 For comparison to an earlier retirement, when the Saturn IB was last flown in 1975 for the Apollo-Soyuz Test Project, the Shuttle development program was already well underway.However, the Shuttle did not fly until 1981, which left a six-year gap in U.S. manned spaceflight.Because of this and other reasons, in particular, higher than expected Solar activity that caused Skylab's orbit to decay faster than hoped, the U.S. space station Skylab burned up in the atmosphere.