August 20, 2009-- The number of Space Shuttle missions NASA can fly is limited by the number of external fuel tanks. Each blastoff needs one, which is discarded and burns up in the atmosphere. Currently, NASA has enough tanks in its inventory for the remaining seven scheduled missions, plus a rescue flight for the last blastoff on the books. That's led to speculation of launching one "final" final mission with the spare tank. That would be quite a story, since there would be no tank for a rescue Shuttle to save that last crew of astronauts if that spacecraft was damaged. It takes three years to build a Russian Soyuz craft, so this may prompt some deep thinking.