Artificial Gravity in Outer Space: Centrifuges and Beyond

One of the most distinct characteristics of life in outer space is the lack of gravity pulling you and everything else down to the floor. Objects and people inside any structure launched into orbit will float around and experience zero gravity. This is not a serious problem in spaceflight today, because missions to date have been either short jaunts in capsules or spaceplanes where the lack of gravity doesn’t make much difference or extended research missions on space stations where studying the effects of zero gravity is one of the mission objectives.

If we are to live and work in space in any serious way, however, that blissful attitude of indifference will need to change. Zero gravity has deleterious effects on human health that are very well documented, the most serious of which is a weakening of the bones in the absence of gravitational stresses. Astronauts upon return from months-long stays in zero gravity find it hard to walk in Earth gravity, even after doing bone-strengthening exercises in space. The bones weakening to adapt to zero gravity probably wouldn’t pose much of a problem if you spent the rest of your life in a zero-g environment, but people would surely want to retain the ability to visit planets and other places that had gravity.

In addition, pumping fluids through pipes is much harder if there is no gravity directing the flow of liquids; this means you cannot take a shower or draw a bath in zero gravity, a serious lifestyle drawback; this would also pose serious challenges to many types of industrial activities. It also means that dust, crumbs, and fluids will float around the habitat instead of gathering on the floor, making the environment much dirtier. Yet another drawback is the fact that carbon dioxide bubbles build up around people when they exhale, necessitating the use of fans to mix the air on board. Even aesthetics are negatively impacted, with long hair floating around the head, posing a hazard and being very hard to style in a way that is attractive. Clearly some method of generating gravity would greatly aid the colonization of outer space, as distinct from planetary surfaces.

Centrifuges for artificial Gravity

Fortunately we have a solution ready to go that has long been very famous in the annals of science fiction: rotating centrifuges. Since real gravity is unavailable in free fall, it can be simulated by using centrifugal force instead. If a ship, station, or habitat rotate this motion of the structure relative to the objects (and people) inside; the people and objects will feel themselves moving toward the (much larger) structure, thus simulating gravity. While a structure of any shape can generate gravity using this method, a spacecraft shaped like a torus (similar to a doughnut) or a cylinder would provide an even amount of gravity across the whole floor of the habitat.

This is why the classic science-fictional space station is the “rotating wheel”; the centrifugal force would pull objects and people toward the outermost inner surface of the wheel, which would serve as the floor. A cylinder is the other basic shape that would work well with centrifugally-generated artificial gravity; in this case the outer walls would be the long curved surface of the cylinder, which would serve as the floor. While more complex shapes, such as hexagons and spheres, are possible, we will stick with more straightforward examples in this post; also, due to the advantages of having even gravity across the floor of the spacecraft the simpler shapes will likely be more common in the future of space settlement and colonization anyway.

One of the advantages of centrifuges is that they can be spun up and down as needed; thrust can be applied slowing the rotation and gravity all the way down to zero if needed or desired, as was done in the 2014 film Interstellar in order for the crew of the Endurance (a spaceship of the rotating wheel variety) to get a better view of the wormhole near Saturn. This flexibility may be used to create almost any amount of gravity on a permanent basis, from a tiny fraction of Earth gravity to many multiples of Earth gravity. This is a formidable advantage of open space over planetary surfaces; on planetary surfaces the amount of gravity is fixed, but in open space if you want a habitat with a different amount of gravity you simply construct a new spacecraft.

One disadvantage of centrifuges is that they need to be very large compared to, say, a capsule in order to work. The ever-helpful Atomic Rockets has an entire page on the subject of realistic artificial gravity. The Coriolis effect curves the path of objects inside a rotating object; Earth’s rotation, for example, is what causes storms to rotate and travel along curved paths instead of straight lines. Coriolis forces act on the path of even the smallest moving objects, but since Earth is very large and spins at a rate of once a day you don’t notice it in everyday life. A small centrifuge, however, is small and would have to spin on the order of multiple rotations per minute to generate Earth-level gravity; at this level of rotation Coriolis effects would be very noticeable. Balls thrown in such a habitat would have their ballistic trajectories severely curved by the rotation; this would also apply to liquids and fluids being poured, as seen in the TV show The Expanse. The liquids curved include the fluids in the inner ear, producing nausea after a certain point; the different amount of force being exerted on different parts of the body would also be severely disorienting.

This is important since all other things being equal the slower the spin rate needs to be the larger the craft needs to be to generate Earth gravity. Fortunately for our speculation here research has been done on the subject. Older studies indicated that people become sick at 3 rotations per minute (rpm), and become incapacitated at 6 rpm; however, more recent research shows adaptation can occur with no feelings of nausea with some training and incrementally increasing the rotation, perhaps up to 10 rpm. All studies done to date agree that anything under 2 rpm is “comfortable”.

SpinCalc is a helpful way to calculate the relationship between rpm, gravity, and the radius of the centrifuge. Creating 1g at 2 rpm, which would provide Earth gravity at the maximum comfortable spin rate, would require a centrifuge 1500 feet in diameter. This is a very large spacecraft by today’s standards, though it would be somewhat smaller than even the smallest classical space habitat designs. Clearly any centrifuge-shaped spacecraft that has comfortable artificial gravity will be large.

If one compromises on the spin rate smaller designs are possible, of course. Going up to the maximum 10 rpm that is possible with training, 1g may be achieved with a centrifuge as small as 60 feet in diameter, less than the International Space Station! Even better, if you are willing to compromise the gravity down to Martian level (0.38g), the centrifuge can be made as small as 20 feet in diameter, as small or smaller than an average-sized house! A single-family homestead roaming in space could easily achieve this level of artificial gravity, although the Coriolis curving of e.g. the pouring of milk in their morning coffee would be extreme. Nevertheless, this exercise does prove that large centrifuges are not necessarily required, lowering the barriers to having a habitat to call your own in outer space.

Even small amounts of gravity may be desirable to avoid situations like liquids and crumbs floating indefinitely in the air; perhaps 0.01g may be suitable for this purpose. People would be able to float along just fine in such gravity, so a centrifuge shape may not even really be needed for such a craft. Nevertheless a centrifuge generating this amount of gravity at 2 rpm could be made as small as 14 feet in diameter. A craft 1500 feet in diameter would only need to rotate at 0.14 rpm to generate 0.01g.

Batons: the Shape for small Spacecraft?

Very small centrifuges would generate normal gravity at the feet but much less gravity at the head, so they probably wouldn’t be very practical. For craft much smaller than a small space habitat another shape, the realistic (in the near future anyway) alternative to centrifuges, would be best; this is a baton-shaped craft, with the “baton” connecting two habitation modules, or one module and one counterweight at the other end. The amount of spin required would be the same as for a centrifuge, except most of the 1g “rim” would be empty space instead of covered by a structure. This is important since a baton-shaped craft would require far less material to construct than a wheel- or cylinder-shaped craft, thus would be cheaper and in any case much better suited for a family or small group.

Another advantage is that, for example, an individual or family that could afford a mansion-sized centrifuge, 60 feet in diameter and having to rotate at 10 rpm, could use the same amount of resources to purchase or construct a baton craft with a mansion-sized habitation module with a mile-wide “baton”, enabling the same creature comforts to be enjoyed at a far more comfortable 1 rpm. This exercise demonstrates that long-term adaptation to zero gravity, ultra-low gravity, or uncomfortable rotation rates would not be required for even single-family spaceships, opening up the possibility that it would become economically feasible for individuals, households, families, and small groups to roam space autonomously while still retaining the advantages of artificial gravity.

Moving up to larger installations that may be beyond the range of individuals or households, and in any case have ample room for large populations, a centrifuge a mile in diameter could generate 1g with about 1 rpm. A classic O’Neill cylinder is perhaps the next level up at 5 miles in diameter: 1g could be generated with 0.47 rpm. To get the rotation rate down to 0.1 rpm would take a centrifuge 110 miles in diameter. The McKendree cylinder, a more recent version of the O’Neill cylinder design based on carbon nanotubes rather than steel, is 580 miles in diameter; producing 1g would require only 0.043 rpm, which is very comfortable. Aside from paths being deflected and the horizon curving upward in the distance you probably wouldn’t even notice much. The theoretical limit of centrifuges’ size using carbon nanotubes is 1200 miles in diameter, which implies 0.03 rpm or so to generate 1g. For reference, generating 1g with one rotation per day, producing a Coriolis effect that is completely unnoticeable, would take a rotating structure (almost surely a torus at this scale) 2.289 million miles in diameter; this is one and the same with the “Banks orbital” concept, featured in the Orion’s Arm worldbuilding project and in the Culture novels.

With all this in mind, it seems likely that when we become a spacefaring civilization large spacecraft will be shaped like cylinders or toruses, while small spacecraft will be modules attached to batons twirling through space. In this way almost every size of spacecraft carrying people may come with artificial gravity, from the smallest homestead to the largest space-borne city.

Gravity through Thrust: constant Acceleration

Aside from centrifuges, toruses, and batons, there is yet another way to generate artificial gravity with near-future technology: constantly generating 1g of acceleration. This is colloquially called “g-force”, the force you experience when a vehicle you’re in accelerates or decelerates; sufficient acceleration can produce 1g of force pulling you down toward the floor, assuming the ceiling is in the direction of the thrust. The propulsion systems we currently use are unable to achieve 1g of acceleration for more than a few minutes, but nuclear pulse propulsion, which can be built using existing technology and will be available in the near future, is capable of accelerating at 1g all the way up to 10% of light speed.

This would be more than sufficient for, say, a journey between Earth and Mars. The distance between the two planets is never any greater than 22 light-minutes, which means 1g of acceleration would need to attain 0.065% of light speed halfway through the trip before decelerating, easily within nuclear pulse propulsion’s power to achieve. A spaceship that used this method of propulsion wouldn’t need to fool around with centrifuges, batons, or rotating wheels because the 1g of gravitational force would be supplied by the propulsion system! This means the ship may be almost any shape and still provide gravity, so long as the floors are perpendicular to the direction of thrust.

A journey between Earth and Neptune, over 4 light-hours, would require acceleration to 2.55% of light speed, still well within nuclear pulse propulsion’s range. For journeys that demand a more constant speed for whatever reason, alternating acceleration and deceleration may be used (with the spacecraft reversing its orientation accordingly).

For installations intended to stay in or near one part of space, like space stations or habitats, generating artificial gravity with this method would be extremely wasteful, as it would be far cheaper and easier to just design it as a centrifuge, rotating wheel, or baton. For spaceships intended to travel between planets or other sites in space at high speed, however, it might be the preferred way to generate artificial gravity. After all, in order to get across the solar system within days, accelerations on the order of 1g would be needed anyway, and it’s simpler to just accelerate at 1g than it is to coordinate thrust and rotation.

For lower-speed spaceships the rotational method of generating artificial gravity would be best; a constant acceleration of 0.01g, which would probably not even be enough to notice, can get a spaceship from Earth to Neptune in 6 months. Earth to Mars would take 2 months at such an acceleration. Thus it seems likely that unless propulsion becomes so cheap the added cost wouldn’t matter in the least (which might actually happen, but probably not for some time), the accelerational method of generating artificial gravity will be confined to the clipper ships and express trains of the solar system.

A final method of generating artificial gravity is using black holes. Although this is real rather than simulated gravity, the far higher density of planetary- or smaller-mass black holes enables Earth-like gravity to be attained for spherical habitats far smaller than planets; the atmosphere would even tend to be pulled down toward the outer surface of the sphere, just like a miniature planet! This is far beyond current or near-future technology, but theoretically could work. This option doesn’t appear much in science fiction, but Orion’s Arm is a prominent exception to this tendency.

Conclusion

Generating artificial gravity would enable far easier, more pleasant, and more comfortable settlement of the solar system, opening up not only other planets but the full vastness of outer space itself for colonization and settlement. Methods exist that enable any size craft, from the largest planet-size habitat to the smallest one-man habitation module, enabling groups of any size or even individuals to make space their abode in Earth-like gravitational comfort.

Aside from the usual centrifuges and rotating wheels, taking advantage of g-forces by constantly accelerating at 1g is another more obscure method of generating artificial gravity that is actually very realistic. A somewhat more fantastical and far more obscure method is using a black hole to compress the volume and mass needed to generate 1g “naturally”.

Far from having to put up with zero gravity in cramped tin cans for long haul trips between planets, the spacefaring civilization of the near future will likely have artificial gravity as a common if not ubiquitous feature. Taking this into account, while using realistic hard-sci-fi-style methods instead of softer methods of generating artificial gravity, would make give great distinction to any futurological speculation or science-fictional setting that used it.

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