On The Way To Home Plate, a possible route and calendar Options

[ustrax]

As one year ago the question is launched:
Will Spirit make it?

http://i16.photobucket.com/albums/b14/ustrax3/hip.jpg

If Ultreya reveals to be nothing worthy then it would be faster, but...

Any other guesses?

[Tman]

NO NO Spirit have to wait here on the top until winter!!!
I want look Gusev's crater rim in its fuller grandeur...
Unless we come directly back again when the winter season begin!!!

[helvick]

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.