# How to explain Earth-like gravity in small asteroids?

I'm working on a sci-fi game about conquering little asteroids (about 1 km diameter). You can go into one of them and make it Earth-like so other humans, animals or plants can live there.

To do this I need asteroids to have Earth-like gravity, but I'm having a hard time trying to come up with a solution that wouldn't make asteroids collapse. I thought about digging a hole through the center of the rock and placing there some kind of "crazy matter", but I think this is too far-fetched even for a sci-fi game.

Any ideas?

Edit: I didn't state it explicitly, but gravity must be provided by some hi-tech device the player must own.

• You're going to need some artificial gravity generators. Some mumbo-jumbo about artificial graviton emission should do the trick. :P – ifly6 Dec 29 '16 at 11:48
• @ifly6: And don't forget to contain the gravity inside some structure, because otherwise your small asteroid will appear to the rest of the Solar System as an Earth-sized planet. – AlexP Dec 29 '16 at 13:18
• From a videogame perspective: adding a special device that needs to be built before anything else can be done only results in an artificial delay/prolongment of the process needed to colonize (or whatever you all it) the new asteroid; include the device as part of your expedition starship/space-probe or whatever is the first thing to touch an asteroid and be done with it; that will do much more for the suspension of disbelief than any crazy hyper-scientifical explanations can ever hope to achieve – dot_Sp0T Dec 29 '16 at 13:24
• @Trap: Yes, of course, Earth mass. – AlexP Dec 29 '16 at 13:27
• When you say "so [life] can live there", gravity is not your only problem. While the gravity can hold an atmosphere to the surface, you have the two-facet problem of how to sustain a breathable atmosphere (the O2/CO2 balance) and the lack of a magnetosphere to protect the surface from the harsher variety of solar radiation. But if you can "make" gravity, you can probably fix those problems too. – Jym Dec 29 '16 at 14:11

The first SFnal solution was suggested by Robert L Forward in his book "Future Magic", where an asteroid of that size is crushed down into a sphere @ 30cm in diameter. At that point, the density is close to that of a neutron star and the surface gravity is @ 1 "g". You might find the surface a trifle small for what you want to do.

Expanding on that idea a bit, you could simply slide your sphere of super compressed matter into the centre of another asteroid. The potential problem is that the immense gravitational pull will likely disrupt the asteroid, and there will be interesting (for certain values of interesting) tidal effects due to the steep gravitational gradient.

Going even farther down the rabbit hole, cosmic strings are flaws in space-time which would theoretically have immense gravitational fields, but are (as the name suggests) string like, and the gravitational fields would follow the string rather than be point sources like a sphere. This provides some interesting effects, such as the string would provide gravity along the outer surface of a pop can shaped construct, but not at the ends (the super dense cosmic string would be a danger to anything at the end points, however). The topology of a "knotted" cosmic string might provide a more spherical (or at least lumpy) gravitational field, as well has securing the ends.

Paul J. McAuley, in his "Quiet War" trilogy, had a fictional technology of "entangling gravitons" and depositing them in the centre of an asteroid or construct to bulk up the gravity field of the object. Presumably, one could make a hoop of gravitons and create a local null gravity zone in the centre as well.

As the author, you are free to select whatever means you want (or simply handwave and don't bother to explain how this is done, much like in most SF TV shows and movies). If it isn't important to the plot, then your best bet is to ignore it.

Place a small, charged black hole in the center of the asteroid. After it carves out a small space, you can hold in position using electrostatic manipulators, and even use it as a power source by dropping trash into it. (Idea courtesy of Larry Niven's Known Space series.)

• Note that you would also have to keep it charged. The hawking radiation would preferentially be charged as it naturally wants to shed the charge, and it will repell like charges so swollow cancelling charges. – JDługosz Dec 30 '16 at 6:28

I am answering late but I would like to give a more reasonable solution. You wouldn't exactly need 1g, but something close to 1g (say 0.5g-0.75g). To get this gravity on such a small body would require two things:

Density of composing matter - I cannot give a specific value, but something dense enough to contribute to gravity on the range of 0.1g to 0.3g is 'safe'. The reason I am saying safe is because having 1g on such a small body means that it is very dense. The misconception of having the earth's mass contracted to such a volume is a bad thought. Mass volume ration applies here. Just because a 1km radius body has a density equal to earth's doesn't mean it's safe for humans or anything else to be close to it. They will be crushed on its surface. Just because a neutron star is 1km in radius doesn't make it less dangerous.

Spin - The asteroid spins at such a speed that on any part of it one doesn't feel 'upside downy'. The speed of spin is of utmost importance since even having 0.5g on such a small body is dangerous. The spin needs to contribute heavily to the overall rest of bodies existing on its (the asteroid's) surface to offset reliance on gravity alone for this.

Explain these two factors clearly in your story and how 0.5g is something a space-faring people can deal with. Story characters can use hooks to the surface for improved stability.

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It is simple. Just tie two asteroids together and rotate.

The asteroids will have to be similar in mass and the connecting cable will need to be very strong as well as sufficiently long but ultimately this should be the most straightforward solution.

Entry point for both will be a station in the middle that should rotate the other way so it stays stationary.

• Won't work. The asteroids will move, but everything on top of them will just stay static (unless they are in the "forward" zone of the asteroids). If you are in the "backwards" zone of the asteroids you will be left floating in the space only until the next asteroid comes by and smashes into you. – SJuan76 Dec 29 '16 at 12:21
• You can only live on the inside/in an attached cabin anyway due to radiation anyway. – Bookeater Dec 29 '16 at 12:22
• Another problem is the cold, assuming the asteroid belt is at the usual distance from the sun. But this answer makes no sense any more after the question was edited to include a high-tech device... – Bookeater Dec 30 '16 at 12:55

It depends on how sci-fi you want to go, realistically it makes 0 sense a asteroid that small have a gravity the same as earth. You could say the asteroid is made of a very dense material so it could fit the mass of an earth like planet in a small asteroid like that. the earth is around(5.972)×10^24 kg a asteroid with a diameter of 1km will be 523600000 square meter (assuming it's round). It would have a density of 1,140603514×10^17 kg/m^3. note, a black hole is 6×10^18 kg/m^3. So forget about the laws of nature for they say no.

I suggest you invent something that can increase the mass without increasing the mass.

• I should've stated it more clearly, but yes, the point is, what to invent that wouldn't seem too crazy to believe? – Trap Dec 29 '16 at 13:14
• @Trap it depends, if all asteroids are already inhabited it wouldn't be weird settlers implemented some sort of artificial gravity system at the center of the asteroid. By the way have you thought about how the people on your asteroids shield themselves from cosmic radiation a asteroid like that wouldn't have a liquid core so it wouldn't have an electromagnetic field. – PDek Dec 29 '16 at 13:34
• Maybe an Earth-like gravity would allow an Earth-like atmosphere that could be somehow tweaked...? I don't really know :) – Trap Dec 29 '16 at 13:53
• @Trap I think making this concept scientifically correct is way to much work I think you should take some artistic liberties to create this game. – PDek Dec 29 '16 at 13:59
• High mass and gravity on such a small object will cause people on the surface to experience different levels of gravity on their body depending on the distance from the ground. – Cem Kalyoncu Dec 29 '16 at 20:01

If you had Earthlike gravity on an asteroid, it would probably crush itself down to a very small size. Asteroids are not solid chunks of rock generally; they tend to be very loosely held together piles of rubble with virtually no gravity. If you had something as heavy as a city sitting on an asteroid being pulled down by 1 g of force, I'm guessing it would punch right through the asteroid. Maybe you could hold the asteroid together with some kind of carbon based super fibers and then spin it to crate centrifugal artificial gravity on the hollowed out interior?

The gravity wouldn't crush the rocks. The thing about most asteroids (non-monolithic ones) is that there's a lot of empty space between the chunks and rocks that make them up, because there's so little gravitational force holding any of it together. If you applied a powerful gravitational force at the center, it would cause all those rocks to compress, drawing them closer together and shrinking the asteroid. At least that's what I would expect; no one's ever actually done it!

I had two more thoughts about this question that might be helpful, without completely altering the premise. The first would be to place some kind of super paramagnetic generator at the center of the asteroid. The induced magnetic field could pull things down in a way that simulates gravity and falls off the further away you get from the asteroid. It wouldn't be a perfect stand-in for gravity, but with a few modifications here and there your asteroid colonists could live in a very similar manner to how they would on Earth, and they'd still have the benefits of low gravity in areas that aren't treated to respond to the paramagnets. (this idea may or may not be scientific nonsense, but you could make it sound plausible)

The other idea would be to construct large centrifuges on the surface of the asteroid that have angled surfaces, sort of like an Indy 500 racetrack. The centripetal acceleration could augment the low asteroid gravity to give however much artificial gravity you need. This would require a ton of energy to do, but it would accomplish the goal.

For a small asteroid to have such gravity, its density would have to be pretty close to a black hole, and in that case it should just collapse in itself. If you want to make this work, you'll have to bend the science a bit. I propose one of the following solutions:

• Make the asteroids out of some material that is not affected by gravity. You could then still have a black hole like core, with earth gravity, surrounded by this material. It will not collapse because it can't be influenced by gravity.
• Place a gravity generator at the core of the asteroid, that produces some new type of gravity. And let that gravity interact differently with materials than the normal one.

Of course scientifically speaking those don't quite make sense, but Sci-fi as the name already says, is fiction.

Some math: The gravitational acceleration of a planet is given by:

$g = \frac{G * M}{r^2}$ where $G = 6.6408 * 10^{-11} m^3 kg^{-1}s^{-2}$

For a asteroid of diameter $1km$ $(r = 500m)$ and a wanted gravitational acceleration of $9.81 m/s^2$ we calculate the Mass $M = \frac{g * r^{2}}{G} = 3.6931 * 10^{16} kg$

And the density is $ρ = \frac{M}{V} = 7.0526 * 10^{7}kg/m^3$

• For a small asteroid to have such gravity, its density would have to be pretty close to a black hole, and in that case it should just collapse in itself. Can you please provide a citation for this? Thanks, and welcome to Worldbuilding Stack Exchange. – HDE 226868 Jan 3 '17 at 14:05
• It was more of an educated guess, but I added some math to the post, to have a look at the numbers. – Adamo Jan 3 '17 at 19:27