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updated 10/22/2010 7:51:18 PM ET 2010-10-22T23:51:18

Less than two weeks before the space shuttle Discovery is scheduled to launch on its last flight, uncertainty still remains as to exactly when NASA's very final orbiter mission will fly, bringing the storied shuttle era to a close next year.

NASA has two scheduled shuttle missions, on Discovery and Endeavour, left to fly before retiring its orbiter fleet in 2011. A third, extra shuttle mission has been approved by Congress and President Obama, but still faces review by congressional appropriators later this year.

But NASA's shuttle program manager John Shannon told reporters Thursday (Oct. 21) that definitive plans have not yet been made as to whether the potential third and final shuttle mission will fly to the International Space Station. [Gallery: Shuttle Discovery's Last Launch Pad Trip]

"I told my team it was 50/50," Shannon said.

Discovery is poised to launch Nov. 1 on an 11-day mission to deliver a storage room and humanoid robot to the International Space Station. The mission will be the 39th and last space voyage for Discovery NASA's oldest flying shuttle.

The shuttle Endeavour is set to follow Discovery's final flight with one last mission of its own in early 2011.

NASA's third shuttle in service, Atlantis, is being prepared to serve as a possible rescue vehicle for Endeavour's flight scheduled to launch Feb. 27. Having backup shuttles ready has been standard for NASA since the tragic 2003 loss of shuttle Columbia and its crew.

The fleet is being retired to make way for new missions to an asteroid by 2025 and Mars in the 2030s. NASA will rely on Russian, European and Japanese spacecraft to ferry crews and supplies to the space station until U.S. commercial spacecraft become available.

One last flight
NASA would prefer to shift Atlantis out of standby status and use it for one last cargo mission to the International Space Station.

To do that, the agency needs approval from congressional appropriators in order to set aside the funding required for the extra mission. NASA's space shuttle program costs about $200 million a month to operate, agency officials have said.

Presently, the final shuttle mission called STS-135 would most likely see Atlantis launch on a summer flight no earlier than June 2011.

"Theres going to be a lot of discussions between now and the summer if were going to fly 135 exactly where were going to fly it," Shannon said. "Does it make sense to delay a bit and get the ISS in better shape?"

The potential third and final shuttle flight will be critical in maintaining the space station, officials said.

"I have to point out how important that flight is to the International Space Station," Shannon explained. "From a logistics standpoint, I think 2012 is going to be a real challenge."

For one, after the shuttles are retired, NASA will lack the ability to ferry large payloads to the station.

Russia's unmanned Progress spacecraft routinely hauls about 2.5 tons of cargo to the station per flight. European and Japanese robotic freighters can larger loads, but not as much as the shuttle.

"My operations guy said one shuttle flight is equivalent to roughly seven Progress flights," Shannon said. "So, getting to fly 135 late is going to give the space station margin to keep six crew (members) up, to keep doing research."

Space supply shortage risks
NASA officials also said that if scheduled demonstration of commercial supply ships are delayed between the launch of Endeavour's STS-134 flight in February and the proposed summer STS-135 flight, the space station could face a dangerous shortage of essential provisions.

According to projections, that challenge would likely come at the tail end of 2011, moving into 2012, said Dan Hartman, Integration and Mission Operations manager for the International Space Station Program.

Yet, the final decision will likely be driven primarily by budgetary constraints.

"As far as how far could we push back the June flight? It all depends on money," Shannon said. "Well work with headquarters on the money, then that will define how far out we can stretch this program."

Still, the agencys hands remain tied until congressional appropriations are complete.

"The plan we have put in place allows us to carry through enough money to be able to keep the program going and make the decision on whether we fly 135 as late as possible," Shannon said. "But we really cant make that decision under continuing resolution we need appropriation."

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