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I've just finished reading Mission of Gravity by Hal Clement, a reasonably hard-scifi novella set on a planet with exceptionally high gravity: 700g at the poles, offset by very rapid rotation to 3g at the equator. The human characters wish to retrieve a MacGuffin from the polar region, but can only barely inhabit the equator, so have to negotiate with the native lifeforms to make the trip for them.

Some of the corollaries and counter-intuitive consequences of the high-grav setting are interesting, but the least believable aspect of the entire story, IMO, is what the humans are doing there in the first place. The given reason (to conduct 'scientific experiments about gravity') seems rather weak, and also not conducive to any sort of long-term human presence. It would be much better if there was some profit to be had from an occupation.

What industrial or chemical processes might be easier or more cheaply undertaken in an extremely high-gravity environment? Assume that access can be provided by portal or somesuch, so no need to pay the excruciating launch cost of trying to get stuff in and out by rocket.

One possible process that springs to mind would be isotope separation, for which we currently use centrifuges; it's understandably hard to find authoritative information on "how to build your own uranium enrichment centrifuge", but my instinct is that they produce centrifugal forces much greater than 700g, so I don't know whether a much slower process not dissimilar to septic tank separation chambers, might still be effective?

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  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – L.Dutch
    Feb 24 at 15:29
  • $\begingroup$ Just to note that with the exception of mining, most if not all of the suggested answers could be done far more easily using a centrifuge. (Either on a lab scale or on an O'Neill cylinder scale.) Don't forget that anything you make on the surface needs to be sent back into space again, and because of the high gravity that either requires a much bigger rocket than anything we've launched from Earth, or a megastructure far more ambitious than a simple rotating cylinder. $\endgroup$
    – N. Virgo
    Feb 25 at 7:50

8 Answers 8

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Frame Challenge: Setting realistic expectations

It is not possible for any astral body to have a surface gravity of 700g.

The highest theoretical surface gravity of any rocky or icy planet is about 3.3g If a rocky planet were to accrete more mass than this, it would become virtually impossible for solar winds to strip enough atmosphere to prevent it from becoming a Gas Giant which you can not stand on. Also the surface of a gas giant rarely seems to get past about 3g either. Past a certain point though, you move out of the "planet" scale into into the "star" scale of things when you hit brown dwarves. These things have the highest surface gravity out of anything that is not a solar remnant topping out at about 100g... so still about an order of magnitude too weak... and way to hot, radioactive, and gassy to actually be a thing you could land on. Even a tiny star is still a star.

As you get past the mass of a brown dwarf, you are looking at main sequence stars that have lower and lower surface gravities because the nuclear forces happening in their cores make them less dense than their smaller, barely reacting cousins.

The only things dense enough to exert over 700g are white dwarfs (>1e6g), neutron stars (>1e11g), and blackholes (~1.6e13g)... but these exert WAY more than 700g at their surfaces.

While you may be thinking, why not just wrap a planet around one of these... that is easier said than done. Normal matter likes to go nuclear when it accumulates on the surface of a white dwarf or neutron star resulting in a Nova or X-ray Burst respectively, and with black holes, stuff just falls past the event horizon never to been seen again. As such, there is not any sort of astrological middle ground in the 700g range.

So, if we're gonna be talking about colonizing a heavy world, we should be clear we are talking about 1.5-3.3g Not 700g... unless the "world" is an artificial megastructure created by a highly advanced alien race or deity.

Rare Earth Metal Refineries

Many different kinds of refining processes from petrochemical processing to metallurgy require heating up a substance and letting the parts separate by density in a process called distillation. This separation becomes much stronger in higher gravity. In many cases, substances have to be separated where the density of elements/compounds is so similar that 1g is not enough to cause significant separation. In a small laboratory setting, we can use centrifuges to simulate >1g environments, but this takes a lot of energy, and becomes impossible from an engineering perspective to do at industrial scale.

One area where this is a huge limitation on Earth is the refinement of Rare Earth metals. Not only are these elements only found in trace amounts, but they tend to be found in ore with very similar density to the other elements that need to be removed, because of this, it can take hundreds of smeltings to refine them. Currently, some of these metals cost thousands of dollars an ounce because of the shear labor and energy required to refine them.

If you were to smelt them in higher gravity, then the elements in a smelt will more completely separate allowing you to extract a thinner, more pure disc of the desired metal reducing the number of smelts you need to do and decreasing the refining energy required.

Light Element Mining

Lithium and Beryllium are both very valuable low density elements. They are not very prevalent in the universe because they are not released en masse by cosmic explosions like novas or supernovas; however, just like how refineries will distill more cleanly in higher gravity, the planet itself will act like a superpowered distillery of the elements in it. Heavy stuff will sink into the core much harder than on Earth and lighter elements will rise more to the surface. So, instead of getting all mixed throughout the crust, what little Lithium and Beryllium this planet contains will be squeezed to the very surface meaning that the topsoil will be far more rich in these rare elements than on a lower gravity planet.

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    $\begingroup$ I'd be skeptical that light elements would be present on such a planet in any significant quantities. Assuming this is a terrestrial world, (and that seems unlikely, since it would readily gather the gas it needed to be a gas giant), it would have to be made up almost entirely of high-density materials. $\endgroup$
    – jdunlop
    Feb 22 at 17:20
  • $\begingroup$ @jdunlop I am going off of the idea that the OP simply means "why would humans settle a heavy world environment?", not the specific example from Mission of Gravity that inspired the question since that would be off topic. There are plenty of mundane super-earth exoplanets that this question could be applied to. $\endgroup$
    – Nosajimiki
    Feb 22 at 17:59
  • $\begingroup$ This is true, but they do call out 700 gravities later in the question, and specify extremely high-gravity. $\endgroup$
    – jdunlop
    Feb 22 at 18:55
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    $\begingroup$ @jdunlop, I think the more important argument here is that a 700G world is impossible to begin with, not what it may or may not be made out of. I've refined my answer to address this. $\endgroup$
    – Nosajimiki
    Feb 22 at 20:19
  • $\begingroup$ You are 100% not light element mining on a planet that heavy. *and dense. When your crust has to be solid Platinum there is not much room left for light elements of any quantity. $\endgroup$
    – ErikHall
    Feb 22 at 20:40
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There is a somewhat frame-challengish answer here - depending on how broadly you define industrial...

Training of Soldiers

Especially if those people are born and raised in the higher gravity environment. Think of certain groups of soldiers from high altitudes who have better Red bloodcell counts - one would presume that growing up from birth in such an environment would lead to higher bone density, stronger muscles etc.

I mean it's a good ol' Sci-Fi trope that supersoldiers are bred on particularly 'tough' planets - Dune, WH40K etc.

But seriously - if you want to justify why there's a 'Scientific research station' in the middle of nowhere with seemingly no point - just stick some military applicability onto it for maximum believability.

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    $\begingroup$ This is definitely more science-fantasy than science-based - assuming human biology can be adapted (probably artificially) to live long-term in high multiples of Earth's gravity, which I wouldn't assume, all of the problems Earth humans are expected to have from long-term living in lower-G environments, (cardiac problems, fluid accumulating in the wrong tissues, skeletal and muscular atrophy) would affect your soldiers when deployed away from their homeworld. Having your army be universally ill is probably not beneficial. $\endgroup$
    – jdunlop
    Feb 22 at 20:59
  • $\begingroup$ @jdunlop - Interesting points - however with a proper troop rotation schedule, you could mitigate the 'long-term' aspect - and if your soldiers are able to carry more weight into Battle (More Ammo, more armor etc.) that could easily offset the additional logistical concerns. $\endgroup$ Feb 22 at 22:06
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    $\begingroup$ Goku and his Sayan bros from Drsgon Ball usually train in higher gravity environments as well to get stronger. $\endgroup$ Feb 22 at 22:21
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    $\begingroup$ Aside from the issue brought up by @jdunlop, you also have to deal with the side effects of these soldiers’ proprioception and instinctual motor control having developed in a high gravity environment. There will be an adaptation period when they relocate to a lower gravity environment, and it will almost certainly not be short. During that period, they’ll be borderline useless as soldiers (their instincts will be off), dangerous for deployment as manual labor (as they will probably underestimate their strength), and likely very accident prone (again, underestimating their strength). $\endgroup$ Feb 23 at 2:31
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    $\begingroup$ @TheDemonLord No, it’s still really dangerous for the soldier, because it means their body is not moving the way they instinctually expect it to. That’s really dangerous in a fight. $\endgroup$ Feb 23 at 12:55
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ErikHall's answer calculates that the density of Mesklin should be about 19.702 grams per cubic centimeter.

And many have said how improbable a world with such a density would be.

As a matter of fact, I asked a question two years ago about the densest know exoplanet.

https://astronomy.stackexchange.com/questions/48615/what-is-the-densest-known-exoplanet

In it I pointed out that the measured mass and radius of Kepler 131 c should give it a density between 25.308 and 152.388 grams per cubic centimeter. And the paper describing it said that the density is way to large and so the mass measurement must be an error.

And the only answer to the question says that the density of Kepler-131 c is unphysically high. It suggests that a better candidate for most dense known exoplanet is KELT-1-b at about 23.7 plus or minus 4 grams per cubic centimeter, or 19.7 to 27.7 grams per cubic centimeter.

https://arxiv.org/abs/1808.04533

This paper reduces the density of what was considered the densest known exoplanet, CoRoT-3~b, from 26.4 to 17.3 plus or minus 2.9 grams per cubic centimeter - or 14.4 to 20.2 grams per cubic centimeter.

So according to that paper, the latest measurements indicated that there weretwo known exoplanets with masses roughly in the range of the 19.702 grams per cubic centimeter of Mesklin.

And no doubt the majority of scientists who learn of those results will assume there are errors in the measurements and that both CoRoT-3~b & KELT-1-b will turn out to have lesser densities when more accurate measurements are made.

But if not...

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    $\begingroup$ There's your scientific research station's raison d'être: "This thing shouldn't possibly be able to exist, and yet it does. We're here to find out what's going on allowing it to exist at all..." $\endgroup$
    – Ralph J
    Feb 24 at 2:59
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Leaving aside what prevents such a planet from instantly imploding into a very small black hole. . . Let me do some math...

So Mesklin is said to have a Polar diameter of 31770 km and a equatorial diameter of 77250 km. The Volume of a Spheroid is given by $\frac{4}{3}\pi a^2b$ where $a$ is the big radius and $b$ the small one. So, $V = 794150480000000 km^3$. We know $g = \frac{GM}{r^2}$ We want to solve for $M$, so $M = \frac{g r^2}{G}$ I think. Of course, we have two radii and g to deal with and I am not about to do the math for a spheroid. BUT the mass is about $1.56\times10^{28} kg$. And the Density $19702 \frac{kg}{m^3}$ Or $19.702 \frac{g}{cm^3}$. Thats about the density of just pure Plutonium. Slightly denser than Tungsten.

So i guess there is your answer, mine like all the Plutonium and Tungsten in the world. Or rather, the 10 densest elements known to humankind. Since that is almost certainly what the whole planet is made of. Or chunks of Neutron star matter.

At this scale, you could also probe General Relativity. Such a massive and compact object would seriously deflect light due to Gravitational Lensing and have noticeable time dilation effects... At least visually you would most certainly see something. Also a great place to test for Frame Dragging.

EDIT; After some more research it turns out a planet like this wouldnt implode at all. Intermolecular force are indeed strong enough to prevent the planet from imploding. Even if they were not, the Planet would be held up by first Electron and then Neutron degeneracy pressure. The same forces holding White Dwarfs and Neutron stars up. Such a planet could also not be made from chunks of either, because White Dwarf or Neutron Star matter isnt stable outside the enormous pressures found on either object.

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    $\begingroup$ Don't forget about critical mass... such a large chunk of any element that high up the periodic table would enter a spontaneous fission reaction LONG before this world could accreate. $\endgroup$
    – Nosajimiki
    Feb 22 at 20:23
  • $\begingroup$ Yeah i kind of looked over that xD $\endgroup$
    – ErikHall
    Feb 22 at 20:40
  • $\begingroup$ Also, you can't exactly get just the 10 most dense elements arbitrarily. Different astral events spray out different ratios of elements, even a Neutron Star explosion mostly spits out period 5 elements; so, you will never have a naturally occurring accretion disk with elements that average more dense than maybe 10-11 g/cm³ $\endgroup$
    – Nosajimiki
    Feb 22 at 21:57
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    $\begingroup$ Forget mining the planet made from plutonium: the moment someone digs up a shovel of material and puts it into a differently ordered pile, the planet might go supercritical for you disturbed the carefully crafted arrangement that prevented it from blowing up. $\endgroup$
    – Trish
    Feb 23 at 1:00
  • $\begingroup$ @Trish Natural nuke for K3 civ. $\endgroup$
    – DKNguyen
    Feb 23 at 3:04
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Entertainment

There are people in the real world that pay actual money to see things like Jackass and team sports. Under higher gravity, these kinds of shows would be even more entertaining to watch.

Waste disposal

There is a kind of trash that you don't want around on your planet, but you also don't want to send it to interstellar space or into a black hole in case you need it right after discarding it. For example, DVD players. You wonder who the [redacted] has those nowadays until your very emotional spouse wakes up someday insisting they want to rewatch the videos from your wedding, and then you have to run to the garbage bin to fetch that player before the garbage truck passes by to take it to the dump.

Store such trash on another planet so it doesn't pollute your own water sources and stuff. The planet having high G discourages scavengers bringing the trash back frivolously.

Private prisons

High G planets are costlier to leave, so escaping a penal colony in one such place should be much harder. Take your prisoners there.

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    $\begingroup$ Jackass yay. Finally it got included in an answer. +1 $\endgroup$ Feb 22 at 22:33
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Filtration is the first thing that comes to mind: high gravity would amplify the differences in specific weight between similar species, making separation more easy.

Think, for an example, to the separation between different isotopes of Uranium. It usually requires turning the Uranium ore into a gaseous compound which is then spun into high speed centrifuges until the slightly heavier isotope is separated from the slightly lighter one: just waiting for the weight difference to make its job would take too much time to make it practical.

Now imagine that, instead of needing a centrifuge spinning at 90000 rpm, you can accomplish the same feat just because gravity is stronger and helps you separate the two isotopes in the same way as you can separate water from oil.

Imagine separating water from suspended soil way faster because, again, higher gravity makes it faster for the soil to precipitate.

And so on and so on.

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    $\begingroup$ I don't see how gravity affects a centrifuge one way or another. Centrifuges typically operate on a vertical rotation axis, otherwise you'd get a lopsided force which was higher at the bottom than the top. Gravity typically operates orthogonal to the direction of centrifugation. A centrifuge merely requires a difference in density, not weight - it would still work in zero-gravity. As I understand, you can use a countercurrent flow to improve efficiency, but that's an induced flow that could not be caused by a constant force like gravity alone. $\endgroup$ Feb 22 at 16:51
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    $\begingroup$ @NuclearHoagie, clarified $\endgroup$
    – L.Dutch
    Feb 22 at 16:53
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    $\begingroup$ My point is that a centrifuge doesn't stratify along the vertical direction, so gravity won't affect it one way or another. Different density liquids will naturally stratify well in high-G, but not at all in zero-G (in which direction could they stratify?). A horizontal centrifuge, on the other hand, works equally well in zero-G as in 1G, or in 100G for that matter, because the centrifuge stratifies orthogonal to the direction of gravity. Weight due to gravity is irrelevant when there is nothing happening in the vertical direction, the centrifuge operates in a horizontal plane. $\endgroup$ Feb 22 at 17:13
  • $\begingroup$ @NuclearHoagie, if you have stronger gravity the also fluid which are, under our gravity, difficult to separate because their specific weight difference is too small, would become separable in reasonable time. Uranium fluoride is just given as extreme example of bi component fluid hard to separate. $\endgroup$
    – L.Dutch
    Feb 22 at 17:21
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    $\begingroup$ A centrifuge is a way of artificially simulating a high-gravity environment with a large radial force that causes separation. In a sufficiently high-gravity environment, just leaving the material in a tank will cause it to separate. I'm just not sure how economically-viable the effect would be. $\endgroup$
    – Stephen
    Feb 22 at 17:54
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Humankind wants to have fast space ships whose acceleration is at least 2 g, maybe 3g. They want to study how the body can be adjusted to such conditions for prolonged time.

Doing so on a planet is safer and possibly also more economical than in an experimental ship, and a centrifuge has its drawbacks. This configuration allows scientists to concentrate on both aspects (the ship and the people) separately.

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Perhaps the best reason for humans to be there is to study the biosphere and the flora and fauna on such an extreme world and to investigate what adaptions have been made to cope, especially the variation and distribution of organisms across huge gravity gradients.

It would also be interesting to contrast and compare the ecological niches found on this planet and the similarities and differences to those on Earth (predator - prey, parasite - host, photosynthesis etc).

If it is possible to communicate with the locals they might be of the greatest interest of all. Perhaps the first example of intelligence beyond Earth? And even if they were not really intelligent, studying the biochemistry resulting from an alien abiogenesis event (and a high gravity one at that) would be a scientific bonanza of unimaginable scale. The whole of biochemistry rewritten from scratch and rewritten differently due to the high gravity environment. What is the same and what is different from the biochemistry we see on Earth?

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