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podcast transcript
The human body is built for gravity. Removing them can cause bones to weaken, muscles to contract, fluids to shift, and even vision to change.
For astronauts who spend months in orbit, weightlessness is not just strange. That’s one of the biggest obstacles to living and working in space.
But there is a solution. It may be a movement problem, or in the future, the solution may be to create artificial gravity by rotating a spacecraft.
Learn more about dealing with weightlessness in this episode of Everything Everywhere Daily.
Let me start by pointing out that “zero gravity” is a general term for the conditions that astronauts experience in orbit, but it is technically incorrect. A better term is microgravity or zero gravity.
There is no gravity in orbit. In fact, astronauts aboard the International Space Station are still greatly affected by Earth’s gravity. At the altitude of the International Space Station, about 250 miles above Earth, gravity is still about 90 percent stronger than at the surface. When gravity disappears, the station will fly straight into space.
What’s actually happening is that the spacecraft and everything inside it are falling together around the Earth.
Orbit is basically a continuous fall. Imagine throwing a ball horizontally. It falls to the ground. The faster you throw it, the farther it will fall. If you throw it fast enough, as it falls, the curve of the Earth will fall down at the same speed. The object continues to fall but never touches the ground.
This is what it means to orbit something.
What an astronaut experiences is the same as what you experience when skydiving.
So “zero gravity” is a misnomer because gravity is still very much present. What is lacking is not gravity itself but the feeling of weight.
By all accounts, weightlessness is fun, at least at first. I can’t say I experienced it except for a few seconds of bungee jumping.
However, the longer you remain in a zero-gravity environment, the more problems can arise. The first problem many astronauts experience is space sickness.
Space sickness is the nausea, dizziness, headaches, and disorientation that many astronauts feel during their first hours or days in microgravity.
This happens because the brain receives conflicting signals. On Earth, the inner ear’s vestibular system uses gravity to determine balance and orientation.
In orbit, the gravitational signal disappears, but the eyes still report movement and direction. The brain must readjust to a world where there is no true “up” or “down.”
Symptoms include nausea, vomiting, cold sweats, loss of appetite, fatigue, and difficulty concentrating. It is similar to motion sickness, but is caused by weightlessness rather than by cars, boats, or airplanes. Most astronauts adapt within a few days, but they may experience similar readjustment problems when returning to Earth gravity.
Long-term weightlessness poses problems beyond nausea. This is a serious biological problem because the human body is built around constant mechanical loads.
Gravity tells our bones, muscles, blood vessels, balance organs, and even fluid distribution how to behave. When removed for months or years, the body adapts in ways that are useful in orbit but dangerous when returning to Earth.
In weightlessness, your body no longer needs to support itself. The legs, hips, and spine cease most of their normal activities. Bones no longer receive the same stress signals to maintain density.
The fluid no longer settles toward the lower body, so blood and cerebrospinal fluid move upward toward the head. The cardiovascular system, vestibular system, eyes, immune system and kidneys all respond to this new environment.
NASA summarizes the main effects as muscle loss, bone loss, increased body fluids, vision problems, increased risk of kidney stones, and cardiovascular disease.
In microgravity, astronauts can lose bone density, and NASA’s 2025 Risk Summary estimates a typical rate of loss of about 1% to 1.5% per month over a 4- to 6-month mission if not responded appropriately.
Astronaut Scott Kelly spent 340 days on the ISS from 2015 to 2016. After returning, he reported skin pain, rashes, flu-like symptoms, leg swelling, balance problems and other difficulties adjusting to Earth’s gravity.
He has a twin brother, and NASA conducted a study on him and his twin to compare what happened to him after the flight. They found changes associated with gene expression, immune response, bone metabolism, body mass, and cardiovascular function, although many returned to baseline after he returned home.
The most important thing used to offset these problems is exercise.
ISS astronauts typically use harnessed treadmills, stationary cycles, and resistance exercise devices that mimic weightlifting. This helps a lot. Modern crews come back in much better shape than early long-term crews. Diet, vitamin D, medications, monitoring fluid intake, and medical imaging may also be helpful.
But exercise is an imperfect substitute. It takes time, requires bulky equipment, stresses joints in unnatural ways, and doesn’t replicate the sustained full-body effects of gravity. It also does nothing to address fluid movement to the head.
The ultimate solution is to replicate gravity.
Many science fiction movies and TV shows use artificial gravity as a plot device because filming in zero gravity is difficult and expensive. Many times it isn’t even explained, and people move around on the deck of the spaceship as if they are on the surface of a planet.
In reality, the only solution to artificial gravity is rotation. There is no known practical machine that can produce planet-like gravity. However, rotating structures can generate apparent forces outward. If you stand inside the edge of the rotating station, your feet will touch the floor. To you it feels like a weight.
The basic equation for creating artificial gravity is: Angular velocity squared × radius.
This means the station could either rotate quickly, be very large, or a combination of the two to achieve Earth-like gravity.
There was a movie depicting such a space station. The movie 2001: A Space Odyssey and the series For All Mankind had rotating space stations.
These are usually depicted as large rotating wheels with spokes and a central docking hub.
But there’s a problem. A rotating station is not exactly the same as standing on Earth. You experience the Coriolis effect when you move your head, throw an object, pour water, climb a ladder, or walk inward toward the hub.
This makes moving objects appear curved from the perspective of people inside the station.
At low turnover this is manageable. High turnover can be nauseating.
An oft-cited rule of thumb in artificial gravity design is that about 1 to 2 revolutions per minute is comfortable for almost everyone, while after adaptation, 3 to 4 rpm is tolerable, with higher speeds becoming increasingly unpleasant.
Because we have never actually built such a space station, the exact limits are debated. However, the lower the rpm, the larger the station must be.
To support Earth-like gravity at just 1 rpm would require a rotating space station with a radius of about 895 meters, or just over half a mile. That’s the radius. Double the diameter.
At 2 rpm, which is also reasonable, a radius of only 224 meters is needed. At 4 rpm you’ll need a radius of about 56 meters, which may require some tweaking.
Of course, you may never need to experience the full force of Earth’s gravity. Simulating the moon’s gravity at 1 rpm requires a station with a radius of 148 meters (485 feet).
To rotate at 4 rpm you would need a reasonable radius of 9.2 meters, or about 30 feet.
This isn’t really a question of physics. It’s a question of engineering and how to actually build something like that in orbit. The first one would be very difficult and very expensive to make.
A rotating space station is currently possible, but difficult. This could become more plausible if the cost of transporting cargo to orbit could be further reduced.
That hasn’t stopped people from thinking bigger. There have been proposals for really huge space stations that would use rotational motion to create artificial gravity.
The Stanford torus is a more ambitious version of the wheel. A large donut-shaped habitat, it is generally imagined to be a space colony rather than a small station. People live on the inner surface of the torus, with the “ground” curving away.
The biggest advantage is habitability. Taurus could offer large, continuous landscapes, neighborhoods, agriculture, and an environment more similar to Earth. Larger radii result in slower rotation speeds.
The Stanford Taurus starts at about 1rpm and goes down from there for anything larger.
But the Stanford Taurus simply has people living on the rim of the wheel. It’s the O’Neill cylinder that can radically expand the amount of living space people can have.
As the name suggests, an O’Neill cylinder is a huge rotating cylinder whose entire interior is usable. Princeton physicist Gerard K. O’Neill proposed a giant counter-rotating cylinder with people living on its inner surface. Cylinders can provide vast habitable areas. The classic concept features alternating windows on the ground and mirrors that reflect sunlight inside.
The biggest advantage is scale. In theory, cylinders could support cities, farmland, and industry. They also have better land use geometries than wheels because the internal surfaces can be very large.
O’Neill cylinders have been featured in the movie Interstellar and the TV show The Expanse. The alien O’Neill Cylinder also plays a central role in Arthur C. Clarke’s book. meeting with llama. A film to be directed by Denis Villeneuve is believed to be in the works, but production has not yet begun.
The theoretical length of an O’Neill cylinder could be several miles, but today we have no idea how to make such a thing.
However, this theoretical idea has been developed to a much higher level. The Dyson Sphere is a massive structure proposed to surround a star and capture some or all of its energy output.
The generally popular image is a solid shell around a star, but physicist Freeman Dyson did not originally propose a solid sphere. His more plausible idea was a huge swarm of solar collectors, habitats, or satellites orbiting a star.
The novel that best expresses Dyson’s ideas is Larry Niven’s 1970 novel. Ring World. It is a huge artificial ring formed around a star, with its inner surface acting as habitable land.
Unlike a spinning space station, it’s not just a little wheel in orbit. It roughly resembles a slice of Dyson’s sphere, a belt millions of miles wide that completely surrounds a star in Earth’s orbit. The rings rotate to create artificial gravity through centrifugal force, and the stars provide light and heat.
Even with such a huge ring, it would have to rotate once every nine and a half days. To produce the same force of gravity, you would have to move more than 38 times faster than the speed of Earth’s orbit around the Sun.
We haven’t yet built a single artificial gravity system for humans in space, so all of these ideas, especially ones as far-flung as the O’Neill cylinder, aren’t even in the planning stages.
But the long-term weightlessness problem is not going away. In the short term, more exercises and additional mitigation efforts may be the solution, but for extremely long missions, this will not solve the problem.
A long-term rotating space station is the cleanest conceptual solution because it attacks the root cause: the absence of gravity. Maybe in the future we will have people in space without having to float for months.
