And that leaves NASA's astronaut program in limbo. Already, depending on whom you ask, some 8,000 space program jobs are threatened -- to say nothing of all the other people whose work depends on them.
"All we know for certain is that the shuttle program will wind down," said Discovery astronaut Alvin Drew during a preflight interview with ABC News. "We don't know what is going to come out of the backside of this thing. I don't have that sense of 'We are leaping into some great new future,' because I don't know what that future is."
Discovery is going on what, if not for the end of the program, would be a routine-sounding supply flight to the space station. It is to deliver a freezer, equipment racks, a water-recycling system, and a robot that NASA could not resist calling R2.
Everything is packed in a bus-sized container called Leonardo. It has been used for supply missions before and brought home in the shuttle's cargo bay. But for lack of future shuttle flights, Leonardo will be left docked to the station so astronauts on board have some extra room.
Mike Moses, the head of the prelaunch mission management team -- which decides whether they try for launch or not -- tried to be philosophical. The Obama administration has called on private industry to ferry astronauts to the space station, but even if they succeed, there will be a gap of several years during which America will not be able to launch astronauts. It will have to depend on the Russian Soyuz program.
"In a perfect world, if you sell your old car, you'd like to know what your new car will look like," said Moses. "But I understand what's going on. The realities are that NASA is on a fixed budget, and without a big infusion of cash you can only do a certain number of things at a time."
So Discovery waits on the ground while anxiety floats in the air.
"There's still a certain amount of disbelief that we're getting to the final launch," said Leinbach. "It's difficult to accept emotionally, but rationally we know it's coming to an end."