A comment by SF. regarding the InSight Mars lander caught my attention:

I really, really wonder why they didn't just include a small compartment with a duster brush grippable by the robotic arm.

Dust on the solar panels is a major issue for Mars landers without an RTG, and InSight does have a robotic arm. So adding a brush of some sort (either fixed to the arm or graspable) sounds like a good idea. It hasn't been done and (AFAIK) no other lander had this either, ever.

What could be the reason for not including a brush? Simple oversight or are there issues that are worse than having dust on the solar panels?

(Secondary question: Would a fan work in the thin atmosphere to blow away most of the dust?)

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    $\begingroup$ Take a look at this: en.wikipedia.org/wiki/Cleaning_event $\endgroup$ – Markus Appel Nov 30 '18 at 12:45
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    $\begingroup$ @MarkusAppel: Thanks, I am aware of these, but these events cannot be relied on. As far as I know you cannot event predict when one is going to happen or how strong it's going to be. Being able to clean the panels when in need seems like a good idea to me. $\endgroup$ – DarkDust Nov 30 '18 at 12:50
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    $\begingroup$ 1. not all of the solar panels area needs to be in reach. A partial solution is still better than none. (plus the brush could be long, think "thin flexible wire with feathers"). 2. The cleaning wouldn't need to be more frequent than the cleaning events - treat it more like a contingency than a standard procedure. 3. The robotic arm already has a standard 'latch' for the two devices it's to deploy. Nothing new would need to be invented. $\endgroup$ – SF. Nov 30 '18 at 13:39
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    $\begingroup$ @SF answer section is below. $\endgroup$ – UKMonkey Nov 30 '18 at 13:58
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    $\begingroup$ @Uwe: I'd be quite surprised if the engineers so obsessed with safety, redundancy and robustness would risk the entire mission on a single cell going bad. BTW, as for interference with experiments, I think lack of power would interfere more. $\endgroup$ – SF. Nov 30 '18 at 16:10

Reasons not to provide mechanical means to clean solar panels on Mars:

  1. and this is the primary reason: Wind on Mars occasionally blows the dust away. This means dust is not a major issue, but a minor one. Spirit and Opportunity functioned for years despite not having dust removal equipment.
  2. Mechanical operations are expensive: you've just added a series of operations that have to be designed and tested to avoid damaging the solar panels, and to avoid interference with the experiments. You've also added weight and complexity which eat into your budget.
  3. Dry dust is abrasive. It's all to easy to scratch the solar panels.
  4. Mars dust is tiny, more like cigarette smoke than the particles we recognize as dust. Any imperfections in the brush will result in dust remaining on the solar panels.
  5. Based on my own experiments, a fan doesn't work well: as soon as you start blowing the dust away, there'll be a cloud around the rover that will be sucked back in by the fan.

NASA is looking into technologies that can clean a solar panel. This has been identified as an enabling technology for crewed missions to Mars. Evidently, NASA does not see this as a necessity for robotic missions up to and including InSight.
These 2 options are being explored:

  1. Electrostatic cleaning: this leverages the static charge of dust particles. By creating an EM field, the dust can be moved to the edge of the panel.

The system takes advantage of the fact that most dust particles, particularly in dry environments, have an electric charge. A transparent electrode material such as indium tin oxide delivers an alternating current to the top surface of the panel. As it swings between being positively and negatively charged, it creates an electric field that repels positively and negatively charged particles. The electric field also helps to impart a charge to uncharged dust particles, allowing them to be quickly repelled as they come in contact with the panel. The researchers have designed the system so that the electric field works its way from one side of the solar panel to the other, gradually moving the dust along until it falls off.

...typically it only needs to be on between two and five minutes a day,

2012 paper on initial tests with a prototype panel showed promising results.

  1. Vibration: this removes the dust by shaking the entire panel.

Using vibrations is simpler and requires fewer modifications to the solar panel, he says. But it does not remove fine particles as well as the electrical field approach.

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    $\begingroup$ Aside that, Wikipedia has recorded that Spirit's solar panel was coincidently benefited from dust devils: On March 9, 2005 (probably during the Martian night), the rover's solar panel efficiency jumped from the original ~60% to 93%, followed on March 10, by the sighting of dust devils. NASA scientists speculate a dust devil must have swept the solar panels clean, possibly significantly extending the duration of the mission. $\endgroup$ – not_Prince Nov 30 '18 at 13:50
  • $\begingroup$ @BoostedNub - Good point! While that shows that the winds on Mars can be beneficial - my first thought was "Woah, 60 to 93% thanks to wind? Why not put a physical arm on there to do that any time." $\endgroup$ – BruceWayne Nov 30 '18 at 16:24
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    $\begingroup$ What that planet needs is a good rainstorm. $\endgroup$ – DrSheldon Nov 30 '18 at 19:27
  • $\begingroup$ Would running the fan for a short period at a time help? $\endgroup$ – Roland Pihlakas Nov 30 '18 at 21:22
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    $\begingroup$ I don't buy your reason for a fan not being a viable solution. Not all of the dust blown off the panels will get sucked back in, because that's just not how fluid dynamics works. Even if it did, at worst you'd just end up with a circulating dust cloud, some of which would settle back onto the panels when the fan stopped, but some would land elsewhere. At best, even a slight wind should carry most of the dust away, especially if you blew it downwind to begin with. There might be other reasons why using fans to blow dust off solar panels on Mars is impractical, but I don't think that's one. $\endgroup$ – Ilmari Karonen Dec 1 '18 at 12:52

Answering the "what would work?" part of the question:

I would use an electret haired brush which barely touches the surface of the solar panels.

Electret filters are very effective in collecting fine particles. The brush is mounted on a simple wiper. Once it collected the brown powder it vibrates downstream to shed the dust of, a bit like a dog after a swim.

  • $\begingroup$ Interesting; is it possible to find a source that describes the existence or use of an electret material as a brush in a similar way? I checked briefly and only found Need electrostatic material to attract pet hair Welcome to Space! $\endgroup$ – uhoh Aug 3 '20 at 7:20

Supplementary answer, which is also somewhat complementary to both @Hobbes's extensive/exhaustive answer and @LouisvanRijn's intriguing answer and shows that their answers were spot-on!

From Forbes.com's Mars Opportunity And Spirit Rovers Could Have Lived Practically Forever With One Tiny Change:

One option would be to install some mechanism such as wiper blades to remove any Martian dust that accumulates. Here on Earth, windshield wipers are so commonplace that it seems like the obvious solution to such a problem. But Mars is very different from Earth in two important ways when it comes to dust accumulating on a surface like a solar panel.

  • Martian dust is extremely small and fine-grained. Atmospheric dust on Mars is approximately three microns in diameter, and adheres via electrostatic forces. You can't just "brush it off" like you would on Earth; much dust would remain.
  • There is no fluid to help on Mars. Water won't remain liquid on Mars, and you can only bring a limited amount of any fluid with you. Dry wiping motions would harm the panel surfaces, making them an inferior solution.

There are better options, though. Wiper blades are heavy, complex, subject to damage at the temperature extremes that Mars experiences, and are easily broken. A second solution would be to have articulated (tilt-able) panels, where you could simply orient them vertically. Since Mars normally has low-level winds that blow continuously, any dust accumulation could be blown off by the natural environment of Mars. In fact, there are "natural" cleaning events that occur on Mars, which we didn't know about when we designed these rovers; the winds occasionally, overnight, will blow large amounts of dust off of the rovers, upping their power output. It was only when Spirit didn't get one for an entire Martian year that it wound up dying.


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