Since the advent of space exploration, we have introduced the new vocabulary in engineering terminology with respect to reliability. The reliability demands of satellite equipment exceed anything ever contemplated in developing of equipment for terrestrial use. Reliability phrases of "four nines" or "five nines" refer to the probability of a component lasting x hours or days, etc. over the length of a space journey. Four nines is 0.9999; five nines is 0.99999. If a piece of equipment in a satellite has 100 parts you might consider how each part contributes to the reliability of the satellite as a whole.
The time interval of the mission of early satellites was modest: 1-2 years. Still, even a one-year interval is 365 x 24 = 8760 hours. Now we have missions of ten years or longer. Clearly, this presents challenges to engineering to provide parts of sufficient reliability to endure the extended mission lifetime.