Gravity. It's what keeps us held to the
surface of the Earth and provides a sense of up and down. When we venture
into space and orbit the Earth, or some other world, the force of gravity precisely
cancels out with the centrifugal force. The sudden and stark absence of any
forces acting upon your body is known as weightlessness. Your body has literally
entered a state of perpetual and stomach-churning free-fall. Most humans
can adapt to this nausea after a few unpleasant hours, but many biological
functions of our body are hardwired for gravity
thanks to billions of years of evolution here on the Earth. the muscles used for
standing posture and skeletal support are no longer used and thus atrophy
losing up to 20% of their mass after a week. Astronauts on the International
Space Station need to exercise for at least two hours every day to combat this
muscle loss. But without gravity our bones also wither with astronauts losing
1 percent of their bone mass each month. The breakdown of bone within our body
saturates our cells with calcium, leading to dangerous calcification of soft
tissues. If we want to live in space for years or decades without severe medical
interventions or genetic modification, some form of artificial gravity is going
to be needed. In science fiction, artificial gravity is ubiquitous, mostly
to save money on special effects. The explanation for this is usually
something like gravity plating - a wholly fictional technology.
But how could we do this for real? The simplest way to create artificial
gravity in the real world is just to accelerate in a straight line. If you
were inside a spacecraft that was not just moving but accelerating at 1g, your
inertia - which is to say your resistance to motion - causes you to be pushed
against the back wall with the same force as that which the Earth pulls you
down right now as you watch this video. In fact, Einstein postulated that the
effect is so similar that there is no experiment you could possibly do on
board that spacecraft to distinguish between being in a
gravitational field versus just accelerating. In other words, inertial
mass and gravitational mass are equal, something famously known as the
equivalence principle in general relativity. This sounds promising,
but accelerating at 1g for months, years or decades would require engines with
vastly greater sustainable thrust than any modern space vehicle. The N star ion
thruster is probably the closest example we've ever built of such an engine.
NASA's Dawn spacecraft which visited Vesta and Ceres used three xenon ion
thruster engines to sustain a record-breaking constant acceleration of
just under 10 millionths of a G. It was an impressive technological feat but it
is a long way of providing useful artificial gravity. Is there any other
way of generating artificial gravity then? The only other proven physics to do this
is rotation. When a car turns a corner you feel yourself pulled to the opposite
side - that's the centrifugal force. Another
example is a fighter pilot pulling a hard turn causing a centrifugal force
several times that of Earth's gravity, or several G, so strong in fact that it can
cause a human to black out. These forces are what physicists call fictional.
Like the case of linear acceleration, they are just the products of
our inertia. They are not fundamental like electromagnetism, for example. Your
body's matter will not change velocity unless acted upon by a force and so if
the aircraft around you moves to one side you will not because of inertia, and
so you find yourself slammed into the wall. But what if the wall wasn't a wall,
what if it was the floor? Take a giant merry-go-round and spin it fast enough
and the occupants would feel pushed against the interior wall. If you did this
in orbit, where there are no other net forces acting, that centrifugal force
to the wall would mimic gravity. You could stand on the wall, feeling the same
downward force but Earthlings enjoy. The idea of a spinning habitat in space is
arguably the most plausible way in which humanity might mimic gravity. It's an old
idea too, toyed with in fictional tales such as Rendezvous with Rama, Babylon 5,
Interstellar and The Expanse. There are quite a few different proposed concepts
for achieving this with real-world physics but today we'll focus on two
foundational concepts that have been the most influential - the O'Neill Cylinder
and the Stanford Torus. We'll start with a cylinder concept,
proposed by Princeton physicist Gerard O'Neill in his 1976 book, called The High
Frontier Human Colonies in Space, O'Neill laid out a strategy for the colonization
of space through asteroid and lunar mining, which included the construction
of a five mile diameter spinning cylinder with a length of up to 20 miles.
The inner wall of the cylinder would serve as the floor for the inhabitants
providing up to 300 square miles of livable area - this is about the same land
area as New York's five boroughs. O'Neill wasn't afraid to dream big
describing a habitat for millions of people with a total mass of several
billion tons. Today it costs about twenty thousand dollars per kilo to launch into
low-Earth orbit, which means that the dry mass of the cylinder would cost about a
hundred thousand trillion dollars in launch costs alone. That's about a
thousand times the world's current GDP. A more realistic plan would be to build
most of the habitat in space through lunar or asteroid mining facilities, which
of course necessitates that those industries exist firs. Even so, a
space-based construct of this scale would be so expensive that likely only
the very wealthiest members of society could afford to live above the clouds.
You would notice a few things that are quite different on board. Tilt your head
up and you could see the curvature of the cylinder and even see your
cohabitants hanging upside down above you. Air pressure and gravity might be
reduced compared to that of the Earth, in order to save upon the cost of spinning
of the cylinder and material strength for the walls. And the artificial gravity
generated by rotation would depend on your altitude, as you ascend towards the
slanders rotational and the gravity would drop off until
eventually reaching zero, perhaps giving you the chance to enjoy
low gravity sports near the axis. The Stanford Torus was the product of a NASA
sponsored summer workshop held at Stanford University in 1975 around about
the same time that O'Neill published his book on the cylinder concept, indeed
O'Neil was the technical director of the Stanford study too. "This is torus the
concept that has evolved from the work of these teams of scientists and
engineers. They believe the huge space colony could be built before the year
2000." The torus is again a spinning structure to give the artificial gravity
needed for comfortable living. Unlike the cylinder only about 1/2 of
the interior wall is now usable land. This could be seen as an advantage since
now you have a potentially natural-looking sky, rather than a
mind-bending landscape of the cylinder imposes. The diameter of the torus is not
grossly different from the cylinder and about one kilometer with an inner tube
diameter of about a hundred meters or so. This gives a living area of about one
third of that of Hell's Kitchen, certainly far smaller than that of the
cylinder. But the smaller design saves on materials, coming in at about 100 times
less mass. Compared to the O'Neill Cylinder, this would decrease the overall cast by an order of magnitude, but it would raise the cost
per square foot by about the same factor. Living on board one might enjoy joking a
couple of laps of the three-mile rim or staring out to the warped horizon at
your neighbors down the torus. When you need supplies you might venture down the
torus spokes to a central hub which experiences far weaker centrifugal
forces enabling low gravity manufacturing or easy docking with
supply ships. When we retrospectively look at these designs they sadly remain
as fantastical today as they did during their inception in the 1970s. Although
we have access to better materials and reduced launch casts that sheer size of
these structures means that they are far beyond our economic capabilities, or
indeed our collective will in the foreseeable future. So why are both of
these structures so large? Could we build a much smaller rotating structure today,
enough for a few astronauts maybe - just to demonstrate a working example of
artificial gravity. Putting aside the issue of shape, cylinder or torus, an
architect has two basic dials that they can control in designing a centrifugal
artificial gravity structure these are the rate of rotation, omega, and the
radius of rotation, R. Clearly, to reduced costs,
we would like to use the smallest R possible since surface area, and thus
cost, should scale roughly linearly with this term. As we make R smaller we need
to spin the habitat faster in order to recreate the required gravity. This is
because the centripetal acceleration equals R multiplied by omega squared, thus
making the habitat 4 times smaller in diameter requires a
spin rate twice as fast. Even ignoring the effect on human occupants rapid spin
rates are generally non-desirable, since they make docking with the station more
challenging and require more energy to enter and exit the rotational frame. This
raises the possibility of humans living in reduced gravity, perhaps equal to that
of Mars, or even the Moon. Research by Harris et at have shown that
accelerations below lunar, about 1/6 of a G, make it difficult for humans to have a
sense of up and down and remain balanced, so this is likely to be a good lower
limit. On the other side, anything above 1G is generally uncomfortable for humans
with 4G leading to complete blackout for example - so putting these together gives
us the following comfort zone for possible combinations of R and omega.
Although a small rotational radius reduces the costs it also diminishes the
usable living space and even head space onboard. Clearly, radii a less than
typical human heights would be a poor choice causing the inhabitants to
constantly crouch and experienced no downward force in their heads but strong
forces at their feet. In other words they'd experience a tidal force of 1G. There
isn't much research about how much tidal acceleration humans find comfortable but
a reasonable guess might be an order of magnitude less tidal acceleration than
centripetal, which would constrain our to be at least 17 meters for average human
height. A 17 meter radius habitat rotating a
very fast rates would indeed mimic Earth-like gravity if the occupants were
completely stationary inside, but any movement inside and they would feel an
additional force that we do not notice here on the Earth - the Coriolis effect.
Artificial gravity of a centrifuge is not the same as that of linear
acceleration, recall that Einstein argued that linear acceleration is
indistinguishable from gravitational acceleration but this is not true for a
centrifuge. There is no equivalence principle here
and the occupants of an O'Neill cylinder, or Stanford Torus, could indeed design
experiments to realize this. The acceleration felt by a crew equals the
following, where the first tterm is the centripetal acceleration upon the crew,
and the second term is the Coriolis acceleration and that last titerm there is
linear acceleration. If the station isn't linearly accelerating then we just have
those first two terms. If you are perfectly stationary r dot, which is
velocity, goes to zero and thus you indeed experience a perfect reproduction
of gravity. But any movement - r dot - in a direction perpendicular to the rotation
vector, described by omega here, will cause this second term to depart from
zero and thus you would feel an acceleration normal to both of those
ingredient vectors. That's the Coriolis effect. Like the centrifugal force, this is what
physicists would refer to as a fictional force - purely a product of living in a
rotating frame of reference. Imagine a cannonball floating around the
center of a rotating O'Neill Cylinder. Let's give it a slight push so it starts
to move down towards the edge of the cylinder. If we were watching this happen
from the outside of the cylinder, through a window say, it would seem to travel in
a straight line - which of course makes perfect sense. But now consider the same
motion from the perspective of a person on the inner surface of the cylinder
from their perspective the cannonball does not travel in a straight line but rather
curves. That is purely a result of their rotating frame of reference though, and
that curving is caused by the Coriolis effect. This effect even plays out here
on Earth, affecting the path of winds in its spinning atmosphere or affecting the
path of a thrown ball on a merry-go-round. The Coriolis effect
actually has two different components on the inhabitants of a rotating cylinder
or torus. The first is what feels like an apparent change in downward surface
gravity, the vertical Coriolis if you will, and the second is a tipping effect
and these deserve separate discussions. The direction of the Coriolis
acceleration depends on the direction in which you move and the direction of the
rotational axis, which is presumably fixed. More specifically, the Coriolis
acceleration equals the cross-product of your velocity vector and the habitat's
rotation vector, meaning that it always pushes in a direction perpendicular to
both vectors. So what does this mean? Well consider inside the habitat you have
three directions of possible motion. If you move along the same direction as the
rotation axis you would feel no Coriolis effect at all. So traveling hundreds of
meters from one end of an O'Neill cylinder to the other is perfectly safe. For the
Stanford torus case, this means that walking to either side of the torus
corridor admittedly a smaller distance has zero Coriolis. This leaves us with
two other directions that must experience Coriolis - one radial and one
tangential. The tangential direction is probably the least concerning and leads
to an apparent change in downward gravity. For example, if you walk along
the O'Neill cylinder in a prograde sense, you effectively increase your overall
rate of rotation, pushing you down harder into the floor and making you feel
heavier. Walking the retrograde does the opposite and causes you to lose weight. In the case of the Stanford torus, this
change in gravity happens when traveling along the three mile circumference
instead. Now if we want to downsize these designs
the question becomes how much change in surface gravity is comfortable. Research
by Neste et al. in 2014 on human subjects show that people are not even
able to perceive vertical changes in acceleration less than about 5% of
surface gravity. The maximum tolerance is less well studied but certainly for a 1G
system a 25% increase in gravity would start to become uncomfortable, based on
centrifuge experiments by Cohen et al. If we take this as a maximum threshold, it leads to the following modified comfort zone when choosing R and omega. With motion along the rotational axis
and tangential direction discussed, let's finally turn to the issue of radial
motion. In both the cylinder and torus, or indeed any centrifugal system for
artificial gravity, this equates to the act of jumping up and down or
equivalently ascending and descending to different decks. During these actions
there is a slight change in the centripetal force itself, but by far the
most destabilizing aspect for human occupants would be a tipping effect
caused by the Coriolis acceleration. Dr Theodore Hall developed a nice way to
visualize this by imagining dropping or throwing a ball vertically rather than
landing at your feet it would curve to the side. In extreme cases, one can even
throw a ball behind you and catch it in front as it whips around due to the
Coriolis. This tipping acceleration has the effect of creating a perceived
incline in the floor during vertical motion. A strong enough tip could cause
you to stumble over while standing up or climbing ladders. A simple way to deal
with this is to have a single deck with no ladders or staircases. Of course
efficient use of space might demand the inclusion of multiple decks, here then
elevators or carefully designed ladders for ascent and descent could be designed
for this environment. Even so astronauts would likely need to be trained to stand
up slowly when getting up. Perceived inclines of an 8% grade or higher would
exceed the maximum slope of most ramps here on Earth. If astronauts stand up no
faster than 1 foot per second this would place the following limit in our design
specifications of a habitat which further constrains our allowed comfort
zone. Coriolis isn't just a problem for
climbing ladders or standing up, it affects your balance even by turning
your head from side to side. When you do this the inertia of the vestibular fluid
inside your inner ear causes a slight delay between the motion of your head
and the fluid within it. The fluid is then pushed back into place by pressure
causing movement of sensory hair cells within the ear.
Now if we do this on board a rotating spacecraft moving your head in one
direction would lead to an increased downward force versus the other. That
would slightly change the distribution of fluid inside the semicircular canals.
This differential gravity may confuse our vestibular system and create a sense
of nausea and sickness. The fluid itself also undergoes a small amount of
vertical motion and thus would be pushed to one side by the tipping Coriolis
effect. This raises the question as to how fast a rotation can we cope with
before this becomes a problem and occupants experience nausea. In most
proposals of rotating habitats, it is these effects of the Coriolis on the
vestibular system - sometimes called canal sickness - which most strongly constrain
the engineering design. These studies usually cite experiments on human
subjects conducted here on Earth using slowly rotating rooms. A variety of
studies come to agreement that at rotation rates below one or two revolutions per
minute, subjects are able to complete complex tasks over prolonged periods. But
above this spin rate, it takes humans days to adapt and beyond six rpm, people
struggle to ever adapt. These experiments guided the design of the Stanford torus
which aims to rotate at one revolution per minute in order to avoid canal
sickness. But to recreate 1G of centrifugal force at just one rpm
demands a very large radius almost a kilometer. Using this one rpm or slower
constraint we can see that the comfort zone is greatly diminished and now it
becomes clear why previously proposed rotational habitats need to be so vast
and thus expensive. But it's important to remember that these experiments do not
recreate the environment of a space station because they cannot remove the
Earth's gravity. Typically the rotation chamber is oriented such that the spin
axis is aligned with the Earth's gravitational field lines the rotation
rates are slow enough that the centripetal force is small and your
sense of down comes from the Earth's gravity - not the centrifuge itself. This
change in orientation, chosen in order to accommodate the Earth's gravity, means
that the Coriolis acceleration act in perpendicular directions to that
experienced by the crew of a rotating spacecraft. So imagine walking around the
floor or moving your head from side to side both of which are lateral motions.
In an O'Neill cylinder, the Coriolis accelerations act up and down during
these activities, slightly affecting the apparent strength of gravity. In contrast
for the rotation chambers used on Earth those same movements lead to a tipping
Coriolis acceleration, trying to knock you over to one side.
A study by Graybiel et al. using this exact set up finds that at 10 rpm even
experienced test pilots are not able to adapt. At this rotation rate, walking
across the room causes a tipping Coriolis acceleration of 0.3 G, making
you feel like you're walking a 30 percent grade slope. What's more if the
rotation chamber subjects jump up and down or stand up sharply, they would feel
no Coriolis effect at all because that is oriented with the centrifuges
rotation axis. This is again in stark contrast to the case of a rotational
habitat in space. In conclusion the Earth-based rotation chamber experiments
may be simulating a harsher environment than that experienced by our prospective
astronauts, or indeed at the very least a very different environment.
Simple lateral motions of the head and body should not result in tipping forces
but this is generally what is happening in these types of experiments. This is
important because in comparing lateral and vertical accelerations
Nestia et al. found that humans have a higher sensitivity to horizontal
accelerations than vertical ones. So these slow rotation room experiments are
certainly worthwhile, but an upper safety limit of 6 rpm may be overly
conservative this is very important because if humans can tolerate Coriolis
accelerations at rotation rates of 10 rpm or higher then Coriolis forces no
longer become a design limiting factor and much smaller feasible artificial
gravity systems could be conceived. For these reasons let's be optimistic and
assume that humans can plausibly cope with 6 rpm environments which gives us
our final comfort zone as depicted here. Let us finally turn to the question of
shape and ask whether aside from the cylinder or torus could there be
opportunities to further optimize our astronaut comfort levels at small
rotational radii. Recall that when astronauts move along the rotational
axis they experience no Coriolis effect accordingly one might be tempted to
design a train like carriage hanging off a tether connected to a counterweight. As
the carriage spins the occupants are pushed to the floor in their quasi
1-dimensional habitat simulating gravity. Such a design aims to provide the least
exposure to Coriolis accelerations. But Coriolis is not the only thing we need
to worry about in designing a safe environment. If this train were too long
and thus massive the rotation axis is no longer the primary axis of rotation but
instead becomes the intermediate axis. Rotation about such an axis is unstable
and subject of violent tumbling as demonstrated on board the ISS with this
T-handle experiment. During this tumbling motion the occupants would be thrown
about the habitat in a clearly unacceptable manner. Tumbling can also
occur for O'Neill cylinders. A cylinder has two principal axes of rotation - one
along the length of the cylinder shown here in blue and another perpendicular
to the ends shown here in red. O'Neill rotates his cylinder about the blue axis
but that's actually the smaller principal axis of rotation. Any slight
disturbance to the distribution of mass inside the cylinder such as people
moving around inside could lead to tumbling due to the rotational
instability imposed. One could imagine pumping water around the cylinder to
redistribute mass accordingly using thrusters or reaction wheels to correct
for any perturbations or even using a second cylinder next door which counter
rotates and that's actually the idea that O'Neill originally
envisaged. Finally, it's worth noting that for the Stanford torus, things are much
simpler this rotation actually occurs now around the primary axis and thus the
structure should be far more stable. Could we see a demonstration of
centrifugal artificial gravity in the near term? The ISS offers perhaps our
best hope being a permanent station for space experiments. In 2011, Mark Holdeman
and Edward Henderson proposed an ISS demonstration of a centrifuge as part of
their grander plan for a space station called Nautilus X. A small rotating torus
would be attached to the ISS recreating partial gravity at an estimated cost of
100 million dollars. Sadly this concept went no further than the initial
drawings though. A few years prior in 2005 Kirk Sorensen argued that the
simplest path would be a habitat attached to a more massive counterweight
via a tether spinning sufficiently fast to recreate artificial gravity in the
lower mass object. Sorensen pointed out that by making the tether retractable
the gravity on board could be controlled at will much like how an ice skater can
extend his or her legs to control their rate of spin. The habitat itself could
perhaps be something like the NASA trans hab concept with several decks and a
higher decks featuring weaker artificial gravity. This is about where things stand
with no concrete plans for a space-based artificial gravity experiment on the
horizon. Long term human presence in space environments will at some point
need to solve this problem either through engineering or biological
solutions or perhaps a combination of the two. Let me know down below what kind
of design you are most excited about to simulate gravity and whether you think
this should be a priority for NASA and other space agencies in the near future.
Thank you for taking the time to watch this slightly different video from us
here at the Cool Worlds Channel. Let me know if you like these styles of
videos and as always thank you for watching, stay thoughtful and stay
curious.
This channel produces some amazing content.