Humans will never go past Mars

Physical limitations will prevent it

Space is really spread out, and we will forever lack the means to get around it fast. Space also happens to be highly inhospitable to human life. For these reasons, I submit that no human will ever go farther than Mars.

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In our last post, we discussed why humans would never colonize Mars. The reasons are not subtle. Colonization of Mars isn’t possible because the planet lacks three major conditions for human life: sufficient gravity, sufficient atmosphere, and any magnetosphere. These are big things that humans can’t engineer around. Although they haven’t, these reasons should render the entire discussion moot. Full stop.

As with Mars, human colonization of space is a central trope of our futuristic thinking. It's taken as granted that someday soon humanity will push out into the solar system to take up residence on other planets, their moons, asteroids, or in free-floating space stations. Mars, in this vision, is a stepping stone to all these other destinations. And these other destinations in turn provide stepping stones to getting out of the solar system. It's a dream that sources inspiration from humanity's history of exodus, discovery, exploitation, and colonization. It's manifest destiny on a stellar — even galactic — scale. If we want to think humanity is on a mission (as we discussed here), colonizing the galaxy certainly fits that bill.

But as with Mars, colonization of space will never come to pass. Stark truths prevent it. In the first instance, there are the big three problems we already discussed in relation to the Red Planet: gravity, atmosphere, and magnetospheric protection from particle radiation. They lack on Mars. And anywhere else in the solar system, they lack way more. Thus if humans can’t make it on Mars, they really can’t make it anywhere else in space. End of story. I could rest my case here, happy that I’ve made it.

But I won’t be that lazy. There are some additional big reasons why space will simply stay out of reach for human inhabitants forever.

The Ghost of Carnot’s three limits of fuel

Space is big (as we’ll discuss below). Getting to it and around it requires us to go really fast, and to do that we need to have fuel. But fuel has limits — physical limits that technology cannot ever sidestep.

The first limit of fuels according to the Ghost of Carnot is energy density: how much energy fits into a unit of mass of fuel, how many joules per kilogram. The energy density of different fuels is not something we can engineer. It’s rather more something written into the laws of the universe, like the speed of light. A molecule of hydrogen (H2) or methane (CH4) will always contain the same amount of energy. The forces that bind them together, and the kinetic energy those forces unleash when they are combusted, do not ever change. Ever.

Hydrogen (H2) is the most energy-dense chemical fuel there is. It holds 142 megajoules per kilogram. We don’t even use H2 for rocketry because of various practical limitations, but it’s important to know its energy density. That is the upper limit on anything humans will ever have to work with when it comes to fuel for space travel. It would take a constitutional amendment to the laws of the universe to get anything better.¹

The second limit of fuels according to the Ghost of Carnot are Newton’s laws, which formulaically relate mass, energy, acceleration, and speed. According to them, a quantity of fuel, x, with a given energy density, y, can only ever accelerate an object of a given mass, m, to a certain maximum speed, z. How much that speed, z, is depends on x, y, and m.

So if we want that object to go faster, we’ve got three levers to pull. 1) We need more energy dense fuel. Or 2) we need to shrink the objects we want to accelerate to a smaller mass. Or 3) we need more fuel. We now know that fuel has a limit on energy density — it’s 142 MJ/kg. And let’s assume humans won’t shrink themselves to get to the stars. Our only lever to ever pull in getting a faster spaceship, then, is adding more fuel.

We can do that, but only to an extent.

The problem is that the more fuel you add, the more massive your ship gets. The more massive it gets, the more energy is needed to accelerate it. The more energy needed, the more fuel is needed, the more massive… it’s a tedious, circuitous differential-equation trap, but it can be formulated and worked out. Konstantin Tsiolkovsky did this in 1903.

Much like our main man Carnot, and long before the space age began, Tsiolkovsky used logic and math to articulate with absolute, mathematical certainty the limiting relationships of fuel, mass, and speed. His so-called “Tsiolkovsky rocket equation” has tyrannized our realistic space ambitions since. The science fiction community just hasn’t taken notice yet.

His equation tells us that the fuel requirements of a ship increase exponentially as the ship’s mass or desired final speed increase linearly. In other words, you need more and more fuel to get less and less of a speed result, until your ability to increase the speed of a ship slams into an asymptotic wall that can never be practically overcome, no matter the ship and no matter the fuel. It tells us even that a nuclear-powered photonically-propulsed rocket under ideal conditions can never go faster than 0.02% the speed of light.²

And all of this, more or less, is to say that our space ships — with or without a flux capacitor — will never go faster than they do now. Not by much anyway.

The third limit of fuels according to the Ghost of Carnot is a rather practical one. Our insurmountable fuel problem just discussed immediately gets twice as insurmountable if we want to not only get to somewhere in space but also to actually stop there.

On Earth, a moving object — like a car or plane or bullet — will come to a stop on its on, or at least it will appear to. It's actually air resistance and friction that do the stopping. But we don't see air resistance and friction. Instead, we intuit that all moving things eventually halt on their own.

They don't though. In space, there is no air resistance and there is no friction. Objects, once sped up, will go at that speed forever unless they're slowed down. The energy put into speeding the object is conserved in the kinetic energy of that object until opposing energy slows it down. And slowing down requires the exact same amount of energy as speeding up.

This means that any space craft that is going to a specific destination — whether an asteroid, moon, or another star — must not only carry enough fuel to accelerate to its cruising speed, it must carry double that amount in order to come to a stop when it gets there.

And if whoever crews that ship would someday like to come home, well then double again your already doubly insurmountable fuel problem. Once you've accelerated then decelerated to arrive your destination, you'll have to point the ship back toward home, accelerate again, and then decelerate again when you get there. In each instance, you'll use the same amount of energy, and therefore fuel, to accelerate and decelerate.³

Conservation of energy is a bitch. And it will limit the size and speed of ships designed for human space navigation forever and ever.

Space is too spread out

The solar system is big. It has some massive objects in it, like the sun and Jupiter and even planet Earth. But the word "big" fails to capture the most impressively unimpressive feature of our solar system and of space in general, which is that it is all really spread out. We focus on what is out there — planets and stars and the like. And we’re used to fun but entirely misleading images of such objects, like this one:

But we fail to see the unimaginably vast nothingness that lies between. The voids of space far exceed in magnitude the objects within them.

If you drew our solar system to scale on a single sheet of paper, it's unlikely you could discern even the dot that represents the sun, it would be so small. But that's hard to intuit. So I leave you two anecdotes to help wrap your minds.

Number one is this video, To Scale: The Solar System, which I've previously shared, and which is well worth a watch.

Number two is the asteroid belt. We imagine this circumstellar disk of small, rocky bodies between Mars and Jupiter to look like the Hoth Asteroid Field from Star Wars: a bunch of boulders and rocks floating around densely but aimlessly. It's not. The average distance between asteroids in the belt is nearly 1 million kilometers, some 80-ish times the diameter of Earth. If you passed through it, not only would you be unlikely to run into an asteroid, you would be unlikely to even see one.

With these anecdotes in mind, let's go to the numbers.

The fastest object man has ever made is the Parker Solar Probe. In 2018, it achieved a speed of 191 km/s (0.064% the speed of light) relative to the sun. But of course it only got that fast because we essentially pushed it over the edge and down into the sun’s gravity well. The vast majority of its speed came from the gravitational influence of a massive star.

The fastest object man has ever made heading away from the sun is Voyager 1. Launched in 1977, it will forever flee the solar system at an impressive 17 km/s. But Voyager 1 didn’t get that fast because of humans. It too had gravity assists. A special alignment of the outer planets that only happens every two centuries allowed the craft to reach that speed. (That’s why the New Horizons probe, launched some 30 years later, will never catch it, traveling at a mere 13 km/s.)

But for the sake of argument, let’s say in the future we’ll have ships that can travel on average at the speed of Voyager 1. The math here becomes very simple. At that speed, these are the times it would take (ignoring the impacts of orbital mechanics) to reach the following objects:

• Mars: 0.15 yrs | 0.29 yrs roundtrip

• Asteroid Belt: 0.47 yrs | 0.95 yrs roundtrip

• Jupiter: 1.2 yrs | 2.3 yrs roundtrip

• Saturn: 2.4 yrs | 4.8 yrs roundtrip

• Neptune: 8.1 yrs | 16.2 yrs roundtrip

• Proxima Centauri (the nearest star): 74,948 yrs | 149,895 yrs roundtrip

We know from our discussion of Mars that four years is about the maximum humans should be exposed to space. Thus only Mars, the Asteroid Belt, and Jupiter could ever potentially be reachable for a human.

These numbers above, of course, assume an average speed that is probably nowhere near actually achievable, account for no time of acceleration or deceleration, and assume that these destinations are at their absolute nearest to Earth. Thus, when it comes to manned voyages, Jupiter is probably ruled out and the Asteroid Belt too. (By the way, Jupiter and its moons, with the exception of only Callisto, are absolutely inhospitable for humans due to the high levels of radiation Jupiter throws off.)

There’s nothing to do in space

Space is really spread out, and we lack the means to get around it fast. Oh and it’s inhospitable to humans. There it all is, the summation of my argument for why human travel beyond Mars will never happen.

Though that argument is soundly underpinned by what we know to be true about this universe, I expect many of my readers to find it controversial and/or irritating and to shrug them off with a “meh”, a “maybe”, a “technology will get there, you’ll see” or some other typical comment of blind faith in the future. What I’m about to query, though, is perhaps even more controversial.

What is there to do in space for humans?

Space exploration sounds so exciting. It's the modern equivalent of more rudimentary exploration of ages past, when ships set sail over the horizon, unsure of where they might land, who they might meet, what riches they might find, what places they might build a city.

But in those cases, even when access to food and water were no guarantees, a ship’s crew could always count on the matrix of yet more basic conditions for survivability to be omni-present: air to breath; atmospheric pressure; gravity; protection from cosmic rays — things so fundamental it probably never occurred to them to question their existence.

Space is not like those unexplored reaches of Earth. There's no access to these fundamental needs, at least not all in one place. In our solar system, Mars comes closest to being habitable for humans, and it still misses the mark by a lot. We discussed that last time.

Every other asteroid, moon, or space station coordinate in the void falls far shorter still.

So even if we could put humans beyond Mars and do it in timely manner, this really big question remains: what will they do there?

Flying around in an empty void just to be there seems super boring. It also happens to be a colossal waste of resources.

Mining asteroids for raw materials to bring back to Earth would be way more costly than doing it here.

Mining asteroids to build cities in space is pointless, since humans can’t stay in space very long.

So if there's no cities to build on Mars or beyond, if all we can do on distant rocks is set up small protective shelters and huddle in them temporarily, if we can send probes and robots to do our science for us — why do humans need to be there at all? What is more, why would they want to?

Nobody on Earth really lives in Antarctica. And nobody lives on Himalayan peaks, or deep under the sea. And that’s not at all surprising. To do so is costly, impractical, solitary, nasty, brutish, and short-lived.

To live in space is all those things, just exponentially more so. If living in Earth’s harshest, loneliest, remotest places makes sense for literally no one on Earth — even though the means to do so do exist — it makes no sense for us to pursue a “dream” of doing that in space, where the conditions are worse and the means to do it probably don’t exist, and probably never will.

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1. Here I’d like to note that I'm not considering other forms of fuel than chemical. None are particularly practical for human space flight. This Wikipedia article (Spacecraft propulsion) offers a good general summary. And this one (Photon rocket) has a good treatment of nuclear-powered photonic propulsion and how it can never achieve more than 0.02% the speed of light.

2. “Photon rocket", Wikipedia

3. It's true that creative orbital mechanics and gravity assists can reduce some of the fuel requirement for acceleration and deceleration. The atmospheres of destination planets, like Earth and Mars can also be used to slow incoming craft. But asteroids have little gravity and atmosphere, so neither benefit exists with respect to them.