Dust Storm- Opportunity EOM, the end of the beginning of a new era in robotic spaceflight Options

[elakdawalla]

John Callas said "If rover is generating less than 22Wh, then it won't have enough power to maintain clock". I'm not sure how to parse this. AFAIK, the mission clock is powered directly from the batteries during sleep and will presumably drain them down to some minimum voltage cutoff. 22Wh per sol would be a little under 1 watt of power, which is a heck of a high power draw for a simple clock.

At any rate, I think it's safe to assume that over the next few days there will be essentially no power generated.

He said that below 22Wh they would likely have a clock fault, so the fact that 22 was the last number reported from the rover made him think it very likely that they had already triggered that fault.

[Gerald]

.... And then there's the possibility that the panels will be so dusty after the storm clears that they won't produce a useful amount of power (although I think that's probably unlikely.)

If the dust will settle uniformly, the panels will be just as dusty as the sky will have been during the storm. So, a cleaning event will be required after or near the end of the dusty sky phase.

[Deimos]

Not strictly true, at least if you're referring to how dusty the local skies were. The very high opacity the rover last saw is quite localized. The planet-encircling dust event, by its nature, distributes dust geographically. When local lifting stops, Meridiani could see rapid clearing that is not dust settling on panels. Then, a long slow period of sedimentation will get things dusty, but I imagine the Meridiani winds will not let too much dust accumulate (Mars may imagine differently, of course).

[marsophile]

New MARCI weather report:

http://www.msss.com/msss_images/latest_weather.html

It includes a link to this "Storm Watch" website:

https://mars.nasa.gov/weather/storm-watch-2018/

[Steve G]

To add a layman's context to the amount of sunlight blocked from the dust storm, if I was standing next to Opportunity, how dark would it be to my human eyes? (Bright as a full moon on Earth? for example)

[serpens]

There are a lot of variables to consider there Steve and the human eye has remarkable adaptability to light intensity. To put this in context the difference in intensity between a really bright sunlit day in a high albedo environment and a moonlit night is around 9 or 10 million to 1. The eye doesn't mind whether it gets its lumens by the bucket load or with a tea spoon provided it gets enough. But since the impact of tau on solar intensity is a negative exponential function with a tau of 10.8+ you probably wouldn't be able to see your hand in front of your face.

[Don1]

OK, so according to the source linked below, a sunny day on earth is about 100,000 lux, while a moonlit night is about 0.25 lux.

At Mars distance the sun will be about 1/4 brightness compared to Earth, so say 25000 lux under clear skies. A tau of 10 means a reduction of about 50,000 fold in the light intensity, so I would estimate about 0.5 lux. This is comparable to a moonlit night on Earth.

https://answers.yahoo.com/question/index?qi...mp;guccounter=1

[Gerald]

If I understand it correctly, tau measurements are performed by considering only the sun or another small apparent source of light.
https://agupubs.onlinelibrary.wiley.com/doi...29/1998JE900017
Scattered light seems to be neglected. This approach tells something about the total incoming light, if tau is small. But for large values of tau, most of the light is scattered. So, it doesn't need to be much darker than it would be, if the whole sky would be as bright as the moon on Earth.

More useful, in this case, would probably be the comparison of about 700 Wh with 22 Wh, which is about a factor of 34 darker than under clear sunshine. Reduce this factor a bit by the dust layer on the panels. It may still be bright enough to read newspapers easily, like on a cloudy or very cloudy day on Earth.

[Deimos]

Exactly! The Sun itself was fainter than a full moon on Earth on Sunday. But in each photon-dust interaction the extinction (which tau measures) is <10% absorption and >90% scattering to sky light. The scattering is ~85% downward, 15% upward. There would be 10-11 extinction events (more like 14-15 at the time Opportunity was awake, with the Sun not vertical). Modeling literally the round numbers I just used, I get 98% loss of light at the surface (i.e., sky light >10,000 times direct sun light); you got 97% (and I believe the panels more than made up numbers).

What did it look like? Other than the decidedly ruddy to brownish tint, it looked like a somewhat overcast day. The sort of mid-afternoon when you can just barely see (or not quite see) the Sun through clouds that stretch from horizon to horizon. When the world would be black through eclipse glasses, but easily visible without. Normal human vision would adapt well--it's not night, moonless or moonlit, or even sunset. Solar panels, however, are not known for being so dark-adaptable. So the drama is real, even if people seem to want to describe it in terms that would be as dramatic for people as the actual situation is dramatic for the rover.

This weekend, I expect tau of several hundred. Not on Mars, but at home, where we expect thunderstorms.

PS, scattered light is not 'neglected'. In photometrically measuring opacity it is hunted and eliminated with extreme prejudice.

[mcaplinger]

Solar panels, however, are not known for being so dark-adaptable.

As a data point, I have solar panels on my house. On a typical day in March they generate about 30 kW-h. During the cloudiest, lowest-production day this March, I got about 3 kW-h, about 10x worse.

700/22 is 32x less production, so this would be a really cloudy day, at least by SoCal standards. (Solar distance is already scaled out so don't complain about that.)

[fredk]

The sort of mid-afternoon when you can just barely see (or not quite see) the Sun through clouds that stretch from horizon to horizon.

Another example that people may have witnessed is being under heavy smoke from forest fires. The sun may be just barely visible as a dull orange disk (so a huge tau), but the landscape will be as bright as some level of cloudy day due to light scattered from the sky (smoke).

I don't know how smoke particle size compares with Martian dust, which would affect the details of the scattering (how much into what angles vs frequency), so the details may be different.

Still it's worth pointing out that so far 22 Whrs/sol is an upper limit...

[hendric]

On the 22 WH just to keep the clock alive, maybe that number is accounting for the stuck IDD heater?

From the paper - "The Mission Clock in the MER rovers uses a custom hybrid crystal oscillator from Q-Tech Corporation. " which however is proprietary, at least at the time the paper was published. The skew of +-10s over 10 sols with temperature is pretty reasonable for a typical +-50 ppm crystal oscillator. (~900,000s for 10 sols, so about +- 10ppm with 10s drift). To get better PPMs clocks in your phones use temperature compensation, ie they adjust the voltage based on temp, with <2 ppm or less commercially available. Of course, that costs power too (~mA), and isn't space rated. A COTS RTC (real-time clock, think wrist-watch in a tiny package) without temperature compensation can be < 1uA though, again not space rated.

[Roby72]

just as a reminder how Mars looked in 2001 fully in dust - shots with the MGS orbiter camera:

https://photojournal.jpl.nasa.gov/catalog/pia03170

[siravan]

Regarding tau and the efficiency of solar panels, another issue is that Oppy's solar panels are of triple-junction type. This allows a wider spectral coverage and improved efficiency (each junction works at a different part of the spectrum). But this increased efficiency comes with a drawback as the junctions are in series. During a dust storm, the light is likely skewed toward red. Hence, not much blue is left and the blue junction becomes the limiting factor.

[mcaplinger]

On the 22 WH just to keep the clock alive, maybe that number is accounting for the stuck IDD heater?

I don't see how, that heater is turned off when the battery controller is turned off, that being the whole point of deep sleep.