As the Space X's starship edges closer to reality, it's time to ask a question if our civilization can colonize other planets in the solar system while being still reliant on fossil fuels.

Could we do this BEFORE transitioning to a civilisation which does not rely on fossil fuels, or to a Kardashev's type I civilization (harnessing all energy of our planet)?

Certainly a start can be made (base on the Moon, maybe base on Mars), but is there sufficient energy left in Earth's fossil fuels (and harvestable Moon resources like hydrogen isotopes etc.) to support solar system colonization? If there is sufficient energy remaining would the consequences of burning the fuel we would need be bad or other reasons?

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    $\begingroup$ Which timeframe are you talking about? Is it decades millenia or millions of years. Given that the sun will output energy for billions of years and that rocketfuel can be made from sea water that would not be the limiting factor. More likelly the limiting factor is if we can get there before our civilization colapses. $\endgroup$
    – lijat
    Commented Jun 19, 2020 at 8:22
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    $\begingroup$ Yes, we can: as we can see from the colonization of North America, building new countries on fresh lands has typically impressive results. It is enough, if only a little, self-sustaining city can be built on the Moon or on the Mars. After that, we can already collapse, the Humanity will survive. (Except if some wonderful world-wide dictatorship, which is a possible development for us on the long term, does not develop interplanetary nuclear weapons.) $\endgroup$
    – peterh
    Commented Jun 19, 2020 at 15:48
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    $\begingroup$ @Dragongeek most methane absolutely is a fossil fuel that makes up a large portion of natural gas which is fracked or released as a by-product of traditional oil drilling. You can get renewable methane through either electrolysis of water and then methanation or more directly through co-electrolysis of water and CO2 (you'll still need a methanation step). $\endgroup$
    – aranedain
    Commented Jun 19, 2020 at 16:47
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    $\begingroup$ @peterh-ReinstateMonica how can you all be blind to the fact that while North America has everything that humans need and want (abundant breathable oxygen, clean water, easily attainable food, fuel, building materials, coal, iron, etc, etc ad nauseum... Mars has none. At all. It's a dead planet, for Heaven's sake! $\endgroup$
    – RonJohn
    Commented Jun 20, 2020 at 0:23
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    $\begingroup$ That there are several well-received fact-based answers shows that this question did not need to be closed for the reason stated, and that it should be reopened. There is no need to prevent the community from writing fact-based answers there. voting to reopen $\endgroup$
    – uhoh
    Commented Jun 20, 2020 at 12:44

3 Answers 3


There are at least two approaches to colonizing a planet or a moon:

  • you can send a relatively small number of people there and assume the population will grow in the way small populations sometimes do;
  • you can send a relatively large number of people there.

Effects of lifting a large number of people

So, considering the case where we plan to send a fairly large number of people, an interesting thing then is to consider whether, with rockets driven by fossil fuels, you can lift a large number of people without doing enormous damage to the remaining population. One particular enormous damage you might do is to cause enough warming that bad things happen in due course. We're already doing this in other ways of course, but it's worth looking at the $\mathrm{CO_2}$ emissions of lifting people off Earth in really large numbers.

To do this properly is quite hard: you need to work out the $\Delta V$ properly, know the $I_\mathrm{sp}$ of your launch system, consider how much fuel gets burnt in the atmosphere (obviously the fuel you burn out of the atmosphere does not count, at all), consider other impacts of burning the fuel (aerosols, which may cause some cooling, other pollutants which may do either), consider the cost of producing launch vehicle, the oxidiser, any fuel which does not dirctly produce $\mathrm{CO_2}$ and so on: it's a big engineering calculation.

So, as an annoying theoretical physics person, I'm going to do a horrible spherical-cow estimation in the hipe that the answer is right to a factor of a few. I'll make the following assumptions:

  • we're lifting people with something like an S-IC, burning RP-1;
  • that system can lift enough to put one person and their required support equipment on Mars (so they need something like 3 times more support equipment to live on Mars indefinitely than the Apollo astronauts did to live, briefly, on the Moon, and I assume that the Apollo stack could have put someone on Mars) (note also it does not matter whether you send the support equipment separately: it still costs you to lift it);
  • There is a production-cost fudge factor, $k$ which accounts for emissions from anything but the first-stage (the S-II should not have emitted anything directly, but probably making its fuel did) and emissions in production. I'll assume $k = 2$ initially.

So, OK, let's do the maths.

  • The S-IC contained $770\,\mathrm{m^3}$ of RP-1, or $770\times 10^3\,\mathrm{l}$;
  • RP-1 has a density of about $0.82\,\mathrm{kg/l}$, so the mass of fuel in the S-IC was about $630\times 10^3\,\mathrm{kg}$;
  • Burning $1\,\mathrm{kg}$ of RP-1 emits about $3\,\mathrm{kg}$ of $\mathrm{CO_2}$.

So the direct amount of $\mathrm{CO_2}$ emitted by an S-IC is about $1.9\times 10^6\,\mathrm{kg}$ (the additional mass comes from the oxidizer!). And with our fudge factor to account for other emissions associated with a launch this comes to $3.8\times 10^6\,\mathrm{kg}$.

So the $\mathrm{CO_2}$ cost per person to Mars is about $3.8\times 10^6\,\mathrm{kg}$.

So the question is: is this a problem? Well, let's assume you're not going to do the small-colony-and-breed thing: let's say you're going to lift a million people this way. The cost of doing that is is $3.8\times 10^{12}\,\mathrm{kg}$ or $3.8\times 10^9\,\mathrm{t}$.

Human emissions at present are somewhere around $10^{10}\,\mathrm{t/y}$, which is extremely non-sustainable, to say the least.

So how bad is the cost of lifting a million people? Well, if you did it in a year it would be very bad indeed. If you did it over a century not so bad. Note also that these costs are borne by the people you don't lift, which, if you lift a million people, is 'almost everyone'. Of course, this is the result of some kind of Fermi estimation process: it may be wrong by a factor of ten or something. But it's good enough to tell you two things:

  • someone should do a real engineering calculation to get real numbers as the results are clearly in the significant-to-catastrophic range;
  • we're definitely not going to be living in some world where a million people a year (a little over one person in ten thousand) go on holiday to Mars or the Moon, until we've solved the emissions problem, and we're probably not going to be lifting a million people at all, unless we're happy to kill very large numbers of the remaining population, where that remaining population is almost everyone.

How many people could we lift?

Another question is: given the known fossil fuel reserves, how many people could we lift, if we don't care about the consequences for the people left on Earth? Again I will use the Apollo stack as my source of information:

  • each S-IC contains $770\,\mathrm{m^3}$ of RP-1;
  • there is a fudge factor, $k = 2$ to account for other fossil fuels used in the production of the system;
  • each rocket puts a single person on Mars.

I will base my information on fossil fuel reserves on BP's Statistical review of world energy, and I'll just focus on oil reserves. They say that there are about $1.7\times 10^{12}$ barrels of crude oil left in proved reserves (I am not sure what 'proved' means but I suspect it means there is more we don't know about). A barrel is $159\,\mathrm{l}$ or $0.159\,\mathrm{m^3}$, so there are about $2.8\times 10^{11}\,\mathrm{m^3}$ of crude oil left.

I'll assume that all of that oil is RP-1. This is absurdly optimistic, but on the other hand I haven't taken any account of the coal-fired rockets. So the number of launches we can do – the number of people we can lift – if we burn all our oil, is

$$ \begin{aligned} N &= \frac{2.8\times 10^{11}}{770\times 2}\\ &= 1.8\times 10^8 \end{aligned} $$

the current world population is about $7\times 10^9$: if we burn all the oil we can lift about 1 person in every 40. Or, in other words, we could lift more than half of the population of the US.

If we did this we would emit $6.8\times 10^{14}\,\mathrm{kgCO_2}$: about $68$ times current annual emissions. The consequences of that would be catastrophic for the people left on Earth.

I am not going to try and work out how much coal it takes to drive a rocket, but presumably you could in theory lift some more people with coal-fired rockets: there is quite a lot of coal I think.

Note that I am not arguing that the $\mathrm{CO_2}$ emissions from Apollo, or from space exploration in general, were or are a problem: they are not, and I am very strongly in favour of space exploration. The emissions from lifting people in large numbers, on the other hand, would be a big problem.

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    $\begingroup$ So moving something like 10,000 to 100,000 people per year off the planet using fossil fuels would be a significant but not overwhelming increase in our CO2 production, but moving everyone off the earth that way within a few years would leave the planet quite damaged? $\endgroup$
    – uhoh
    Commented Jun 19, 2020 at 11:58
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    $\begingroup$ @uhoh: I haven't done the sums but I assume that there simply are not enough fossil fuels to lift everyone. There seem to be estimates that there are about $1.7\times 10^{12}$ barrels of crude oil left: a barrel is 159 litres, so about $2.7\times 10^{14}$ litres: at $770\times 10^3\,\mathrm{l}$ per person that would lift about half the world's population, at the cost of catastrophic damage. There might be enough coal to lift the rest. $\endgroup$
    – user21103
    Commented Jun 19, 2020 at 12:14
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    $\begingroup$ great answer, what if we would factor in the extended "life support" (materials, fuels transported from Earth etc.) over the years that any space colony would need to become at least energy independent ? (colony going fully independent could take thousand of years, like terraforming Mars etc.) $\endgroup$
    – ijon_tichy
    Commented Jun 19, 2020 at 12:21
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    $\begingroup$ @ijon_tichy: That would make it worse: probably much worse. My aim really was to make the number as small as I thought I could get away with, because I don't want to be seen as being alarmist. It really needs someone to do a proper engineering estimate, which would need funding as it would take months of work I think: I hope someone is doing something like that, if not someone should apply for a grant :-) $\endgroup$
    – user21103
    Commented Jun 19, 2020 at 12:27
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    $\begingroup$ Yep. Seems to me then that we cannot treat space movement as a solution to our fossil fuel woes. We've gotta fix this civilization right here, or die trying. (Note that "seed 'n breed" civilization on Mars, the Moon, etc. takes a long time before you have enough to be fully independent and self-sustaining from Earth, so Earth's gotta hold up for that haul.) $\endgroup$ Commented Jun 20, 2020 at 1:31

I don't believe we can effectively colonize other worlds while dependent on fossil fuels for the simple reason that the worlds that we'll be colonizing won't necessarily have fossil fuels. You could build a remote outpost or research station, but to have something that you can really consider a colony requires that settlement to be able to live at a level somewhat similar to the civilization back home. That means a fossil-fuel-dependent civilization could only reasonably expand to locations that had their own fossil fuel supplies.

To address the point in your last paragraph, it's extremely unlikely that an interplanetary colonization mission would deplete our fossil fuel reserves. "Reserves" usually means "what's currently economical to extract". History has shown that advances in technology will allow us to locate new reserves and to access reserves previously thought to be inaccessible. It's more likely that you'd run into environmental problems before you deplete the supply of extractable fuel.

A few important caveats:

I use the term "fossil fuels" because that's what was in the question, but this is really about hydrocarbons. On Earth, fossil fuels are our primary source of hydrocarbons and the two terms are used somewhat interchangeably. In the context of space exploration, though, these are two very different things. Some places in the solar system (like Saturn's moon Titan) have literal lakes of liquid hydrocarbons that aren't derived from fossils.

Also, don't forget that fossil fuels are more than an energy source. Energy production accounts for the majority of hydrocarbon uses, but a significant portion (15-25%, depending on who you ask) is used as an ingredient in all sorts of products like asphalt, lubricants, inks and paints, synthetic materials of all types, medicines, cosmetics, etc etc. Even if a civilization gets 100% of its energy from non-hydrocarbon sources, they'll still be dependent on those hydrocarbons for non-energy reasons. Lack of hydrocarbon availability on the destination planet would severely hamper a settlement's ability to be self-sufficient. Relying on imported materials means your settlement's max size is partially limited by the rate at which you can import critical supplies. Due to the distances and logistics involved, these exports will necessarily be limited in volume and too expensive to be a primary source.

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    $\begingroup$ The problem with the hydrocarbons on Titan is that you also need some oxidizer so the fuel actually gives you usable energy. Shipping that in from Earth is also not sustainable. $\endgroup$ Commented Jun 19, 2020 at 23:02
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    $\begingroup$ The first paragraph comes so close to the One True Answer to the question. Not only are there no fossil fuels on Mars and the Moon... there's NOTHING on Mars and the Moon worth anything for colonists. $\endgroup$
    – RonJohn
    Commented Jun 20, 2020 at 0:27
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    $\begingroup$ That seems excessive - there's quite a bit of stuff on Mars, and some on the Moon, that can be used for many purposes. Mars has water, CO2, iron ore, etc. $\endgroup$
    – ikrase
    Commented Jun 20, 2020 at 21:59
  • $\begingroup$ Hydrocarbons are a convenient ingredient for those things, but they aren't actually necessary. For example, it's possible to make plastic just from CO2 and water: docs.wixstatic.com/ugd/…. The only things you actually need are the elements hydrogen and carbon. $\endgroup$
    – Pitto
    Commented Nov 6, 2020 at 5:58

The colonies will need non-fossil-fuel power sources. Free O2 is rare in the inner solar system. I don't know if there are meaningful reserves of oxygen ice in the outer system - but shipping oxygen from the Kuiper belt to warmer colonies (like, say, Titan) would be impractical regardless. Without O2 or another suitable oxidizer, gasoline is just a useless gas/fluid/jelly/rock. A civilization with colonies on the Jovian satellites is a civilization that's happy with nuclear power or has gotten very good at energy-efficiency so they can use solar that far out. Possibly both.

Therefore, a civilization that's managed to get that far will also be a civilization that's not completely reliant on fossil fuels. In theory, parts of the civilization could still rely on them, but the question of why you bother with gasoline when the rest of the system gets by fine on solar and nukes is going to keep coming up.

There is no need to become a Kardashev Type I civilization - it's an arbitrary measure of energy use, which is only really relevant if you're trying to find large and inefficient civilizations by pointing telescopes at them. It's no more valuable than making sure my electric bill is over €1024,00 a month would be.

We need to move away from fossil fuel dependence because they're causing a great deal of ecological damage (which will, in turn, kill a lot of people), but there is no reason to make space exploration wait until after we've stopped depending on fossil fuels. In fact, space colonies may be good testbeds for applied ecology (useful for helping us mitigate global warming) and medium nuclear reactor designs (no ecosystem to trash on the Lunar surface, so we can plonk a couple dozen reactors there and see if anything goes wrong.)

There is certainly a risk of Earthly oligarchs depleting resources on a quixotic attempt at space colonization, but they're currently doing just fine at depleting resources on other, stupider things.


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