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boredcircuits t1_j1t2mx2 wrote

It's not a "life expectancy." A better term is "minimum requirement." Basically, they tell the contractors that everything needs to last at least X years, and everything is designed around that. Fuel tanks, redundancies, etc.

Compare this to a consumer device, where the specifications might have an average life requirement. For example, a phone manufacturer might specify a battery that degrades 20% after 400 charge cycles. What they care about is the average: some will live longer than that, and some will be less. That's ok because they'll just warrantee the bad batteries.

But you usually can't just say "oops, sorry" and replace a faulty battery in something that goes to space. (Hubble's mirror being the notable exception.) So NASA requires everything to satisfy the minimum mission requirements. We shouldn't be surprised when something outlives the minimum, really.

Also, NASA has a knack for extending missions as hardware fails. Finding unplanned uses for mostly-operational hardware. Look up Kepler for a great example.