Submitted by MoreGull t3_11dhit7 in space

Summary: The Jovian moon Callisto is the best body in the solar system to consider for future human colonization.

Reasons: It's got a relatively high surface gravity.

It's unique in the solar system among large objects that it is undifferentiated, meaning there will be large mineral deposits at or near the surface.

It is outside Jupiter's massive radiation belt, which for reference does envelop the larger moon Ganymede.

There is water ice easily available on the surface. This would provide not just water but also oxygen.

Close proximity to the asteroid belt and the trojan asteroids around Jupiter, thus any asteroid mining efforts would have essentially limitless supply.

And the future is in orbit around Callisto, going to the surface to mine minerals and collect ice.



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UmbralRaptor t1_ja8n4ul wrote

Assuming we get extremely good at building habitats, maybe.


MoreGull OP t1_ja8nimj wrote

No doubt. Any talk of colonization assumes a highly functioning orbital economy well in place around Earth.


Ok_Champion6840 t1_ja9yhah wrote

Just leave Europa alone. Europa is off limits.


3SquirrelsinaCoat t1_ja8wgp0 wrote

The challenge with all colonization is the motivation. In theory, your idea makes sense. But that's a long way to go unless colonization is heavily motivated. To build so far away, even with future tech that allows us to get there in say a couple months, the builders would need a huge reason to go for it. I'm thinking Earth becoming uninhabitable or certain groups being under threat if they remain on Earth. I cannot think of a grand reason why we would go so far unless Venus cloud cities are proven useless, Mars isn't workable, and the Moon is for whatever reason off the table.

But if we're building orbital stations, then what does it matter where we put them? And if gravity isn't a factor for an orbital station, why do we care about the surface gravity of the body we are orbiting?

I like your idea a lot, cool premise for a story, I can't think of a reason it would ever come to pass.


MoreGull OP t1_ja8xcet wrote

Economics are the only realistic reason. Imagine a huge deposit of uranium is discovered near the surface on Callisto, easily mined. That would create an industry around the effort, which, due to distance, would create colonization.

Economics, mining specifically, are the only reason we will spread out into the solar system. Other than a scientific probe here and there....


_Bl4ze t1_ja9cd7h wrote

Does that necessarily imply colonization, though? We might just send robots to mine things. Sure, there's minutes of delay because those pesky little photons can't be whipped into going any faster, but we can automate a lot of it and it's not like the task involves particularly quick reaction times, seeing as the mineral isn't going anywhere.

And with an remote-controlled miner, you don't have to worry about all the extra dead weight of the hairless monkeys and their habitat and food and water and needing to bring them back before their bones rot from too little gravity.


MoreGull OP t1_ja9d4vi wrote

It's a question of how capable the tech is I suppose. Can robotics easily reproduce people?


Kantrh t1_jaaig9u wrote

The energy cost to bring that uranium back is immense though.


MoreGull OP t1_jaaivwk wrote

What If uranium is super scarce on Earth....


Kantrh t1_jaakrwi wrote

Going all the way to Callisto to mine it still wouldn't make sense.

Uranium is actually one of the more common elements in the crust. It's just about finding an economically viable concentration and going to Callisto isn't. Aside from the fact that the surface is covered in Ice


jeffh4 t1_jaa05n3 wrote

Beside economics, the other overriding reasons in science fiction for establishing a space colony are fundamentalist regimes and fundamentalist religious groups.

Actually, add "extremely rich" to the beginning and "who whole-heartedly embrace technology" after then end of the two groups named above. We're unlikely to ever see the an Amish barn raising on Callisto. :-)


isleepinahammock t1_jabfjxy wrote

Ok. Here's the plan:

  1. Spend centuries fully terraforming Callisto into a paradise with verdant Earthlike conditions on its surface.

  2. Populate it exclusively with Amish settlers.


Keh_veli t1_jabqz2b wrote

I don't know how Earth could become so uninhabitable than humans would be better off elsewhere. Even after an all out nuclear war Earth would still be the most habitable place in the solar system. If we have to live inside habitat domes or whatever, it's a lot easier to build them here than in space.


RoyalFalse t1_ja9kb90 wrote

Is there a protocol in place to have this reasonably considered? If so, what should we call it?


GeorgeOlduvai t1_jaazn6l wrote

Project Hope Hubris?

Edit - I'm so happy a few people understood this reference.


spacetimeguy t1_ja8x79q wrote

I agree 100%. Callisto gets less radiation than the Earth does.

Robotic craft could also mine the other Jovian moons for raw materials, possibly including hydrocarbons.

It's an excellent vantage point for deep-space observations and close-up study of Jupiter and its other moons. The only real challenge is energy. Solar is way too weak out there.


MoreGull OP t1_ja8xo9y wrote

Solar energy would still work on Callisto, though, like you say, it would be weak. But all the hydrogen fuel you need is right there on the surface...


rksd t1_jab1zke wrote

In the form of water which you need to crack which takes energy.


bluestraveller42 t1_jabu7i0 wrote

Then to extract energy from the hydrogen you can either burn it ( just recombine with the oxygen you just dissociated from) or put it in a (fusion) reactor.


rksd t1_jabvua8 wrote

I forgot about the possibility of fusion but that seems like a later on mission since we haven't even cracked practical breakeven fusion here yet.


schnazzychase t1_jacow6y wrote

There actually have been successful experiments in fusion that result in a net gain. The news just doesn't talk about it.


spacetimeguy t1_jacs8y0 wrote


rksd t1_jad7x4c wrote

I'm aware which is why I used the word "practical". When we have a city of say, a million people getting most of their power from a fusion reactor then I might get chubby about the prospect of us operating a fusion reactor the better part of a billion kilometers from here.


MoreGull OP t1_jab2b0p wrote

Indeed. A self contained nuclear reactor.


DonTeca35 t1_ja97h6x wrote

You haven’t played the game or dead space have you? You really want to get infected with whatever roams there 😂😂😂


theycallmeqtip t1_ja9y9rg wrote

I'm glad I wasn't the only one who immediately thought of that


PB_Mack t1_jab90tw wrote

I think the Moon would be better. It's so much closer, and way more economical.


MoreGull OP t1_ja8q1zp wrote

I mention Callisto specifically as I think it is a far better prospect than Mars. And the only advantage the Moon has is proximity to Earth.


chirop1 t1_ja8srtd wrote

The moon is a training ground


lochlainn t1_ja9bry2 wrote


The moon will probably be our orbital industry hub or downtime location long term; we just don't know enough about the interaction with lower gravity yet.

If we can survive on a more or less permanent basis at lunar gravity, it's easier to base there for orbital work: shallower gravity well with no atmospheric drag and closer geostationary orbit for less control lag. Remote work and even shift crews from the moon to lunar orbit make much more sense than from the bottom of Earth's gravity well.

If lunar gravity isn't sufficient for the human body over the long term, it'll still extend our ability to stay in space. We'll just need more crews and more energy to turn them over faster.

And in either case, lunar water and metals will probably be the first source of significant orbital construction material we tap.


MoreGull OP t1_ja9cjxr wrote

Why the moon at all?


lochlainn t1_ja9g620 wrote

Because that proximity advantage, for the bootstrapping phase of orbital industry, is enormous.

Like the previous poster said, the moon is going to be our training ground. But it's also more than that. It's an entire orbital mine with a low orbital Delta V requirement. Lunar orbit is much less expensive to achieve from the surface than Earth orbit. Remote orbital processing will have far less signal lag. Sending crews up in shifts and rotating them down for in-gravity recovery is a fraction of the price it would be to Earth.

We know that microgravity is ultimately catastrophic to humans. We don't know how much gravity it takes to remediate or prevent the damage.

So the moon gives us the perfect, "low" effort testbed. Without the knowledge we can only get on the moon (gravity effects, how to build safe living structures using native materials, how to build low-energy processing facility in low gravity, how to maintain a long term closed ecology of food, waste, and heat), we'd be going into any longer term missions blind.

There are only 3 options for orbital industry; haul it up from Earth, get it from the moon, or get it from a near Earth asteroid.

Near Earth asteroids are, until we develop the capability to actually alter their orbit, effectively a remote deep space mission with pass/fail criticality. There's nothing we could learn from one that we can't learn from the moon, while learning everything else already mentioned at the same time.


MoreGull OP t1_ja8tvdu wrote

Indeed. I think that's its main role for us, other than dedicated scientific pursuits.


Fortissano71 t1_ja99mht wrote

The moon is the tutorial. Once unlocked, we can move on to the higher levels with greater dangers and distances. /s


MoreGull OP t1_ja99xyq wrote

We can still build our orbital infrastructure.


ObviousGazelle t1_jaaiwqt wrote

You forget mars has an atmosphere. Thin, but it's there. The question when choosing the two will be this: Can humans build radiation shielding good enough to live permanently on Mars, and of not then what are the trade offs because Callisto has no atmosphere for aerobraking... So which challenge is solvable first?


Hosni__Mubarak t1_jab4b0g wrote

Can’t you use the orbital mechanics of Jupiter to aerobrake yourself?


MoreGull OP t1_jaaj5ce wrote

Indeed. Another reason to skip Mars entirely.


Dexel_Roosh t1_ja9k8xv wrote

Okay but, I like Ganymede’s name more. Although ngl this is the first time I’ve heard of Callisto and it’s kind of growing on me. Alright I’m in agreement with you.


tsunami141 t1_jaawl1l wrote

Your sudden change of heart has convinced me. Ok we’ll go with Callisto.


CardboardSoyuz t1_jaa2by2 wrote

I agree Callisto has promise but the delta-V to get to the surface of Callisto from LEO is like 28 kps, and the delta V to get to the surface of Mars is about 10 kps and Callisto is no help with the aerobraking.


Shrike99 t1_jabbz5s wrote

Where'd you get 28km/s?

My handy dandy Delta-V map says LEO to Callisto's surface is only ~14.2km/s. I know Delta-V maps aren't super accurate but I find it hard to believe it's that wrong.

For reference, the map also puts LEO to Mars surface at 9.5km/s, which is within a reasonable margin of error of 10km/s.


Reddit-runner t1_jacl8ss wrote

>the map also puts LEO to Mars surface at 9.5km/s,

And that's only if you pretend aerobraking doesn't exist.

With aerobraking it's only 5km/s max. Including landing.


relic2279 t1_jabpcwj wrote

> It is outside Jupiter's massive radiation belt, which for reference does envelop the larger moon Ganymede.

On the surface of Callisto, you'd experience about 10x-12x the background radiation that you'd get here on Earth. That's manageable with strong radiation attenuating glass (according to this). However, many of these cosmology & space blogs ignore galactic cosmic rays/radiation (GCRs). Glass won't protect you from those.

According this this study, the amount of GCR hitting Europa's surface is reduced thanks to being inside the Jovian magnetosphere - this prevents a significant fraction of the GCR spectrum from reaching Europa's surface. But this is a double-edged sword, Callisto is less protected from GCRs since it's further away from Jupiter's protection. In fact, the study goes on to say:

> "In addition, the cut-off rigidity threshold is expected to decrease in energy as one moves outward in the Jovian magnetosphere. Therefore, the GCR contribution to surface radiation processing may be more significant at these bodies, particularly at Callisto, which is bombarded by magnetospheric particles at a much lower rate than Europa, and which does not possess Ganymede's intrinsic dynamo magnetic field."

GCRs aren't a huge issue if you're taking short jaunts to the Moon or a space station, but that (combined with radiation, solar flares/ejecta/particles, etc) isn't something you'd want to experience long term. And it's not just about raising your risk for cancer by orders of magnitude, it can make you sick/unwell which puts the entire mission at risk. See here and also here.

In this study about dealing with radiation on a potential manned-trip to Mars, they find the "overwhelming health risk" comes from GCRs. This is because they cascade and create secondary radioactive particles and so on. In fact, he thicker the material they pass through, the more secondary particles there will be in the cascade. In condensed matter (liquid or solid) the shower of a 1 GeV proton continues to grow until it reaches about 200 g/cm2.


MoreGull OP t1_ja92egl wrote

3 year trip to Callisto orbit, 2 years on site, 3 years back.... You think people would sign up for a 10 year plus committment?


lochlainn t1_ja9bzgo wrote

People boarded the Mayflower, and every other colony ship to the New World, without even the guarantee of a return trip or survival at all.


jeffh4 t1_jaa1hyk wrote

As long as we don't have the stupid premise of Garden of Rama, then I'm fine.

What was that, you ask? A selection committee was created to select only the finest of the finest 2000 of Earth scientific minds to take the return journey to the Rama Node. However, they can't get that many so they fill in the extras with ... convicts. Not only convicts but convicts with life sentences. The story then proceeds to not bother to mention the few scientific minds chosen to go on the mission.

Number one in my "all time disappointment sequel" list. Number two is the sequel.


FlingingGoronGonads t1_jab0duk wrote

OP, it's nice to see someone thinking about the Galilean moons, and Callisto in particular. All of these worlds are interesting and they fly under the radar quite a bit, so I'm happy to see this post. There was even an announcement last week about Callisto seeing aurorae, which I never saw coming.

I want to first ask about your last point - what do you mean by "the future is in orbit around..."? I understood the second half of the sentence, but not the first.

Re: point #5: Mars is much closer than the Jovian system to the Main Belt, and the vast majority of known asteroids in the inner system. There are some interesting asteroid groups closer to Jupiter (e.g. the Hilda family, or the Trojan clouds that the Lucy mission will be investigating), but Mars has the advantage there in terms of proximity.

Re: point #2: Yes, Callisto is pretty undifferentiated, but not completely so - from what I understand (see papers like this or this) it does have some internal structure. I mention this because I don't think we know enough about weird objects like Callisto (or Ceres, to give the only other remotely comparable example I can think of) to speculate on the ease or difficulty of finding minerals in accessible quantity near the surface. All of our detailed geology experience is from well-structured/differentiated worlds (Earth/Luna/Mars), and we don't know that undifferentiated/messy mantles like Callisto's won't be worse at concentrating ores. I'd be speculating if you asked me about magmatic or hydrothermal processes in such places.

All of that being said, I would humbly like to add one point to your set of arguments: Callisto's location in the Jovian system seems like a huge advantage to me. Jupiter is a fantastic source of magnetic energy and light elements, and the other moons in the system are excellent targets for exploration, especially if you've a base camp on reasonably stable Callisto. Science aside, I wonder if there are resources available on the other Galileans that Callisto may lack, bolstering the overall case. I personally imagine Callisto would be part of a "second wave" of solar system bases/international scientific villages, but this is interesting to think about, for sure. Sign me up for the first expedition!


ICLazeru t1_jab4ci0 wrote

Sounds like it could work provided fusion gets off the ground as an energy source.


Decronym t1_jabpx2g wrote

Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:

|Fewer Letters|More Letters| |-------|---------|---| |GCR|Galactic Cosmic Rays, incident from outside the star system| |GeV|Giga-Electron-Volts, measure of energy for particles| |LEO|Low Earth Orbit (180-2000km)| | |Law Enforcement Officer (most often mentioned during transport operations)| |SoI|Saturnian Orbital Insertion maneuver| | |Sphere of Influence|

^(4 acronyms in this thread; )^(the most compressed thread commented on today)^( has 18 acronyms.)
^([Thread #8630 for this sub, first seen 28th Feb 2023, 07:30]) ^[FAQ] ^([Full list]) ^[Contact] ^([Source code])


PhotonicSymmetry t1_jad272c wrote

I agree with your points OP. I think over the long term, the primary place of human habitation is going to be in orbital habitats (particularly O'Neill cylinders). However, Callisto is as you say one of the strongest candidates for a place for human habitation in the solar system - if not the strongest.

Callisto as the primary hub of the Jovian system seems like the most likely outcome. I think over the medium to long term, the Jovian system becomes the largest economy in the solar system - exporting tons of raw resources to other settlements across both the inner and outer solar system. It will likely get overtaken by Neptune eventually, but Callisto as THE economic juggernaut of the solar system for a period at least is not at all far fetched.


MoreGull OP t1_jad3apc wrote

Indeed. Resource extraction would be the main focus. Also serving as the "gas station" of local space travel. Which given the vastness of the Jovian system could be mostly local.


Real_Affect39 t1_jadb3tv wrote

I actually made this argument in a uni paper I wrote recently, Callisto is the only Jupiter of Moon that can be rationally colonised, even sending robotic missions to land on Europa would be insanely difficult due to the radiation belts.


MoreGull OP t1_jadbzpn wrote

Indeed. The radiation from Jupiter is intense. I think the current spacecraft in orbit around Jupiter (Juno) has its orbits planned to spend as much time as possible outside the radiation belt.


AtomicPow_r_D t1_jadkin3 wrote

(I got the "Europa is off limits" line, from Arthur C. Clarke. Nice.) Callisto is not entirely within the protective shield of Jupiter's magnetosphere, which might mean it is bombarded by the sorts of things that are unfriendly to living creatures like us. So it could be very difficult. Mercury, which does have a magnetosphere, is too close to the Sun, whose solar wind overpowers it at that range. So the Solar System is not very cooperative in this regard.


MoreGull OP t1_jadkqf3 wrote

Callisto is in fact blessed to not be within Jupiter's protective shield.


volcanopele t1_jae9180 wrote

Except every time you come back into the hab there is the brown soot that you can't get off your boots so it gets tracked everywhere...

But seriously, about the water ice. Yes it is easily available on the surface, certainly more so than say the Moon, but in many places it is buried underneath a lag deposit of [insert non-ice component name here]. So the easiest exposures are on steep slopes.


MoreGull OP t1_ja94lu5 wrote

To add to this discussion: Mars is a joke. I can't see any realistic benefit at all for human efforts directed at Mars.


lochlainn t1_ja9d3e0 wrote

Other than scientific inquiry or barring some specific mineral wealth, I agree. Everything we need can be found floating in space except for a gravity well to live in. If we're capable of living in space, why deal with an atmosphere that does nothing but add to the energy cost of leaving it?

The future entirely depends on just what we discover about human adaptation to microgravity, likely from experimentation on the Moon. If we can remain healthy and especially reproduce in fractional gravity, other planets have much less appeal than the moons and asteroids that don't require a huge energy expenditure to reach.


dgames_90 t1_jacpg4t wrote

Mars is a pipedream cultivated by sci-fi and scam artists.

Terraforming Venus would be a lot better but much harder techwise.

A bunch of juvian and saturn moons could also be interesting since they have gravity and readily available water.

anyway colonizing any of those is so far out we will have to see how tech develops.


MoreGull OP t1_jacpswl wrote

I think terraforming is far out as to be basically pointless to discuss. Could it be done? Sure, theoretically. Will it anytime soon? Not likely.


PhotonicSymmetry t1_jad3ps5 wrote

Hey, I wouldn't call it a pipe dream cultivated by scam artists. Mars has appeal purely for the exploration - just like any other planet. Humans will surely set foot on Mercury too if we don't eliminate ourselves sooner. Mars will never be terraformed and terraforming Mars is a terrible idea. But a reasonable sized human settlement for scientific purposes with a decent number of orbital habitats is in the cards. Over time, I expect most people living within Mars SOI will be living in orbit and perhaps going down to the surface to work.


dgames_90 t1_jae1txj wrote

>Mars will never be terraformed and terraforming Mars is a terrible idea.

isn't musk selling this idea? colony with MILLIONS of people?

he isn't the only one, MarsOne comes to mind among many others.

I totally agree with what you say, having small bases like in the artic, or simple mining colonies is feasable and interesting, but the talk here is fully functional independent colonies. that's not gonna happen anytime soon (200++ years).


djellison t1_jabjb0i wrote

>It's got a relatively high surface gravity.

Venus, Mars, Mercury, Io, our Moon, Ganymede, Titan and Europa all have higher.

>It is outside Jupiter's massive radiation belt

As are Venus, Mars, Mercury, our Moon

>There is water ice easily available on the surface

As there is at Mars and some would argue on certain parts of the Moon

>Close proximity to the asteroid belt

So is Mars.

>and the trojan asteroids around Jupiter,

They're actually a long way from Jupiter.

>The Jovian moon Callisto is the best body in the solar system to consider for future human colonization.

For what purpose?


dgames_90 t1_jacq1j2 wrote

>For what purpose?

expansion of the human race. increase survival of the species in case of disaster.


djellison t1_jad6t03 wrote

Yeah - Callisto is an awful place to do any of that.


anotheroutlaw t1_ja8ozfw wrote

How many generations of work and technological advancement in physics, engineering, biology, chemistry, materials science, medicine, etc. do you think it would take to colonize Callisto? Off the top of my head I would say 500 generations.


ObligatoryOption t1_ja8rdug wrote

> 500 generations

10,000 years? I think you err far to the pessimistic side.


anotheroutlaw t1_ja93skk wrote

I studied history. Periods of human enlightenment are short lived and interspersed between long periods of difficulty. To actually colonize a hostile object beyond Earth would require a level of cooperation and scientific focus never-before seen in human history.


ObligatoryOption t1_ja9565z wrote

Humanity today bears little resemblance to what it was 10,000 years ago.


anotheroutlaw t1_ja95zeb wrote

And the humanity that colonizes another celestial body will bear little resemblance to us.


ObligatoryOption t1_ja96mv6 wrote

If you don't think the current generation will see humans on Mars, you're in the minority.


anotheroutlaw t1_ja97z62 wrote

And in 1969 everyone thought we’d be on Mars by the end of the century. That being said I am hopeful we make it to Mars in my lifetime. But I also know that overseas wars and the military will always siphon the incredibly large majority of our tax dollars. Geopolitics can derail achievement in space at any moment.


MoreGull OP t1_ja9hw31 wrote

If you are looking foward to first steps, I agree. It will be awesome when a man steps on the surface of Mars.

But other than being cool it's pointless.


MoreGull OP t1_ja95afn wrote

Or a highly incentivized profit....


anotheroutlaw t1_ja969ii wrote

Whatever profit is to be made will not be enjoyed by those who start this kind of work. They will be long dead. The initial cost of this work would be in the trillions.


MoreGull OP t1_ja986r1 wrote

The key would be there are profits to be made every step of the way....


doctorclark t1_ja996qo wrote

The true profit was the 499 generations of human civilization we met along the way.


MoreGull OP t1_ja8pj5u wrote

I think we could do it right now if we committed ourselves as a species. Which we won't, and aren't even remotely close to, so.... I'd say it entirely hinges on commercializing space. When it becomes profitable to engage in space based endeavors is when colonization of other places enters the realm of realistic.


Fit-Capital1526 t1_ja8ti7j wrote

500? I mean it’s entirely possible in the next 200 or so years. Tunnelling is a pretty good industry for some nations, and is likely to only improve considering the protection an underground structure would offer on the Moon and Mars

And that would be 8 generations at worst. At the moment, major hurdle is getting a manned mission to Callisto


anotheroutlaw t1_ja933wf wrote

You think we can colonize Callisto in 200 years???


Fit-Capital1526 t1_ja93kbb wrote

As the OP pointed out. Callisto is the easiest of the Jovian moons to visit. It is also ideal as a jumping off point for the rest of the Jovian moon. With predictions about the idea we will have put people on Mars by the 2100s. The idea of a semi-permanent or permanent base on Callisto being built in by 2200 is pretty likely


anotheroutlaw t1_ja93ys0 wrote

OP didn’t say build a base. He said colonize.


MoreGull OP t1_ja98x0s wrote

1000 people living and working above and on Callisto - is that Colonization?


anotheroutlaw t1_ja99yx4 wrote

Sure, I think reproducing as well. But you may have had different ideas when you said colonize. Creating a mining colony is very different than creating a colony for people to establish themselves for generations.


MoreGull OP t1_ja9a704 wrote

If it takes 3 years to get there and 3 more to get back, it will become a real colony if people persist at the location.


anotheroutlaw t1_ja9bjdi wrote

People can’t just persist there. Survival alone requires technological feats never-before seen in human history. You need to raise the temperature hundreds of degrees, you need oxygen, and you need to account for psychological factors like a lack of sunlight or knowing certain death is a certainty outside the human created environment in which you live.

You can’t just drop people off and say “see you in three years!”.


MoreGull OP t1_ja9cul9 wrote

I know, that's why I make a big deal of it. Can people actually live - with families, with lives above and beyond whatever job they are out there for....


anotheroutlaw t1_ja9ieku wrote

I think it’s a great question you posed and one with exploring, no pun intended