Interestingly, a thorough risk assessment of the Shuttle was done much later by NASA (near the end of the program) and it concluded that the risk of losing a Shuttle in the pre-Challenger era to be much higher than 1 in 100, closer to 1 in 10. Many people look at Challenger and Columbia as instances where the Shuttle program hit a patch of bad luck. In reality the Shuttle program has been extraordinarily lucky, there were many other close calls, some not well publicized, that came within a hair's breadth of causing loss of crew and vehicle (STS-1, STS-8, STS-9, STS-27 being examples of such). It was always a tricky bird to fly, and in the early days there were about half a dozen different things that could kill it outright with a shockingly high probability (not just the SRBs or foam/ice strikes on the TPS, also the APUs (which caught fire and exploded on one flight), the computer (which was completely inoperable just before landing on another flight), the SSMEs (which came close to causing loss of the orbiter once or twice), and other components). Over time some of the systems were improved to such a degree that they were no longer serious risks, but the whole system was so complicated and there were so many elements of risk that even at the end of the program many substantial risks still remained.