No one can be sure if this arrangement will endure. Tensions over events in Ukraine have strained relations between Washington and Moscow to the point that a Russian deputy prime minister, Dmitri O. Rogozin, said recently that American astronauts might be denied access to Russian launch vehicles. Americans want to go to the space station? Let them do it “using a trampoline,” Mr. Rogozin said.

Over 30 years, there were 135 American shuttle launches and returns, and 133 were successes. Perhaps inevitably, it is the other two that people remember best.

Those missions turned into catastrophes for different reasons. On Challenger, an O-ring seal failed on a rocket booster, causing a breach that let loose a stream of hot gas, which ignited an external fuel tank; 73 seconds after liftoff, the shuttle broke apart over the Atlantic. With Columbia, a piece of insulating foam broke off from an external tank during the launch, and struck the left wing. The damage proved severe enough that when the craft re-entered the Earth’s atmosphere after two weeks in space, hot atmospheric gases were able to penetrate the wing structure. The shuttle became unstable. It broke apart over Texas and Louisiana.

The two disasters led to investigations, to the removal of senior officials, to promises of institutional change, to undying expressions of sorrow. Among the remorseful were engineers who had warned that headaches like eroded O rings and missing foam created risks, but who could not persuade their immediate superiors to act. One former NASA engineer, Rodney Rocha, lamented to Retro Report that he wished he had been more aggressive in making his concerns known to those at the very top. Observing the chain of command looms large in the world of engineering, Mr. Rocha said, adding, “I will regret, always, why I didn’t break down the door by myself.”

In theorizing about what went wrong in these two disasters and in others, some social scientists have observed that certain circumstances may well be beyond anyone’s control. Even before Challenger, in 1984, a sociologist named Charles Perrow put forth the concept of “normal accidents,” by which he meant that in a technologically complex operation something is bound to go wrong at some point. These systems are made up of so many tightly linked parts, Mr. Perrow said, that even a seemingly minor glitch could lead to a cascade of woes that make cosmic failure almost unavoidable. An example for him was the near-meltdown in 1979 at the Three Mile Island nuclear power plant in Pennsylvania.