Artificial Gravity

Gravity. It is what keeps us glued to the surface of the Earth and gives us a feeling of up and down. When we venture into space and orbit the Earth, or some other world, the force of

gravity

is canceled out precisely by the centrifugal force. The sudden and marked absence of forces acting on the body is known as weightlessness. Your body has literally entered a state of perpetual, stomach-churning free fall. Most humans can adapt to this nausea after a few unpleasant hours, but many biological functions in our body are programmed for

gravity

thanks to billions of years of evolution here on Earth.

The muscles used for standing posture and skeletal support are no longer used and therefore atrophy and lose up to 20% of their mass after a week. Astronauts on the International Space Station need to exercise for at least two hours every day to combat this muscle loss. But without gravity our bones also wither and astronauts lose 1 percent of their bone mass each month. The breakdown of bones within our body saturates our cells with calcium, leading to dangerous soft tissue calcification. If we want to live in space for years or decades without serious medical interventions or genetic modifications, some form of

artificial

gravity will be necessary.

In science fiction,

artificial

gravity is ubiquitous, mainly to save money on special effects. The explanation is usually something like gravity coating, a completely fictitious technology. But how could we really do this? The easiest way to create artificial gravity in the real world is to simply accelerate in a straight line. If you were inside a spaceship that was not only moving but accelerating at 1 g, your inertia, that is, your resistance to motion, would cause you to be pushed against the back wall with the same force that the Earth pushes you toward. below. now while you watch this video.

In fact, Einstein postulated that the effect is so similar that there is no experiment that can be done aboard that spacecraft to distinguish between being in a gravitational field and simply accelerating. In other words, inertial mass and gravitational mass are equal, something known as the equivalence principle in general relativity. This sounds promising, but accelerating at 1 g for months, years, or decades would require engines with a sustainable thrust far greater than that of any modern space vehicle. The N Star ion drive is probably the closest example we have ever built of such an engine. NASA's Dawn spacecraft that visited Vesta and Ceres used three xenon ion thruster engines to maintain a record constant acceleration of just under 10 millionths of a G.

It was an impressive technological feat, but there is a long way to go to provide artificial gravity useful. So is there any other way to generate artificial gravity? The only other proven physics that can do this is rotation. When a car turns a corner, you feel like you're being pushed to the opposite side - that's centrifugal force. Another example is a fighter pilot making a sharp turn causing a centrifugal force several times greater than Earth's gravity, or several G's, so strong that it can cause a human to pass out. These forces are what physicists call fictitious.

As in the case of linear acceleration, they are just products of our inertia. They are not fundamental like electromagnetism, for example. The matter in your body will not change speed unless acted on by a force, so if the plane around you moves to one side, you will not do so due to inertia, so you will crash into the wall. But what if the wall wasn't a wall, and what if it were the floor? Take a giant merry-go-round and spin it fast enough and the occupants will feel pushed against the interior wall. If you did this in orbit, where no other net forces act, that centrifugal force on the wall would mimic gravity.

You could stand on the wall and feel the same downward force, but earthlings enjoy it. The idea of ​​a rotating habitat in space is possibly the most plausible way humanity could mimic gravity. It's also an old idea, played with in fictional tales like Rendezvous with Rama, Babylon 5, Interstellar, and The Expanse. There are quite a few different concepts proposed to achieve this with real-world physics, but today we will focus on two fundamental concepts that have been the most influential: the O'Neill cylinder and the Stanford torus. We'll start with a cylinder concept, proposed by Princeton physicist Gerard O'Neill.

In his 1976 book, The High Frontier Human Colonies in Space, O'Neill laid out a strategy for space colonization through asteroid mining. and moon, which included the construction of a rotating cylinder five miles in diameter with a length of up to 20 miles. The inner wall of the cylinder would serve as a floor for the inhabitants, providing up to 300 square miles of habitable area; This is approximately the same area as the five boroughs of New York. O'Neill was not afraid to dream big when describing a habitat for millions of people with a total mass of several billion tons. Today, launching it into low-Earth orbit costs about $20,000 per kilo, meaning the dry mass of the cylinder would cost about $100 trillion in launch costs alone.

That's about a thousand times the world's current GDP. A more realistic plan would be to build most of the habitat in space through lunar or asteroid mining facilities, which of course requires those industries to exist first. Still, a space construction of this scale would be so expensive that probably only the wealthiest members of society could afford to live above the clouds. You'll notice some things that are quite different on board. Tilt your head up and you can see the curvature of the cylinder and even see your companions hanging upside down above you. Air pressure and gravity could be reduced compared to those on Earth, to save the cost of spinning the cylinder and the strength of the wall material.

And the artificial gravity generated by the rotation would depend on your altitude, as you ascend towards the rotational slanders and the gravity would decrease until eventually reaching zero, perhaps giving you the opportunity to enjoy low-gravity sports near the axis. The Stanford Torus was the product of a NASA-sponsored summer workshop held at Stanford University in 1975 around the same time that O'Neill published his book on the cylinder concept; in fact, O'Neil was also the technical director of the Stanford study. "This is the concept that emerged from the work of these teams of scientists and engineers. They believe that the huge space colony could be built before the year 2000." The toroid is again a rotating structure to provide the artificial gravity necessary for comfortable living.

Unlike the cylinder, only about half of the inner wall is now usable earth. This could be seen as an advantage, as you now have a potentially natural sky, rather than what is imposed by the hallucinatory landscape of the cylinder. The diameter of the toroid is not very different from that of the cylinder and approximately one kilometer with a diameter of the inner tube of approximately one hundred meters. This results in a living area about one-third that of Hell's Kitchen, certainly much smaller than that of the cylinder. But the smaller design saves materials and has about 100 times less mass.

Compared to the O'Neill cylinder, this would decrease the total mold by an order of magnitude, but would increase the cost per square foot by about the same factor. Living aboard, one can enjoy jokingly taking a couple of laps around the three-mile edge or gazing across the warped horizon at one's neighbors on the torus. When you need supplies, you can venture down the toroidal radii to a central shaft that experiences much weaker centrifugal forces, allowing for low-gravity manufacturing or easy docking with supply ships. When we look back at these designs, they are sadly still as fantastic today as they were during their inception in the 1970s.

Although we have access to better materials and reduced releases, the sheer size of these structures means they are far beyond our financial capabilities. , or even our collective will for the foreseeable future. So why are both structures so large? Could we build a much smaller rotating structure today, perhaps enough for a few astronauts, just to demonstrate a practical example of artificial gravity? Leaving aside the question of shape, cylinder or torus, an architect has two basic dials he can control when designing a centrifugal artificial gravity structure: the speed of rotation, omega, and the radius of rotation, R. Clearly, to reduce costs. , we would like to use the smallest possible R, since the surface area and therefore the cost should scale approximately linearly with this term.

As we reduce R, we need to spin the habitat faster to recreate the required gravity. This is because centripetal acceleration is equal to R times omega squared, so making the habitat 4 times smaller in diameter requires a spin rate twice as fast. Even ignoring the effect on human occupants, fast turning speeds are generally undesirable, as they make docking with the station more challenging and require more energy to enter and exit the rotating frame. This raises the possibility of humans living in reduced gravity, perhaps equal to that of Mars or even the Moon. Research by Harris et al has shown that accelerations below the Moon, about 1/6 G, make it difficult for humans to get a sense of up and down and stay balanced, so this is likely a good limit. lower.

On the other hand, anything above 1G is generally uncomfortable for humans and 4G leads to a total blackout, for example, so putting them together gives us the following comfort zone for possible combinations of R and omega. Although a small rotation radius reduces costs, it also decreases usable living space and even headroom on board. Clearly, radii smaller than typical human heights would be a poor choice, causing inhabitants to constantly crouch and experience no downward forces on their heads, but rather strong forces on their feet. In other words, they would experience a tidal force of 1G. There isn't much research on how much tidal acceleration humans find comfortable, but a reasonable assumption might be an order of magnitude less than centripetal tidal acceleration, which would force us to be at least 17 meters for the average human height.

A 17 meter radius habitat rotating at very fast speeds would mimic Earth-like gravity if the occupants were completely stationary inside, but any movement inside would feel an additional force that we don't notice here on Earth: the Coriolis effect. The artificial gravity of a centrifuge is not the same as linear acceleration, remember that Einstein argued that linear acceleration is indistinguishable from gravitational acceleration but this is not true for a centrifuge. There is no equivalence principle here, and the occupants of an O'Neill cylinder, or a Stanford torus, could design experiments to do so. The acceleration felt by a crew is equal to the following, where the first term is the centripetal acceleration of the crew, the second term is the Coriolis acceleration, and the last term is the linear acceleration.

If the station does not accelerate linearly, then we only have those first two terms. If you are perfectly stationary, the r point, which is the velocity, reaches zero and therefore you experience a perfect reproduction of gravity. But any movement (r point) in a direction perpendicular to the rotation vector, described here by omega, will cause this second term to move away from zero and will therefore feel an acceleration normal to both ingredient vectors. That's the Coriolis effect. Like centrifugal force, this is what physicists would call a fictitious force: purely a product of living in a rotating reference frame.

Imagine a cannonball floating around the center of a rotating O'Neill cylinder. We give it a light push so that it begins to descend towards the edge of the cylinder. If we were watching this happen from the outside of the cylinder, through a window for example, it would appear to travel in a straight line, which of course makes a lot of sense. But now consider the same motion from the perspective of a person on the inner surface of the cylinder. From his perspective, the cannonball does not travel in a straight line but in curves. However, that is purely a result of its rotating reference frame, and that curvature is caused by the Coriolis effect.

This effect even occurs here on Earth,affecting the path of winds in its rotating atmosphere or affecting the path of a ball thrown on a merry-go-round. In reality, the Coriolis effect has two different components in the inhabitants of a rotating cylinder or torus. The first is what feels like an apparent change in descending surface gravity, the vertical Coriolis if you will, and the second is a tilting effect and these deserve separate discussions. The direction of the Coriolis acceleration depends on the direction in which it is moving and the direction of the axis of rotation, which is presumably fixed.

More specifically, the Coriolis acceleration is equal to the cross product of its velocity vector and the habitat's rotation vector, meaning that it always pushes in a direction perpendicular to both vectors. So what does this mean? Let us consider that within the habitat there are three possible directions of movement. If you move in the same direction as the axis of rotation, you will not feel any Coriolis effect. Therefore, traveling hundreds of meters from one end of an O'Neill cylinder to the other is perfectly safe. For the case of the Stanford toroid, this means that walking to either side of the torus corridor is true that a smaller distance has zero Coriolis.

This leaves us with two other directions that we must experience Coriolis: one radial and one tangential. The tangential direction is probably the least concerning and leads to an apparent change in downward gravity. For example, if you walk along the O'Neill cylinder in a progressive direction, you effectively increase your overall rotational speed, pushing you down harder onto the ground and making you feel heavier. Walking retrograde does the opposite and makes you lose weight. In the case of the Stanford torus, this change in gravity occurs when traveling along the three-mile circumference. Now, if we want to reduce the size of these designs, the question is how much change in surface gravity is comfortable.

The research by Neste et al. in 2014 on humans show that people are not even able to perceive vertical changes in acceleration less than about 5% of surface gravity. The maximum tolerance is less studied, but certainly for a 1G system a 25% increase in gravity would start to become uncomfortable, based on centrifugation experiments performed by Cohen et al. If we take this as the maximum threshold, it takes us to the next modified comfort zone by choosing R and omega. Having analyzed the movement along the axis of rotation and the tangential direction, let us finally turn to the question of radial movement.

In both the cylinder and the torus, or even in any artificial gravity centrifugal system, this amounts to the act of jumping up and down or, equivalently, ascending and descending to different platforms. During these actions there is a slight change in the centripetal force itself, but by far the most destabilizing aspect for human occupants would be a tilt effect caused by Coriolis acceleration. Dr. Theodore Hall developed a nice way to visualize this by imagining dropping or throwing a ball vertically instead of landing at your feet, since it would curve to one side. In extreme cases, one can even throw a ball behind you and catch it in front while spinning due to Coriolis.

This rollover acceleration has the effect of creating a perceived tilt in the ground during vertical movement. A strong enough tip could cause you to trip when standing or climbing stairs. An easy way to fix this is to have a single platform with no ladders or ladders. Of course, efficient use of space might require the inclusion of multiple platforms; In this case, elevators or carefully designed stairs for going up and down could be designed for this environment. Still, astronauts would probably need to be trained to stand up slowly when getting up. Perceived grades of a grade of 8% or greater would exceed the maximum grade of most ramps here on Earth.

If astronauts lift at no more than 1 foot per second, this would place the next limit on our design specifications for a habitat that further limits our allowable comfort zone. Coriolis is not just a problem climbing stairs or standing, it affects balance even when turning your head from side to side. When you do this, the inertia of the vestibular fluid within the inner ear causes a slight delay between the movement of your head and the fluid it contains. The fluid is then pushed back into place by pressure, causing the sensory hair cells inside the ear to move.

Now, if we do this aboard a rotating spacecraft, moving the head in one direction would generate a greater downward force compared to the other. That would slightly change the distribution of fluid within the semicircular canals. This differential severity can confuse our vestibular system and create a feeling of nausea and discomfort. The fluid itself also undergoes a small vertical motion and would therefore be pushed to one side by the Coriolis effect. This raises the question of how quickly we can deal with a turnover before this becomes a problem and the occupants experience nausea. In most rotating habitat proposals, it is these Coriolis effects on the vestibular system – sometimes called canal disease – that most strongly limit engineering design.

These studies often cite human experiments performed here on Earth using slowly rotating rooms. Various studies agree that at rotation speeds less than one or two revolutions per minute, subjects can complete complex tasks for prolonged periods. But above this spin speed, it takes humans days to adapt and beyond six rpm, people struggle to adapt. These experiments guided the design of the Stanford torus, which aims to rotate at one revolution per minute to prevent canal disease. But to recreate 1G of centrifugal force at just one rpm requires a very large radius, almost a kilometer. Using this constraint of one rpm or slower, we can see that the comfort zone decreases considerably and it is now clear why the previously proposed rotational habitats must be so vast and therefore expensive.

But it's important to remember that these experiments do not recreate the environment of a space station because they cannot eliminate Earth's gravity. Typically, the rotation chamber is oriented so that the axis of rotation is aligned with the lines of the Earth's gravitational field, the rotation speeds are slow enough that the centripetal force is small and the sensation of descent It comes from the Earth's gravity, not from the centrifuge itself. This change in orientation, chosen to accommodate Earth's gravity, means that the Coriolis acceleration acts in directions perpendicular to that experienced by the crew of a rotating spacecraft.

So imagine walking on the floor or moving your head from side to side, both lateral movements. In an O'Neill cylinder, Coriolis accelerations act up and down during these activities, slightly affecting the apparent force of gravity. Unlike the rotation chambers used on Earth, those same movements cause a Coriolis acceleration that attempts to knock you to the side. A study by Graybiel et al. Using this exact setting, it is found that at 10 rpm not even experienced test drivers are able to adapt. At this rotational speed, walking across the room causes a Coriolis acceleration of 0.3 G, making you feel like you're walking down a 30 percent slope.

What's more, if the subjects in the rotation chamber jump up and down or stand up abruptly, they will not feel any Coriolis effect because it is oriented with the axis of rotation of the centrifuge. This again contrasts with the case of a rotational habitat in space. In conclusion, experiments with Earth's rotation chambers may be simulating a harsher environment than that experienced by our future astronauts, or at least a very different environment. Simple lateral movements of the head and body should not cause tilting forces, but this is generally what happens in these types of experiments. This is important because when comparing lateral and vertical accelerations Nestia et al.

He discovered that humans have a greater sensitivity to horizontal accelerations than to vertical ones. Therefore, these experiments in slow-rotating rooms are certainly worthwhile, but an upper safety limit of 6 rpm may be too conservative. This is very important because if humans can tolerate Coriolis accelerations at rotational speeds of 10 rpm or more, then Coriolis forces no longer become a design limitation. factor and much smaller feasible artificial gravity systems could be conceived. For these reasons, let's be optimistic and assume that humans can cope with 6 rpm environments, giving us our final comfort zone as shown here. Let us finally turn to the question of shape and ask whether, apart from the cylinder or the torus, there might be opportunities to further optimize the comfort levels of our astronauts with small rotation radii.

Remember that when astronauts move along the axis of rotation they do not experience any Coriolis effect, so one might be tempted to design a train-like carriage that hangs from a strap connected to a counterweight. As the carriage rotates, the occupants are pushed to the ground in their almost one-dimensional habitat that simulates gravity. This design aims to provide the least exposure to Coriolis accelerations. But Coriolis isn't the only thing we need to worry about when designing a secure environment. If this train were too long and therefore massive, the axis of rotation would no longer be the main axis of rotation, but would become the intermediate axis.

Rotation around such an axis is unstable and subject to violent falls, as demonstrated aboard the ISS with this T-handle experiment. During this falling motion, occupants would be thrown around the habitat in a clearly unacceptable manner. Drops can also occur on O'Neill cylinders. A cylinder has two main axes of rotation: one along the cylinder shown here in blue and one perpendicular to the ends shown here in red. O'Neill rotates his cylinder around the blue axis, but that is actually the smaller principal axis of rotation. Any slight disturbance in the distribution of mass within the cylinder, such as the movement of people inside, could cause a fall due to the imposed rotational instability.

One could imagine pumping water around the cylinder to redistribute the mass accordingly using thrusters or reaction wheels to correct any disturbances or even using a second cylinder next to it that rotates in the opposite direction and that is actually the idea O'Neill originally envisioned . Finally, it is worth noting that for the Stanford torus, things are much simpler: this rotation actually occurs now around the primary axis and therefore the structure should be much more stable. Could we see a demonstration of centrifugal artificial gravity anytime soon? The ISS offers perhaps our best hope of being a permanent station for space experiments.

In 2011, Mark Holdeman and Edward Henderson proposed a centrifuge demonstration on the ISS as part of their broader plan for a space station called Nautilus of 100 million dollars. . Unfortunately, this concept did not go beyond the initial drawings. A few years earlier, in 2005, Kirk Sorensen argued that the simplest path would be a habitat attached to a more massive counterweight by a belt that rotated fast enough to recreate artificial gravity on the lower-mass object. Sorensen noted that by making the strap retractable, the gravity on board could be controlled at will, much like how an ice skater can extend his legs to control his turning speed.

The habitat itself could perhaps be something like NASA's transhab concept with multiple decks and higher decks with weaker artificial gravity. It's about where things stand with no concrete plans for a space artificial gravity experiment on the horizon. Long-term human presence in space environments will at some point need to solve this problem through engineering or biological solutions or perhaps a combination of both. Let me know below what type of design you are most excited about for simulating gravity and if you think this should be a priority for NASA and other space agencies in the near future. Thank you for taking the time to watch this slightly different video than ours here on Cool Worlds Channel.

Let me know if you like these styles of videos and as always, thanks for watching, stay tuned and curious.