There were six STS orbiter vehicles, of which five were operational for space flight:
- OV-099: Space Shuttle Challenger
- OV-101: Space Shuttle Enterprise
- OV-102: Space Shuttle Columbia
- OV-103: Space Shuttle Discovery
- OV-104: Space Shuttle Atlantis
- OV-105: Space Shuttle Endeavour
Challenger and Enterprise were used for tests, with Challenger later refitted for space flight. Columbia was the first to fly, on April 12, 1981 and Endeavour the last one to have its maiden flight on May 7, 1992.
Technology progressed during the 11 years between Columbia and Endeavour and the orbiters received various upgrades during service. As a consequence, they were not identical, making them (un)suitable for specific missions:
- Columbia kept its internal air lock: this made it suitable for working Hubble service missions and launching the Chandra space telescope, but this plus other differences made it too heavy for ISS missions.
- Atlantis was fitted with equipment for Mir missions.
I'm interested to know what the major differences between all of the five flight-ready orbiters were, i.e. differences that affected their suitability for certain mission types. This would include things like unique equipment installed to facilitate certain missions (like the Atlantis example), but also structural differences that were a result from more advanced technologies or design applied to the newer orbiters (such as with Columbia, that had a heavier wing and fuselage spars).