During the Space Shuttle program, several missions were canceled. Many were canceled as a result of the Challenger and the Columbia disasters. Many early missions were canceled due to delays in the development of the shuttle. Others were canceled because of changes in payload and missions requirements.
Canceled due to the late development of the Space Shuttle
In 1972, NASA's planners had projected for 570 Space Shuttle missions between 1980 and 1991. Later, this estimate was lowered to 487 launches between 1980 and 1992. The details of the first 23 projected missions, listed in the third edition of Manned Spaceflight (Reginald Turnill, 1978) and the first edition of the STS Flight Assignment Baseline, an internal NASA document published in October 1977, are presented below.
Later in the development process, NASA suggested using the first manned space shuttle mission, STS-1, as a sub-orbital test of the Return to Landing Site (RTLS) flight profile devised for emergency abort scenarios. Columbia would have launched from Kennedy Space Center, then executed a 180-degree turn at a speed of 8,400 kilometres per hour (5,200 mph), or 6.7 times the speed of sound, in order to land at the Kennedy Space Center runway. The mission was canceled when astronauts refused to fly it, having deemed the plan to be too dangerous. STS-1 commander John W. Young recalled that "I said no. I said let's not practice Russian roulette, because you may have a loaded gun there. So we didn't."