Settle Luna or Mars

One of the most passionate debates within the space settlement community is whether humans should focus on Luna or Mars first.  Because we obviously can’t walk and chew gum at the same time.

I’m going to say something controversial now. It might even upset you if you’re firmly in the “Mars Now” camp. I apologize in advance for any hurt feelings but hear me out before you grab a pitchfork and torch.

Mars is a short-term, and medium-term, dead-end.

Don’t get me wrong. Mars is a fantastic and mysterious place that humanity should definitely explore. We should conduct science, extract resources, build cities, and make it a human world. We should look for evidence of life. However, settling Mars is not going to enable humans to expand throughout the Solar system and, eventually, the galaxy. At least not for a long time. I’m talking generations.

The problem is gravity.

Mars has a gravity that is about 38% that of Earth (3.711 m/s² vs 9.807 m/s²). That’s good (for transporting people and goods). But it’s not as good as Luna’s gravity of 1.62 m/s². Add in Mars’s thin atmosphere and the simple fact is that it will take more energy to lift things from the Martian surface to orbit than from the Lunar surface. More energy means more cost (fuel, larger and sturdier vehicles, etc.). Mars has resources that Luna and other settlements will need. But they can probably get them at a lower cost from other places. Maybe Phobos and Deimos. Possibly near-Earth asteroids. Definitely from Earth. The costs associated by Earth’s deeper gravity should be off-set by its proximity to Luna and its substantial industrial capacity. Or maybe not. I’m not an economist. But it just doesn’t seem very efficient, to me, for Mars to export raw materials. And there’s not much Mars can make that Luna couldn’t. A Mars industrial base will almost exclusively service Martian needs.

A Lunar industrial base, however, could ship manufactured goods to an early Mars settlement. Probably at a lower cost than stuff launched from Earth (it really depends on what the stuff is). A Lunar industrial base can also service cis-Lunar space with both raw materials and manufactured goods. Luna could build stuff and send it to near-Earth asteroids, the asteroid belt, and beyond. Mars won’t be able to do any of that for a long time.

Distance is the other big disadvantage for Mars. The average distance between Earth and Luna is 382,500 km [17]. The average distance between Earth and Mars is 225 million km [18]. It took the Apollo missions almost 3 days to travel from Earth to Luna (and another 3 to get back). A low energy Hohmann transfer orbit from Earth to Mars can take around 9 months, each way [19]. The faster trajectories, using current technology, still take 400 to 450 days but require significantly more energy [19]. With our current chemical rockets, more energy for travel means much less payload delivered. So, we can either go fast and light or slow and a little heavier.

What can happen during a 9-month trip from Earth orbit to Mars orbit? Lots of things. Radiation is a constant threat. Radiation protection adds mass to the vehicle. More mass is more protection for the crew but less payload delivered to Mars. It’s a difficult balance, but the reality is that the crew will be exposed to a lot more radiation than any astronauts before them. Other threats include (but are not limited to) vehicle and equipment failures, catastrophic impacts (empty space isn’t always empty), and crew issues (multiple people in a small space a long way from home with no hope of rescue performing a high-stress mission). Add the fact that there is an awful lot we still don’t know (about long-duration space missions, Mars, etc.).  

Distance is also a problem because it disadvantages Mars in the cis-Lunar economy. You know, whenever we get around to building it. Goods coming from Mars are going to take longer and require more energy than stuff coming from Luna. Time equals money, so they say. The substantial communications lag caused by the immense distance between Earth and Mars will make participating in Earth’s remote work economy more difficult as well.

The current NASA program is Moon to Mars. This means we go to Luna, learn what we need to learn, develop the necessary tech, and then send people to Mars. Luna becomes a “proving ground”. There’s nothing in the plans about creating a Lunar industrial base. Nothing about permanent settlements. And nothing about building a cis-Lunar infrastructure and economy. Mars is still the ultimate goal.

A Lunar proving ground does make sense though. Mars is more similar to Luna than Earth in several important ways. Although Mars has more of an atmosphere than Luna, it’s still very thin. Mars has a maximum atmospheric pressure of 9 millibars [11], while Earth’s atmospheric pressure (at sea level) is 1,013.25 millibars and Luna’s is 1.33e-11 millibars [12]. Sure, Luna’s atmosphere is a really good vacuum. However, Mar’s 9 millibars is so low that it requires the same equipment that Lunar Homesteaders will need to survive.

Like Luna, Mars lacks a protective magnetic field. Couple that with the very thin atmosphere and you end up with a lot of radiation at the surface. NASA estimates that the average Mars surface exposure will be 1.7 mSv/day [13]. This is about the same exposure as the International Space Station [13]. The Health Physics Society recommends a maximum limit of 50 mSv/year with a cap of 100 mSv over a lifetime [14]. NASA career radiation limits are based on age and sex. Females that are 25 years old have a cap at 1,000 mSv [15]. Males 55 and over have a cap at 4,000 mSv [15]. Some quick math shows that young females would only have 588 days of Martian surface exposure before hitting the NASA radiation cap. That’s not even a Martian year (687 days). Obviously, Martian residents are going to need substantial radiation protection if they plan to stay for the long-term. Just like Lunar residents, although the Lunar surface is exposed to more radiation. I couldn’t find any good data on how much more because, apparently, we don’t have good data on it [16]. One estimate I found is a max of 380 mSv/year from Galactic Cosmic Rays (GCR) alone [16]. Those of us stuck on Earth get about 2.4 mSv of GCR each year but that doesn’t count against lifetime caps [16].

Unfortunately, the reverse isn’t true. Mars would make a terrible proving ground for Lunar tech. Or any other place. Mars is a unique place and tech designed for its environment will have to be substantially modified to work elsewhere. Luna, on the other hand, is a lot like any other airless rocky (and probably icy) body. Most Lunar tech can be used in a lot of other locations with minimal modifications. Thus, making Luna the universal proving ground for tech in the Solar system.

Costs

  • A lot less if wealthy billionaires fund it. None if we convert Lunar Homestead tech into Martian Homestead tech.

Results

  • If we’re lucky, a couple of “flags and footprints” missions. We need a good chunk of luck because the first crew has a very real possibility of dying on the way to Mars. And that would probably be the end of the program as the American public, and politicians in particular, are risk-adverse. We’ll also need some luck because Congress will need to keep funding the program at appropriate levels over many election cycles.
  • If we’re really lucky we might end up with the beginnings of an outpost. Not necessarily a settlement because crew will still be rotated back to Earth. If we’re really, really lucky we’ll have a continual presence on Mars, although the outpost will almost assuredly be completely dependent on regular (and expensive) shipments from Earth for its survival for many years.  

Settling Mars would be an epic adventure. We would be bringing life back (maybe!) to Mars. And creating a new planetary home for life from Earth. However, if our goal is to spread life (all life, not just human) throughout the Solar system (and beyond) we need to create a Lunar civilization with a robust industrial base. Luna first, then the rest of the Solar system. Including Mars.

References

[11] Mars Fact Sheet (https://nssdc.gsfc.nasa.gov/planetary/factsheet/marsfact.html)

[12] Lunar Sourcebbook, pg 635 (https://www.lpi.usra.edu/publications/books/lunar_sourcebook/)

[13] Mars Radiation Environment –what have we learned? (https://www.nasa.gov/sites/default/files/atoms/files/mars_radiation_environment_nac_july_2017_final.pdf)

[14] Health Physics Society FAQ (https://hps.org/publicinformation/ate/q10031.html)

[15] Space Radiation NASA ebook (https://www.nasa.gov/sites/default/files/atoms/files/nasa_space_radiation_ebook_0.pdf)

[16] Radiation exposure in the moon environment (https://www.sciencedirect.com/science/article/abs/pii/S0032063312002085)

[17] Distance to The Moon (https://www.nasa.gov/pdf/180561main_ETM.Distance.Moon.pdf)

[18] How far is it to Mars (https://sservi.nasa.gov/articles/how-far-is-it-to-mars/)

[19] Wikipedia – Human mission to Mars (https://en.wikipedia.org/wiki/Human_mission_to_Mars)

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