This is no cave: Humans will never colonize Mars

Three big, basic facts about Mars will render it forever out of reach for permanent human settlement

In my prior post, I made the bold claim that humans would never colonize Mars. The feedback was not kind. I got booed. I got called unimaginative. Skepticism all around.

What is more, this week the Ibsen-esque “master builder” Elon Musk announced plans to construct his first gigabay in Texas, a structure designed to house up to one thousand of his rockets per year and to serve in colonizing Mars.

Like gravity, though, logic will prevail…eventually. Below here is my logic for why Martian “colonization”, in the full sense of the word, will remain forever out of reach.

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This is no cave

In the Empire Strikes Back, Han Solo and and his crew steer the Millennium Falcon into the Hoth Asteroid Field in order to evade Darth Vader’s Death Squadron. Seeking a hiding place, Han directs his ship to a large asteroid¹, aka the “Big One”, which was several orders of magnitude larger than the Falcon, but many orders of magnitude smaller than a moon or planet. Seeing a cave opening, they fly the ship in, land it, and then proceed to disembark and look around.

As it turns out, they weren’t in a cave exactly. They were in the belly of the Exogorth Sy-O, a giant space worm living in the cave. It’s a fun plot twist, and it’s absurd. But that’s science fiction. And scenes like this, where people disembark from ships on other planets, moons, and asteroids, reflect a common trope across the genre.

What’s more absurd, but never discussed, is the way in which Han and Leia and Chewie landed the ship, got off, and walked around. Star Wars takes place in a galaxy far far away, but it’s still in this universe and subject to its physical laws. Thus two things — at least — are problematic.

Given the relatively smaller mass of the asteroid, its gravity would have been much less than 1% of Earth’s, or practically non-existent. Far from calmly landing and calmly walking around, the ship and its crew, respectively, would have awkwardly struggled to stay rooted to any surface and to move around with ease. Instead, they would have floated around, grasping to change position with purpose in any direction — kind of like this guy.

That’s problem one.

Problem two is that while the crew accommodates their air supply with flimsy breathing masks, the rest of their bodies are exposed. Even in the worm’s belly, with its jaws agape to open space, local atmospheric pressure around the Millennium Falcon would have approached zero. The human body can’t survive such an environment for very long. Water would vaporize, causing swelling within the body and rapid escape and cooling of water from the eyes, nose, and mouth. Water and dissolved gas in the blood would also vaporize in the blood stream, blocking the circulatory system. With the interior of their bodies over-pressured relative to their new surroundings, gas would immediately force its way out of the stomach and bowels, inducing vomiting and defecation simultaneously. Seizures and many other problems would also ensue before death arrives within a few minutes.² Not the stuff of family-friendly movies.

But out of sight, out of mind. We don’t see gravity and we don’t see air pressure. So while these two things are problematic, they don’t seem problematic when we watch on a TV screen, and we can enjoy Star Wars blissfully ignorant of the impossibilities of its storyline.

Our romance with colonizing Mars, and after Mars the planets and stars beyond, is kind of like this. Images of the Martian surface beamed back to Earth show us a desert-like landscape, not unlike the North American southwest or the Arabian desert.

Image credit: NASA/JPL-Caltech/ASU

It’s easy to picture a human city there in our minds. Yes, we all know you can’t breathe the air, so humans would have to wear suits to venture outside. But surely that’s manageable. Otherwise, there’s nothing apparent in these images to stymy the vision.

But as Han Solo himself realized, things aren’t always as they appear. “This is no cave,” he exclaims, and Mars is no Earth. There are indeed things that stymy this vision, things that we can’t see in a photo but are nonetheless very physically real and important for human life. Things that will prevent us from ever realizing the dream of cities on Mars.

The big three

Visions of Martian colonization range from small underground or indoor settlements to full terraforming of the planet, with eventual lush forests and blue oceans. Thus some visions are more ambitious than others and with greater challenges. But they’re all impossible for humans for three reasons.

1. Gravity. All humans live in an environment with 1 g of gravitational force. It’s an absolute constant in our existence, so much so we’re hardly aware of it. Few humans have ever been to space (or unstrapped in the interior of a free-falling airplane) to experience a lesser gravitational force. Our complex biology, the result of hundreds of millions of years of evolution, is finely attuned to this 1 g.
   

   The gravity at the surface of Mars, though, is only 0.38 g, or 38% that of Earth. It might be fun to jump higher and walk lighter in our step, but too much time in low gravity bears significant challenges for human life, including bone loss, muscle atrophy, and cardiovascular, sensory, and motor deconditioning. It’s true that exercise can counteract some of these — partially and for a while — but not forever. And there are some impractical suggestions that might also help, like having humans in low-g scenarios regularly stand in a centrifuge for several hours a day.
   
   In the end, though, it’s not enough. As this systematic review³ of the matter concluded, “Partial gravity exposure below 0.4 g seems to be insufficient to maintain musculoskeletal and cardiopulmonary properties in the long-term”, and as this one⁴ concluded, “adequate countermeasures appear to be unavailable.”

2. Air pressure. Even less so than gravity do we notice air pressure in an image. On Earth, air compresses us with a force of 14.7 pounds per square inch. That’s actually quite a lot (imagine a 14.7 pound weight bearing down on one square inch of your skin). Luckily the atmosphere surrounds us on all sides and bears at us equally in all directions and dimensions, so our incompressible water-based bodies don’t really feel it. But it is there. And it’s important.

   
   Mars has practically no atmospheric pressure⁵ — about 0.09 psi or 0.6% of Earth’s. In that near-vacuum environment, humans exposed to the outdoors without a pressure suit would experience all those ugly symptoms described above (swelling, simultaneous vomiting and defecation, blocked circulatory system, etc.). So the thought of walking around outside on Mars with just a breathing mask will never happen.
   
   As importantly, water at such pressure can’t exist in liquid form. Liquid water quickly evaporates and melting ice sublimates into a gas.
   
   In the less ambitious visions of colonization, where everything is happening indoors in a controlled environment, maybe this isn’t a big deal. But it certainly puts the kibosh on terraforming. And those oceans on Mars…not happening.

3. Magnetosphere. One of Earth’s greatest blessings (at least for those that evolved to live on it) is its magnetosphere. We can’t see it (other than indirectly via the Northern and Southern Lights), and we certainly can’t sense it. But it’s really really important for life, and for the conditions that make life possible in the first place.

   
   The magnetosphere is a magnetic field surrounding our planet, generated by the slow churn of the planet’s interior molten iron core. It serves to deflect away from the Earth’s atmosphere the solar wind, a shower of energetic charged particles ejected from the sun. Without it, we wouldn’t be here, and neither would our atmosphere. Biological matter can’t long sustain the barrage of these particles. What is more, without a magnetosphere, the solar wind would directly bombard our atmosphere, blowing it out into space.

   Mars hasn’t had a magnetosphere for about four billion years. As such, its surface is forever exposed to torrential downpours of particle radiation. Any life we intend to put there — whether human, animal, or plant — would have to be forever shielded from it to survive. Even with the shielding afforded by space ships and space suits, current studies estimate four years is the maximum appropriate exposure for humans.⁶

   How do you terraform a landscape and expect to have forests when it all has to be indoors and under thick protective shields?

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There are other problems with Mars. It’s really damn cold. It’s inhospitable. Its atmosphere lacks oxygen. It’s very far away. The cost of getting there is ungodly. I could go on.

But gravity, air pressure, and magnetosphere. Those are the big three, the problems that render all the others moot. Engineer and invent and bloviate away, you’re not getting around them.

Don’t get me wrong. Humans are capable of putting humans on Mars. And humans, for short durations of a few years, can endure there. I fully expect before the end of the 21^st century “Outpost Musk” will host a few temporary Martians. It may even have a permanent crew of rotating residents, similar to Antarctic research stations. (By the way, it would be far easier to colonize Antarctica or the Himalayan peaks than to colonize Mars, but we seem to have no intention of doing those things).

But nothing more ambitious than that is possible.

There is no way to rectify the lack of gravity — unless you think we can double or triple the planet’s mass. But where would that matter, amounting to about half of Earth’s, come from? And how would we agglomerate it onto the planet?

There is no way to rectify the lack of magnetosphere — unless you think we can heat up the planet’s core and melt it. But where would that enormous amount of energy come from? And how would we direct it into the planet’s core?

There is no way to increase atmospheric pressure. We we would have to add more matter still to the atmosphere to increase its density and pressure while also increasing gravity to hold it all close to the planet’s surface. Where would the gaseous matter come from? And even if we had it, and could add it to the atmosphere, how would we protect it from the solar wind, ever intent on blowing it back into space?

No technology serves here. No AI or quantum computing, no billionaire’s bravado will ever prevail in fixing these incontrovertible, inconvenient physical facts. They will never change.

If your vision of a Mars “colony” is my Outpost Musk described above — a small, heavily shielded or underground research station offering a tiny, just bearable existence for temporary human residents — I’ll grant you that. Anything more is pure science fiction.

1. Technically, this asteroid is the accretion shell of the Exogorth Sy-O, at least according to starwars.fandom.com.

2. Gosline, Anna. “Survival in Space Unprotected Is Possible--Briefly.” Scientific American, Scientific American, 20 Feb. 2024, www.scientificamerican.com/article/survival-in-space-unprotected-possible/.

3. Richter, Charlotte, et al. “Human Biomechanical and Cardiopulmonary Responses to Partial Gravity – a Systematic Review.” Frontiers, 4 June 2025, www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2017.00583/full.

4. Jones, Harry W. “The Partial Gravity of the Moon and Mars Appears Insufficient to Maintain Human Health.” International Conference on Environmental Systems, 12-15 July 2021, https://ntrs.nasa.gov/api/citations/20210019591/downloads/ICES-2021-142.pdf

5. So the movie The Martian, which is perhaps the most realistic Martian sci-fi story in wide circulation, has a flawed premise. Mars’s atmosphere is far too thin to generate a major dust storm with force sufficient to convey heavy particulates or to heavily damage a structure.

6. Dobynde, M.I., et al. “Beating 1 Sievert: Optimal Radiation Shielding of Astronauts on a Mission to Mars.” Space Weather, 7 August 2021, agupubs.onlinelibrary.wiley.com/doi/10.1029/2021SW002749.

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