[X] My Assistant

[Cugel]

Does anybody know how much time Oppy actually can spend on driving on a single sol?

I remember that when she travelled from Eagle to Endurance crater it was no more than 1.5 hours per sol (or so), due to power limitations. I have no idea what the current power situation and battery status of Oppy allows these days...

[edstrick]

Every time you wish for a solar panel cleaning system, consider 1.) weight, 2.) complexity, and ask: "Which scientific instrument on these rovers would I trade in return for the mass and volume for a dust-removal system?".

[Nirgal]

Every time you wish for a solar panel cleaning system, consider 1.) weight, 2.) complexity

then what about a simple "windscreen wiper" ?, lightweight (maybe 0.1 percent of rover mass), negligible complexity and quite effective

[dvandorn]

A "windscreen wiper" (or "windshield wiper," as we Americans call it) woud end up, over time, scratching the coatings of the solar cells so badly that they would lose effectiveness. (Try tossing fine rock dust on your windscreen and clearing it with the wipers -- you end up with thousands of tiny scratches in the glass. And that's glass, which is more resistant to such scratching than the materials from which solar cells are made.)

You would also have to design your solar cell surfaces so that they have no cracks or crevices, and nothing that protrudes up above them -- they would have to be very flat, smooth surfaces. Ever seen a solar cell array? They're not all that smooth -- and making them smooth would probably add weight, too.

I think the best dust-clearing device you could come up with would be a small compressor that takes in Martian air, compresses it into a bottle, and (when you need a cleaning) is commanded to blow the air out through vents positioned to duct it across the arrays. Or perhaps just a bottle of compressed CO2 instead of the compressor. A compressor would let you clean your arrays as many times as you want over the lifetime of the vehicle, while a pre-loaded CO2 bottle would only be good for "X" number of cleanings.

But Doug is right -- such a system would likely weigh as much as one of the major sensor packages you want to put on your probe. So you either have to 1) accept the sacrifice of one of your sensors, or 2) increase the mass you can land on the surface by *just* enough to include your cleaning system. Since the second option makes the whole mission more expensive, you'll probably end up getting stuck with the first.

And while you're deliberating on solar cell array cleaning systems, you have to face up to the fact that any solar-powered Martian lander could have its power cut down to below-survival levels by a planet-wide dust storm, whether or not you can clean the arrays. If a really energetic global dust storm occurs and the atmospheric opacity increases beyond a given level for too many sols, a solar-powered probe may simply die. And you've sacrificed your sensor for nothing.

So I'm inclined to agree with Doug -- for longer-duration missions, RTGs are the way to go. For shorter-duration missions, solar power without array cleaning is acceptable, since you're going to get your primary science return very early on. Besides, it seems like Mars is willing to provide enough wind to clean off solar arrays at frequent enough intervals to make extended missions for even these short-duration probes more likely.

There are ways of "tweaking" solar-powered probes, though, to try and get the longest life out them. For example, I believe the MERs use tiny radioisotopic heaters within their main "boxes" to keep the electronics and batteries comfortably warm. Those provide heat without using electricty, thus reducing power consumption needs. The MERs only have electric heaters in places where you can't easily use a passive heating system, like on the IDD. I'm sure there are other ways tweak solar-powered landers to enhance their lifetimes that are both easier to implement and weigh a lot less than solar array cleaning systems.

-the other Doug