I’ve read in several news articles like this one that the Mars rover has a life expectancy of three months. I know the vehicle is solar powered and some nuclear material generates some heat so the batteries can operate, so what exactly is the limitation? I can understand the batteies not lasting forever, but only three months?
Does dust accumulate on the solar panels? What are the factors in play?
I would vote for temperature changes bringing about structural fatigue (electronics mostly), and the generally harsh operating environment of Mars (fine abrasive sand blowing all over the place). I doubt that dust on the solar panels is the limiting factor because that would be pretty easy to fix.
I also believe that as the seasons on Mars change to “Martian autumn and winter” the intense cold becomes too much for its small plutonium powered heaters to keep it warm.
WAG
Although it is solar powered the stregnth of the sunlight hitting Mars is weaker than the sunlight we get here. So the panels are not able to charge as well as you might think.
Bingo. Dust builds up on the solar panels, along with everything else, basically mucking things up.
They couldn’t have equipped the solar panels with wipers?
Wipers were not added mainly for two reasons:
(1)A wiper system was never tested – so its possible the wipers could end up scratching the solar panels, or collecting dust themselves, and just adding more dust on top of the panels
(2)Wipers would add weight to the whole probe, which would limit the amount of information gathering tools that could be placed onto the probe.
From what I’ve read it seems that the biggest problem is the extreme changes in temperature that occur between day and night, as Phage said. The stress of the temperature changes is expected to cause a breakdown at some point after 90 days.
There is a very large (minivan sized) Rover planned for launch in 2009 that will have a nuclear power source. That rover is planned to last for several years. So this would argue that the main limiter now is battery/solar cell life.
On the other hand, with a nuclear power source you can afford to run heaters to keep the thing warm, so I imagine thermal cycling is also a big life limiter for the smaller rovers.
The issue does seem reduced power from the solar panels…
From this site:
It seems like temperature fluctuations aren’t as much of an issue because of the “warm electronics box”:
Although off-topic, I also found this item interesting talking about the computing power of the Rovers:
IIRC, the Mars Pathfinder probe lasted some 45 days past its “expiration date” (no cite, sorry.) So it’s not a guaranteed thing that the new probe will fail right at the 90-day mark.
NASA tends to be very conservative about their lifetime estimates. Almost everything that doesn’t fail immediately lasts longer than expected.
The two Voyager probes have also greatly exceeded their expected useful lifetimes.
There’s an element of chance here. If conditions are milder than forecast Spirit may last considerably longer than 90 days. If it falls in a hole or has some other accident, well, we may get less than three months out of it.
I wouldn’t be surprised if sometime after 90 days it starts to degrade, but still retains some abilities. Like, if it loses one camera it has others.
The Viking landers were powered by heavy duty RTGs (Radioisotope Thermoelectric Generators) instead of solar cells which enabled it to function for over six years after landing.
The temperature fluctuations and the dust are dead on, but the other thing to consider is solar background radiation (not the available light, BTW). While the rover was in transit, it was shielded by probe carrying it, but now that it’s mobile it’s susceptible to solar flares.
Mars has about 1% of the atmosphere earth does, and that doesn’t shield the surface very well. This is something that will have to be overcome for the astronauts who go there.
Hasn’t that problem already been overcome by astronauts who went to the moon?
The astronauts who went to the moon took quite a few rads in the process. I read somewhere, though I don’t remember where so I can’t dig up a cite quickly, that the lunar explorers have shown a somewhat elevated rate of cancer. However, since there’s fewer than twenty of them, it’s not really an epidemiologically reliable sample. I do, however, clearly remember seeing a micrograph of the interior of one guy’s helmet, and all the tiny little spikes that were created when cosmic rays shot through the material.
Re the OP, it’s important to remember just how freakin’ hostile the Martian environment really is. You have an extremely low-pressure environment (0.1% of Earth at sea level, IIRC), temperature fluctuations, cosmic rays, dust that’s as fine or finer than talcum powder, and more. And those dust storms, when they get cranked up, can cover the entire planet for months at a time. Nasty business, really. So we can test devices under most of these conditions separately, at least on a limited basis, but trying to test all of them at once is damn near impossible. Really, it’s something of an engineering miracle that we know what we’re doing enough to get something that works on Mars for an hour, let alone ninety days.
For some background, read Managing Martians by Donna Shirley, who worked on (and later supervised) the rover program that put Pathfinder/Sojourner on Mars a few years ago. There’s a lot of detail about engineers seeking out particular components that match certain specifications, and requesting additional customizations over and above the Earth-standard tolerances. It’s not just a matter of going to Wal-Mart and getting a drill, a camcorder, and a Sony Vaio and hooking it all together. You’ve got custom-engineered battery packs, specially fitted transmitters, high-tolerance wiring, hardened computer memory, lots of moving parts and mechanical bits carefully sealed against the incursion of extremely fine dust, and more, and it’s all got to fit in a bruisingly tiny little space.
Regarding just the dust: The Science Channel had a thing on within the last couple of days where a U.S. Geological Survey scientist put on a space suit and stood in the middle of an artificially generated dust devil, in an attempt to see what might happen if an astronaut on Mars got caught in a dust storm. The surprising thing was that within minutes, a buildup of static electricity caused by all that dust whipping around him and against itself caused his suit to develop a charge, and his visor was quickly coated with blinding dust. And within ten minutes, all of his equipment had failed.
And that’s here on Earth, minus the vacuum, the background radiation, the extreme cold, and everything else.
Sam Stone, on the topic of that big rover, do you have any other information on that? I can’t find much through google.
Oh, I forgot to ask, to what extent could the Viking project work at after its projected death-date? Did it break down little by little, instrument by instrument, or was it driving around in circles until it zapped out?
I may be misreading what you’re asking, but Viking was not a mobile device; it was a land-and-stand probe that analyzed only what it could reach with its little T-rex arms. 
So, no little circles for Viking. Or were you referring to something else?
I thought is was a budget thing. It costs a certain amount to keep the program staffed and all that per day (did I read 2 million?) and there’s only so much money budgeted for the program. I think additional funding to continue will be provided on an as-needed basis.
I’ll poke around for some cites if I get the time. Anyone else read anything like this?