Phoenix Sun, Sun elevation at Phoenix lander site Options

[jmknapp]

Here's a chart showing the elevation of the sun at noon and midnight at the Phoenix lander site for the next year or so:



During one of the press conferences it was mentioned that later this year the situation goes "energy negative" and it's lights out for the lander. What are the odds it could wake up next year when the sun returns? Also, when the sun is only up for a small portion of the day, is it possible to put the lander to sleep and just wake it up for brief periods to use whatever battery power it can get?

[Skyrunner]

I'd say the odds are pretty remote...not to say null. Yes there is a mode on Phoenix which attempts to revive phoenix once it has enough energy hitting it's solar panels and it should also be possible to turn off much of the hardware to conserve power. The MERs however are in a much more benign environment than Phoenix is. Already temperatures never top -30C and get as low as -80C therefore phoenixs electronics shall need almost constant heating. Temperatures will only worsen after the primary mission. So we than get lower temperatures (=higher power requirement to survive) and less power input and both effects are a lot worse for MPX than the MERs.

Furthermore the MERs have small radioisotope heaters which Phoenix has not (??)

If the temperatures go really cold during that negative power phase ice could start to build up and wreak havoc with all sorts of things on Phoenix.

Not much hope I have in getting her to wake up.

[djellison]

I think of this survival as like the NEAR survival parked on Eros. They shut it down a few days after landing, then 22 months later, tried again to see if it was still alive after long periods of total darkness and cold soak. It's just not a survivable thing for complex electronics etc to handle. Phoenix will only have 3 months of TOTAL darkness, then about 6 months of partial sunlight before returning to an environmental situation like is has today. The solar arrays might have snapped off, the joints and soldering failed etc etc. If it DOES survive - it would be astonishing. And it is worth checking, just in case.

Doug

[helvick]

Nice chart Joe.

The solar panels have approximately 4.2m^2 of collecting surface (as far as I can tell but I have no reliable source for that) and assuming that they were built using Triple Junction PV tech that predates the MER's they should be approximately 20% efficient. Taking a rough ballpark of the solar constant @ mars being 600watt/m^2, assuming Tau ~0.5, say the efficiency of the overall power system between the panels and the batteries is ~80% then right now Phoenix should be good for around 2kW hours of power per Sol if the back of my envelope is working.

I must fix my solar power calculator and pump in the Phoenix numbers to see what the power extinction rate is actually going to be but if my memory serves me correctly things should be AOK through the 90 Sols and probably pretty good for a further 90 to 100 but at that stage the power will have started plumetting very rapidly.

I believe that the on board batteries are 25Amp\720Watt hour units.

I don't believe that the lander will be able to survive the winter but I really hope it can survive long enough to get images of the CO2 frost accumulating around it.

[jmknapp]

...right now Phoenix should be good for around 2kW hours of power per Sol if the back of my envelope is working.

Good envelope work! I guess from what you say it's kind of an exponentially worsening situation, with higher power demands and failing sunlight. I wonder what the office pools at JPL might have the under/over lights-out date at--Christmas?

I don't believe that the lander will be able to survive the winter but I really hope it can survive long enough to get images of the CO2 frost accumulating around it.

An English/philosophy-major friend says "it would be neat if it wakes up next year, free from the fetters of its masters."

[Steve G]

I would imagine that the frozen CO2 ice will either crush it, or, when solid, rip it apart, or boiling off, blow it apart. The fact that the landler’s solar arrays were designed for an equatorial mission doesn't help. I suppose they could have gone further over budget by having panels that could track the sun, but most of the science will be accomplished during the first 90 days.





An interesting future mission with RTGs would be to have a survivable spacecraft designed to watch the full seasons and have the ability to crawl over the frozen ice and protect itself from explosive sublimination.



Superb chart. Thanks so much

[helvick]

Phoenix Solar Power.

They are lots of assumptions in this but hopefully it captures the general trend reasonably well.
Solar Cells - 20% conversion efficiency.
Effective Cell Area - 4.2m^2.
Array Tilt - 0.5deg [ facing due south ]
Power loss rate due to dust deposition - 0.2%/Sol.
Local surface albedo 0.23
Atmospheric losses - Tau constant at 0.5
Power Management system efficiency 80%.

Y-axis is kWhr per Sol, X-axis is the Phoenix mission Sol #.

The blue line is the net power available per sol, it is the total of the cyan (diffuse) and yellow (direct beam) values.

I've no idea how much power Phoenix actually needs to operate but unless she is very unlucky and has a huge degradation in atmospheric quality or has a bunch of dust land on the arrays she should be still be generating ~80% of her initial power levels at the end of the primary mission.

[charborob]

I don't believe that the lander will be able to survive the winter but I really hope it can survive long enough to get images of the CO2 frost accumulating around it.

Does anyone know at what temperature C02 will start freezing at the Phoenix site?

[MahFL]

Does anyone know at what temperature C02 will start freezing at the Phoenix site?

-125 C I believe.

[MahFL]

The web page with the sundial disappeared, anyone know what happened ?

[helvick]

I found a press release from ATK, the manufacturers of the solar arrays, that says that each of the two panels will [initially] generate "770 Watts" and are 1.2 meters in diameter. 1.2m in diameter makes for a total surface area of 6.93m^2 so the stated solar panel area of 4.2m^2 seems about right if it refers to the actual collection area of the cells themselves. The 770 Watt number is odd though. If the units are incorrect and they are actually meant to say was 770 Watt Hours then the cells themselves must be only about 13% efficient. If they genuinely do mean Watts then assuming that the panels actually do generate 770 Watts at some stage then the efficiency of the cells would need to be about 85% efficient, which is certainly not correct.

The 770 Watt Hour assumption seems most likely to me so I've amended my power chart to reflect that. Without any info on what sort of power levels Phoenix needs to survive at each stage of the mission the numbers still don't really tell us much.

As a quick indication of the accelerating rate at which power will drop off:

CODE

Sol 0: 1540 W Hr 100%
Sol 138: 1155 W Hr 75%
Sol 180: 770 W Hr 50%
Sol 219: 385 W Hr 25%
Sol 249: 154 W Hr 10%
Sol 264: 77 W Hr 5%
Sol 277: 38 W Hr 2.5%
Sol 287: 15 W Hr 1%
Sol 305 0 W Hr

[nprev]

...impressive work, Helvick, as always!

That 13% efficiency figure is certainly odd, though. Are they trying to say that the cells would optimally function at 85% on Earth, but the solar flux on Mars is (IIRC) around 45% on average with respect to Earth due to distance & atmospheric dust? Might be some sort of conversion loss in the power distribution system as well.

[djellison]

the %'ge is the %'ge of electricity produced as a function of the total radiation in watts / sq metre from the sun at that location

Doug

[helvick]

The 770 Watt Hour number as a per Sol amount of power is the only one that really makes much sense.

If they really meant that the panels could generate 770 Watts then they would each generate at least 5 kiloWatt Hours per sol, which just seems way too much to me, after all MSL is only going to need around 2.5 from its RTG.

On top of that the value is not actually possible as far as I can tell. The solar constant at Mars (the amount of solar energy per M^2 at Mars' current distance from the Sun) is about 490 Watts / M^2. If you ignore all potential losses in the chain such a panel would need to be built of cells that had a 75% conversion efficiency rate. Once you add in atmospheric losses and the power distribution system losses then you'd actually need magical cells that were >100% efficient.

My calculations tell me that the 770 Watt Hour value means that the cells must be around 13% efficient but my calculations could be wrong. The model that I use for diffuse insolation gets less reliable when the sun angle is very low and it's possible that I'm substantially overestimating the amount of diffuse power available, if I were to discount diffuse insolation then that number would be around 20-25% which brings it into line with the MER solar cells.

[mcaplinger]

The 770 Watt Hour number as a per Sol amount of power is the only one that really makes much sense.

http://atk.mediaroom.com/index.php?s=press...es&item=821

"Each Ultraflex array unfolded like an oriental fan into a circular shape 2.1 meters in diameter and will generate 770 watts of power from sunlight at the distance Earth is from the sun. Since Mars is approximately 1.5 times farther from the sun, the solar arrays will produce less than half the power possible on Earth."

My guess is that they meant both panels together (6.92 m2) generate a peak of 770 watts at 1 AU without atmospheric losses, which would put the efficiency at around 23%, which seems believable.