How to increase a moon's gravity?

I am building a sci-fi universe where humans have colonized the moons of Jupiter. They are very small and thus have low gravity. In fact, some of them have gravity equal to less than 5% of Earth.

What would be a believable sci-fi way to artificially increase the surface gravity to make the moons more hospitable?

• Hello and welcome to Worldbuilding S.E! Do I understand correctly your question, you want your moon's gravity to increase artificially right? – Mr.J Feb 1 '19 at 2:57
• @mr.j that is correct! Thank you for the clarification – Dilettanter Feb 1 '19 at 2:59
• Sadly though, the most believable sci fi way of increase a space object's gravity is by creasing its mass, making sure that the core also grow in size too, I'm not too sure what kind of satellite provide greater gravity (e.g earthlike planet with iron core, gas giant with ice core or a water giant with a rocky core, etc etc...). It would be better to if we know why would you want a planet or a satellite to increase its gravity, thank you and good luck! – Mr.J Feb 1 '19 at 3:06
• Because low gravity makes habitability much more difficult. And if we must colonize the gallelian moons, then how could we overcome their weak gravity? Even something like "walk around with magnets on your feet on magnetic floors" is one way around it, albiet a rather lame one. – Dilettanter Feb 1 '19 at 3:23
• Put a tiny black hole at the center of your moon. – Pasqueflower Feb 1 '19 at 10:38

13 Answers

The answer, unfortunately, is "artificial gravity," and don't explain it.

Speaking from a purely scientific perspective there is no known substitute for mass. The only known way to cause gravity is to have mass (or an ungodly amount of energy which is equivalent to said mass). We simply don't know of anything else.

In science fiction landscapes, artificial gravity is simply too useful to not have. We use it all over. Nearly every major science fiction show set on a spaceship has it. Only a handful dare to deal with microgravity in its true form. There's plenty of handwavy reasons for it, but most shows seem to merely assume that artificial gravity works, and move on with the plot. Some will rely on centrifugal force to create the artificial gravity, but that only applies on stations where your people operate on the inside. Since you're on the outside of a planet, that won't work. That being said, if you were operating a mining expedition, you might be able to spin the planet and have "gravity" face the other way (for a short while, before the rocks of the planet break up and fly off).

If you want to add some pseudoscience, feel free to say something about gravitons. That's the hypothetical particle in QM that is supposed to be responsible for gravity. However, nobody has been able to figure out how to work the math for gravitons, so we don't even know how to look for them, much less create them. Still, as far as believability goes, it's at least the name scientists use when exploring gravity.

• "How do the artificial gravity generators work?" "They work quite well, thank you very much!" – Rocky Feb 1 '19 at 17:51
• "or an ungodly amount of energy which is equivalent to said mass" just to put it in perspective, in at least one answer it was mentioned it woulf take many years worth of the output of the sun to achieve a gravity like Earth's. A type II civilization wouldn't be able to pull this off. – Renan Feb 1 '19 at 18:16
• The Expanse is an example of a show which makes the microgravity part of the plot instead of trying to paper it over with "artificial gravity" – taylor swift Feb 1 '19 at 18:48
• "In science fiction landscapes, artificial gravity is simply too useful to not have. We use it all over. Nearly every major science fiction show set on a spaceship has it." : Only really true for television & film, the written word doesn't have the same special effects production cost issues as films. – Pelinore Feb 1 '19 at 20:05
• Spinning the planet will only work if it's a metallic asteroid or something - an object that is held together by its own physical cohesion. Most natural astronomical objects are held together by gravity alone and would fly apart instantly if you spun them fast enough to cancel out their gravity. – IndigoFenix Oct 24 '19 at 6:42

Two Options

Actual Increased Gravity

Gravity is a product of mass and in the case of a human and a moon, to make the gravitational attraction increase one may either increase the mass of the human or the mass of moon.

1. Increasing the mass of the moon would create a situation most similar to "Earth conditions." One could do this by dumping mass on the outside, or by removing parts of the innards and replacing with more dense material. Either of these options are well outside the capabilities of our current civilization, and even for a civilization that was ready to colonize the Jovian system this would almost certainly be prohibitively expensive or impossible. A bigger problem would be that increasing the mass of Jovian moons would change the orbits of said moons. The largest Jovian moons orbit in stable resonance and adding mass to some/all of them would destroy this stable condition, leading to more erratic orbits and possible collisions between moons or with Jupiter.
2. Increasing the mass of the astronaut/settler is less commonly suggested. For instance, if a settler on some moon experiences 1/3g, and that person wears garments that effectively distribute double their weight across their body, they will feel 1g in their normal movement such as walking or moving their arms. (EDIT: As a few commenters have pointed out, this is not really a true statement. One might feel roughly 1g when performing vertical motions like raising arms or jumping, but the increased inertia from the added mass will make most movements like walking feel awkward and difficult.) This will not help with things like changing in the shape of the eyes or deterioration of the heart, but it would help a lot with muscle atrophy. This would also not help with other aspects of low-g environments such as drinking fluids, moving objects, etc. For more detail see this.

Artificial Gravity

By artificial gravity I mean simulation of gravity via an "apparent" force. This is most commonly done in science fiction by having people live in rotating habitats shaped as a cylinder or wheel. The (centrifugal) force is outward against the outer edge, which essentially become a floor that curves around. You can visualize and learn more about these schemes here and here. This could be done either on the surface or in orbit:

1. On the surface, the only way to keep the artificial gravity constant would be to rotate with a rotational axis perpendicular to the surface. Otherwise when rotating away from the surface one would feel lighter, then heavier as they approached the surface again (ferris wheel configuration). So if rotating with an axis perpendicular to the surface (merry go round configuration), combined with the moon's gravity, this would make the "floors" of your rotating habitat feel tilted. This is because force would be generated outward (parallel to the surface) but the gravity field would act downward (toward the surface). Therefore you'd need to compensate for this tilt created by the two different force components (actual gravity from the moon and the apparent centrifugal force) by tilting the floors of your rotating cylinder habitat. See the diagram below (massively out of scale):
2. In orbit, a space habitat would be in freefall, thus experiencing what we call zero-g (like on the ISS). In this case, a rotating habitat could provide a constant 1g by spinning at the right speed for its size. If reaching the surface for mining/farming/building is necessary, it can be done by shuttling or using some future tech such as a space elevator.
3. There is a third option that I think is even less likely but possible. Habitats could be built in the interior, with the floor being the outer surface (think like being upside down in a cave). The moon could then be accelerated rotationally, until the spin simulates an outward force of 1g plus whatever the moon's gravity is. This would allow for living in subsurface dwellings at 1g, but there are big issues: First, the energy required to increase the moon's spin this much would be massive, and it's unclear how exactly this would be done besides literally strapping rockets to the moon. Second, the outward force would be most apparent at the equator, and go to zero at the poles. So in a strip at the equator this is workable, but as one moves away in latitude the effect decreases noticeably. Third, anyone on the outer surface would feel an outward force of 1g (think like if gravity on Earth reversed and you had to hang on to keep from flying off). This relates to the former issue in that it would be most drastic at the equator and not apparent at the poles. Fourth, it's up for debate whether a moon would even stay in one piece rotating fast enough to provide 1g outward. The whole thing might fly off in a bunch of chunks. This idea is commonly proposed for colonizing small asteroids, but would not be a great idea for, say, Io. Though maybe for some of the much smaller outer moons this would be more valid.

What's Likely?

It seems that you really want settlers to live in 1g on the surface of these Jovian moons. It is really unlikely that this setting would ever happen without handwaving some material that "magically" increases density/gravity. In all likelihood your settlers would settle (hah) for living in reduced gravity on the surface and deal with the health effects, or live in 1g in a space habitat and transfer to the surface when necessary.

• Two things to consider regarding the "actual increased gravity": Increasing the moon's mass would certainly lead to massive "moonquakes" which would need some time to settle before you can start to raise any buildings. Second, increasing the mass of the inhabitants would have an unwanted side effect: increased inertia (since gravitational mass = inertial mass). Maybe you would feel the ground like on earth, but e.g. walking would not feel normal. Starting, stopping, taking turns would be totally different. – Dubu Feb 1 '19 at 10:11
• Don't forget the Starcraft way: adding constantly burning, upward pointing thrusters to the space suits. Wasteful? Yes. Dangerous? Oh, yes. Impractical? Of course yes. Looks cool? Hell, yes! – zovits Feb 1 '19 at 14:32
• Good job pointing out that you can increase the mass of the astronaut with weights. I skipped over it in my answer because I knew about the limitations of that approach, but when I saw what you wrote I thought, "Dang, I should have mentioned that!" – Cort Ammon Feb 1 '19 at 14:39
• I think this was by far the best answer, you covered every possible approach I can think of & your conclusion is the right one +1 – Pelinore Feb 1 '19 at 20:47
• So... at 5% gravity, I’d need to wear a ~2000kg suit to sort of simulate 1g. Does my car count as a suit? – HopelessN00b Feb 2 '19 at 1:40

Make the moons smaller.

Gravity is proportional to mass and inversely proportional to the square of distance. Two moons of the same mass but of different densities would have different surface gravities because their radii were different. If one moon were made of ice and one of iron the density of the second moon would be about eight times the first; the radius about half and the surface gravity about four times as great.

So how to make the moons smaller? Two sci-fi ways that spring to mind are:

1. Crush the moon. Enclose it in an ultra strong shell (perhaps a force field?) and compress the contents. This makes your moon smaller, but would require a lot of energy to do.
2. Make the atoms smaller. Atoms are mostly empty space, their size determined by the size of their electron shells which in turn are determined by the mass of their electrons. If you were to replace the electron with a different particle with the same charge but a higher mass it would make the atoms and thus the moon smaller. In terms of known physics muons have the same mass as an electron and about 207 times the mass: an atom formed by replacing electrons with muons will be 207 times smaller. Unfortunately it would also not last long as muons decay in a matter of microseconds, but if you could find some way to stabilize them or find some other particle with the right properties then you just need to set up a muon factory on the surface of the moon and pour them in until enough mass is compressed to give the desired results.

Both these methods have the disadvantage that compression releases a lot of heat so your squashed moon would have to deal with that somehow. Also you loose a lot of surface area - if you halve the radius to increase the surface gravity by a factor of four then you will also reduce the surface area by a factor of four.

• Relevant xkcd: what-if.xkcd.com/68 – Jens Feb 1 '19 at 10:00
• As mentioned in the XCKD comic, if you went with this approach, there could be substantial tidal forces, which have a tendency to rip large objects apart. Including some engineering crafted to deal with this effect would add a great deal of believably to a world build by handwaving the process of throwing the moon in a large trash compactor! I haven't yet had to think about what a skyscraper looks like on a planetoid small enough to have tidal effects on such scales. It could be fun! – Cort Ammon Feb 1 '19 at 14:42
• Compressing a moon is hard science (barring the method of crushing it?) but reducing the size of a whole moons atoms goes way beyond into high fantasy scifi doesn't it? (+1 for the first though, I like that one). – Pelinore Feb 1 '19 at 20:11
• Cort, the XKCD version is talking about a very tiny planet - Little Prince sized. I'm talking about taking one of the Jovian moons and reducing its size until it has gravity that approaches that of Earth. Yes the tidal gradient would be steeper than on Earth, but not so much as to cause any great issues. – Barry Haworth Feb 2 '19 at 3:59
• @Pelinore the notion of reducing the size of atoms using muons has been around for a while, and has I think been done in practice. I first read about it in a 1987 Scientific American article about muon catalysed cold fusion, the notion being that bombarding hydrogen (or probably deuterium) with muons allows the creation of "mu-molecules" where the muon replaces the electron. This brings the nuclei of the atoms close enough together for fusion to occur. See Wikipedia – Barry Haworth Feb 2 '19 at 4:11

As others have already said, the only 2 ways are to decrease the radius or increase the mass of the moon.

Both of them come with problems. Increasing the mass of the moon in an extent that brings the gravity closer to earthlike, means that you would also change it's orbit and or orbital velocity. Decreasing the radius means less surface to live on.

So why change the moon when you can change the people living there way more easy?

Have people be genetically engineered to keep their muscle and bone structure the same way they would be on earth and you have some nice advantages. Those people could easily lift heavy stuff, they could propell flying machines with only their muscles and so on.

All it takes is some treatment that makes sure bones and muscles stay healthy + a bit of engineering newborns to keep genetical drift at bay.

• I'm not sure they're going to be propelling flying machines on any of Jupiter's moons, given that their atmospheres are many orders of magnitude thinner than Earth's. Perhaps they've been terraformed with Earth-like atmospheres, but I don't believe OP has specified that anywhere. – F1Krazy Feb 1 '19 at 12:27
• Hello elPolloLoco! This was a completely reasonable frame challenge to the OP's question, although it would have been nice if you had fleshed out the idea of a genetically engineered solution. I'm sure you'd agree that genetics aren't magical and there would also be problems with maintaining Earth-like muscle tone on an unchanged moon. Nevertheless, the frame challenge provided both elements required on this site: (a) you identified a problem with the premise of the OP's question and (b) offered an alternative solution to achieve the OP's goals. Thanks! – JBH Feb 1 '19 at 16:24
• @F1Krazy though it's a moon of Saturn, it seems that human-powered flight is within reason on Titan. I'm totally trusting Randall Munroe here, but the relevant xkcd is entertaining: what-if.xkcd.com/30 – ben Feb 1 '19 at 19:35

Have you considered (electro-)magnetic forces?

My first thought was that if the colonists had something magnetic on them, they could be pulled toward the surface by electromagnets, for which I have three working ideas:

• A non-toxic compound that can be magnetically attracted is spread through their body, much like some elements that are used forensically to identify where someone grew up and get in our system through the food chain but aren't there for any specific purpose. This might even already exist, though we probably don't like putting weird stuff inside us and wasting a lot of energy to create artificial forces for such purposes. This has the great advantage that by spreading evenly through the body we'd perceive gravity perfectly fine (as it would also be in the instruments near our ears that tell us which way is up, a set of three canals per ear filled with a liquid if I'm not mistaken)

• Metallic implants, spread across the body, this would probably not give us the complete feeling of gravity unless we also tampered with the canals in the ears. Should be doable, though maybe not very comfortable

• Suits with metal woven into them. This would give us a much more even distribution of the forces than implants at regular intervals, and we could again modify the ear canals. These also have the advantage of being adjustable to different conditions, either by just swapping them for another suite in a different environment, or by even being powered and creating small electromagnetic fields themselves. This would also allow them to respond to the following issue:

Other than losing power to the electric field, which would essentially bring you back to microgravity, a far more powerful electric field would equate to bone-crushing gravity, and that would, in fact, be a very interesting hack I'd like to see in a story some day.

An other point is that if you make the implants smaller and compensate by increasing the number, if you keep going you'll end up with the first solution.

Then I remembered a passage from a book I started reading (or rather, hearing, as an audiobook) a while ago, here's what I remember, maybe someone else can correct anything I got wrong. I think I came across this one in Michio Kaku's Physics of the Impossible, where he theorizes on ways to implement some sci-fi ideas in the (not particularly) near future, based on what we know.

The author was saying that sufficiently large electrical fields would manage to magnetize (not sure it's the right term) a lot more than the usual metals, and this would allow artificial gravity. More specifically I believe he was referring to levitation, actually lifting us off the ground. This would require enormous amounts of energy, and would possibly affect us in other ways, but maybe fusion reactors would permit it.

On a side note, I should note that you can get electronics to work under extreme magnetic forces, and it would be conceivable that they are all manufactured so (fluctuations would be more of a problem but they'd be a problem for humans too with my solutions so presumably there won't be any). So this should not stand in the way of the rest of your tech.

• I haven't read Physics of the Impossible, but he's almost certainly talking about diamagnetism. It's a highly-effective way to levitate frogs, but since different materials are affected differently, it's not usable as a substitute for gravity (you might feel Earth-normal gravity, while the sheet of paper you just set down is almost unaffected, and the steel screwdriver you left in your pocket when you went through the airlock goes shooting up towards the ceiling). – Mark Feb 1 '19 at 22:30
• Yes, diamagnetism does ring a bell. I did not consider the fact that different materials will be affected to different degrees, but I gather the main reason we care so much about gravity is that our body weakens if it doesn't have to support itself, no? Losing muscle mass, bone density in the legs and eventual atrophy should be averted if you're pulled to the floor diamagnetically. Of course getting screwdrivers back from the ceiling is another matter, but maybe we can just agree to put something of an appropriate material in everything we need to keep down, and avoid materials that shoot up. – user3079666 Feb 2 '19 at 17:30
• The problem is that "materials that shoot up" include almost all of the structural metals. Iron, nickel, cobalt, and most steels are ferromagnetic, while stainless steels, aluminum, tungsten, and magnesium are paramagnetic. This rather limits what you can put in your magnetic-gravity living quarters. – Mark Feb 2 '19 at 21:40
• Good point.... If you could regulate it so that the forces are similar to gravitational though, it should put the same stress onto the structure as an earth environment. How easy that is to do, I don't know (I admit we've reached the end of my knowledge of electromagnetism), but being a sci-fi setting maybe we can be a bit loose on the details. Also being a sci-fi setting, the author may be willing to compromise for futuristic alloys that can cope better with these effects, it's the author's choice but it's probably not the most far-reached compromise. – user3079666 Mar 1 '19 at 19:03

The surface gravity of a spherical body is governed by the following equation:

$$g \propto \frac {m}{r^2}$$

Thus, there are two ways to increase the gravity.

1. Add more mass $$m$$.
2. Decrease the radius $$r$$.

Adding more mass

If a moon has 5% of the surface gravity of the Earth then you'll have to add many more moons of material in order to increase the gravity to Earth levels. At this point you're no longer modifying the existant moon. You're building a new bigger moon.

Decreasing the radius

You're better off decreasing the radius. If you decrease a moon's radius while keeping the mass constant then the surface gravity increases. You can do this by transmuting the moon's light elements into denser elements. Human beings can already transmute some elements on a small scale. If generalized to the right elements and scaled up, then this process could increase the surface gravity of a planet or moon.

Option 1:

Depleted uranium shell on the moon's surface, of sufficient thickness to add the mass required, but that would add cost to whatever activity needs to break the surface. Plus the increase radiation hazard, but you're on a moon with no atmosphere or magnetic field. Alternately, a better, but more expensive, solution would be to drive shafts down to the core of the moon and dump the depleted uranium there.

Option 2:

Rather than increasing gravity across the entire surface of the moon, do it only in enclosed, inhabited biospheres. Use high pressure pumps to circulate air by drawing in in from the bottom and releasing it at the top of the biosphere dome. The atmospheric pressure plus the downward velocity of the air should act as a suitable simulation of gravity.

If the gravity on the surface of the moon is only 5% of Earth's gravity, it's an airless moon so outside you need (probably heavy) pressurized suites. This will help with the perceived weight (but not with improving movement rates, which are mostly due to ground friction.

On the other side, inside you could have undergound shelters/bases with metal floors and magnetic harnesses worn on the body (this has a lot of drawbacks though).

Criss-cross your planetoid with gigantic underground particle accelerators, whose purpose is to generate collisions in the 126.0 ± 0.6 GeV/c² range, and thus artificially generate short-lived Higgs Bosons to temporarily increase the interaction between nearby matter and the Higgs field.

This means that the rock inside your planetoid will be providing a gravitational effect consistent with a higher level of mass than actually exists in situ.

If you can shrink and arrange these artificial Higgs generators into some sort of Halbach array, then you have artificial gravity deck-plating for your ships too.

How about the idea that humans remain as power hungry as ever. So they found a way to harness the power of a contained black hole that can be created in the core of a moon. Depending on the size of the black hole you can different power outputs thus different gravities can be created?

Since we are talking about moon colonization, I assume humanity has some advanced technology. Introduce black hole generator technology/device which can generate, contain and grow black holes under very well controlled conditions. Then install such a device at the center of the moon and at this point you have two courses of action: a) if you want to keep the moon size, feed off-moon materials to thegenerator, e.g. space junk or nearby asteroids. This, however, would required a lot of mass to be brought to the moon. b) if you dont need the surface area, strip away outer moon layers and feed them to the generator until the size is small enough to gave the desired gravity. This would required considerably less mass to be fed to the generator.

Creating artificial gravity on the surface of a planet or moon is possible without resorting to pseudo-magic. Ever been in a Gravitron? It's a carnival ride where, as the donut-shaped capsule spins, you're plastered to the wall by a centrifugal force of three Gs or so.

As Ben pointed out, this force combined with the natural gravity of the moon makes the floor, if perpendicular to the axis of rotation, feel tilted. Luckily there's a simple solution: tilt the floor. To oversimplify, if you tilt the floor at 45 degrees, and calibrate the centrifugal force to equal the moon's natural gravity, you will have doubled that gravity and the floor will feel flat. For any given ratio of natural to centrifugal gravity, you can tilt the floor such that it's perpendicular to the apparent combined force. (Technically, it should be a slightly concave shape, because the centrifugal component diminishes relative to the natural along the down/in direction, but maybe don't worry about that.) Keep in mind that the bigger the wheel, the better/less nauseating it is.

This results in a long, narrow, circular space with any desired g-force, which would probably make a better running track than full-time habitation. Make the colonists run daily laps to keep their bodies in shape, and let them hop around in low-g the rest of the time... That will keep their Earth muscles in shape, while keeping costs down and avoiding the need for imaginary technology.

In this case in order to increase the gravity of the moon it is technically by increasing the mass by doing so you have increased it's gravity another method is also by decreasing the speed of it's rotation

• What do you mean by "decrease the speed of its rotation"? The moons of Jupiter and tidally-locked and don't rotate on their axes, and I don't understand how flowing their orbital rotation would increase their gravity. Could you edit your answer to clarify? – F1Krazy Feb 1 '19 at 19:22
• @F1Krazy : ^ I'm just guessing from context here, but presumably he's assuming some gravity is countered by the centripetal force of the bodies rotation & by slowing or stopping it's spin less gravity will be lost to this apposing force? – Pelinore Feb 2 '19 at 2:22