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Podcast Transcripts
The human body was built for gravity. Take it away, and bones weaken, muscles shrink, fluids shift, and even vision can change.
For astronauts spending months in orbit, zero gravity isn’t just strange; it is one of the greatest obstacles to living and working in space.
Yet there are solutions. It might be a matter of exercise, or, in the future, the solution may be to create artificial gravity by spinning a spacecraft.
Learn more about dealing with zero gravity on this episode of Everything Everywhere Daily.
Let me start right out by noting that while “Zero gravity” is the common term for the condition astronauts experience in orbit, it is technically incorrect. A better term is microgravity or weightlessness.
Gravity is not absent in orbit. In fact, astronauts aboard the International Space Station are still strongly affected by Earth’s gravity. At the International Space Station’s altitude, roughly 250 miles above Earth, gravity is still about 90% as strong as it is at the surface. If gravity vanished, the station would fly off in a straight line into space.
What is really happening is that the spacecraft and everything inside it are falling around Earth together.
An orbit is basically a continuous fall. Imagine throwing a ball horizontally. It falls to the ground. Throw it faster, and it lands farther away. Throw it fast enough, and as it falls, the curve of Earth drops away beneath it at the same rate. The object keeps falling, but it never hits the ground.
That is what it means to orbit something.
What astronauts experience is the same thing you would experience if you went skydiving.
So “zero gravity” is a misnomer because gravity is still very much present. What is missing is not gravity itself, but the sensation of weight.
By all accounts, weightlessness is fun, at least initially. Outside of a few seconds of bungee jumping, I can’t claim to have experienced it.
However, the longer you are in a weightless environment, the more problems can arise. The first problem that many astronauts experience is space sickness.
Space sickness is the nausea, dizziness, headache, and disorientation that many astronauts feel during their first hours or days in microgravity.
It happens because the brain receives conflicting signals. On Earth, the inner ear’s vestibular system uses gravity to help determine balance and direction.
In orbit, that gravity cue disappears, while the eyes still report motion and orientation. The brain has to recalibrate to a world with no true “up” or “down.”
Symptoms can include nausea, vomiting, cold sweating, loss of appetite, fatigue, and trouble concentrating. It is similar to motion sickness but is caused by weightlessness rather than by a car, boat, or airplane. Most astronauts adapt within a few days, though they can experience a similar readjustment problem when they return to Earth’s gravity.
Long-term weightlessness has problems beyond being nauseous. It is a serious biological problem because the human body is built around constant mechanical loading.
Gravity tells bones, muscles, blood vessels, balance organs, and even fluid distribution how to behave. Remove it for months or years, and the body adapts in ways that are useful in orbit but dangerous when returning to Earth.
In weightlessness, the body no longer has to support itself. The legs, hips, and spine stop doing much of their normal work. Bones no longer receive the same stress signals that tell them to maintain density.
Fluids no longer settle toward the lower body, so blood and cerebrospinal fluid shift upward toward the head. The cardiovascular system, vestibular system, eyes, immune system, and kidneys all respond to this new environment.
NASA summarizes the major effects as muscle loss, bone loss, upward fluid shifts, vision problems, increased risk of kidney stones, and cardiovascular deconditioning.
In microgravity, astronauts can lose bone density, and NASA’s 2025 risk summary estimates a typical loss rate of about 1% to 1.5% per month during four- to six-month missions if not adequately countered.
Astronaut Scott Kelly spent 340 days aboard the ISS in 2015–2016. After returning, he reported sore skin, rashes, flu-like symptoms, swollen legs, balance issues, and other difficulties readjusting to Earth’s gravity.
He has a twin brother, and NASA did a study of him and his twin to compare what happened to him after his flight. They found changes involving gene expression, immune response, bone metabolism, body mass, and cardiovascular function, though many returned toward baseline after he came home.
The main thing used to offset these problems is exercise.
ISS astronauts typically use a treadmill with harnesses, a stationary cycle, and a resistive exercise device that mimics weightlifting. This helps a lot. Modern crews return in far better condition than early long-duration crews did. Diet, vitamin D, medication, hydration monitoring, and medical imaging also help.
However, exercise is an imperfect substitute. It takes time, requires bulky equipment, stresses joints in unnatural ways, and does not reproduce gravity’s continuous, whole-body effects. It also does nothing to solve the fluid shift to the head.
The ultimate solution would be to try and replicate gravity.
In many science fiction movies and TV shows, artificial gravity is used as a plot device because filming weightlessness would be challenging and expensive. Many times it isn’t even explained, and people walk around on the decks of spaceships like they were on the surface of a planet.
In reality, the only solution to artificial gravity is rotation. There is no known practical machine that can generate gravity like a planet. But a rotating structure can create an outward apparent force. Stand inside the rim of a spinning station, and the floor pushes against your feet. To you, that feels much like weight.
The basic equation for creating artificial gravity is angular velocity squared × radius.
That means a station can get Earth-like gravity by spinning fast, by being very large, or by some combination of both.
There have been some movies that have depicted such space stations. There was a rotating space station in the movie 2001: A Space Odyssey and in the series For All Mankind.
These are usually depicted as large rotating wheels with spokes and a central docking hub.
However, there is a problem with this. A rotating station is not exactly the same as standing on Earth. When you move your head, throw an object, pour water, climb a ladder, or walk inward toward the hub, you experience Coriolis effects.
These make moving objects appear to curve from the perspective of people inside the station.
At low rotation rates, this is manageable. At high rotation rates, it can be nauseating.
The rule of thumb often cited in artificial gravity design is that around 1 to 2 revolutions per minute would be comfortable for almost everyone, while 3 to 4 rpm may be tolerable after adaptation, and higher rates become increasingly unpleasant.
The exact limit is debated because we have never actually built such a space station. But the lower the rpm, the larger the station must be.
To be able to support Earth-like gravity at only 1 rpm, you’d need a rotating space station with a radius of about 895 meters or a little more than a half a mile. That’s the radius. Double it for the diameter.
At 2 rpm, which is also reasonable, you’d need a radius of only 224 meters. At 4 rpm, which might require some adjustment, it would need a radius of about 56 meters.
Of course, it might not be necessary to experience the full gravity of Earth. If you wanted to simulate the moon’s gravity at 1 rpm, it would require a station with a radius of 148 meters (485 feet).
If you are willing to spin at 4 rpm, you’d need a radius of a reasonable 9.2 meters or about 30 feet.
This really isn’t a question of physics. It is more a matter of engineering and how you could actually build such a thing in orbit. The first one would be extremely difficult to build and would most likely be very expensive.
A rotating space station is currently possible but difficult, and it might be more plausible if we can reduce the cost of transporting cargo to orbit even further.
That hasn’t stopped people from thinking even bigger. There have been proposals for some truly enormous space stations that use rotational motion to create artificial gravity.
The Stanford torus is a more ambitious version of the wheel: a large donut-shaped habitat, usually imagined as a space colony rather than a small station. People live on the inner surface of the torus, with the “ground” curving up in the distance.
Its major advantage is livability. A torus can provide a large continuous landscape, neighborhoods, agriculture, and a more Earth-like environment. Its large radius allows slower rotation.
A Stanford torus would start at about 1 rpm and go down from there if it were even larger.
However, a Stanford torus would simply have people living on the rim of a wheel. Something that would radically expand the amount of living space people could have is an O’Neill cylinder.
An O’Neill cylinder, as the name would suggest, is a gigantic rotating cylinder with its entire interior available for use. Princeton physicist Gerard K. O’Neill proposed enormous counter-rotating cylinders, with people living on the inside surface. A cylinder could provide a vast habitable area. The classic concept features alternating strips of land and windows, with mirrors reflecting sunlight into the interior.
The big advantage is scale. A cylinder can, in theory, support cities, farmland, and industry. It also has better land-use geometry than a wheel does because its inner surface can be very large.
O’Neill cylinders have been shown in the movie Interstellar and in the TV show The Expanse. An alien O’Neill cylinder also plays a central role in Arthur C. Clarke’s book Rendezvous with Rama. There is supposedly a movie in the works that is to be directed by Denis Villeneuve, but production hasn’t started yet.
The theoretical length of an O’Neill cylinder could be miles long, although we have no clue how to build such a thing today.
However, this theoretical idea has been taken to an even higher level. A Dyson sphere is a proposed megastructure that would surround a star and capture some or all of its energy output.
The usual popular image is a solid shell around a star, but physicist Freeman Dyson did not originally propose a rigid sphere. His more plausible idea was a vast swarm of orbiting solar collectors, habitats, or satellites surrounding a star.
The best fictional representation of Dyson’s idea was in Larry Niven’s 1970 novel Ringworld. It is an enormous artificial ring built around a star, with its inner surface serving as habitable land.
Unlike a rotating space station, it is not a small wheel in orbit. It is more like a slice of a Dyson sphere: a band millions of miles across that completely encircles a star at roughly the orbit of the Earth. The ring rotates to create artificial gravity through centrifugal force, while the star provides light and heat.
Even with such a gigantic ring, it would be required to rotate once every nine and a half days. It would have to move more than 38 times faster than Earth’s orbital speed around the Sun to produce the same gravity.
We have yet to build a single artificial gravity system for humans in space, so all of these ideas, especially ones as far-out as O’Neil Cylinders, aren’t even in the planning stages.
But the problems of long-term weightlessness aren’t going away. In the near term, the solution will likely be more exercise and additional mitigation efforts, but that won’t solve the problem for extremely long missions.
In the long run, rotating space stations are the cleanest conceptual solution because they attack the root cause: the absence of gravity. Maybe, someday in the future, we’ll have people in space who won’t have to spend months floating around.