Final answer:
The three Apollo missions were canceled due to reasons like an explosion in Apollo 13's service module and issues concerning the increased complexity of later missions. Human error and technical challenges in space exploration also contributed to these cancellations.
Step-by-step explanation:
Three Apollo missions were scrubbed for different reasons. The Apollo 13 mission was aborted due to an explosion in the service module. The last three Apollo missions were planned to explore more geologically interesting locales on the Moon with increased levels of scientific exploration. Unfortunately, they did not all succeed in landing. Issues ranging from technical failures, human error, and changing mission objectives led to the scrapping of Apollo missions. One such human error, although not directly related to the Apollo program, was the loss of the Mars Climate Orbiter due to the failure to convert English measures to metric values. Safety and the challenges associated with space exploration have always been factors in the planning and execution of these ambitious missions.