I wonder what the expected lifetime of the crafts were when they were designed back in the 70s. Maybe they thought the RTGs would outlive the other components?
My thoughts exactly -- they probably didn't bother putting a whopping big 100-year RTG on there because they didn't figure the electronics would last long enough, or they figured they'd run out of attitude propellant long before then.
You could look up the planned mission duration, but all this would tell you would be the duration that they were absolutely designed to work for -- kind of like how the Mars rover missions were 90 sols, but in actuality they've gone over 2,000 (albeit with solar panel dust issues, busted wheel motors, etc).
Well I mean, Voyager 1, launched in 1977, has been on the "interstellar mission" since 1980 (32 years) and is planned to go into the 2020's (total of more than 40 years). That doesn't quite work out to the (2000-90)/90 ratio (though the distance traveled by Opportunity is an even more impressive ratio), but I think it is pretty damned impressive nevertheless. It was definitely engineered to last a lot longer than the Jupiter/Saturn observation mission required; I feel like both came from the same "school" of engineering and mission planning.
This sort of thing is a natural consequence of high cost and high risk missions. You have a multi-billion dollar mission, and it's doing something unprecedented and using a brand new design and construction. When you build something like that what tends to happen is that you try to keep the mission scope reasonable and then you build the vehicle to have a very high (say >95%) chance of satisfying that minimum mission requirement. But what happens when, say, you create a Mars rover with a 95% chance of working for 90 days you'll likely end up engineering a vehicle which might have, say, a 50% chance of lasting for 5 years. If you set out to do that from the start it would be a non-starter, because a 50% mission success rate is too low for a multi-billion dollar mission.
Not at all, actually. The whole interstellar mission aspect of the mission was entirely a beneficial side effect. In fact, the only part of the mission that was planned from the start was the Jupiter and Saturn encounters. The mission ended up becoming an outer Solar System "grand tour" due to a fortuitous planetary alignment, but only Voyager 2 was able to visit Uranus and Neptune. The interstellar mission was entirely unplanned and was funded mostly because it has been relatively inexpensive and for various reasons the Voyager craft have suitable instruments for studying the magnetic, particle, and dust environment in the outer Solar System and interstellar space.
