Surely if it's nuclear powered using a fuel with a known and predictable halflife plus a known and controlled reaction rate they will know, if not exactly then reasonably accurately, how long it will remain operational for?
While here on Earth we humans were recovering from celebrating another solar orbit with the usual fireworks, drunkenness, and canoodling, the Curiosity rover yesterday ticked over its 500th Martian day on the surface of the Red Planet. Mount Sharp on Mars Mount Sharp, Curiosity's next target "Goals for 2014: Finish driving …
I don't know off hand what isotope its RTG is using, but I'm going to guess their design lifetime is based on wear and tear and mechanical failure, not the power supply. So they've got a pretty good idea of when it will fail based on power but they probably don't expect it to last as long as its RTG.
"I don't know off hand what isotope its RTG is using"
Plutonium-238. It's been a long time since NASA tried any other RTG or radioscopic heater material. Its standard RTGs and heating elements are entirely built around Pu238.
"but I'm going to guess their design lifetime is based on wear and tear and mechanical failure, not the power supply."
Yep. The Spirit and Opportunity Rovers suffered increasing mechanical failures in their arms and drive trains. Spirit eventually got stuck in soft sand at a non-optimal angle for solar charging and froze up over the winter, which is arguably a power supply failure, but by that point it had two dead wheels.
Predictable but has a wide margin of error. That figures given for the life of a nuclear plant is the length of time they can guarantee a certain level of power output.
The rover also has a couple solar panels to power its control circuitry and some of its basic equipment, so it could theoretically keep going indefinitely reporting back the weather, levels of radiation and pictures or whatever else they have the power budget for.
> The rover also has a couple solar panels to power its control circuitry and some of its basic equipment
I'm pretty sure it doesn't have any solar panels (where are they on the rover? and why waste the extra weight when you have an RTG?). You might be thinking of the cruise stage, which was solar powered...