NASA managers Sunday deferred making a formal decision on whether to reschedule the delayed shuttle Endeavour for launch Wednesday or press ahead instead with launch of the agency’s $583 million Lunar Reconnaissance Orbiter mission aboard an unmanned Atlas 5 rocket.

But with both missions facing tight launch windows, Mission Management Team Chairman LeRoy Cain said the agency’s preference was to launch Endeavour on Wednesday, if possible, to maximize the number of launch opportunities for both programs.

“If shuttle goes first on the 17th, then the most opportunities we can give LRO is two, and that would be on the 19th and 20th,” Cain said. “If LRO goes first on the 17th, then the most opportunities we could get for the shuttle is one opportunity, and that would be on the 20th.”

A final decision on how to proceed must be made Monday to provide enough time for the Air Force Eastern Range, which provides required tracking and telemetry support for all rockets launched from Florida, to set up its systems to support one launch or the other.

But Cain said if no additional problems develop, and if work to repair a leaky hydrogen vent line umbilical plate on Endeavour’s external tank goes smoothly, NASA likely will opt to press ahead with an attempt to launch the shuttle at 5:40:50 a.m. EDT Wednesday.

The forecast for Wednesday calls for a 70 percent chance of good weather for the shuttle’s pre-dawn launch window and a 60 percent chance of acceptable conditions for the lunar orbiter’s window Wednesday afternoon.

If LRO is not off the ground by June 20, the flight will slip to the end of the month. If Endeavour is not off the ground by June 20 or 21 at the latest, the shuttle launch will be delayed to July 11 because of temperature constraints related to the space station’s orbit.

Endeavour was grounded during fueling overnight Friday when the gaseous hydrogen vent line umbilical on the side of the shuttle’s external fuel tank began leaking potentially dangerous vapors as the hydrogen section of the tank was filled.

The work is expected to be finished early Tuesday.

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