It's sort of like how companies often make conservative projections because if you make a crazy high projection and almost reach it that's a "failure" to shareholders even if you did exceptionally well.
> It's really a good sign that these things keep outliving their specified life
> span, though I also would assume that NASA's life span quotes are
> probably conservative to avoid the political fallout of "failure."
NASA designates a science goal, which often includes a detailed plan of what to explore. The administration designates a cost budget, a time budget, and a mass budget. These specs then get sent to engineers (at e.g. JPL) who do the best they can to achieve the science goal within the cost, time, and mass constraints. Sometimes they can build a craft that will outlive its initial 90-day science mission by 14 years. Often that is because they got lucky with conditions, i.e. wind cleaning dust off solar panels.
In no case have they built a craft with no specific goal, then declared it's operational life expectancy, and then went and found a mission to do with it.
What about the SLS?
I don't think the message you responded to said that they built the craft then declared the operational life expectancy.
NASA could have designated a science goal which needed 22 flights, and had the same cost, time and mass budget associated with it. Clearly that goal would have been achievable with some probability given that it was achieved. They didn't. The comment in question is speculating about the reasons why NASA might have chosen 5 flight as the target instead of 22 or 222.
So when NASA set a target minimum lifetime goal, it puts them quite far into the bathtub curve, long after all that initial risk goes away.
If it makes it to it's target lifetime, chances are it will last a lot longer.
Why is this one lasting so long? Presumably we know the radiation conditions on the surface so testing more units in a lab is a better guide than one plucky helicopter.
That way at least if something goes wrong with the non-rad-hardened equipment, it could be rebooted or debugged using equipment which is rad-hardened
I'm no scientist but that's my take on the matter.
My trusty SPARCstation had a 3 year extended warranty when bought, but it's still going since 1996. Not getting as much usage as it got when young, but she's a very reliable worker.
Isn't that the case with all space agencies? It is usually the best case scenario playing out in most cases, like this one. Even with the JWST, I can safely predict it will last as long as the Hubble, if not longer.