Power on Mars

[Lt-Col A. Tack]

Power on Mars

Part 1: Dust devils clean up solar arrays

Jun 16 2008 10:44AM

Some of you may recall a post at the end of May where I listed questions I had posed to Yardney Technical Products, which made the lithium ion battery used in the Mars Phoenix lander. I expected to get routed through Yardney’s PR department which would act as a go-between with the Yardney engineers. Well, I don’t even know if Yardney has a PR department, but they do have a dedicated applications engineer, Bill Yalen, who spent lot of time and research answering my questions – and judging from the times he sent his email replies, usually pretty late in the evening.

Since his answers are so detailed and interesting, I have divided them up into three PowerSource posts.


Q) What is the lowest temperature that the pack can reasonably withstand?

Yardney Technical Products Lithion battery [Yalen]: The Phoenix Mars Lander battery packs are rated for operation down to -20 degrees C (-4F), and the rigorous testing that we do on all of the battery cells includes cycling down at that level. The Mars Exploration Rover battery cells are rated down to -40C, but their operating voltage and energy capacity are greatly reduced under such conditions, which is one reason for the heaters (see below). I suppose we’ll find out how low they can really survive when Martian winter kicks in (which will be after the designated mission period is over).


Q) Do the Mars Rovers use battery power to heat the electronics?

[Yalen]: Yes – see: http://marsrover.nasa.gov/mission/spacecraft_rover_temp.html : “…the rover's "vital organs" must not exceed extreme temperatures of -40º Celsius to +40º Celsius (-40º Fahrenheit to 104º Fahrenheit).

The rover’s essentials, such as the batteries, electronics, and computer, which are basically the rover’s heart and brains, stay safe inside a Warm Electronics Box (WEB), commonly called the "rover body." Heaters are packed inside the rover body, and like a warm coat, the WEB walls help keep heat in when the night temperatures on Mars can drop to -96º Celsius (-140º Fahrenheit).”

For pictures of the WEB, including the location of the batteries (which, as for Phoenix, were developed here at Yardney),
see http://marsrover.nasa.gov/mission/spacecraft_rover_body.html .

Don’t get too excited about extrapolating the extraordinary performance of the Mars Exploration Rovers to the Phoenix Lander. It’s true that MER was also planned as a 3-month mission which has, of course, instead kept on operating for well over four years now.

We’re all very proud of that success but, as I understand it, a key driver in that original mission duration projection was that by that time dust accumulation on the solar panels would block solar energy and so reduce available power too low to continue operating. Atmospheric dust and solar panel accumulation were, in fact, significant factors but there were also unanticipated beneficial effects of swirling winds which helped by clearing dust away. See also:

-a remarkable dust devil action sequence http://marsrovers.nasa.gov/spotlight/spirit/images/20051121_PIA07863.gif
-related power systems info http://marsrovers.nasa.gov/spotlight/spirit/20051121.html


Q) The temperatures are colder in the Martian polar area than where the Mars Rovers are operating. How much colder?

[Yalen]: Martian Climatology is not really my area of expertise, but I can refer you to the following published sources:

See http://marsrovers.nasa.gov/spotlight/20070612.html :
“Nighttime temperatures on Opportunity's solar panels fell within a fairly stable range of about minus 90 degrees C. (-130 degrees F.) to minus 100 degrees C. (-148 degrees F.) most nights.” Spirit's solar arrays have seen winter lows of about minus 110 degrees C (-166F).

See: http://mars.jpl.nasa.gov/msp98//lander/science.html :
“Carbon Dioxide, which makes up 99% of the air on Mars, turns to solid, or "dry ice", when it freezes at 148 ° Kelvin (-193 °F). When these temperatures are reached in the atmosphere or on the surface during winter, the atmosphere itself begins to freeze onto the ground. Each winter, a seasonal polar cap of carbon dioxide is deposited at the pole.”

Both Rovers are at relatively low latitudes that can reach temperature highs that are – by Martian standards – relatively balmy compared to the deep cold of the poles in winter darkness. Phoenix is much further from the equator than the Rovers. As noted, it will get very, very cold when winter comes and the continuous sunlight of the polar summer gives way to regular daily sunset cycles and then ultimately the continuous darkness of polar winter (after the current mission has run its course). But also keep in mind that the Phoenix site is currently in summer and getting lots of sunlight. The minimum temperature reported for Phoenix Sol (Martian Day) 1 was -80C (-112F) (see http://phoenix.lpl.arizona.edu/ ) which is actually not as cold as the rovers have experienced.

Next up: The likelihood of Phoenix rising from the frozen ashes

Link (http://www.edn.com/blog/1470000147/post/1070028307.html)


Technical Editor Margery Conner's PowerSource streams the latest developments in electronic power design and related technologies.

[Lt-Col A. Tack]

Power on Mars

Part 2: The likelihood of Phoenix rising from the frozen ashes

Jun 17 2008 10:47AM

Yesterday’s post, based on a series of questions with application engineer Bill Yalen of Yardney Technical Products, supplier of the lithium ion battery for the Mars Phoenix lander, covered some basic information on the Martian weather, and how the battery was protected in the earlier Mars Rover missions. Today’s post continues with the affect winter temperatures at the Martian poles may have on the Phoenix’s power system.

Q) Is it possible that the Phoenix battery might be able to withstand the extreme winter cold because it doesn’t have to operate during the winter and can just hunker down and go completely dormant, unlike the Rover batteries which I assume are cycling on a daily basis regardless of the temperature?

[Yalen] Storing Lithium-ion batteries at cool temperatures can be beneficial. But not down on the order of -200F, when the atmosphere itself freezes solid!

Q) Can the solar arrays be folded away for the winter so that they might have a better chance of withstanding the winter weather and winds?

[Yalen] I’m not in a position to answer that definitively but personally I very much doubt it. That would require an entire retraction mechanism to re-stow the panels and also presumably charge up a mechanical energy store (e.g., a spring) to drive future redeployment, since there would be no battery power at that point to drive an electrical system. (That also means that it would have to be triggered by a thermal and/or mechanical sensor/latch of some sort, so such an effort would be quite involved.)

Considering the kinds of challenges that have been overcome, I wouldn’t necessarily rule out the possibility that it could have been done, but all of that would be expensive to develop, take up valuable weight and space, and add risk to the entire mission -- while serving no purpose other than supporting the vanishingly small long-shot chance that anything else could survive the long dark cold winter under the dry ice pack. So, I do doubt that that was done, but you could check with Lockheed Martin or the University of Arizona (see below).

Q) As you can tell, all of these questions relate to whether it’s reasonable to hope that the Phoenix lander might come back to life after the Martian winter. I thought I heard the Lockheed–Martin representative make a comment [during the webcast of the landing] that the mission would last for “at least” 90 days, and based on the Rovers incredible lifetime, it seems like a longer mission might be possible.

[Yalen] Quoting Peter Smith of the University of Arizona, the Phoenix mission Principal Investigator: “…we've had to come up with a well-insulated container to hold our electronics, which only work down to certain temperatures, and then we put in heaters to keep those electronics above that temperature at all times.

…as winter comes to the spacecraft and the sun sets, it gets extremely cold -- so cold that it actually freezes out the carbon dioxide atmosphere into dry ice. And you get a layer of dry ice that actually encases the spacecraft, and no solar energy for the heaters.

And so, at that point, the electronics would be stressed past the point where they're guaranteed to work and it'd be a miracle if they survive through that winter, but we may try and listen in the spring and summer of the next year just to see if it did. I suspect it won't.”
see http://www.nasa.gov/multimedia/podcasting/phx_webcast-transcript.html

So, some small additional time beyond the basic (i.e., “at least”) 90 days? That might seem reasonable to hope for, but no guaranties. As for any hopes that Phoenix will ‘rise from the ices’ to wake up next spring and keep going? Well, we can all hope for that miracle, but I think Peter Smith is your best source on that: “I suspect it won't.”

Next up: Martian Power Part 3 (and the last) – Really low temperatures for a lithium ion battery

Link (http://www.edn.com/blog/1470000147/post/1750028375.html?nid=3351&rid=727330865)

[Mr. Bunny]

Pretty interesting.

[Lt-Col A. Tack]

Power on Mars

Part 3: *Really* low temperatures for a lithium ion battery

Jun 18 2008 11:25AM

This post wraps up the last of my Q&A with Bill Yalen, lead project engineer with Yardney Technical Products, supplier of the lithium ion batteries used on the Mars Phoenix lander. Here’s Part 1, and here’s Part 2.

Because of my interest in the chances of the Phoenix surviving the Martian winter, Bill addressed the effect of extreme low temperatures on the battery.

“We have hard-frozen Li-ion cells in the past and verified that they could be thawed and recover. To do that, however, they should be at medium state of charge, whereas the Phoenix battery would presumably have discharged deeply due to lack of solar power… Unless there was some way to command it to drop off the power bus earlier, while it did still have that partial charge… – And then there would have to be a way to bring it back on later, in the spring. That’s a pretty tough path to follow. Also, to survive freezing, the cells would have to be completely unloaded – any electrical load at all on the battery during the freeze would cause damage and make it unrecoverable. So, theoretically – perhaps something could be designed to do that. Practically, for Phoenix – not likely.

So just how cold is “hard frozen”? Is it the freezing temperature of Co2 (-78.5 C)? Again, he gave a detailed explanation of the battery’s tests and their implications.

“One way we know that Li-ion cells can – under certain conditions - survive a hard freeze is the result of a test chamber malfunction during low temperature testing. Instead of cycling as needed to maintain the desired test temperature, the chamber cooling system ran continuously and drove the temperature far below the intended test profile. Since it was obliviously an invalid test, it’s not clear that we retained test records, as would otherwise be the case. We do know that the result was definitely cell freezing, which means it was below -60 deg C (-76F) which is approximately the temp at which the electrolyte freezes solid. Yet after thawing, functionality was recovered.

“Another way we have learned about cell freezing is that there is a lab diagnostic technique that involves freezing cells in liquid Nitrogen, which would mean at least down at -196C / -321F. Since most of whatever physical degradation caused by freezing has already happened when the electrolyte changes phase from liquid to solid around -60C, going down further doesn’t have much more effect. We do know that cells frozen that cold can also recover – but only if the state of charge and load conditions that I mentioned last week are met. That would be highly unlikely for Phoenix (although, you have gotten me thinking of some ideas that might be interesting to brainstorm for the future…).

“A word of caution: When considering the phenomenon of CO2 freezing, remember that the environment is Mars, not Earth. If CO2 freezes here at -78.5C at 1 atm pressure, it would have to be colder than that to freeze at the much lower pressure on Mars. According to http://mars.jpl.nasa.gov/msp98/lander/science.html , that’s 148 Kelvin -125C / -193 °F).”

Tomorrow -- One last Martian post: A downloadable Mars sunclock

Link (http://www.edn.com/blog/1470000147/post/510028451.html?nid=3351&rid=727330865)