“Acceptable risk” is used to gauge the reliability of all pieces of equipment and components of a space shuttle. Space travel in itself is dangerous and can never be guaranteed to be completely safe. Instead, NASA and its contractors must minimize the degree of risk as much as possible. The acceptable risk, then, is the smallest degree of risk allowable for flight. 

Initially, the slight erosion of the O-rings was deemed within the limits of acceptable risk. As the damage worsened—proving not to be an anomaly—the likelihood of the O-rings failing completely increased and thus should not have been deemed within the parameters of acceptable risk. Rather than push for a new solution to seal the rocket joints, NASA gradually widened its definition of acceptable risk, thus deeming the O-rings to be fail-safe even though they weren’t.

O-rings were large pieces (12 feet in diameter) of circular rubber that were needed to seal the joints of rocket boosters. Due to their enormous size, the rocket boosters could not be assembled at the plant where they were manufactured and were instead shipped in pieces to Cape Canaveral and assembled there. The presence of the O-rings ensured that each separate section of the rocket booster was airtight, preventing dangerous fuel from leaking out.