The Ring

Let's say, for the sake of argument, we have a small, sort-of-heavy moon, with little geological activity, orbiting a gas giant. Let's take Ganymede. It has around 0.12Gs of surface gravity. This is great for launching materials and rockets, but it's horrible for human physiology, and probably plays more hell with Keidran biology for that matter.

If we want permanent homes in space, we need a full earths gravity, or close to it. What I propose is a rotating station. (groundbreaking, right?) But instead of just building a big space ring, what if we built a ring around the equator of a moon.

A train.

But why do we have to build a normal train? Why not make one thats 20 meters wide? 40 meters? Why not 200 meters wide? A massive, modular ring.

The Habitat

This home away from home would be a a massive, modular habitat that spans the entire equator, perpendicular to the axis of rotation of the moon it is built on. It would ride on electromagnetic suspension rails with high-temperature superconductors.

To board the station, a second, parallel service track would run next to the ring, with tram cars able to slow to a stop at stations around the moon, and then speed up to the same speed as the ring so people and cargo can board it.

But why stop with one? We could cover the surface with them, but let's start with 7 tracks. If we spun all of them up to the same speed, the ones closest to the poles would have less gravity and worse coriolis. This is actually a good thing as it allows cargo to be moved with less energy.

A happy side effect of this is that if the rings are spun opposite to the rotation of the planet, it will slow down, possibly stopping. This is great if you wanted to build docks on the poles, with ships not needing to equalize with the station spin, or dock directly to the rings themselves.

In a capitalist economy, the outer rings will be the cheapest, dirtiest, most industrial parts of the habitat, probably housing cargo and freight. The ring cars would be three stories tall, with a service level above and below, handling electricity, life support, agriculture, water purification and industrial systems.

The middle level could house a tram system that ran the entire circumference of the station, being also a null-G test ground, as the tram would slow almost to a stop relative to the planet.

And lastly, around the rings could be a carbon fiber mesh that could catch runaway equipment if anything where to go wrong.

The Question

What are the flaws? How can it be made more realistic? How practical is this approach compared to a regular space station?

  • $\begingroup$ The 1 g requirement for human habitability is far from certain. 0 g means no up and down at all, that's a lot more alien than just 0.1g. The requirement gets even murkier if one considers genetic enhancement or cybernetics. $\endgroup$ Dec 31, 2022 at 16:43
  • $\begingroup$ This is a cool idea but it's quite perverse. You've got a body large enough to have appreciable gravity, but rather than exploit that for free, you spend a huge amount of resources to create an opposed artificial gravitational force that exists only on artificial structures built around the body. This is very much the hard way, and a capitalist society wouldn't permit so much capital or resources to be wasted. Rather, they would just make people tolerate the low natural gravity. $\endgroup$
    – Tom
    Dec 31, 2022 at 20:19
  • $\begingroup$ @Tom good point, but for one, Ganymede is only an example, these trains would actually be easier to build on smaller, less dense moons. $\endgroup$ Dec 31, 2022 at 20:45
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    $\begingroup$ "Unfortunately, capitalism cant really force evolution to make us better at coping with adverse effects" Have you seen the pharmaceutical industry? Sure, it's a difficult problem to get to grips with, but so is cancer, or Alzheimer's, or vaccinations, and that hasn't stopped anyone. $\endgroup$
    – Cadence
    Dec 31, 2022 at 23:54
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    $\begingroup$ @Cadence... well... runs out of arguments your point is valid, but so is the rule of cool! As long as its not stupidly unbelievable, I'll take it! :3 $\endgroup$ Jan 1, 2023 at 0:26

3 Answers 3




Gravity pulls inward, centrifugal force flings outward.

To get a full gee the train will first have to cancel out Ganymede's 0.12 gees. That begs the question, "why even put it on Ganymede? Why not build it in empty space where there is no gravity to cancel out?"

If the train is moving at Mach 16+, what could possibly be the benefits of having it on Ganymede? So you get see the gloomy landscape blur by at hypersonic speeds out the window? Would people ... actually want that?
To do anything on the surface, you'd have to slow down from 5.5 km/s. No small feat. If the train is instead a space station in orbit, the delta-v tax to Ganymede would be much, much less, merely the orbital velocity of the space station (a couple km/s, tops). You could achieve 1 gee centrifugal force with a much smaller ring, less design complexity, orders of magnitude less materials. Then you have to worry about maintaining a 8,300+ km long track, checking for cracks/meteorite impacts/abrasion, powering it, etc. There are an ungodly number of failure modes and things that could go wrong which all end with Everyone Dies.

In my opinion, the track material is put to better use in an array of space elevators and to leave the habitation to rotating rings/cylinders in orbit, if it's truly impossible for humans to survive long-term in low-gee environments despite medical advancements, regular exercise, and possible genetic conditioning.

  • $\begingroup$ Your point about failure modes is a good one. If anything goes wrong with a conventional rotating space station, maybe people are thrown into the floor or walls, but if anything goes wrong here, you hit the surface of Ganymede at several thousand kps. That's a heck of a risk assessment. $\endgroup$
    – Cadence
    Jan 1, 2023 at 0:04

Kim Stanley Robinson proposed a similar thing in his Mars trilogy... a habitat on Phobos used a train to provide artificial gravity. That was slightly simpler than your train, by virtue of only providing Martian gravity (0.38 gees), travelling at under 200m/s, only being ~60km long and only having to overcome Phobos' feeble half-milligee gravity, etc.

What are the flaws? How can it be made more realistic?

The largest one, perhaps obviously, is safety. Consider environmental safety. Building such a thing on the surface of an airless moon means you need radiation shielding, and quite a lot of it if you're expecting to shield against things like high-energy galactic cosmic rays. On Earth you're sitting underneath ten tonnes of atmosphere and a planet-sized magnetic field. On Ganymede, there's negligible protection from its atmosphere and feeble magnetic field and its proximity to Jupiter still provides a somewhat hostile radiation environment. Add to that the risk from meteorite impact, and you can see that the surface isn't a place you'd want to do this sort of construction.

Now, Ganymede is relatively low density, and its crust is expected to be both pretty thick and mostly made of ice. There's plenty of scope for building monstrous nuclear powered (presumably fusion, probably using deuterium mined from Ganymede) tunnelling machines to carve out deep subsurface spaces that are extremely well shielded from the hostile environment of the surface, and even well protected against all but the largest of impactors and those could presumably be tracked by astronomers and dealt with via various means.

This doesn't eliminate other safety hazards to do with the train hitting stuff, stuff falling off the train, etc, though if there's a roof over the tunnel you can at least avoid the issue of being able to shoot orbiting spacecraft using stuff thrown out of your much-faster-than-escape-velocity vehicle.

How practical is this approach compared to a regular space station?

Given Ganymede's surface gravity of 0.146 gees, in order to generate an apparent Earthlike gravity inside you need a centrifugal force of 1.146 gees... this manifests as a linear habitat speed of ~5.44km/s. A habitat in microgravity doesn't have this issue, but even if it had the same radius as Ganymede (a fearsome engineering exercise) it would still have a linear speed at the rim of 5.08km/s. Any debris impacts have the potential to be catastrophic, so you'd need substantial armoring in either case.

Now, the strength of the centrifugal force is proportional to the square of the angular velocity and the strength of the coriolis force is merely proportional to the angular velocity. If you half the radius of rotation whilst keeping artificial gravity levels the same, your angular velocity (and hence the strength of coriolis effects) goes up by a factor of $\sqrt 2$ and your linear velocity at the rim goes down by the same factor (increasing fall and impact safety).

Project Rho, as always, has some good material on the subject of coriolis effects, including this nice diagram:

A diagram of the "comfort zone" of habitats spun to provide artificial gravity, with gravity limits between .3 and 1g, and rotation limit at 2rpm, showing that large habitat radii are needed to minimize coriolis effects and maximise comfortable gravity. For 1g, a minimum radius of 75m is proposed to remain below the 2rpm threshold

You can see that the "comfort zone" (as derived from a bunch of papers, but no actual in-space experience yet) extends down to quite small radii. You don't need something as big as Ganymede to stop people suffering from coriolis effects like nausea and dizzyness.

Of course, in a small habitat coriolis effects will be a lot stronger than your Ganymede-sized track. A useful blog post on oikofuge.com gives a formula for the horizontal deflection of $d$ an object dropped from radius $r$ to radius $R$ in a rotating reference frame: $$d = R\left [ \sqrt{\left ( \frac{R^2}{r^2} - 1 \right )} - \arccos \left ( \frac{r}{R} \right ) \right ]$$ which is conveniently unaffected by the actual rotation rate of the object. I won't derive it for you here, but it is correct and only needs pretty simple trigonometry to prove it to yourself. You can see that for a 2km radius station, an object dropped from 2m to the floor gets a ~6cm deflection to anti-spinwards whereas on your Ganymede train it would only get a deflection of ~1.6mm which is basically negligible for humans. How much you care about this is up to you. From the point of view of story-telling or game-playing flavor, coriolis forces can add all sorts of interesting effects, IMHO.

Anyway. It might actually make a lot more sense to build hollow rock- or ice-clad habitats from asteroids, moonlets or ring material, with sufficient exterior thickness to protect against impacts and radiation. It is hard to judge the relative complexity of the engineering projects, but at the point where you're considering making multiple 5+ km/s, 16000 km-long train tracks (the other kind of rail gun) it doesn't seem implausible that you could make a bunch of 1-2km radius space habitats of this kind, and they be not just simpler than your circum-ganymedean train but safer, too... the outside rim of a 2km radius habitat is only going at 140m/s, and so is somewhat less vulnerable to impacts, and things flung off will be slightly more easy to retrieve.

Speaking of which,

And lastly, around the rings could be a carbon fiber mesh that could catch runaway equipment if anything where to go wrong.

You don't "catch", you "attempt to minimize destructive effects of the impact and spallation". You'll be needing Whipple shields.

  • $\begingroup$ Wow... thank you! I'd say... for one, Ganymede was an offhand example, but now that you talk about it, it would make a lot of sense to tunnel in and build these massive train-rings kilometers under the ice. If you did that, these trains could actually be a lot bigger, and if dug deep enough, could give a marginal reduction to gravity you have to fight to break even. It would also reduce the radius of the track, making it smaller and thus easier to build. So yes! Subterranean Trans-Ganemydean Grav-trains it is! (just rolls off the tongue!) $\endgroup$ Jan 2, 2023 at 18:23
  • $\begingroup$ AND! It frees up the surface for more of those domed farms. Or whatever else you want the surface for. Fan-friggin-tastic! $\endgroup$ Jan 2, 2023 at 18:24
  • $\begingroup$ For reference, humanity would have access to a small (12~ exawatt max) dyson sphere, described in my other post. Enough power at their disposal to be truly terrifying and awe striking, but not so far that they seem 'foreign' or 'alien' $\endgroup$ Jan 2, 2023 at 18:27

At such a large radius the trains need to move extremely fast to simulate earth gravity. If my calculations are correct 19366km/h. That's a bit faster than current trains and probably not feasible due to engineering issues.

For down-scaling: reducing the radius by a factor of 100 reduces the speed by a factor of 10. For example, with a radius of 263m (instead of Ganymede's 2.634.000m) the speed drops to 193km/h. That would be fine even with current day technology.

Meanwhile reducing apparent gravity has little effect; at Ganymede's radius 0.1g would still require 8583km/h. Still too fast to be trivial and the resulting acceleration would be less then simply using Ganymede's gravity.

For a strictly realistic approach, building such trains on celestial bodies with radii around 3000m or less would be the way to go. Then again, if you want those trains around Ganymede you can simply declare that for your story having a speed of (whatever fits your story best) is sufficient. Being able to ignore a factor of 1000 or more here and there is the main reason we have any kind of science fiction after all, which is a good thing, too.

(I used formulas from here.)

  • $\begingroup$ Well... If you were to build a space habitat just as big, you would still need those kinds of speeds. $\endgroup$ Dec 31, 2022 at 16:49
  • $\begingroup$ Well for one, no atmosphere for drag per se. Secondly, the only real concern is structural stress, as the hanging track and trains themselves would need to deal with the centrifugal force of creating a full gravity. $\endgroup$ Dec 31, 2022 at 16:50
  • $\begingroup$ And once its moving, it will likely never need to be slowed down again. $\endgroup$ Dec 31, 2022 at 16:51
  • $\begingroup$ It's not going to be at all easy, but could it be done? How does it compare to The Expanse, where they hollowed out an entire asteroid and spun it up $\endgroup$ Dec 31, 2022 at 16:54
  • $\begingroup$ And also, it would be nice to have a full gravity, but we could run our train at only a half-gravity, or less... $\endgroup$ Dec 31, 2022 at 16:54

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