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.
"We're going to see how the processing goes," Cain said. "If we have some good fortune, if we have some good weather, or at least not too much bad weather, in the next 24, 36 hours, then we think it's achievable for us to get to a (shuttle launch) on the 17th."
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 t
he 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.
Some of the supercold liquid hydrogen propellant inside the tank constantly turns into a gas that is routed overboard through a vent line to a flare stack near the pad where it is harmlessly burned away. The vent line attaches to the tank at an umbilical plate that pulls away at liftoff.
During an attempt to launch the shuttle Discovery last March, a gaseous hydrogen leak in the umbilical plate triggered a four-day delay. Engineers were unable to duplicate the leak under ambient conditions--it only occurred when cryogenic hydrogen was filling the tank--but after replacing a critical internal seal, the umbilical worked normally and Discovery was able to take off.
A virtually identical scenario is playing out with Endeavour. Engineers were unable to duplicate the leak after the tank was drained and mission managers decided to press ahead with a seal replacement.
Launch Director Pete Nickolenko said engineers discovered small areas where the seal in question appeared to have pulled away from the external tank slightly, possibly due to exposure to cryogenic conditions. The gaps seen are similar to those found during troubleshooting of the leak that grounded Discovery in March.
Engineers are trying to figure out what caused seal problems in two of the last three shuttle flights, but in the meantime, "we are on the path of installing a new quick-disconnect and a new flight seal,"
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