Florida.

A Wednesday launch for Endeavour would occur at 5:40 a.m. EDT (0940 GMT) from a seaside pad the Cape Canaveral, Fla.-based spaceport with favorable weather expected. NASA?s two new moon probes - the Lunar Reconnaissance Orbiter and a smaller spacecraft that will intentionally crash into the moon to look for water ice - could then launch on Friday, Cain said.

The schedule is tight. If foul weather delays any work on Endeavour or its launch pad by even a few hours, the shuttle may not be ready to launch on Wednesday. If a delay occurs soon enough, such as overnight tonight or early Monday, NASA could swap launch days with the moon mission and try to loft Endeavour on June 20.

?The schedule that we?re moving toward is very tight,? said NASA launch director Pete Nickolenko. ?But it is achievable.?

Gas leak repairs

Endeavour was slated to launch early Saturday from the Cape Canaveral, Fla., spaceport and begin a grueling 16-day space station construction flight. The shuttle?s seven-astronaut crew plans to deliver an external experiment porch for the station?s $1 billion Japanese Kibo lab. Five spacewalks and a one-man swap for the station?s new six-person crew were also planned.

But a leak of gaseous hydrogen forced NASA to postpone the launch attempt since the extremely flammable gas posed an explosion risk to both Endeavour and the seven astronauts aboard. Repairs to the vent line leaking the hydrogen should be complete by early Monday.

The leak is similar to one that delayed the launch of Endeavour?s sister ship Discovery earlier this year during NASA?s STS-119 mission in March. Shuttle technicians replaced the hydrogen vent pipe that siphons excess gas away from the Discovery?s 15-story external tank and the mission successfully lifted off after a four-day delay.

While NASA is confident a similar fix will work for Endeavour, shuttle managers remain perplexed by the gas leak. Engineers suspect temperature changes caused by a shuttle?s super-chilled cryogenic fuel may lead to gaps in the vent pipe, but they still do not know for sure.

?We?ll change it out. It will either work or it won?t,? Cain said of the vent line repair. ?If it doesn?t, then we certainly aren?t going to go launch.?

Endeavour?s mission is NASA?s third shuttle flight of 2009 and the second this year dedicated to space station construction. A May mission by the shuttle Atlantis flew to the Hubble Space Telescope.

Commanded by veteran spaceflyer Mark Polansky, Endeavour?s six-man, one-woman crew is the first to the visit the space station since the outpost doubled its population up to six people last month. The astronauts are remaining in quarantine to avoid getting sick while NASA decides their launch plan.

When Endeavour docks, 13 people - the highest number ever - will be living and working aboard the station at the same time.

Launch date toss-up

The difficulty surrounding Endeavour?s launch date revolves around an overlapping schedule on the Eastern Range, a rocket launch range that extends eastward out over the Atlantic Ocean. NASA shares the range with the nearby Cape Canaveral Air Force Station, which is where an unmanned Atlas 5 rocket is poised to launch the two moon probes.

Typically, the Eastern Range requires at least two days to switch between a shuttle flight out of the Kennedy Space Center and an unmanned rocket launch from the Air Force station. Clouding the issue even more are the different launch windows for Endeavour and the unmanned moon probes.

Endeavour?s window to launch toward the space station closes on June 20, when sun angles and heating conditions become unfavorable for a shuttle linked to the outpost. NASA could extend that window to the June 21, but that would require shortening Endeavour?s busy flight by a day and cutting one of the five spacewalks in order to leave the station in time, Cain said.

The Atlas 5 rocket, meanwhile, has a parallel four-day window (June 17-20) in order to launch on a trajectory that would allow its Lunar Reconnaissance Orbiter payload to arrive at the moon four days later. The orbital mechanics line up every two weeks, but the lunar mission?s launch has already been delayed several times since October 2008.

If Endeavour cannot launch this week, NASA?s next opportunity to launch the shuttle would come on July 11, when sun angles and heating conditions at the space station, mission managers said. That slip should not seriously impact NASA?s plans to launch eight more missions before retiring the shuttle fleet in 2010 to make way for its capsule-based Orion successor, but NASA remains hopeful the shuttle can fly on Wednesday.

?A lot of things have to go our way,? Cain said.

SPACE.com is providing continuous coverage of STS-127 with Staff Writer Clara Moskowitz and Senior Editor Tariq Malik in New York. Click here for mission updates and SPACE.com's live NASA TV video feed.

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