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On The Way To Home Plate, a possible route and calendar
Bert
post Aug 26 2005, 09:03 AM
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QUOTE (Bill Harris @ Aug 26 2005, 01:48 AM)
I was thinking that the Sun passed the Southern Hemisphere Summer solstice
at LS 270 (Aug 16) and the Autumnal Equinox occurs in  January 2006.  The power level may not become critical for many, many months [...]
IMHO, power-wise, Spirit might hurry up with that descent into the basin.

Spirit's latitude is 14.57° S. Mars's tropic of Capricorn is at 25.19° S.

The latitude at which the Sun reaches zenith at noon is moving north from 25.19°S to 0° between Aug 2005 and Jan 2006. Some day in between, Spirit will have its "zenith day". After that day, the Sun will be in the northern sky and Spirit will be better on the north-facing far slope of the basin.
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slinted
post Aug 26 2005, 09:35 AM
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QUOTE (Bert @ Aug 26 2005, 01:03 AM)
IMHO, power-wise, Spirit might hurry up with that descent into the basin.
*

I couldn't agree more! They've got a tight window to take advantage of the southern sun power boost by getting down onto the southern slope, then run to get down the hill and across to a north facing slope by the middle of November (14-16th is when the JPL horizons system says the sun will switch from south to north overhead at midday).
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ustrax
post Aug 26 2005, 09:41 AM
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QUOTE (slinted @ Aug 26 2005, 09:35 AM)
I couldn't agree more!  They've got a tight window to take advantage of the southern sun power boost by getting down onto the southern slope, then run to get down the hill and across to a north facing slope by the middle of November (14-16th is when the JPL horizons system says the sun will switch from south to north overhead at midday).
*


Yes! Yes! Dowhill in a hurry!
Helmets on! ph34r.gif
and... wheel.gif wheel.gif wheel.gif


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Nix
post Aug 26 2005, 10:37 AM
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Maybe they're counting on regularly positioning Spirit on small north-facing slopes while going down and while investigating the area, there seem to be enough of those places down there. Also, I guess it's another few weeks/months until there is a threatening shadow cast by HH?

Sorry Ustrax, I somehow don't see them going down that fast..though I'd like it, once there's been enough data gathered from up here!

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Bert
post Aug 26 2005, 11:29 AM
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QUOTE (NIX @ Aug 26 2005, 10:37 AM)
Maybe they're counting on regularly positioning Spirit on small north-facing slopes while going down and while investigating the area, there seem to be enough of those places down there. Also, I guess it's another few weeks/months until there is a threatening shadow cast by HH?
*
I think Spirit can stay quite some time on the basin floor, but IMO it has to start descending as soon as possible because it's hard to estimate when it will reach the floor.

The far slope offers the most options to Spirit for the winter. Staying on the north side of one of these knolls down there might be a solution but it might get "stuck" there, insolation-wise. Meanwhile, the HH shadow will creep closer and closer ... ohmy.gif
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Tman
post Aug 26 2005, 12:04 PM
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NO NO Spirit have to wait here on the top until winter!!! ph34r.gif
I want look Gusev's crater rim in its fuller grandeur...
Unless we come directly back again when the winter season begin!!!

cool.gif rolleyes.gif


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Burmese
post Aug 26 2005, 01:01 PM
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They could climb one of the southern hills, timing to arrive in winter when there should be less dust in the air. In the tradeoff between dust Vs a slightly lower hill, I think such a pan would put the HH pan to shame.
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helvick
post Aug 26 2005, 02:37 PM
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The insolation\orientation question is not all that straightforward because of the way insolation varies with Tau particularly with the odd ball Martian atmosphere. Apologies for the length of this - if you want to skip it my conclusion is that orientation will not be an issue unless we still find ourselves crawling down the side of the hill as Winter rolls in and the route choice will be determined by traversibility and science concerns, not power budget.

Right now insolation is potentially very near it's max as we're just past mid summer. Tau plays two roles in affecting insolation - the simplest effect is that as it rises it makes the atmosphere more opaque and thus reduces direct beam irradiation (~exp(-tau)). Direct beam irradiation is also affected by the panel's physical orientation. So to maximise power from the direct beam you try to keep the panel aligned to the sun, or at the very least normal to the Noon Zenith. The corollary of that is that as Tau rises so does diffuse irradiation and that is still effective at generating power via the solar cells. Diffuse irradiation is by definition much less dependant on the orientation of the panel.

Albedo and a bunch of other factors (latitude, scale height, pressure ....) play a role here but it's approximately true to say that when Tau rises past about 0.6 diffuse irradiation plays a more important role than beam irradiation as far as generating power from solar panels is concerned and as Tau rises past 1.5 direct beam accounts for < 25% of total irradiation and falls rapidly thereafter.

Additionally Tau tends to be higher in SH Summer and Autumn, lower in Winter\Spring. So diffuse irradiation tends to be significant and often the dominant component in Summer\Autumn. Moreover the risk in Summer\Early Autumn is that Tau will rise not that the total insolation will be low (which is the big problem in Winter) so the risk of getting caught out by a sudden reduction in power in Summer is less dependant on the orientation of the Rover and mostly due to fluctuations in Tau regardless of the orientation of the Rover.

In SH Winter Tau tends to be low (~0.2-0.3) and 75%+ of total irradiation comes from direct beam. This happens at the same time that total theoretical power is lower because it's winter so the orientation of the panels wrt the Solar Zenith becomes very important. This is what we saw around the time of Spirit Sol 204, total insolation was low (Late Autumn), Tau was quite low (0.28) and orientation was temporarily oriented badly due to driving constraints and daily power output power dropped to 288 Wh.

Provided Spirit's Panel output is still at or above 800Wh per sol the variations of +-5-7% in power output that changes in orientation would cause are not a problem. If the power budget was as tight as it was late last Autumn then even minor benefits accrued from re-orienting the Rover would be needed but the current situation leaves the Science team free to choose the most interesting routes rather than the most power safe, at least for now.

Of Course a Big Bad Global Dust Storm could whip up, increase Tau to 5 and then all bets are off even for Spirit but for now things look pretty good.
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ilbasso
post Aug 26 2005, 02:56 PM
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Another factor affecting the amount of solar power available is Mars' relatively eccentric orbit. Mars varies in distance from the Sun from 128.6 million miles at perihelion to 160 million miles at aphelion - a variation of 24%. Mars reached perihelion in early August this year, and it is now drawing gradually farther from the Sun. This time next year, Mars will be 24% farther away from the Sun, so the amount of solar power available will drop according to the inverse square law.


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RNeuhaus
post Aug 26 2005, 07:09 PM
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Up to now, the topic is going very interesting. Thanks to the forum, there is ample audience of views.

I knew about Tau (indirect radiation) that in the summer is the worst time than the ones in winter which is the best. The other factor plays an important influence to solar power is about the Mars' excentric orbit around the sun, perihelion and aphelion (direct radiation).

The other factor, I am sure that this summer session there will be more winds (lots of Dust Devil) and according to the pictures, the prominent wind direction comes from south. I am afraid that the direction of wind might change from south to north when the session becomes Winter that the wind coming from north, through the slope won't be so strong as the ones coming thru from the south. It is very probably that on the south slope, Spirit will pick good ascendent wind to wipe out the dust from their power solar panels.

Now about the selection of route. I know that the aim is to follow the route of most science and geological interest after trading with others factors such as traversavility, power management and the usefulnes of MiniTES, Navcam and Pancam. The others, X Alpha Particle, and Mossbauer instruments will be reserved their use for the most intrigating sciences spottings.

I am still doubt that by going downward way will require less electrical power than by climbing. It looks like that the difference will be very small.

Up to know, the topic would be about traversability. By seeing the pictures looks like that there is no slope since the plane of picture is plane. It is hard to evaluate about its inclination. Maybe, someone can map topography surface from Summit at the north side up to another summit at the south.

Nobody and me too have not thought about what is the best place for refugee of these ladies when their legs are exhausted....Spirit at Home Plate and Oppys at Victoria.... So in the future, these places will become an museum for the future visitors....
Maybee, now it is too early.

Rodolfo
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dvandorn
post Aug 26 2005, 07:37 PM
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One of the greatest conundrums of exploring Mars via solar cell is that the times of highest planetary insolation (SH summer) are also the times of highest atmospheric tau, because the greater insolation drives greeater atmospheric instability, which kicks more dust into the air and raises tau.

And yes, I wouldn't be at all surprised if the prevailing winds at Gusev were to shift from southerly to northerly as SH fall and winter approach. On Mars, the seasonal variations see the *air itself* precipitating out and forming the polar caps, so the theoretical highest volume of gaseous air on the planet occurs when the total extent of both polar caps is at a minimum. Which ought to happen at the equinoxes. However, as one cap sublimates down and the other builds up, the CO2 that forms the caps *must* travel, as flowing air, from one pole to the other -- so that would tend to drive northerly winds as the northen polar cap forms and the southern sublimates, while the reverse should be true when the northern cap shrinks and the southern cap grows.

-the other Doug


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RNeuhaus
post Aug 26 2005, 07:51 PM
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One word of caution, the slope of Columbia Hill toward Ultreya (very steep) might hide some gullies sad.gif . So be watchfull on these...

Rodolfo

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To learn a bit about Gullies on Mars
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helvick
post Aug 26 2005, 10:53 PM
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QUOTE (dvandorn @ Aug 26 2005, 08:37 PM)
...On Mars, the seasonal variations see the *air itself* precipitating out and forming the polar caps,...
*


Doug - this is both a very sobering reminde of just how different a place Mars is and a fantasically evocotive description of an alien world I would give almost anything to experience in person.
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