Spinning to make artificial gravity is a classic element of hard sci-fi. And it’s been imagined from ringworlds the size of the solar system, to asteroids, to tiny ships just a few meters wide. So I wanted to see if I could put their scale into perspective but also understand the dynamics between the different variables of spin gravity.
One of the smallest practical examples is the Discovery One from 2001: A Space Odyssey. At the front there is a sphere that has a centrifuge inside. That centrifuge rotates at about 5 RPM and creates 0.
17g—the same amount of gravity as the moon. There are 4 interdependent variables for spin gravity. And you can engineer you’re station by changing these variables.
The first two I've already mentioned. Angular Velocity or “spin rate” shown in RPM and centripetal acceleration or gravity level or shown in g’s. There is a maximum spin rate for human comfort, the maximum where people won't be dizzy and sick.
And 5 to 6 RPM has been suggested by studies as the maximum. But even that is still pretty fast, and it’s probably going to make some people sick, which is accurately depicted in the movie Interstellar. The ship Endurance rotates at 5.
5 RPM and poor Romley must have run out of Drmamine. (“Can we stop the spinning? ”) But it produces more gravity than the Discovery with nearly the same RPM, because it has a larger diameter.
When you increase the diameter, it slows down the spin rate needed to make the same amount of gravity. That is the third variable. Math uses radius, but I will be using diameter because it makes more sense for people.
Another way to increase the diameter is to use a tether. And that’s what the ship Hail Mary uses in Andy Weir's book Project Hail Mary. It flips the crew compartment 180 degrees, extends to 100 meters, and spins at 4.
2 RPM to create 1g. But that’s still pretty fast. 1 RPM has been suggested as a reasonable limit for the average person.
And that's what the Space Station V, also from 2001, rotates at. Just under actually. And it produces the same point .
17g as the Discovery at that speed. But . 17g is a little low for comfort and maybe human health.
So to get a reasonable level of gravity and RPM’s, we have to go even bigger. The Babylon 5 space station is 800 meters in diameter and also rotates at an even 1 RPM, but produces point . 45g because of its 800-meter diameter.
The OPAS Behemoth from the Expanse is 960 meters in diameter, and rotates a little slower at . 75 RPM, and produces . 3g.
In the Expanse . 3g is the standard for gravity in the outer planets and under thrust, but in real life there are no studies showing longer term low g health implications more than one year in zero g. So it’s not yet known what the minimum is.
Cooper station from Interstellar is never shown from the outside and its size and specs can only be speculated on from images in the film, and so it has been hypothesized to be 1. 6 kilometers in diameter, but if we make it just slightly larger, we have settings for an ideal cylinder: 1. 78 kilometers in diameter with 1 RPM and 1g of gravity.
Rama from Rendezvous with Rama by Arthur C. Clarke is 18 kilometers in diameter on the outside (it’s 16 on the inside. ) It rotates at .
25 RPM, completing a full rotation once every 4 minutes, which produces . 56g on the inside. From here on out, I’m going to switch from RPM to the time it takes for one full rotation because after the diameter get’s large enough, the RPM’s don't matter anymore.
Exactly what that diameter is is debatable, but we’ve definitely passed it by this point with Rama. What does become the limiting factor is the fourth variable: the rim speed, or the tangential velocity. Rama’s Tangential Velocity is 527 miles per hour.
And this is a great example of sci-fi using science to create drama. In the book, the tangential velocity becomes a problem. But 527 miles per hour is small potatoes compared to what's coming.
The Elysium space station from the movie of the same name is a Stanford Torus 60 kilometers in diameter and spins at 1g, completing a full rotation in 5 minutes 47 seconds. Its tangential velocity is 1,213 miles per hour! Ceres is the largest object in the asteroid belt, and in The Expanse is a space station that was spun up.
And I didn't realize this, but it's 7 times larger than the death star at 970 kilometers in diameter. The show really didn’t capture just how massive it is. Ceres It is the largest object in the asteroid belt and makes up 25% of the mass of the entire asteroid belt but is not actually an asteroid anymore it has been reclassified as a dwarf planet….
I have no comment. Ceres has a natural gravity from its mass of point . 029g but in The Expanse, it was “spun up” by the Tycho corporation to produce .
3g. No small feat. There is a reason why most of these are rings or cylinders and not spheres.
When spheres spin, the centripetal acceleration is concentrated on the rotation axis. Not only does it diminish as you move closer to the core, but it also diminishes as you move away from that axis. In the show, there is a graphic showing the tunnels inside Ceres in a spiral fashion, but that would be absolutely massive if they were that big, and it doesn’t really make any sense spin gravity-wise.
It is also at this point that the tangential velocity starts to become a problem. For man-made stations, the stresses produced by these ludicrous velocities probably would exceed current material strengths and wouldn’t be possible with current material science, and, as for ceres, it's questionable whether this asteroid–planet is compact enough to maintain its structural integrity under spin load. We don't know how dense Ceres is yet, but scientists are starting to learn that most asteroids are loosely packed, and if you spun one up with a tangential velocity of twenty-six hundred miles per hour it might just disintegrate.
The Moon is three thousand four hundred and seventy-six kilometers in diameter produces gravity the old fashion way, from mass, making 0. 17g. Alpha Halo from Halo is probably the most famous ringworld.
It’s 10,000 kilometers in diameter, but it does not spin for gravity. It has artificial gravity handwavium generators that produce the gravity… Why is it a ring then? Good question.
Probably because it looks cool… I’m done with Halo. The Earth is 12,756 km in diameter and it spins at about 1,000 miles per hour at the equator, and its gravity is, you guessed it, 1g. And also just here for scale, the original Halo installations, known as the Senescent array, were 30,000 km in diameter.
If you’ve never heard of the Culture Series by Ian Banks, it's a legend among sci-fi, and in it most of the galaxy's population lives on what are called “orbitals. ” Which are ringworlds of various sizes. There are two known orbitals with detail: Masaq and Vavatch.
Masaq is only 6 thousand kilometers wide, and Vavatch is 35 thousand kilometers wide. But Masaq is 3 million kilometers in diameter and Vavatch is 4. 46 million.
They have surface areas equal to 118 earths and 960 respectively. And the reason why orbitals come in different sizes is because their spin variables are scaled so that one revolution creates one standard day and one standard gravity for the preference of whichever species is living on them. This is because a natural day-night cycle can be created by just tilting the ring 1 or 2 degrees off of the ecliptic plane and the ring naturally casts a shadow on half of itself.
Only one variable, the diameter, is listed canonically, so we can’t calculate what these are spinning at, but what if we make a hypothetical ring with earth standards? An earth ring with a 24 hour rotation period and 1g would be 3. 7 million kilometers in diameter, and its tangential velocity would be over three hundred thousand miles per hour.
And suspiciously Vavatch is suspiciously 20 percent larger in diameter… But all of these are small scrith compared to the OG: Larry Niven’s Ringworld. This thing is a true behemoth, standing at 1. 6 million kilometers wide and with a radius slightly larger than the Earth's orbit around the sun, at 1 hundred and 50 million kilometers, making a total diameter of 3 hundred million kilometers.
To produce just under 1g, the Ringworld rotates around the star once every 9. 37 days, and its tangential velocity is 2. 7 million miles per hour… It has a surface area of 965 trillion square kilometers, almost a quadrillion, which is equal to over 3 million earths.
It’s so ridiculously huge that all of the worlds from Known Space have a 1 to 1 map of themselves on the ring… with plenty of space to spare. Also, the day-night problem couldn't be addressed by tilting a ring that encircles the star completely. So he devised a shade system in which 20 shades, each 1 by 2.
5 million miles in size, would rotate around the inner radius of the ring and produce a day-night cycle from their shadow Such a ridiculously huge ring is well beyond any science we have, but nevertheless Larry Niven, like Andy Weir, got suggestions from his fans about the scientific accuracy of the Ringworld. At the Worldcon convention in 1971, MIT students chanted, “The Ringworld is unstable! ” Because they identified that an object rotating around a star is not in a gravitational orbit, it has an equal pull on all sides and wouldn’t naturally be in equilibrium.
Eventually, it would drift one way or the other, colliding with the star. Niven then addressed this in the next book adding Bussard Ramjets around the ring to maintain its position and creating some drama for the story, too. I won’t spoil it.
There is one more and it’s a new contender for the biggest boi in hard sci-fi megastructures. Heaven’s River from the Bobiverse by Dennis E Taylor. Heaven’s River is a Topopolis.
A Topopolis is a rotating space station like the cylinders previously shown, but stretched out all the way around a star. But Heaven’s River says, “Hold my beer,” and ties that into what is called a Torus knot. Going around the star three times, in a helical fashion.
And in true Bob fashion, the author says, “There was no engineering reason for that, by the way, I just thought it was fun. ” Its diameter is slightly larger than the ringworld, at 321 million kilometers. But it's not actually this thick.
The diameter of the cylinder is only 180 kilometers. Which is still pretty big, but from this distance you can't see it. So let’s zoom in.
It rotates inside a static shell, and I put Rama here for scale. And as for the total surface area, doing the math, it would be over 600 billion square kilometers, which is about 3,300 Earths. Still ridiculous, but not quite the ringworld.
The Ringworld still holds the crown for the most ridiculous megastructure, but this is still pretty cool. There are dozens, if not hundreds, of other space stations. Let me know if I missed anything big (pun intended).
I have to thank my viewers for recommending the Bobiverse; I didn’t know about it until I started making these videos, and it's right up my alley as a socially isolated nerd. If you have any other sleeper sci-fi recommendations, let me know in the comments. And don’t say Warhammer.
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