Today’s topic, Rotating Habitats, is going
to be a rather long one by the standards of this series thus far, so we’re going to
jump right in. On the off chance this is the first of my
videos you’ve ever seen though, you’re strongly encouraged to turn on the closed
captions, my voice takes a bit of getting used to. So our subject today is Rotating Habitats,
and the first thing to understand about rotating habitats is that it is a huge zone of options,
all linked by only one common denominator: Centrifugal Force. If you’re in a place that has no gravity,
and you want some gravity, the only two ways we currently have to do that is to either
pile a ton of mass together for its natural gravity or to fake it with ‘spin gravity’,
essentially to use centrifugal force to mimic gravity. Odds are if you’re watching this video you
already know what centrifugal force is, we all encounter this force on a regular basis. You’ve probably heard it referred to as
a fictional force as well, or more accurately as one which does not exist in an inertial
reference frame, but for our purposes it’s real enough. It’s real enough because it lets us hold
objects down against a surface like there was gravity even though there isn’t, and
so long as the vessel you’re spinning is decently sized, basically bigger than a football
field, the mimicry of gravity holds for most biological purposes. So we can take a big ring, or cylinder, or
torus, or anything else with radial symmetry like a sphere and spin it around and the sides
become a floor you can walk around on. You can even jump up and down and land where
you’re supposed to as the fake gravity keeps working even when your feet leave the floor. You won’t quite fall straight down due to
Coriolis Effect but for any normal human leap on any decently sized rotating habitat you’d
never be able to tell you missed your mark without highly precise equipment. This gives us our first issue with using rotation
to fake gravity though. That Coriolis effect can be a bit disorienting
on humans as it acts on the inner ear to cause dizziness and nausea. As best as we can tell anything beneath 2
RPM, 2 rotations per minute, doesn’t affect anyone, and we expect people could adapt to
rates of 20 RPM or higher. It’s basically akin to motion sickness though. Problem is, a slower rotation, or fewer RPM,
results in weaker gravity. That’s fine for a space station, we can
get away with picking astronauts who are less sensitive to the effect and go with less gravity. You could get away with having a metal can
in space 30 feet in diameter spinning 10 times a minute and producing half gravity for astronauts. That’s probably okay for some Mars Mission
where they need to adjust to lower gravity anyway and you can pack a lot of Dramamine
along for the motion sickness. But this video isn’t about space stations
or ships, it’s about full blown habitats. Places that comfortably simulate what we’re
used to. So we’re not interested in anything that
doesn’t produce normal Earth gravity in a comfortable way. To get higher gravity at a slower rotation
we need to make the rotating structure wider, and if you want Earth gravity provided under
that 2 RPM threshold then your diameter is about 1500 feet or 225 meters. This is basically the minimal threshold for
building mock environments since the idea is comfort, you can go wider, but you don’t
really want to go skinnier. You can’t go too much wider though because
the wider these things get, without decreasing the simulated gravity, the more stress is
put on them. For steel the usually assumed maximum is on
an order of a diameter of several miles, for stuff like Kevlar or Carbon Nanotubes it’s
much higher and is a lot like the problem we discussed way back in episode one regarding
space elevators. Essentially the breaking length of a material
in normal gravity tells you the maximum circumference of a rotating habitat made of that material
simulating normal gravity because it’s the same thing. Since you’re operating in the vacuum of
space besides the initial energy to get it spinning you don’t need to add much more
to keep it spinning. That’s why mechanical flywheels in a vacuum
are such an attractive option as batteries. No air drag to slow them down. Which means you can sack some of your gravity
for emergency power too. While their diameter is controlled by the
strength of the building materials, and the amount of gravity you want, the length of
the habitat is not, you can go anywhere from a thin ring to an arbitrarily skinny cylinder. So that’s the basic intro, how the fake
gravity works and what the control factors are. When we talk about rotating habitats in any
long term sense, beyond just avoiding health ailments for astronauts, we’re talking about
doing something that mankind has never truly done before, and that’s make more living
space. Oh, we’ve built some fake islands, cut into
mountainsides, or built vertically from time to time but as a whole, while we’ve made
land and sea more livable to us, we’ve never added to it. Earth is our only world and its size does
not change. If you want to add more people you can improve
your farming technology and in the video on Fusion we discussed some of the ways you can
use that, if you’ve got that, to really push out your maximum sustainable population,
often called your carrying capacity, without wrecking your ecology or reducing everyone
to a lower standard of living. There’s some other ways to push that even
further we’ll look at in the future but ultimately you can just only pack so many
people on a planet comfortably before you run out of space. Rotating Habitats give us a way to increase
that space. The classic version of this is called an O’neill
Cylinder, and its 20 miles long and 5 miles wide, about as wide as you’d comfortably
want to make something like this out of steel. That means its internal surface area is 314
square miles. For comparison that’s about half again as
big as Guam or a third the size of the State of Rhode Island or a quarter the size of Long
Island New York, and almost identical in size to the island nation of Malta. So an O’neill Cylinder is not a small object. And you can go larger, titanium would roughly
let you quadruple that, and stuff like Graphene could hypothetically let you make things on
par with continents. You can also connect the things together,
like a string of sausages or in various other configurations. So that material strength issue isn’t all
that strong a control factor on your true interior size since they can be linked to
fairly seamlessly create one greater structure, even if it would be more like an island archipelago
than a vast continuous plain. You can also go bigger by having multiple
levels, the lower ones having slightly higher gravity than the higher ones, which is actually
true on Earth too though much less noticeably. You can only go so many levels before even
just the waste heat of lighting the place would make it uncomfortably warm even with
an array of radiator fins on the cylinder. In space you can only get rid of heat by radiating
it away, same as how our planet gets rid of its own heat. In and of itself that’s the basic intro
to what rotating habitats are and what the basic issues with them are. Now let’s get into some of the more fun
aspects as well as some of the challenges. The first and most obviously big one is cost,
which is way worse right now when we have to drag every ounce of building material up
into space at phenomenal costs. We already talked about that in the prior
videos though, and space is full of asteroids we can cannibalize too. If you feel like we’re going invent fusion
one day, that we’re going to get way better at automated manufacturing and 3D printing,
and you think we’ll get one or more of those cheaper launch systems built that we discussed
in previous videos, then we can skip cost for now. Needless to say building new living area from
scratch is a pretty major endeavor. But if you’ve got all three of those things
you can do it. Heck you don’t even need fusion but it saves
the effort of screwing around with mirrors to bounce sunlight in to the habitat or transparent
sections or needing to keep them fairly close to the sun, meaning you can use those asteroids
out in the bet without having to either drag them close to the sun or creating giant parabolic
mirrors to bouncing light in. We should start this section then by discussing
one common misconceptions about rotating habitats, and that’s the idea that you can see one
spinning. Most of the images or videos of these I’ve
put up so far, or that you can see elsewhere, always show them spinning. Usually when someone talks about building
them inside hollowed out asteroids they will say they spun the asteroid. That last is especially wrong since only the
largest asteroid have any really noticeable surface gravity and they’re all basically
wads of gravel loosely held together. Spin one up to Earth gravity and it will fly
apart. But the notion of using hollowed out asteroids
is on the right trail, because all that rock under your feet between you and space provides
nice shielding from radiation and meteorites. Here’s the thing though. You don’t need your exterior shielding to
spin any more than you need the casing for a centrifuge or washing machine to spin. In fact it’s pretty damn dumb to do that. Space ships with rotating sections won’t
have some big hub you can see turning from outside, just some superstructure that doesn’t
spin that it’s nested inside. That way your superstructure shielding isn’t
under all sorts of strain from spinning when it’s taking hits, and what’s more everything
that hits a rotating object is going to either add or subtract some of that spin speed to
its relative strike speed, damage is pretty much synonymous with raw kinetic energy, which
goes with the square of velocity, even though half as many objects are striking slower and
half faster, you still take more damage. So you don’t see rotating habitats spin
since inside you’re spinning with it and can’t tell and outside it’s surrounded
by some non-rotating superstructure, or possibly one rotating considerably slower in the opposite
direction. This shielding material doesn’t necessarily
need to be rock, or ice, or metal either. You could use the most common substances in
the universe, hydrogen and helium, as shielding. Hydrogen is also one of the best shields against
cosmic radiation, pound for pound. So you could surround your rotating hab with
a non-rotating superstructure full of hydrogen tanks and other layers of shielding as seen
appropriate. On a ship you can use that hydrogen as fuel,
and you can also use your air and water reserves as more shielding. Radiation doesn’t really hurt them and better
a micrometeor knock out a bit of your reserves than to knock out you. But in the context of asteroid mining we would
presumably use the slag. The thing is, you don’t really need to hollow
out an asteroid. If you come across any of the roughly million
or so asteroids in our solar system that are around a mile wide that’s really not a good
approach. It’s not hard, shoveling rock on even a
big asteroid with decent gravity is like shoveling packing peanuts, and even on the largest,
Ceres, generally considered a dwarf planet now not an asteroid, you could bench press
a truck without breaking a sweat. One these smaller ones, the mile across kind
that outnumber the big named ones thousands to one, you could kick around boulders the
size of your house and your big problem mining is you’d need to erect a dome over you to
keep the debris flying off into space. Asteroids generally don’t tend to be one
solid chunk of rock you’d need to cut either, many are basically wads of gravel. Nothing you build inside needs to be terribly
sturdy either, your typical asteroid is so small and with such weak gravity that even
under hundreds of feet of material the pressure isn’t strong enough to crush an empty beer
can, so you don’t really need to shore your tunnels up like you do when mining on earth. So why wouldn’t you hollow one out then? Well in a nutshell because it’s intensely
wasteful of material. Let’s say you come across some conveniently
spherical rocky asteroid a mile across and want to use rock as your shielding. Fact of the matter is anything much beyond
a dozen or so feet is going to stop micrometeors with ease and drop cosmic radiation to near
nothing. Here, on Earth, over your head, is about 14
pounds per square inch of air or 10 tons per square meter. That’s roughly comparable in mass to being
under 10 meters of water or 3 to 5 of typical rock, so you’ve got as much raw mass between
you and space with thirty feet of rock as you do down here on Earth. But let’s say you want a hundred feet of
protection of rock, way more than is needed to protect you from anything but a direct
nuclear strike. You’d still have only used about 3 or 4%
of that mile wide asteroid, and a much smaller percentage on a bigger asteroid. And the rest, all hollowed out, it just air
surrounded by a thin layer of dirt, water, and steel. What do you do with the rest of that raw material? Well you could ship it all off elsewhere but
rock is really only valuable for making habitats once you’ve stripped out the valuable stuff
like platinum, gold, iridium, and so forth it doesn’t have much export value. Truth be told with asteroid in this size range
it’s probably easier to mine it if you spread it out anyway so you might want to just make
the whole asteroid into one much bigger hollow sphere 5 or 6 times wider and then just slowly
replace what you mine over the year with hydrogen tanks. In the long run, in a fusion economy, you’d
want to trade away excess minerals for larger quantities of hydrogen stored in exterior
tanks that slowly replaced that rock as shielding. As discussed in the fusion video you could
light up and power a rotating habitat for billions of years with less hydrogen then
you’d use just normally shielding it from cosmic radiation. So you can take that tiny asteroid and turn
into a nice big sphere with a rotating habitat inside it and lots of zero-gravity storage
or industrial spaces or smaller additional cylinders, maybe to used for hydroponics. When dealing with a bigger asteroid you can
either break it up into multiple spheres or if you don’t want bigger cylinders you can
arrange your cylinders into various geometric shapes touching each other at the tips. This brings up another point. These things don’t have to be the same radius
the entire length, you can taper them at the edges and the gravity will fall off as it
gets more slender. You can also put in dips and rises in the
shell to let you get away with taller hills and deeper lakes without needing to put tons
and tons of dirt and water inside. Similarly new materials like aerogel, that
are incredibly light weight and sturdy, could be used below the topsoil to help. We don’t generally dig much more than a
few meters deep on Earth nor do most roots go much deeper, so there’s now real need
to have hundreds of meters of dirt and rock in these things. Lighting for the inside would either be provided
by mirrors coming in through the cylinder caps or preferably by fusion powered lamps
putting out their light only in those frequencies we can see or that plants use, that helps
cut down on waste heat letting you do multiple layers without sacrificing the aesthetics. And the upward curving horizon can be dealt
with in part by just disrupting the flatness with hills and valleys, though on very large
rings you wouldn’t even see that. Big difference, and the hardest one to deal
with, is that the sky isn’t blue and cloudy, it’s your neighbors, and the stars in the
night sky are their porch lights. You can get some of that blue with lots of
lakes as opposed to grass and forest since water really is blue, but if people really
wanted that blue sky effect you’d probably want to nest another smaller thinner cylinder
inside to fake a sky, preferably a bit more elaborately than just painting it but that
would presumably work. When you’re building land many meters deep
over a thick steel shell building a giant LCD TV overhead isn’t really that much of
a stretch either. And again if you’ve got fusion to power
these things you can build them anywhere. Around planets from the smaller moons or rings,
out in Oort Cloud, out in interstellar space. They’re fairly mobile too though not ideal
as spaceships since they’ve got so much superfluous mass in the name of comfort. As we discussed in the Rogue Planets video,
interstellar space is littered with junk, there might be more planets and asteroids
between two stars than around either in their solar systems. Maybe a lot more. These things are more than big enough to support
sufficient gene pools even if technology didn’t give us a lot of easy workarounds to genetic
bottlenecking. So just as example if some ideological or
religious group here on Earth decided they wanted their own sealed off place they could
grab any of the millions of asteroids or comets kicking around our solar system and turn it
into a habitat able to support a million or so people indefinitely, or even several thousand
if they were of a bit of techno-primitivist bent. These being effectively low-grade space ships
you could set your course for deep space and leave other people behind if you found the
core civilization too undesirable to share space with. Nor do you have to build it all at once. You start with a small cylinder and either
make it longer with time or just add more cylinders. You could even drag in mostly empty prefabricated
ones and arrange them outside the asteroid then just build a thin shell around the whole
thing and disassemble the asteroid for exterior shielding and fill dirt for the habs. In terms of how many of these we can make
in our solar system that all just depends on how thick you want your dirt, since again
you can use hydrogen as your real exterior shielding. If you disassembled all the rocky planets
in the solar system to make habs with about 10 meters thick of dirt and hull you could
get away with fabricating an amount of these equivalent to a few million Earth’s worth
of living area. Less dirt, more living area, more dirt, less
living area. If you’re using that dirt as your main source
of food, rather than mostly hydroponics, a population a few million times our own, if
not, if it’s really more for gardens and lawns and some dedicated habitats as wildlife
preserves, than maybe a hundred times as dense. Okay, we’ve looked at the more plausible
ones. Let’s close out by reviewing some of the
bigger and often more famous designs. As I mentioned earlier if you’re working
with metals like steel, or even titanium, you can just only make these things so wide. Once we discovered carbon nanotubes and graphene
we set our sights a lot higher and came up with two called the Bishop Ring and the Mckendree
Cylinder. These are things with circumferences on the
order of a thousand miles, not just ten or so and they are big enough to nearly be considered
planets of their own. Same concept as before, just bigger. But even before we discovered carbon nanotubes
we already had two rather well known fictional examples. The smaller, more recent, and less well known
of those first appeared in the late Ian M. Banks 1987 novel Consider Phlebas and we call
it the Banks Orbital. What’s noteworthy about this ring is the
rather specific spin rate. It rotates once every 24 hours. Meaning that if you turn it on its side facing
the sun it will replicate our normal 24 hour day night cycle without needing artificial
lighting or mirrors. You can even give it a little tilt to simulate
seasons. Of course you need what we call an airwall
many miles high to keep the air spill out of the thing but the object is so huge you’d
barely even see those and you’d probably sculpt them as fake mountains. You get the same sky, day and night, as on
a planet, and the horizon is so far off all the air in between would probably hide it
so you just saw a thin bridge over head. In order to achieve that 24 hour spin rate
and produce earth-like gravity the Banks Orbital has to be a very specific size. For any given planetary gravity and day length
there is only one unique diameter that will work. An Earth Banks Orbital would be roughly 2
million miles in diameter, and it can be as wide as you want but the wider you make it
the brighter your night sky since the sunlit side will glow. Even one just a thousand kilometers wide is
going to make the nights brighter than a full moon. One that wide would have a couple hundred
Earth’s worth of surface area though. Again you can make them wider but at the cost
of brighter night time skies and since the nice thing about these is how closely they
replicate Earth, since it’s already got a couple hundred time more living area than
Earth, you might as well just build a second neighboring skinny one rather than make it
wider. The obvious issue with building ones of these
is the material stress. Nothing, not even Graphene, comes close to
being strong enough not to be ripped to shreds. Nor could any type of molecule ever do it. In theory some sort of material like Neutronium,
the loose concept for some material held together by the strong nuclear force that binds atomic
nuclei together, could maybe pull it off but the usual method in science fiction is a handwave
to force fields. The next and better known, and also older
and larger design, is Larry Niven’s Ringworld. These are just under a hundred times wider
in diameter than a Banks Orbital and wrap a star entirely. They require an even stronger material than
a Banks Orbital does and since they always face the sun you have to put up shades to
block the light that orbit at some spacing and rate to produce a 24-hour day. And that just has you go from high noon to
midnight in short order, though you could get around that by making the edges of the
shades translucent especially to red light, to mimic twilight. Banks Orbitals don’t have that issue, they
have a natural day and night with regular old twilight and dawn. That’s one of the reasons why the concept
is pretty popular even though it’s newer and smaller than the idea of a Ringworld. Otherwise they’re much alike, and much akin
to the Bishop Ring. You have airwalls to keep your atmosphere
in. Ringworlds can be arbitrarily wide too but
usually we put the number at around a million earth’s worth of surface area or more. They have stability issue, and they’re spinning
at nearly half a percent of light speed meaning you’ve really got to worry about debris
hitting them, but realistically if you can build the thing in the first place those kinds
of problems are pretty insignificant. Kinda like worrying about if you’ve got
enough power outlets in the kitchen on an aircraft carrier, it matters but it’s just
not that big a hurdle compared to floating a hundred thousand tons of steel on water. About the only thing the Banks Orbital has
to worry about that a Ringworld doesn’t is tidal forces, the thing is big enough that
the part near the sun gets yanked on more than the part farther from the sun but that’s
not necessarily a bad thing since if give you tides, another thing rotating habitats
wouldn’t have unless you brute forced it by having attached cisterns that pumped some
water in and out of the habitat on appropriate times. Both of these are very popular designs but
not really in the realm of currently plausible science. Amusingly it is typically in the realm of
doable in most space operas and scifi like Star Trek which is one of the reasons why
it often seems a bit strange the dudes are always squabbling over planets when they could
just build these things instead. Back in the realm of plausible science, but
similar immense in size, is another object popularized by Larry Niven that also showed
up in one of Bank’s novels called a Topopolis. You might recall earlier I mentioned you could
connect rotating habitats together at their ends like sausage links, this one goes one
better and avoids some of the problems with that by just having one insanely long habitat
that doesn’t resemble a ring, or cylinder, or even a skinny pencil but is more like a
giant spool of wire. And you just wrap it around a star as many
times as you want, or if it isn’t solar powered, around whatever you want like some
gas giant you’re mining for the hydrogen to fuel the fusion reactors to light the giant
thing. It could be steel, some miles in diameter,
or graphene, some hundreds of miles in diameter, and arbitrarily long until you ran out of
raw materials to build it anyway. There’s literally no difference between
them and the shorter O’Neill or McKendree Cylinders. No tricky engineering or anything like that. They’ve not show up much in fiction though,
which has always surprised me. Personally I always like to think of them
having some super long river running down the whole length for millions or billions
of miles. Even though all these things can only be built
by high tech, often clarketech, civilizations they always seem to make people think of them
as inhabited by lower tech civilizations of more of a fantasy than science fiction bent. Medieval not high-tech, and I’m not really
an exception, the Topopolis is rather neat for the option of being one giant coastline
of port cities. The Topopolis is as big as it gets for rotating
habitats that are a single piece and don’t require inventing new science, but they’re
not the end of the story. Earlier I showed a couple ways of linking
these things together in groups and it might have occurred to you at the time that a direct
connection like that has some problems. The most obvious being if you connect a spinning
cylinder to a sphere that isn’t spinning with it you’re going to start leaking air
or have gears grinding on each other or both. That’s a serious issue with the classic
rotating habitat exposed to void but there’s two work arounds. The first is a plasma window or similar technology,
that I discussed in the last video as way to keep air from leaking into evacuated tunnels
at the end of launch loops. It can work the opposite way too, keeping
air from leaving pressurized tunnels. The second we’ll touch on in a moment. First let me hit on one point, if you’re
connecting multiple cylinders at the same junction then that junction really can’t
be spinning to produce gravity itself, another reason you’d probably taper these cylinders
near the end so that gravity ebbed off slowly for those entering the spheres. You could however fill them with air just
fine so birds could fly through. In theory land critters could learn to maneuver
in zero gravity and you could line the edge with easily gripped, or clawed, materials
and arrange a constant outward air pressure to blow things back against the sides of the
sphere. That doesn’t help sea life if you want fish
to be able to migrate between habs though and we do often think about using rotating
habs as a way of making truly protected wildlife reserves so overcoming that is worth consideration. You’d almost have to have two big pipes
running out of each hab with pressure pushing water in through the one and out though the
other so things could swim between, but it could be done and could also work in tandem
with faking some tides and currents. Rotating habitats aren’t really ideal for
deeps seas either but you also really don’t need much gravity for marine life, just enough
to make sure stuff goes the right way so slower spinning habs mostly full of salt water and
much deeper is an option, with the lower apparent gravity the pressure rises slower too and
so they can be much deeper. If you saw the rogue planets video and remember
me mentioning the idea of vertical reefs this would be another applicable cases. You’re always going to want a nice supply
of reserve water and water is very plentiful in this universe, so you might prefer to put
it to use as an ecological niche rather than just as a protective ice sheath for habitats. That protective sheath brings us back to our
other fix for leaking air and water. Remember that our spinning cylinders are not
exposed to outer space directly. They have a non-rotating exterior layer around
them. That can be welded right onto the junction
sphere, nice and air tight. If it isn’t rotating then you can just let
a bit of leakage occur where the rotating section meets the connecting junction sphere
because you can pump that back to near vacuum. Running a vacuum pump in gap between the rotating
section and the stationary sheath, and adding a bit more spin to the cylinder to make up
for a bit of loss to air drag in the near vacuum, is fairly energy intensive but it
doesn’t even get into the ballpark of the kinds of power needed to light and heat these
things normally, and all that drag and pumping would end as heat anyway. So with those exterior sheaths we don’t
need to worry much about leaks where moving parts connect and that increases our options. We can do more than long sausage chains or
even fairly two dimensional layouts and go for 3D. So long as you taper the cylinders down before
jamming them into a junction sphere you can cram them together fairly tightly and these
junctions spheres with no gravity of their own don’t need to be very large and they
can also have exterior access to actual space through the usual airlock mechanisms. You can, from the 2D angle, lay yourself out
wide mesh grids like ribbons and fill the gap in between with solar panels if you either
don’t have fusion or want to take advantage of the free supply in a sun. This is one of the ways you can go about creating
a Dyson Sphere, or Partial Dyson Sphere if your raw materials run out, by just wrapping
these ribbons all the way around a star then doing another ribbon cocked at a different
angle and so on, until you have a sphere. Unlike the Ringworld they only need to be
moving at normal solar orbital speeds because they get their entire gravity from spinning
locally, rather than around the entire star. Such combined structures, possessing thousands
if not millions of times as much living room as a planet, let you get away with devoting
whole planets worth of space to things like natural habitats for all the flora and fauna
we have here on Earth while still devoting the super majority of it to human-centric
interests. It’s also a lot easier to protect a rotating
habitat from invasive species or careless campers. Taken as a whole, as we close out for the
day, rotating habitats offer us the advantage of millions of times more space than we’d
ever get just terraforming planets and are doable inside the laws of known science. Plus as we’ve seen they can be made very
comfortable to mankind and quite safe and secure, arguably a lot more than planets are. Unlike planets you can choose your own day
length and temperature and climate and gravity, and while as we saw in the terraforming video
there are ways to do that with other worlds too it’s a heck of a lot easier with these
sorts of constructs. This is, fundamentally, why many of think
that vast swaths of rotating habitats are more likely in mankind’s future than endless
terraformed worlds. So this concludes all the prepwork we needed
to finally get to the video on interstellar colonization. Once we finish that up we’ll be returning
to the megastructures series to look at another type of artificial world, this time with real
gravity, in Shell Worlds, and from there probably move on to the slightly more fantastic Discworlds. Our next stop on the habitable planets series
is going to be a look at Double Planets. If you want alerts when those videos come
out, click the subscribe button, and if you enjoyed this video, hit the like or share
buttons and try out some of the other videos. Questions and comments are welcome down below,
and as always, thanks for watching and have a great day!
