Undated-AP -- It doesn't look like NASA will build a replacement for the space shuttle "Columbia."
The space agency used spare parts to build "Endeavour" as a replacement for "Challenger" after it exploded in 1986.
But NASA can't do that this time around, and the shuttle's former chief engineer, Donald Emero, says the next generation of reusable spacecraft is at least ten to 15 years away.
That leaves a fleet of three shuttles, 18-year-old "Discovery," 17-year-old "Atlantis" and ten-year-old Endeavour.
The status of the next shuttle mission, scheduled for March first, is undecided at this point. That mission is supposed to be a visit to the space station.