> Initially designed to last five years NASA tech often seems to outlive its initial mission length by a massive margin. The Mars rovers spring to mind. It's incredibly impressive, and almost embarrassing! Surely this isn't accidental. Is the kit massively over-specced? Do the uncertainties and risks necessitate such a depth of redundancy that when stuff goes kinda smoothly the thing lasts 9 times longer than it was…
That's a major incentive to over build things. Engineers also love making things better, so, your workforce is defacto onboard with that mission.
And then, there's the issue that, basically every long term mission to space requires bespoke spacecraft. That makes things very, very expensive, but also, presents a requirement to engineer your way around unknown mission requirements. They know what they want to do, but, they don't really know how it'll work in reality. They can test some things, sure, but it's impossible to know every variable.
For instance, you're building a bridge with a 100ft span that's 50ft above the ground at the highest, in an area with a maximum wind speed of 50mph, and a maximum load capacity of 2000 tons of traffic moving 65mph. Now, that's basically enough information to build that bridge. Now imagine that, you're asked to build that same bridge, but, you don't know how fast the traffic is moving, that's more difficult. Now, in addition to that, you don't know how much wind loading you have to deal with, more difficult still. Now imagine that, your load capacity isn't certain either.
Could you still build the bridge? Of course you could, but, you'll have to build it with what you think are reasonable requirements. You might do some research into those requirements, but you also might not be able to. Where you end up is, the bridge you build is going to be over built, likely by a significant margin, if you desire to build a successful bridge.
This is the issue with designing spacecraft, you have more questions about requirements than you have answers, and sure, we have more answers than we used to, and the available pool of knowledge has only increased, but many points of uncertainty still remain. Not an unusual engineering problem, we'll get there eventually. It was about 100 years of thinking for us to learn to fly at all, another 100 years to learn how to do it well, and there's still plenty of room for improvement. Space flight will be much the same, and eventually we'll have the space equivalent of the honda civic