Whr Figures, From various sources Options

[helvick]

If I read the dust loss plot correctly the _rate_ of loss is plotted as opposed to the cumulative loss due to dust, right?

Trying to add/subract the curves didnt' quite work. Looking at sols +/-830-851 noticed ~0 dust loss (but not 0 loss due to dust).

You're correct - the Dust Loss number is a rate not an absolute value - it's an estimate of the rate of change in efficiency of the power generating system from Sol to Sol. ie the power is reduced by that percentage each Sol.

To take an example period the Dust Loss [Rate] is 0.03 from Sol 853 to 882.
On Sol 855 the relative efficiency of the panels compared to Sol 1 is 53.6106%.
On sol 856 it is 53.5945%.
The Sol 856 number is 99.97% of the Sol 855 number. You don't add the values directly, you reduce them by that percentage so you get an exponential decay type curve eventually rather than a straight line, or would be if it lasted long enough and was constant.

[Nirgal]

From Sol 811 to Sol 853 panel efficiency decreased by ~0.08% per sol.
From Sol 853 to Sol 877 panel efficiency decreased by ~0.10% per sol.
From Sol 877 to Sol 891 panel efficiency decreased by ~0.33% per sol.
From Sol 892 to Sol 899 panel efficiency decreased by ~0.41% per sol.

If that rate continues then Spirit will be in a very bad way. After mid winter insolation increases at less than 0.4% per Sol and power generation capacity would just continue to decline rather than picking up again as summer rolls in.

Hi helvick,

do you also have the dust accumulation loss rate estimates for Opportunity ?

(and what's the current WHR generated by Oppy right now (must be around 400 Whr, right ?)

I'm concerned by your observation that even with spring season coming, the negative effect of the dust loss rate seems to "overtake" the positive effect of increasing solar insolation

If for Oppy it's similar to the spirit figures (0.1-0.3 % per Sol) then this could limit the remaining
lifetime to less than 100 Sols ... just barely sufficient to reach Victoria at all and then
leaving not much time for doing the science there
( of course we could hope for future cleaning events but, unfortunately we can not rely on that to actually happen ...)

[jamescanvin]

Helvick.

One thing that struck me last night. The amount of incident sunlight absorbed by dust on the panels must be quite inclination dependent. (Think of the size of the shadow of a dust grain, or the path length through a thin dust film as the the angle varies) Maybe this is the reason for the power falling off faster than expected?

If this is the case then as we appoach the solstice the rate of change of this effect should decrease and even better, give more power than expected as spring arrives.

Does this make any sense?

James

[helvick]

Helvick.
One thing that struck me last night. The amount of incident sunlight absorbed by dust on the panels must be quite inclination dependent. (Think of the size of the shadow of a dust grain, or the path length through a thin dust film as the the angle varies) Maybe this is the reason for the power falling off faster than expected?

Interesting idea - this certainly would make sense for beam insolation. The assumption that I made was that the "loss" in power due to dust could be described by a simple efficiency percentage as that seemed to be what the mission planners had thought. Now that I think about it it doesn't make a lot of sense to think it could be so simple.

I think I need to try to model this to see how it might effect power throughout the day and as the Sun's altitude changes from Sol to Sol.

Diffuse light (which accounts for around 30% of power generated at the moment) is definitely not affected in the same way - by definition it comes from all incident angles after all.

[Nirgal]

Hi Helvick,

do you also have current dust accumulation loss rate estimates for Opportunity ?

(see also my former posting above)

[helvick]

Hi Helvick,
do you also have current dust accumulation loss rate estimates for Opportunity ?
(see also my former posting above)

I haven't carried out the same exercise for Oppy yet - I'll try and post it later this evening.

[RJG]

Diffuse light (which accounts for around 30% of power generated at the moment) is definitely not affected in the same way - by definition it comes from all incident angles after all.

I'd agree that it is not affected in quite the same way -but the effect will also will affect diffuse light. That element of diffuse light arriving vertically will be affected less than the element of the diffuse light arriving obliquely.

I'll leave you to model it!

Rob

[helvick]

I'll leave you to model it!

Aargh! True there is an effect and it will change the distribution of light on the panel but I have a gut feeling that it will generally cancel itself out. Reflection will play a fairly significant part in it too. Hmmh, some more thinking is in order.

Anyway quick back of the envelope calculations for the beam case show that for the best case situation (midday sun directly overhead) a 33% physical covering of dust would cause a performance degradation of ~45-50%. Exactly the same amount of dust in mid winter (with a maximum solar elevation around 63deg) would cause a degradation of 70-75%.

However that also assumes completely non reflecting dust grains. If they reflect anything (which they do) then a significant percentage of the reflected light will end up becoming available to the panel which could even increase the efficiency of the panel at low solar angles if the dust was quite reflective. Not a simple task to model well I think.

Interesting. Got to think about this more and do some real digging into last years data to see if I can spot any trends.

[helvick]

Just found some great news from the folks at JPL - they definitely disagree with my guess that dust deposition rates were high and getting worse. From todays Spirit update

The deepest part of the Martian winter - that is, the Martian winter solstice - will be on Aug. 8, 2006. The lowest amount of solar energy the rover is expected to receive is 275 watt-hours per sol (a hundred watt-hours is the amount of electricity needed to light one 100-watt bulb for one hour). The rover typically spends at least one sol recharging the batteries following each sol of heavy science activities.

For that to be true the dust deposition rate at the moment would need to be very low (~0.05% per sol). Clearly that's good news.
So to celebrate I've effectively trashed my earlier attempt at the power chart and mapped out a possible version using the above data, a much more friendly dust loss rate from the published table for Spirits panel efficiency for her first year, and including two cleaning events (corresponding to the sol 421 and 520 events).

That would allow for quite a bit of activity (ie like trying to move) from around Sol 1100.

Even without any cleaning events under these conditions she would have enough juice to survive fairly easily and be able to keep up the current programme of long term observation while waiting for a bit of a clean.

[helvick]

Hi Helvick,
do you also have current dust accumulation loss rate estimates for Opportunity ?

The problem with Opportunity is that there is very little data - the last update I can find that mentions power was the 400whr number at the end of May.
Using the same assumptions as for Spirit above I get the following for Opportunity at the moment. Power has just about hit its lowest level for this winter (~370) and will stay around there for a while (say around Sol 925\930) and then will slowly creep up to ~420 at around Sol 1080 and then begin to decline back to 300 whr as Tau increases with SH Spring\Summer. Any cleaning events will significantly change that of course.

[Nirgal]

The problem with Opportunity is that there is very little data - the last update I can find that mentions power was the 400whr number at the end of May.
Using the same assumptions as for Spirit above I get the following for Opportunity at the moment. Power has just about hit its lowest level for this winter (~370) and will stay around there for a while (say around Sol 925\930) and then will slowly creep up to ~420 at around Sol 1080 and then begin to decline back to 300 whr as Tau increases with SH Spring\Summer. Any cleaning events will significantly change that of course.

Thanks Helvick !

It's encouraging, that even with your conservative estimate and even without cleaning events,
there seems to be sufficient power for Oppy until at least around Sol 1150

BTW.: I really love those excellent diagrams of yours

[helvick]

More good news on the power front, this time for Oppy - from todays JPL update:

Over the past 50 sols the team noticed a gradual cleaning of the solar panels similar to a more-sudden cleaning event experienced one Mars-year ago in "Endurance Crater." Removal of some of the accumulated dust on the panels allows greater production of electricity from sunlight. Opportunity's solar panels are now producing just over 500 watt-hours per sol.

So Oppy is in very good shape and getting better. The following chart shows that even without any further cleaning Opportunity should have enough power generating capability to last through to next winter.


I think I'm going to break out the Laphroaig and have a wee dram to celebrate.

Spirit hasn't been so lucky (yet) but they have confirmed that her power generating capability is not expected to fall below 275 watt hours which is very good news.

[Nirgal]

More good news on the power front, this time for Oppy - from todays JPL update:

So Oppy is in very good shape and getting better. The following chart shows that even without any further cleaning Opportunity should have enough power generating capability to last through to next winter.


I think I'm going to break out the Laphroaig and have a wee dram to celebrate.

Spirit hasn't been so lucky (yet) but they have confirmed that her power generating capability is not expected to fall below 275 watt hours which is very good news.

Yippie !!

this is very good news !
Thanks Helick, for the prompt power chart update ...

[ElkGroveDan]

I think I'm going to break out the Laphroaig and have a wee dram to celebrate.

I see you like to celebrate in style.

[dilo]

Great news, helvick. I think we definitively need another mission extention, something like another earth year (up to sol 1350). This would allow Opportunity to complete Victoria Crater exploration and, hopefully, Spirit will be able to climb the Mc Cool hill (but probably we need a cleaning event too!).