Spirit cleaner? Options

[Jeff7]

Most excellent indeed.


The new Mars forum question:

"How many MERs will be working when MSL lands?
0, 1, or 2

[edstrick]

Tau values were made by Viking Landers based on pre-programmed solar brightness observations. When the sun wasn't visible at exposure settings used, the values are lower limits. I *think* values of optical tau over 6 were indicated. Scene brightness levels dropped DRAMATICALLY, with darker horizons than higher skys and with more or less no shadows. Many/most of those images were badly underexposed Of course, once new exposures were selected and upllinked, the opacity dropped a lot, and the images were badly whited-out.

Thermal infrared tau values were measured from orbit with the infrared thermal mapper, a multi-channel radiometer and are about 1/10th (as I recall) the optical values.

Pre-Viking data are poor. Mariner 9 data was re-analyzed but I don't recall results and the worst of the storm was 2 1/2 OR 3 MONTHS before Mariner arrived.

[helvick]

I don't have precise data but again there are approximation formulae in "Solar Radiation on Mars - Update 1991" J. Applebaum, G.A. Landis. (NASA Technical Memorandum #105216) for both of the 1977 global storms that yield optical depths by latitude and LS. Both of these peak at around 5.

That should equate to around a 70% loss in insolation which in turn should equate to about 1.5-2 f/stops from an exposure POV. That would be noticable shouldn't have caused dramatic under/over exposure of images so I wonder if the model underestimates the values? Any ideas?

[Guest_Analyst_*]

Spirit's Solar Power Levels Continue to Rise
Another cleaning has occured. Power has risen to 738 watt-hours!!

This should be enough for the next winter (on a slope), shouldn't it?

Analyst

[helvick]

738 Whr means she is still losing about 25% due to dust. Last summer at peak when she was generating 956 Whr the dust loss factor was around 5%. So she's getting cleaner but I don't think that surviving the winter is guaranteed yet. She bottomed out at 280Whr in mid winter at Low Ridge so right now we'd be looking at around 220Whr for the minimum next winter assuming nothing else changes (and dust deposition follows last years pattern).

[helvick]

Tau measurements estimating the amount of dust in the atmosphere rose from 0.69 to 0.75. (Perfectly clean solar arrays would have a dust factor of 1.0, so the larger the dust factor, the cleaner the arrays.) Electrical energy rose to 738 watt-hours.

Interesting mix up of terms there - Tau and Dust Factor are not the same thing, looks like someone left out part of the sentence.

[dvandorn]

738 Whr means she is still losing about 25% due to dust. Last summer at peak when she was generating 956 Whr the dust loss factor was around 5%. So she's getting cleaner but I don't think that surviving the winter is guaranteed yet. She bottomed out at 280Whr in mid winter at Low Ridge so right now we'd be looking at around 220Whr for the minimum next winter assuming nothing else changes (and dust deposition follows last years pattern).

Remember, too, that we're not just talking about a pretty straight-line graph in which the amount of dust entrained in the atmosphere relates directly to the amount of dust deposited on any given surface. There are a lot of microclimatological effects right along the surface that greatly impact dust deposition rates. If we were to park Spirit on top of El Dorado for a winter, for example (and I know, it would never happen, it faces the wrong way, etc.), you'd likely never survive because the hills force a turbulence that drops dust out of the air selectively onto the El Dorado formation.

IMHO, Spirit spent last winter in a place that tends to collect a little more dust than other places within reach. If she can claw her way up the side of a hill that faces into the prevailing winter winds, as opposed to sitting in a "deposition sink," she might have a better shot at surviving her next winter. But if she stays exclusively in the Home Plate area and doesn't get very far from it, the deposition rate may be just high enough to kill her without an extraordinarily vigorous cleaning event just before the onset of winter.

-the other Doug

[centsworth_II]

If Spirit makes it to next winter, I think they'll take
a lot more care getting her into the best position
possible, as opposed to the rushed job last winter.
I wonder how far the positioning last winter was
from "perfect", how much better could they do
strictly from an angle-to-the-sun standpoint.

[djellison]

They didnt want to do a rush job last time - remember they were trying to get to McCool hill which would have been amazing - but the stuck wheel, and then the Tyrone sand trap...things went against the rover so they had to get somewhere, fast.

Doug

[centsworth_II]

They didnt want to do a rush job last time...

I didn't mean to imply that they planned it that way. I'm presuming
that the result was a less than optimal, but still adequate, positioning
for wintering. My question is, how much better could they do next
winter in terms of positioning for solar energy gathering? Is there a
lot of room for improvement, or did they get pretty close to optimal
positioning in spite of the difficulties last winter?

[djellison]

Well - with a stuck wheel they can't really climb onto the sorts of slopes that would be much better than Low Ridge haven. 20-25 degrees would have been better I would have thought - but the rover just can't get to that sort of terrain any more.

Doug

[BrianL]

Even if the slope is rock with no significant sand to drag through?

Brian

[djellison]

I'd say so, yeah. You drop not only 18% of your traction, but add a whole lot of drag even on rock. Slightly moot point given that there isn't a lot of exposed rock without sand involved near Spirit.

On the other side of the planet they've said that if they go into Victoria, then have a wheel fail while inside, it would be very hard to get out again.

[fredk]

But fortunately getting to a steeper slope may not mean having to drive uphill. If we find ourselves still on Homeplate by late fall, it should be easy to drive from the top of HP partly down the north edge, which would face the right way. I don't recall any figures for those edge slopes, but I suspect they're better than what we had at Low Ridge haven - was that only 10 degrees?

[centsworth_II]

Strategy for getting Spirit on 25 degree slope:
Climb a rise up a less steep slope, then
descend on the steep side and stop.

Looks like good slopes in yellow circle.
Spirit should be there in time to winter.