A nearly full Moon sets as the space shuttle Discovery sits atop Launch pad 39A at the Kennedy Space Center in Cape Canaveral, Florida, in the early morning hours of Wednesday, March 11, 2009.

Thisarticle was updated at 7:05 p.m. EDT.

CAPECANAVERAL, Fla. - NASA's shuttle Discovery and its seven-astronaut crew willhave to wait at least a few more days to rocket into orbit after a last minute gasleak prevented a Wednesday launch attempt.

Missionmanagers called off Discovery?slaunch attempt after detecting a leak of gaseous hydrogen in a line thatruns into the shuttle?s attached external fuel tank. NASA is now targeting a launch for no earlier than March 15 at 7:43 p.m. EDT (2343 GMT). The extra time will allow engineers to take a look at the leak and make necessary repairs, mission managers said in a briefing after the launch attempt.

The leakoccurred in a gaseous hydrogen vent line that runs into Discovery?s 15-story fueltank to keep its supply of liquid hydrogen propellant adequately pressurized. NASA space shuttlesuse super-chilled liquid hydrogen propellant and liquid oxygen oxidizer to feedtheir three main engines during launch.

NASAofficially scrubbed the launch plan at 2:37 p.m. EDT (1837 GMT). Discovery wasscheduled to lift off from theseaside Pad 39A here at NASA's Kennedy Space Center at 9:20 p.m. EDT (0120March 12 GMT).

"Teamsare going to get together and assess the repair options," said NASAspokesman Allard Beutel.

The leakwas discovered at 2:15 p.m. EDT (1815 GMT), about two hours after ground crewsbegan filling the shuttle's massive orange external fuel tank. The problemforced them to halt fueling immediately. Weather forecasts had predicted a pristine 95percent chance of favorable flight conditions for tonight?s launch attempt.

A newdelay

Wednesday?slaunch delay is the latest in a seriesof setbacks that have postponed Discovery?s STS-119 mission for a month. Theshuttle was initially slated to launch on Feb. 12, but concerns with suspectfuel control valves in the spacecraft?s main engines prompted additional delaysso engineers could replace them.

Like theleaky gas hydrogen line that thwarted Discovery?s Wednesday launch, the shuttle?sthree fuel control valves are also designed to maintain the proper pressure insidethe liquid hydrogen fuel reservoir of the orbiter?s attached external tank. Asimilar valve on the shuttle Endeavour cracked during a November 2008 launchand NASA wanted to be sure a similar problem did not pose a risk to Discoveryand its crew.

Discovery?svalves were replaced twice, with mission managers deciding earlier this monththat the shuttle was fit to fly.

Commandedby veteran shuttle flyer Lee Archambault, Discovery'sseven-astronaut crew will launch toward the International Space Stationcarrying the outpost's final pair of U.S.-built solar wings and the last segmentof its backbone-like main truss. The14-day mission will also ferry Japanese astronaut Koichi Wakata to thestation, where he will replace NASA astronaut Sandra Magnus as a member of theorbiting lab's three-person crew. Wakata is Japan's first long-durationastronaut and is due to return to Earth later this summer.

Set tolaunch spaceward aboard Discovery with Archambault and Wakata are STS-119 pilotTony Antonelli and mission specialists Joseph Acaba, Steven Swanson, RichardArnold II and John Phillips.

Discovery?stwo-week mission will mark NASA's first shuttle flight of the year.

IfDiscovery is unable to launch Thursday, a third attempt is possible on Friday.NASA has until March 17 to launch the shuttle before standing down due to apreviously scheduled Soyuz flight to ferry the new Expedition 19 crew to thespace station. If the shuttle cannot launch by March 17, NASA plans to standdown until after the space station crew change, with the next launch windowopening on April 7.

SPACE.comis providing continuous coverage of STS-119 with reporter Clara Moskowitz atCape Canaveral and senior editor Tariq Malik in New York. Click here for missionupdates and SPACE.com's live NASA TV video feed. Live launch coverage beginsat 4:00 p.m. EDT (2000 GMT).