For those of you who don't read the comments, Rupert Goodwins just asked how long the MER missions are likely to last. I asked the same question of Mike Sims at NASA Ames a couple of days ago, and he said that were now pretty convinced that they would way outlast the nominal 90 sols; 200 or 250 sols currently seems possible. Things will slow down towards the end because of seasonal shifts in the amount of sunlight (and the chill of the nights), because of dust on the panels and because of battery capacity dropping, but science could be going on well into the autumn. This is pretty exciting, particularly because it raises the possibility that Spirit may get somewhere really neat. Bonneville crater, by all accounts, is a bit of a bust. But if the rover lasts long enough not just to get to one or two sites in the hills but also to do some serious work on outcrops there --something that Opportunity is showing us takes time -- it would be a real triumph.
More time for more exploration and more data is obviously good news, but it's worth noting that there are downsides. For one thing, keeping the team together for that long is going to cost money, which will have to be negotiated and, possibly, come from some other part of the Mars budget. Another is that the longer people are there, the more chance there is for fatigue to strike; and the chance of mistakes being made, including mission-critical mistakes, will go up accordingly. Keeping up the current tempo for 90 sols is one thing. Doing it for two and half times longer -- for most of a year -- is another.
Human error would be an entirely normal way for such a mission to end, and stopping early just because of the risk of human error would obviously be silly; it's just a risk you have to face, all the time trying to alleviate through sensible procedures for checking commands, enforced holidays, and other sorts of grown-up management. The worry is that unlike other ways for the mission to end, human error might play badly in the media and public opinion, which could have an effect on the program's future. If someone makes a mistake that's understandable if you know the circumstances but looks kind of stupid if you don't, things could look bad. Or maybe I'm just being paranoid, and everyone would understand.
Anyway, it looks like there's a lot more to come.