Why SpaceX won’t turn us into a multi-planetary species 

Fighting the laws of physics yields at best logarithmic progress, and rocket propulsion technology forms no exception

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Anyone announcing the successful sale of tourist trips around the moon would attract ridicule and laughter. Unless your name is Elon Musk. In that case the announcement amounts to nothing more than a logical and rather modest step towards Musk’s promise of getting a million people to live on Mars.

One might question why humanity would be interested to colonize an inhospitable planet. Sure, rising CO2 levels in Earth’s atmosphere do pose a challenge, and this issue might make few of us long for a second planet. But when given a choice between a planet with a CO2 level a notch above 0.04% and and a planet with an atmosphere consisting 96% of CO2, the choice seems pretty clear to me. Risks other than rising CO2 levels are no different: trading Earth for Mars means going from bad to worse.

But let’s set aside the question for the need to migrate humans to Mars. I want to focus instead on the plausibility that Elon’s company, SpaceX, can indeed pull off its promise of human emigration to Mars. I am going to cut Elon some slack: I will not insist on the aggressive timeline he put forward (Mars colonization starting in 2024). So the question is: can the rocket technology utilized by SpaceX ultimately be expected to deliver Mars colonization?


Have a look at above diagram. It shows historical records for the distance humans have moved away from Earth’s surface. This distance, the altitude, is measured in Earth diameters. Note that the vertical axis covers a huge range in altitudes with each tick mark representing a factor 1000 increase. On the right hand side it is indicated which altitudes correspond to LEO (Low Earth Orbits), to Moon travel, to Interplanetary Travel, and to Interstellar Travel.

Two technologies are shown: 1) lighter-than-air balloon technology, and 2) chemical propulsion rocket technology. You don’t need to be a rocket scientist to spot that both technologies are characterized by a short initial period of rapid progress, followed by a long period of painstakingly slow progress. The transition between both regimes occurs when engineers hit upon fundamental limitations to the technology. When past the transition, progress is still feasible but this progress is logarithmically slow and typically realized by brute-force attacks.

The human altitude records for balloon technology starts in 1783, when the Montgolfier brothers launch a balloon on a tether. In it is Jean-François Pilâtre de Rozier, a chemistry and physics teacher. De Rozier stays aloft for almost four minutes at a height of 24 m, and makes it safely back to Earth. The altitude reached is modest, but the first airborne human is a fact. From that point on record after record gets broken. In less than a year the Montgolfier brothers 10-fold and again 10-fold the altitudes reached by humans. In the process De Rozier, the first airborne human, also becomes the first fatality in an air crash. But this doesn’t stop progress, and altitudes of a few kilometers are reached. But then, just two years after the first airborne human, progress slows down considerably. It takes more than two centuries for the next 10-folding of altitudes to take place.

From a physics perspective it is clear that once higher altitudes get reached, balloon builders start combating thinner and thinner atmospheres. Every next step in increasing altitude requires an exponential increase in volume-to-mass ratio of the manned balloon. It is this challenge that causes progress to slow down.

The human altitude records established with rockets follows a similar curve. This curve starts off with Yuri Gagarin’s 1961 space flight reaching a record altitude of 325 km. This makes Gagarin the first human officially leaving Earth’s atmosphere and reaching empty space. A few years later, also Gagarin, the first man in space, dies in a test flight. But progress in reaching higher altitudes is fast, and less than a year after Gagarin’s death the first humans orbit the Moon thereby establishing an altitude record exceeding that of Gagarin by as much as three orders of magnitude.

But then progress in reaching farther from Earth stalls. Two years later, in 1970 the human altitude record gets improved, but only marginally. The record of 401,056 km still holds today, almost half a century later. If we are optimistic and assume SpaceX is successful next year (!) in improving upon this altitude record, we get the rocket altitude curve as shown above.
Also here, from a physics perspective it is clear what is happening. With chemical propulsion technology the exhaust velocity for rockets is limited to 4.4 km/s. Given this limitation, reaching farther and attaining higher speeds (in rocket scientist speak: obtaining a higher delta-v) requires an exponential increase in the fueled-to-empty mass ratio of rockets. It is this challenge that makes progress stall.

Setting aside technical details, just eyeballing the rocket technology progress curve, drives down the conclusion that deep space exploration by humans is well past the era of rapid progress. This immediately raises the question “what technical miracle is SpaceX counting on?” It is clear that what SpaceX needs is a novel propulsion technology generating exhaust velocities well above the 4.4 km/h mark. This would create a novel curve in above plot that would flatten out at much higher altitude values. Yet, the sobering news is that SpaceX’s ITS (Interplanetary Transport System) is not based on any novel propulsion technology. ITS is based on the same chemical rocket propulsion technology that is responsible for the absence of progress in the above plot. It is the propulsion technology that proved extremely useful in bringing humans and payload into LEO. The technology can even be stretched to bring humans to the moon and back. But counting on it to colonize Mars carries the flavor of counting on balloons to bring humans into space. Won’t happen.

14 thoughts on “Why SpaceX won’t turn us into a multi-planetary species ”

  1. In the balloon part of history you have people trying to go higher and being prevented by physical / technical limits.
    In the rocket part we just didn’t try, we got to the moon and then stopped pushing for political / economic reasons not technical reasons. Where is the altitude record that goes 10% further than the moon? it doesn’t exist, not because it can’t be done but because we couldn’t be bothered. Or would you claim that the extreme technical limit of chemical rockets just happens to be the same as the orbit of the moon?
    And then to take the two curves and say look they are the same shape so they must be limited for the same reasons is just very poor reasoning.

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    1. 1) The 400,000+ km altitude record was achieved in 1970 by Apollo 13.
      2) both in the balloon development and in the rocket development people went higher and higher because it was easy to do. Obviously, both races stalled when limits were reached.
      3) in the case of rocket technology, delta-v (instead of altitude) is a better measure for achievement. And yes, the delta-v required for moon landings is in the ‘logarithmic progress branch’ of chemical rocket propulsion.
      4) pushing balloons into the upper atmosphere and pushing rockets to higher delta-v both require exponential increases in investments. The physics (buoyancy in exponentially thinning atmosphere and the rocket equation for delta-v) is compelling, and it is no coincidence both curves look the same.

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  2. What could be possible alternatives?
    A space elevator following Arthur Clark?
    A space slingshot?
    Or ground based lasers pushing a light sail?
    Hitching a ride on a comet following Jules Verne?

    The crucial part seems to be that the trust should be delivered from the outside. The space craft should not carry the stuff needed to generate the thrust (energy/mass), at least not until it can carry anti-matter and generate push using high speed particles, photons, or even neutrinos.
    (but I do not believe a neutrino sail will work 🙂 https://physics.le.ac.uk/journals/index.php/pst/article/viewFile/932/645 )

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    1. What is needed (what would create an entirely new curve in above plot) is the development of a low-weight propulsion system with high specific thrust (high exhaust velocity). For instance, nuclear thermal propulsion ( https://en.m.wikipedia.org/wiki/Nuclear_thermal_rocket ) is known to be capable of achieving exhaust velocities twice as large as those achievable with chemical propulsion technologies. However, without exception, such propulsion technologies bring their own technical challenges, and all of them are much less mature than chemical propulsion technology.

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  3. *sigh* Your method is to create an empirical model to predict the future. You make no effort to understand the underlying mechanism, despite the fact there is clearly a well known mechanism driving it and you couldn’t be bothered to look at it. Whenever your method works, it will leave you right for the wrong reason, and you will be deluded enough into thinking you should try it again somewhere else.

    Apoapsis (“altitude” as you call it) increases waay faster than exponential WRT velocity, and fuel requirements only increase exponentially with velocity. Refuelling in orbit eliminates any relationship between fuel and craft size. Perhaps refuelling in orbit is the big breakthrough you think we need, but you argue fusion is needed. I suppose you’re the only guardian as to what constitutes a big breakthrough.

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    1. Anyone painting a grandiose picture for the future faces the task to make plausible how this vision extrapolates from current trends. For Elon’s Mars colonisation vision this link is entirely missing. That is the main message I aim to get across.

      Again, the plot is placing in a historical context the hard fact that progress in bringing humans deeper in space has stalled. Nothing more, nothing less. Yet, the physics behind space exploration is unambiguous and is discussed. Higher delta-v requires higher exhaust velocities. Betting on optimising the fuel-to-payload scenario (I.e. refuelling options) is following the road of logarithmic progress. You can try to mystify this issue by bringing in side issues and adding jargon (“apsis” by the way is the wrong jargon as in orbital dynamics in the limit of light test masses it refers to distance to the center of a celestial body; why deploy wrong jargon when the right term (“altitude”) is available?), but that doesn’t change the end conclusion: we lack the rocket technology to render humans a multi-planetary species.

      I get the impression you agree with my end conclusion, but you don’t like the simplicity and hand-waving nature of my argument (“leave you right for the wrong reason”). So be it.

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      1. I’m not arguing for or against your conclusion. Your argument sucks. It boils down to citing statistics and acting like it makes some incontrovertible conclusion. It’s made worse coming from a person with your background. People look at this handwaving and think it makes a solid tool for reasoning. Ray Kurzweil does this all the time and it annoys me to no end.

        Optimizing fuel-to-payload *is* logarithmic progress, but what I’m saying is the relation between altitude and velocity overtakes it. Every diminishing increase in fuel-to-payload gets you farther at an accelerating rate, until other factors like time come into consideration. So even if we were to accept the handwaving, altitude isn’t even the right metric to be using.

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      2. “until other factors like time come into consideration” – that is exactly the point. There is no “until”. Flight time *is* a major consideration when you want to get a million people to Mars. Suggest you calculate the delta-v required for the 60 day trip to Mars as proposed by Mr. Musk.

        One last thought: Ray Kurzweil makes incredible extrapolations to grandiose claims. I am warning against grandiose claims that find no basis in credible extrapolations. Taking a contrarian position to my warning seems worthy of being classified by the label “Kurzweilian”.

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      3. Replying to your comment elsewhere: It is not the point, early ballonists weren’t stopped because it would take too long to get up higher.

        Kurzweil is ridiculous because he make those grandoise claims on flimsy logic, extrapolating from current trends and acting like it’s something more significant. He has gotten away with it because businesses have their own deadlines and their is nothing in the laws of physics that stops businesses from achieving them. Your conclusion is different in spirit, but you are using the same reasoning, and arguments are evaluated by their reasoning, not on spirit of their conclusion.

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      4. “Flight time *is* a major consideration when you want to get a million people to Mars. ”

        And the amount of fuel is too. India needed 200 tons of fuel to send a 3,000 pound probe to mars. The journey took over 300 days. Now, try do that with a million people with life systems and also fuel for landing.

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  4. This article reminds me of Feynman’s rule: “there’s a principle that a point on the edge of the range of the data – the last point – isn’t very good, because if it was, they’d have another point further along” (quote from the end of the chapter entitled “The 7 Percent Solution” in “Surely You’re Joking, Mr. Feynman!”).

    The point brought up here I would classify as a weaker version of Feynman’s principle: the absence of data points beyond the record achievement – despite ample time passing by – should not render the record itself doubtful, but does give us a warning that we have likely hit upon a fundamental limitation.

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