Recognizing the premium for any mass on the mission, I've wondered why some sort of panel-duster (perhaps a spinning flail such as you'd find in a carwash) wasn't designed in.
Still, the longevity and data have been truly amazing.
You'd still have the same problem though: how do you keep a motor lubricated in the partial pressure of a pretty good vacuum? How do you stop the duster's brushes falling out or wearing out?
Not saying it's not possible, but if you consider things like reaction wheel failures, there's a lot of "might last a long time, might die in a year or two" problems in any motor in space.
There are plenty of other motors on the landers, presumably the rovers haven't been telekinesing merrily over the marscape. The brushes would be a far lower duty-cycle item.
Depending on the brush design: something sufficient to clear the panels periodically (say, based on noted solar degradation and/or self-inspection photography) might work. Clearly, incidental wind has proven sufficient.
And there's a lot of engineering work on Earth which could go into testing out the design. As I said: it's just curious that this seems to have been left to chance. Though nobody really expected the rovers to run as long as they have.
> There are plenty of other motors on the landers, presumably the rovers haven't been telekinesing merrily over the marscape. The brushes would be a far lower duty-cycle item.
But far more exposed, though. If not the motors themselves, then the mechanics of the wiper arm.
More so than camera servos or other arms on the probes?
Again: while high-reliability, low-pressure lubrication is a challenge, it's clearly not an insurmountable one. Providing a means to ensure your only source of energy remains viable would be useful. Failure of the dusting system of itself wouldn't result in a mission failure. Success of it could prolong it, and/or allow for smaller collectors (as I recall the solar panels were oversized to allow for possible dust accumulation).
I'm thinking of some sort of rotary duster which might pass horizontally above the panels. Less a windshield wiper design than a dual-arm rocker.
As for abrasive sand, if you're whisking it off lightly enough, you should avoid most problems. Coatings might address that. And you've got the choice (addressing grandparent): unscratched PV under mountains of sand, or lightly scratched PV exposed to sunlight?
Yes, you know, I had forgotten about the rush, and I worked at JPL during that time. Obviously not on the Mars rovers, or I would have remembered ;-). Curiousity, the current rover, missed its original launch window and had to be delayed -- but by then, our confidence in ability to land spacecraft on Mars had been restored, so there was patience.
The really funny thing is, after an article patiently explaining the constraints (need to manage risk after losing two Mars-bound spacecraft, time line for Earth-Mars launch window, weight on the airbag lander) there are still people (see the comments in the article) who want to solve the problem with the methods dismissed in the article above their comment.
E.g., use "a can of compressed air" ("put it in a hollow leg, it would add no more than a few grams"), or a "beater bar" or "squirrel fan". It's like bikeshedding, but by people who don't get that it is a spacecraft and not a bike shed.
It's one thing to ask a question ("Why didn't they...") and another thing to assume the engineers missed something obvious ("Fools! They should have...").
Still, the longevity and data have been truly amazing.