Imagine that we somehow manage to maintain frequent trips Earth <-> Mars. I'm not sure what fuel we use but I've read somewhere that it's methane due to its cheap cost.

As the trip goes on for 6 months, fuel is being used and wasted into deep space. Eventually, in a few centuries (I guess), we will run out of fuel and matter. I know it is not a meaningful amount but the problem is still here and I think it needs to be solved in order to achieve a powerful space exploration.

I'm just curious. Can someone enlighten me?

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    $\begingroup$ you may find answers to Can our civilization colonize solar system while reliant on fossil fuels? very helpful $\endgroup$
    – uhoh
    Jun 25, 2020 at 0:47
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    $\begingroup$ In cold war, quite a few countries (even today's North Korea) spent so much resources on military that the whole population were starving. But today, all resources goes to consumptionism that anything else is just rounding error. So if Mars colonization evolves into either military arms race or a new form of consumption, this could be a non-neglectable possibility. $\endgroup$ Jun 25, 2020 at 7:08
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    $\begingroup$ We'll run out of resources just keeping X-billion skin-sacks of mostly water alive. $\endgroup$ Jun 25, 2020 at 11:49
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    $\begingroup$ I think this is a good question but it's close enough to this one to essentially be a duplicate. In particular: if you use fossil fuels yes, you eventually run out, but long before that the climate impact is catastrophic. None of this matters, at all, for lifting relatively tiny numbers of people (tens to perhaps thousands). $\endgroup$
    – user21103
    Jun 25, 2020 at 12:35
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    $\begingroup$ @CarlWitthoft, the correct term is "ugly bags of mostly water". $\endgroup$
    – Mark
    Jun 25, 2020 at 22:42

6 Answers 6


It's a fair question - and unfortunately the answers here circle around but don't quite aim straight at the nail of what I think the OP is after, which is fundamentally about rocket fuel. Because, no matter how many resources you have on Earth, when it comes to the specific question of launching rockets, you pretty much have to do that from Earth-bound resources, and a rocket launch uses up quite a bit. And getting bunches of precious metals doesn't help if you don't have any fuel left because those things ain't fuel!

For example, the SpaceX Falcon Heavy consumes around 411 megagrams (tonnes) of fuel per launch, and once you're past the atmosphere, you can consider whatever part of that is ejected, spent. And this uses some of the most abundant materials - hydrogen and oxygen - which are easily derivable from water via electrolysis, though kerosene also appears to be involved, and when you get to hydrocarbon fuels, then you have all the well-known problems of their limited supply.

So yes, if you use them enough, in theory eventually you will run out of rocket fuels so that our current technology will be unable to launch anything more. However, the trick is that this form of usage is actually very minimal.

The "ideal" space-based infrastructure, at least as I would envision it, would use rockets only for transporting humans off of Earth - spacecraft for interplanetary transit would always be kept in space, where they could use forms of propulsion that would be impractical on Earth (such as electric plasma rockets, fusion rockets, nuclear explosives, etc.) for various reasons but which would be very useful in space for traveling about, and for these fuels, you have virtually unlimited supply. Thus, you're not even necessarily talking launches on the size of a Falcon Heavy once you've got enough stuff set up "out there" that you can, say, mine asteroids and other space resources.

And thus this comes to the other answers' points - the relevant point I'm trying to make here is to call the attention to the inevitable Earth-based input that must still remain for human-to-space transport. And the answer for that depends on what fuels we're talking. If we're talking the liquid hydrogen/oxygen fuel which, by the way, is what most prior missions have used, it's as abundant as the oceans, and that is around $1.38 \times 10^{18}\ \mathrm{Mg}$, which, even if we had as many launches per year as airline flights, about 36 million, at (say) a 200 Mg per-launch cost so around 7.2 billion Mg ($7.2 \times 10^9$) of fuel per year, we're still talking on the order of 190 million years to remove it all.

That said, this interval is actually a bit of a surprise - the Earth in theory has about 1 billion years of habitable time left with doing nothing, and this carries the seeming implication we would be able to strip it bare of ocean (thus destroying the habitability) with rocket launches before that time. The time for hydrocarbon fuel will, of course, be far less than this at least if we're talking only naturally-occurring hydrocarbons and not, say, synthesis from $\mathrm{CO}_2$ and $\mathrm{H_2O}$ in artificial processes powered by ultra-high-density energy sources like nuclear reactions.

Nonetheless, considering how that "kicking shit down the can" is what currently is killing us with climate change, if we can foresee it, then we need to think about it, I say.

Note, of course, this is likely not that soon, because a rocket does burn a sizeable amount of the fuel in-atmosphere, leaving water vapor and/or carbon dioxide exhaust to return (though there's also the issue then of the solar UV flux at the top of the atmosphere photolyzing the water vapor and releasing the hydrogen), so likely there would still be sizeable amounts of water remaining after and so it is more reasonable to suspect we would not exhaust the supply, but I don't have the chops to figure just how much that would or wouldn't be.

In any case, we should, I'd say, probably want a plan in place to get off of using rockets in maybe the next, say, 300 years or so (10 gigaseconds) in favor of things like on-ground launchers that use the mass of the Earth as reaction mass.


Most of the propellant expended in sending a spacecraft to Mars immediately returns to Earth -- the fuel and oxidizer are combusted, combining into (typically) water vapor, CO2, and other simple compounds -- and ejected out the back of the rocket at high speed. The six month trip to Mars is "coasting", with only very small amounts of fuel used for course correction. In order to make frequent round trips to Mars feasible, methane and oxygen will be extracted from Martian atmosphere and/or surface material, and similarly, the vast majority of that expenditure will go straight back to Mars.

So it should be clear that only a very small fraction of the fuel needed to go to Mars and back will be "lost in space".

Currently, over 2,000,000,000 tons of natural gas (which is mostly methane) is burned on Earth per year. If and when SpaceX's Starship/Super Heavy goes into operation flying to Mars, it might use around 1000 tons of methane per flight; at 20,000 flights a year it would account for 1% of worldwide methane consumption.

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    $\begingroup$ Looks like wolfram alpha lied to me about natural gas density (maybe LNG instead of gas?), I’ll update. $\endgroup$ Jun 25, 2020 at 13:08
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    $\begingroup$ Immediately thought of "The Martian Way" en.wikipedia.org/wiki/The_Martian_Way $\endgroup$ Jun 25, 2020 at 13:18
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    $\begingroup$ Some comparison to natural mass gain (through impacts) and loss (hydrogen reaching escape velocity) would be interesting. $\endgroup$
    – Michael
    Jun 26, 2020 at 10:33
  • $\begingroup$ I didn't know that. But it kinda makes sense. Most of the thrust is inside the gravitational pull of the Earth, therefore, the largest part of the fuel is spent here on Earth, returning back to the atmosphere. Didn't think of that. Thanks! $\endgroup$
    – MigDinny
    Jun 28, 2020 at 2:14

There is far more material to be gained from space exploration than will be lost from Earth in collecting it.

A primary reason to explore space is to exploit mineral and organic resources that occur in abundance off earth. Within the "few centuries" you mention, the net change of mass on Earth may very well be positive due to an influx of precious metals and other resources.

These can be collected by either:

  • not distributing physical resources out in deep space in the first place (as the real issue is not "using up" resources but rather distributing them such low concentrations that they can not be recollected) using light sails, laser propulsion, and/or rail guns.
  • using local fuel sources for transportation back to Earth.

As far as fuel components are concerned, Carbon containing asteroids are abundant, Jupiter is 90% Hydrogen, the moon is 45% Oxygen, and water is found throughout the solar system on planets, comets, and moons.

Here is a good video by futurist Issac Arthur laying out why "running out of things" is not a realistic long term concern.

  • $\begingroup$ I particularly liked your first sentence. I was so close-minded when I asked this question. We surely will be benefitting by resource mining more than wasting fuel. Apart from that, a question remains: I thought the only way to achieve propulsion in space was by "throwing away" matter in high speed (linear momentum). Is there a way to propel without matter? $\endgroup$
    – MigDinny
    Jun 28, 2020 at 2:12
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    $\begingroup$ Matter is just a vessel for energy as far as propulsion is concerned. Other (mass-less) ways to transfer energy to a spacecraft are (in vacuum) using photons (which still have momentum) or (on a celestial body) by basically pushing off the body using mechanical means (space elevator, cannon/rail gun, orbital ring, ect.) $\endgroup$
    – johnDanger
    Jun 29, 2020 at 15:28

Eventually, NASA is planning on making a moon base and they will then make rocket fuel out of the water there. We could also make fuel out of the oceans which would solve this problem.

Here are some links for this:



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    $\begingroup$ This is more wishful speculation than realistic solution $\endgroup$ Jun 25, 2020 at 11:50
  • $\begingroup$ @CarlWitthoft - Well, making rocket fuel from ocean water is not wishful speculation. $\endgroup$
    – Pere
    Jun 25, 2020 at 12:06
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    $\begingroup$ @Pere the water is not the fuel, it is the storage medium. Other energy like solar is required to split the H from the O to make the "Fuel". The energy must be available from another source first to separate them so you can burn them and combine them again. $\endgroup$ Jun 25, 2020 at 15:20
  • $\begingroup$ Thanks for the answer and comments, it led me to post this new question Most efficient method of storing energy with water on the moon? $\endgroup$ Jun 25, 2020 at 16:04
  • $\begingroup$ "We could also make fuel out of the oceans" ...Pretty sure that's exactly the kind of dystopic future the querent was concerned about avoiding. $\endgroup$ Jun 25, 2020 at 16:47

The problem that you have to 'waste' matter to travel to space is one that is simply not solvable for the current rocket technology. There are technologies that will allow acceleration in space, like a light sail which makes use of the momentum of photons to move a space vehicle, but to escape the gravity of a planet, there is just no way it would cut it.

That being said, space travel opens the doors to -literaly- a universe of possibilities of gathering resources, the closest one at the time being asteroid mining. At a very close radius in our space-back-yard lies a collection of resources, just floating around in orbit, waiting to be collected. Not only that, but since the resources are already in space, it is possible to simply build our vehicles up there, so we don't even have to worry about gravity to travel. For manned travel we still would need to get up there of course, so there is still the problem of leaving the planet.

It is no easy task to mine asteroids however, since they have little to no gravitational pull, 'landing' on an asteroid is not a thing. Trying to make contact pushes it away since there isn't any friction or drag to hold it in place, so mining is a very shaky process, that being said there are promissing developments in the area, and at this point it just requires a substential funding.

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    $\begingroup$ I should also note that there is the possibility of building a space elevator, but to build such a structure that could remain stable requires a very strong material. Graphene nanotubes are a viable material, but they are very difficult to mold into a desired shape, and they are difficult to create. A space elevator would of course require an 'astronomical' amount of it. $\endgroup$ Jun 28, 2020 at 6:23

Respectfully, I see the question as limited at origin. We are trying to construct a "plan b" for massive extinction events, that even the Permian has annihilated 96% of life, the last, more discussed has influenced more than 70% of species. Allowing US to evolve. You know we have no defense against some types of these extinction level events. Meteors or comets from the Port clouds or already orbiting in atypical orbits can change vectors due to interacting to the great gas giants or even by the action of a little member, unmapped or not from the belt. We have fusion power ogives enough to end us many times. The sun has critical MCE and all Corona Mass Ejections are dangerous because EM earth field is no god...

So, please, focus on environmental themes like the water of your flush, the North pole beaming a sea again or the South transforming sea level and change in a while, like a blink of a eye and all coastal cities are affected. Methane and other gases can be harvested everywhere. But if you don't evolve technology to have two bases outside our planet, I say Moon and Mars or eventually Europa or the other with great volcanic activity classified as 5 class within the solar system, If you have an extinction level event... The next writing his problem in a forum could be a water breeding intelligence within 400 hundred million years... Perspective, is all I ask. Elon is a little to ingenious, but, how many there are? Are we doing what to help? I am a SpaceX supporter from its beginning.

  • $\begingroup$ "We are trying to construct a "plan b" for massive extinction events," Who's "we"? $\endgroup$ Jun 28, 2020 at 1:14
  • $\begingroup$ Well, all you've said is true but nothing makes my question unfeasible. I'm still aware of mostly all the ways we, as a society, have to end our world and, of course, all natural events. Priorities aside — this is still a forum where I can ask and get answers. There is no such thing as "focus on environmental things". Only world leaders must focus, not us. We are meant to be curious and make our own research based on our interests — not forgetting we are still citizens of the world and must do everything possible not to end it soon. Thanks for the answer, anyway. $\endgroup$
    – MigDinny
    Jun 28, 2020 at 1:57

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