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For as long as we’ve looked up at the night sky and realized that there were other stars and planets out there beyond our own, they’ve captured our imaginations and made us wonder what was possible. How different or similar are they to Earth? Have any of them ever had life on them, and if so, what is (or was) that life like? Can we travel to those worlds, set foot on them, and engineer a habitat that allows us to survive there? Ideally, it wouldn’t just be for a brief time, but rather could potentially result in the establishment of a long-term human presence on a world other than Earth.
There are many obstacles to a human colony on another world, of course, but some of the biggest problems we’d undoubtedly face arise from the differences between the properties of the planet we evolved on and adapted to, Earth, and any other world. In the case of Mars, it’s smaller, drier, has a thinner atmosphere, and is significantly more distant from the Sun than Earth is. But could the biggest barrier to the whole endeavor of colonizing Mars be its low surface gravity? That’s what Seth Goldin wants to know, writing in to ask:
“With all the SpaceX hype around trying to establish a permanent colony on Mars, I’m mystified as to how no one seems to be discussing the long term health effects of lesser gravity on the human body. It seems like an absolute deal breaker.”
While there’s certainly a large difference between difficult and impossible — and I’d hesitate to call the endeavor completely impossible — there are certainly challenges to living in low gravity that we understand better than ever here in 2026. Here’s what we’d have to overcome.
Shown here, an astronaut aboard the International Space Station demonstrates the conservation of angular momentum. A consequence of the symmetry of rotational invariance, the astronaut spins faster only because a torque is applied to him from another astronaut’s body. The amount of angular momentum transferred to the spinning astronaut is canceled out by the equal-and-opposite amount of angular momentum transferred back into the other astronaut-ISS system.
Credit: NASA/International Space Station
The human body is a fragile thing: something we regularly fail to notice here on Earth, whose environment we’re well-adapted to. It’s easy to take for granted some of the remarkable properties that Earth possesses, however, including:
a breathable, oxygen-rich atmosphere,
that’s thick-and-significant, providing sufficient but not crushing pressure,
with an abundance of liquid water at its surface,
with an ozone layer at the top of the atmosphere, blocking a significant fraction of UV rays,
all of which is surrounded by our planet’s magnetic field,
at a distance of 150 million km (93 million miles) from the Sun,
with a mass and radius that leads to an acceleration, at Earth’s surface, of 9.8 m/s² ( 32 ft/s² ),
among other properties.
When people speak about colonizing other worlds, they implicitly account for many of these aspects when it comes to planning. They imagine an enclosed environment, where they artificially control the pressure inside to be similar to pressures found on Earth. They imagine adding oxygen to the proper concentration, and having the ability to have large stores of liquid water within it: essential to human life. They imagine having some type of shielding to protect from ultraviolet radiation and from cosmic particles: something the atmosphere or natural magnetic field of another world may not provide. And they imagine artificial heating-and-cooling to keep things at habitable temperatures for humans: something that no other world in our Solar System naturally provides.
Although Earth and Venus are the two largest rocky objects in the Solar System, Mars, Mercury, as well as over 100 of the largest moons, asteroids, and Kuiper belt objects have all achieved hydrostatic equilibrium. Ganymede and Titan are larger than Mercury, but Callisto, at 99% of Mercury’s size, has just one-third of Mercury’s mass. All told, all known objects with diameters greater than 800 km are in hydrostatic equilibrium. The small sizes and masses of these worlds, relative to Earth, ensure that they all have substantially lower surface gravities.
Credit : Emily Lakdawalla. Data from NASA / JPL, JHUAPL/SwRI, SSI, and UCLA / MPS / DLR / IDA, processed by Gordan Ugarkovic, Ted Stryk, Bjorn Jonsson, Roman Tkachenko, and Emily Lakdawalla
But the last aspect — of having a different gravitational acceleration from what it is at Earth’s surface — is something that’s much more challenging to overcome. Our gravity at Earth’s surface leads to a particular acceleration, but on other solid-surfaced worlds in our Solar System, the acceleration isn’t just different, it’s generally significantly less. For example, the surface acceleration on:
Venus is 8.87 m/s² , or 90.4% of Earth’s,
Mars is 3.71 m/s² , or 37.8% of Earth’s,
Mercury is 3.70 m/s² , or 37.7% of Earth’s,
the Moon is 1.625 m/s² , or 16.6% of Earth’s,
or in interplanetary space is 0 m/s² , or 0% of Earth’s.
While the acceleration on the surface of Venus is comparable to Earth’s, the other rocky worlds of our Solar System have greatly reduced gravity: closer to zero gravity than they are to Earth’s actual gravity.
It isn’t so easy to artificially raise these values, either. Many examples of artificial gravity exist, with the simplest being sustained linear acceleration (like a continuously thrusting rocket) or using centripetal force for rotational motion. Linear acceleration, however, requires a continuous expenditure of fuel and a straight line to keep traveling in: something you won’t find on a planet. Rotational motion is easier to maintain from a physics perspective, but because humans get their sense of balance from fluid in their inner ear, continuous rotation leads to nausea, disorientation, and loss of balance: severe vestibular side-effects . So far, these limitations have all been prohibitive.
While terraforming an entire world, like the Moon, may be a monumental task, a smaller, more easily reachable goal would be to create a series of airtight and watertight domes that could be built and inhabited individually. This pathway toward terraforming could be done a little bit at a time, enabling us to build up our way to an inhabited, colonized world. However, the long-term effects of low gravity on the human body remains a significant unknown.
Credit : European Space Agency
With no serious proposals for artificially enhanced gravity as part of a crewed mission to Mars ( or the Moon ), that leaves any proposed mission with the prospect of simply enduring this significantly reduced gravity: not just temporarily, but potentially forever. We haven’t had humans spend large amounts of time in reduced gravity to study the effects on their bodies, but we have had plenty of astronauts and cosmonauts spend time in the effective zero-gravity environment of outer space. Sure, they still accelerate as they orbit the Earth, but the spacecraft accelerates too: leaving them to experience weightlessness relative to their environment.
The longest period any one person has consecutively spent in space is 437 consecutive days, which belongs to Valeri Polyakov , who set the record aboard Mir from January of 1994 to March of 1995. What was remarkable about Polyakov’s missions (he had previously been to space for 240 days before his record-breaking flight) was how resilient his mental health was. His cognitive functions did not decline. His mood and stress levels only elevated upon a change in environment: when leaving Earth and returning to Earth, but after a few weeks they returned to normal. After his first extended mission, his first words upon his return were, “We can fly to Mars.” Polyakov, along with others aboard Mir in the late 20th century, proved that humans are indeed able to maintain a healthy mental state during long-duration spaceflights .
This photograph shows Cosmonaut Valeri Polyakov on February 6, 1995: thirteen months into his record-setting stay in space, aboard the Space Station Mir. The photograph was taken from NASA’s Space Shuttle Discovery (STS-63) during a rendezvous with the Mir Core Module.
Credit : NASA
But what about physical health?
It turns out, as our question-asker anticipates, that there are severe effects that a human body in greatly reduced gravity experiences: not just immediately, but long-term as well. With so many of today’s modern ambitions for humanity focusing on space colonization, it’s the long-term effects that are most concerning. When NASA discusses the cumulative negative effects of spaceflight on the human body, they use the acronym RIDGE :
Radiation, including energetic forms of light from the Sun and also cosmic and solar particles,
Isolation, including the isolation that comes with being in a confined space, with or without other people,
Distance, and in particular the distance from planet Earth,
Gravity, especially the reduced (or zero) gravity that comes with being off of Earth’s surface, and
Environment, and specifically with spending time in a hostile, closed environment.
Even by just confining ourselves to the “G” aspect of RIDGE — gravity — it turns out the physicial and physiological effects that a low-gravity environments has on the human body are severe, profound, and numerous. While some of these effects might seem intuitive to you, others can be quite surprising, as they surprised both the astronauts who experienced them and the scientists who studied their effects.
If you take a 1 kg mass and place it on the surface of the Earth, it will register 2.2 pounds (9.8 newtons) on a scale. That same mass, on Mars, will have only ~38% of its Earth weight, even though its mass remains unchanged.
Credit : VectorVoyager & Maester Uegly/Wikimedia Commons / Big Think
Here on Earth, gravity serves many purposes for our bodies from a biological perspective.
It orients us, giving us a strong sense of up-and-down, and is essential to our proprioreception : knowing where our body begins-and-ends and understanding how the forces we apply to it relates to our body’s motion.
The resistive force it provides allows activities like sitting, standing, walking, and running to strengthen our bones and muscles.
The “downward” force on our internal organs helps provide us with our stable equilibrium: something that gets disrupted on, say, a roller coaster, when we feel our stomachs “rise” and an accompanying, familiar sensation of nausea.
It helps us, when we assume a reclining position, achieve sleep: something that NASA’s “pillownauts” did far less successfully, at a downward angle, as they simulated the effects sleeping in weightless environments.
And it helps our bodily fluids settle into an equilibrium position; as the gravitational equilibrium of our environment is disrupted, so too is the distribution of fluids within the body.
In reduced (or zero) gravity environments, our ability to do all of these things — things we do routinely and without thought on Earth — is degraded significantly, placing enormous, continuous, and cumulative stresses on our bodies that we aren’t subject to here on our home world.
The NASA Pillownaut program was designed to keep participants in a bed tilted downward, with their heads below their feet, at a six degree angle, for long durations of time. This simulates many of the physiological effects, including fluid shifts, that astronauts in microgravity also experience.
Credit : DLR (German space agency)
Perhaps the most severe and significant change that occurs to a human is the severe atrophy of the muscles in our body and the deterioration of the skeleton: spaceflight osteopenia . On average, for every one month that astronauts spend in space, they lose over 1% (more like around 1.5%) of their total bone mass: a cumulative effect that shows no signs of slowing down up to the 14 month limit that we’ve observed it. According to a 2001 NASA study , this doesn’t just lead to bone loss, but an increase in calcium ion levels in human serum: presenting the same negative effects as the condition of hyperparathyroidism does on Earth. For long-term exposures to low-gravity environment, this can result in irreversible skeletal damage. Exercise, despite its implementation aboard the ISS, does not mitigate these effects .
Similarly, astronauts lose muscle mass quickly: between 10-20% of their lean muscle mass within mere weeks. On Earth, gravity provides the physiological effect known as mechanical loading: where the body must bear the weight of its own mass. Without that load, your body is basically being told, “your heavy, weight-bearing muscles aren’t needed,” which leads to:
a decay of your slow-twitch muscle fibers, costing you not only strength, but eroding your endurance,
an imbalance in how your body uses protein, with the rates of protein breakdown and degradation remaining unchanged while the muscle-building (protein synthesis) pathways decrease significantly,
and the degradation of your body’s most important muscle, the heart, as it does not have to pump against gravity, leading to reduced circulation with unknown effects beyond a duration of 14 months.
It’s possible that devices like ARED, the Advanced Resistive Exercise Device , could mitigate some of these effects, but the benefits are small and the equipment is massive and difficult to transport.
ESA astronaut Alexander Gerst works out on the Advanced Resistive Exercise Device (ARED) aboard the International Space Station. Despite significant and intense interventions, only slight progress has been made in maintaining lean muscle mass and high bone densities in astronauts during long-duration trips away from Earth’s gravity.
Credit : NASA
Another major effect is, without a large gravitational force pulling your fluids “down” in your body, they tend to shift upward: to the head and within the head itself. This leads to the phenomenon of space blindness , putting added pressure on the eyes and causing vision problems. Back pain is frequent, as the spine typically lengthens without the influence of gravity by around 5 cm (2 inches). The active breakdown of bones and muscles leads to a severely elevated risk of developing kidney stones, as well as an ongoing problem with dehydration due to frequent necessary urination. A whopping 75% of astronauts suffer from nasal congestion and sinonasal illnesses , as the body has difficult draining the fluids one’s sinuses without gravity.
For a trip to Mars, a crew will experience an added complication over just being in a zero-gravity environment. This is because there’s also a big difference between the (effectively weightless) journey to Mars, which will take at least 6 months for any crewed mission, and the adjustment of moving on a low-gravity (but still, at ~38% of Earth’s, substantial-gravity) world. According to NASA , this can affect:
your sense of spatial orientation,
your hand-eye and head-eye coordination,
your balance,
your ability to walk,
your sense of motion sickness,
orthostatic intolerance (where you can’t maintain your blood pressure upon standing up),
and much more. In microgravity, astronauts also have a much lower production rate of red blood cells , resulting in space anemia . The persistence of this condition in low-gravity environments, between 0 and 9.8 m/s² , remains unknown.
On Mars, bare-rock structures hold onto heat far better than sand-like structures do, meaning they will appear brighter at night, when viewed in the infrared. A variety of rock types and colors can be seen, as dust clings to some surfaces much better than others. In the reduced-gravity environment of Mars, the human body will no doubt behave differently than it does either in zero-gravity or Earth-gravity environments, although we are not sure in what ways or by how much.
Credit : NASA/JPL-Caltech/MSSS, Mars Curiosity Rover
However, there’s something additional that needs to be considered: the only astronauts we’ve ever studied have been astronauts who have, ultimately, returned to Earth. If the goal of a human colony on Mars is different — to go there, stay there, and never return — the consequences of many of these conditions may not be as important. After all, if you are no longer well-adapted to living on Earth, then that might not be so consequential in a scenario where you don’t return to Earth at all.
But the effects will still be real. Your body will adapt to reduced gravity, but not every adaptation it attempts will be successful. For some examples:
Your heart will not be as atrophied as in zero-gravity, but in reduced gravity, that will likely result in your blood pressure being too high in some parts and too low in other parts of your body.
Your digestive system may still give you a continuously nauseated feeling, and this may result in a greater risk of acid reflux, which could have long-term consequences for your esophagus.
As your metabolism slows to accommodate the reduced muscle and bone mass/density, this may also lower the amount of energy available to your brain, and so long-term, not only may your coordination decrease, but your ability to problem-solve.
It’s important to recognize this is a gray area: we have studied zero-gravity environments very well, but our only study of reduced gravity environments was from lunar gravity during the brief Apollo-era missions.
The Lunar Roving Vehicle was included on the last three Apollo missions and enabled the astronauts to travel greater distances and explore more diverse regions of the Moon than they were able to on foot alone. The longest reduced-gravity mission during that era was Apollo 17, enabling astronauts Jack Schmitt and Gene Cernan spend a record 75 hours (just over 3 days) on the lunar surface. Longer-term effects of reduced gravity on the human body have never been studied.
Credit : NASA/Jack Schmitt/Apollo 17
Some things we are certain will happen: food will taste different, the human sex drive will decrease, your gag reflex will intensify and will be triggered by (terrestrially inoffensive) smells, and many of the problems that appear in microgravity will also appear in reduced gravity environments. But in reduced gravity:
some things will be more similar to how they are on Earth than they are in zero-gravity,
some things will be more similar to how they are in zero-gravity than how they are in Earth’s gravity,
and some things will be entirely different than in either environment.
We don’t know which problems will persist, intensify, and worsen over time, and we don’t know which problems your body will adapt to: reaching a new equilibrium that is different from what it is on Earth, but that allows for long-term thriving. And although we’re certain that shifting your environment like that will result in epigenetic changes , we don’t know how those changes will impact any offspring that are produced after being in that new environment.
Certainly, there will be swift divergences between the type of human that exists on Earth and the type of human that would result after several generations had lived on Mars, and after even a few generations, returning to Earth and “going back to normal” may not be a thing that can even occur. But this doesn’t necessarily mean that building a human colony on Mars — or in any reduced gravity environment — is a dealbreaker; it only means that there will be obstacles to the colony’s survival, including obstacles that we won’t discover until we try and do it. It may or may not be doable with our current understanding of reality, but the only certainty is that if we don’t make the attempt, failure is assured.
Send in your Ask Ethan questions to startswithabang at gmail dot com !
This article Ask Ethan: Does Mars’s gravity prohibit human colonization? is featured on Big Think .
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