Saturday, March 18, 2017

Thinking about Mars: Energy

Elon Musk is determined that mankind go to Mars and eventually settle there.  Put aside the myriad problems with just getting there and then being able to land people and equipment sufficient to survive for even the roughly two years before a return trip works out with the orbits of the planets.  (You can return very quickly instead, within 30 days.)  A number of sticky problems remain.  Gravity, for example, poses an unknown problem.  The low (38% of Earth’s ) gravity may create health problems and even make raising children on Mars impossible.

We can quantify the energy situation and discuss solutions. Begin with the fact that Mars has no forests from which to cut wood for fires.  It never had extensive forests or even a single plant from which to derive vast resources of coal, oil, and natural gas.  On Earth, mankind tamed fire and so could settle the entire planet while feeding those fires with wood.  Later, steam engines fueled the Industrial Revolution, and those engines were fired by wood and then by coal.  Transportation quickly came to depend on fossil fuels.  And so it goes.

Even if you had wood or fossil fuels on Mars, which you decidedly do not, you could not burn them.  The very thin atmosphere cannot support combustion, and it consists almost entirely of carbon dioxide, a gas that has been used to put our fires here on Earth.  On Mars, you have no burnable fuels and no oxygen with which to burn them.  Most of our energy on Earth still comes from fossil fuels burning in air (and creating lots of carbon dioxide).

Our future Martians must seek alternative energy sources.  Solar energy jumps out, but put that aside for last while considering other alternatives first.

What about wind?  Windmills are becoming a significant energy source on Earth.  Mars has high winds and lots of empty space.  Before going further, note that the windmill structures cannot currently be made on Mars.  It lacks the mining, refining, and manufacturing infrastructure.  Creating this infrastructure will certainly take lots of time and, more importantly, energy.  Mining uses massive amounts of energy.  Even exploring for ores take energy.  Furthermore, the equipment for mining must come from somewhere.  Without the ability to mine, refine, and manufacture, how will you make mining equipment?  It’s a Catch-22 situation.

Some suggest that you might use microbots.  Let thousands of mining bots dig up the ore.  Thousands more refining bots can turn ore into metals.  Yet more bots will form the metal ingots (possibly tiny) into usable forms that will be constructed into machines by manufacturing bots.  Putting aside that we just don’t have that technology yet, you run into another problem right away.  The laws of thermodynamics stand squarely in front of you and cannot be broken.

Digging takes quite a bit of energy whether you use it in tiny bits or all at once.  Just because your bots are small doesn’t absolve you from delivering enough total energy to the assembled bot horde to do the job.  Giant machines approach could improve efficiency or be less efficient, but the microbots certainly won’t improve efficiency by an enormous factor.  You must have the energy before you can make the energy.

The same logic applies to the refining, forming, and final assembly of the windmills.  Unfortunately, those windmills will produce only minute amounts of power.  The air on Mars is way too thin, at about 1/100 of that on Earth, to turn an ordinary windmill.  Winds on Mars do exceed 100 mph at times.  The force of those winds amounts to the force of a 10 mph wind on the Earth.  You will not have a good return on your energy investment in windmills on Mars until terraforming raises the air pressure significantly.

Another energy source that dates back far on Earth is the power of falling water.  Too bad that Mars has no liquid water, flowing or otherwise.  You won’t be building water wheels on Mars.  The same ideas apply to tidal energy.  Mars has no oceans to have tides.  Were Mars to get oceans, it has no large moon to create tides, and the Sun is too far for significant tidal motion.

Mars has a hot core that might yield geothermal energy.  Plate tectonics no longer move the surface of Mars.  It no longer has volcanoes that once were very active.  Olympus Mons on Mars, the tallest planetary mountain in our solar system, is a dead shield volcano that may have been active for centuries, building up its height from the same mantle plume because the crust was no longer moving.  Today, you’d have to drill to enormous depths to obtain geothermal energy.  That drilling would take a very large amount of energy and pipe.  Geothermal energy on Mars will remain impractical for a long time, perhaps forever.

Nuclear generators, fission and fusion, create ample supplies of energy.  Of course, fusion energy remains in the future but might be solved in a decade or so, a time that could coincide nicely with setting Mars.  Due to uncertainties regarding how large and complex a fusion power plant would be, planners cannot figure out how to move a plant to Mars and set it up.  Old-fashioned fission plants can be made rather small and could be set up on Mars in small craters to provide shielding for settlers and thus avoid the enormous containment shells we see here on Earth.  Special robots may service the nuclear power plants.  Fission power makes some sense if the components can be safely lifted to Mars and landed there and assembled without significant danger.  Unless fusion power comes soon enough, fission power plants will probably arrive on Mars before the end of the century.

For now, the only practical nuclear power source is the RTG, radioisotope thermoelectric generator.  This device uses capsules of plutonium or other radioisotopes that become quite hot from their radioactivity.  The heat powers thermocouples that generate electricity from the temperature difference between the hot plutonium and the outside.  Placing the external half to the thermocouples outside the habitat will increase the temperature difference and so increase the power generated.  Plutonium-238 is the most commonly used radioisotope and has a half-life of 87.7 years.  A Pu-238 RTG will have more than 80% of its original power after 25 years.

RTGs have a side benefit of generating heat to keep settlers warm in their habitat.  As long as the capsules do not break, they are very safe and have even been used to power pacemakers.  Their radiation is alpha rays (helium nuclei) that are readily stopped even by a sheet of paper.  Even so, plutonium is the most deadly element in the Periodic Table.  A mere speck can kill if inhaled due to induced lung cancer.  RTGs have long been used to power spacecraft.  Engineers understand the technology well.  Recent design improvements mean that they are almost certain to be a part of any Mars program.

Because a typical RTG generates around 100 watts of power (and around ten times as much heat), they will not be sufficient for anything other than auxiliary and backup emergency power in a settlement.  A more robust power source must be used to ensure survival and the possibility of exploration.

Only sunlight remains for discussion.  A Mars settlement or even a temporary Mars base will require this power source.  Using solar power on Mars poses a number of problems that must be solved.  Most solar panels weigh too much.  Flexible solar panels with a plastic substrate might be selected to avoid the extra mass.  However, the technology for making them results in less efficient panels.  We must hope that breakthroughs in solar technology over the next decade will produce light and efficient solar systems.

The sunlight on Mars is less strong than on Earth due to the additional distance to Mars, about half as strong as on Earth.  Even though the atmosphere is very thin, the omnipresent dust attenuates the sunlight and makes the efficiency of solar panels even more important than on Earth.  On Mars, you won’t be able to afford extensive solar structures to tilt the panels toward the Sun.  They take too much mass.  Instead, you’ll be rolling out the flexible panels directly on ground that has been cleared of rocks.  Dust will settle on the flat panels and must be cleared regularly.

Energy storage poses another serious issue.  Today’s battery technology has too much weight per watt-hour of storage.  Prohibitive costs for lifting all of those batteries to Mars could end the program before it has begun.  Once again, we must hope for new energy storage technologies, hypercapacitors and/or new battery chemistry, if a settlement on Mars ever can be self-sustaining.

Despite the claims of Mars One that it can put settlers on Mars with today’s technology and have them survive there indefinitely, the necessary technologies for energy production and storage will only allow them to eke out a bare survival existence.  A robust, developing settlement must have more capability than any energy technology currently in production can readily provide.

We can visit Mars with our current energy technologies, but new ideas are necessary to live there and build a new society.  Given the rapid pace of development in energy, we may have them by the time we land the first settlers on Mars, but it’s not at all certain.

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