The Gravity of the Situation
Keeping Your Feet on the Ground in The Expanse
I’m a pretty big fan of The Expanse — both the series of novels and novellas by James S.A. Corey and the TV adaptation of the first six of the nine novels. It remains the only screened adaptation of a beloved work of fiction that I can think of, where the original books and filmed adaptations actually improve the experiences of consuming both versions in equal measure. Reading the books enhances the TV-watching experience and the show adds depth and character to the books. I wrote about this several years back and it remains true.
The show, in particular, has earned a reputation for scientific accuracy. The physics of spaceflight are taken quite seriously in The Expanse and the show-runners go to great lengths to be faithful depicters of realistic science concepts like orbital mechanics, the effects of working in a vacuum, and especially the effects of acceleration in realistic space battles. I say the show, in particular, because while the books obviously also take the science seriously, that’s not all that unusual in the realm of science fiction novels. But TV sci-fi still mostly reduces the “science” part of science fiction to hand-wavy technobabble and oft-used tropes like faster-than-light travel, universal English-use, and artificial gravity.
And gravity is what I want to talk about today. I don’t think many non-book readers really get how gravity works in The Expanse, which is fine — I don’t think it’s all that critical a thing to understand in order to follow and enjoy the show, but I also think it’s wonderful how the series addresses the issue without resorting to a conceit that in the future they’ll somehow figure out a way to generate “artificial” gravity fields on spaceships.
How do those artificial gravity fields work, by the way? Like in Star Trek, is it deck-by-deck on the Enterprise, or does the whole ship have a gravity field? How much energy does it impart to the ship itself? Like, are they constantly using thrusters to keep the Enterprise from drifting in the direction of the artificial gravity field? Alas, Star Trek isn’t about answering pedantic questions of scientific accuracy — almost no science fiction on TV is. Then there is The Expanse.
There is no “artificial gravity” in The Expanse. The perception of gravity experienced by the characters in the series is all derived from natural, real-world phenomena. There are three distinct methods by which gravitational effects are explained in the world of The Expanse:
Mass-derived gravity
This is what you are experiencing right now (unless, I suppose, you’re reading this from the ISS, but even then you are technically “falling” toward the Earth due to its gravitational influence on you — you just keep missing). The Earth is a whole bunch of atoms held together by their own gravity and all that mass creates a “gravity well,” which you and I experience as “1G” of force (or 9.8 meters-per-second-squared of acceleration toward the ground). In The Expanse, the only people in Sol System who experience mass-derived gravity are those people who live on relatively large, rocky planets or moons. The Earth and Mars are gravity wells that produce enough mass-derived gravity for humans to walk around in (though the pull of gravity on Mars is only about 38% that of Earth). Also, some of the larger moons have been colonized and colonists there experience a reduced gravitational force, but enough to live there long-term. These moons include Ganymede, Titan, Callisto, Io, and Luna (the five largest moons in the system).
Acceleration-derived gravity
Not to be confused with “gravitational acceleration,” which is the acceleration of an object in free fall within a vacuum, in The Expanse, acceleration-derived gravity happens inside spaceships undergoing a constant rate of acceleration (made possible in-world by the Epstein nuclear fusion drive). People traveling inside such a ship undergoing acceleration feel the exertion of force pulling them in the opposite direction of travel. We feel a minute version of this in an accelerating car when we are pushed gently back in our seats as the car speeds up. A plane taking off from a runway offers a slightly more dramatic flavor of the effect of acceleration, but the sensation is short-lived as the plane quickly reaches its cruising speed and the acceleration mostly stops for the duration of the flight. In The Expanse, ships undergo constant acceleration, traveling faster and faster at each passing moment. This has two important physics benefits in the show/books: first, it means these ships get going really-really fast, which is how they are able to traverse the vast distances between planets and moons in mere days and weeks instead of the years that such journeys would take using conventional rocketry and by taking advantage of orbital shortcuts (which is how we get our space probes around the system IRL). Second, the constant acceleration means you can dial in how much gravity you want to experience on board during the journey — the more you accelerate, the heavier you’re going to feel and the faster you will arrive at your destination. In The Expanse, ships moving at 1G of acceleration are pretty zippy. Most standard cruising speeds typically produce .5G or .3G to accommodate Martians and Belters who are unused to the relative crush of 1G.
There is a problem with constantly accelerating during your trip though: you need to slow down or you’ll blow past your destination at Ludicrous Speed. Fortunately, deceleration offers the exact same experience for the traveller inside the ship, so the pilot shuts off the main drive and everyone experiences momentary microgravity free-fall as the ship is flipped around using tiny thrusters and the drive cone (along with the “floor” inside the spaceship) is aimed toward the destination and then reignited, slowing the ship at the same rate it had been accelerating. So if the ship took four days of constant acceleration to reach the half-way point of the journey, it will need to “flip-and-burn” and then constantly decelerate for an additional four days in order to safely arrive at its destination.
This sort of acceleration-derived gravity is not “artificial.” In fact, from a physics standpoint, it is no different from mass-derived gravity. Einstein called this the “Equivalence Principle” in his General Theory of Relativity, which states that no physical measurement or experiment can distinguish a reference frame that is accelerating (relative to an inertial frame) from one that is placed in a uniform gravitational field (like the surface of a planet). Taking this a step further, it’s not inaccurate to say that the gravity we feel on the surface of Earth is actually acceleration-derived gravity — the surface of the Earth is constantly accelerating upwards at 9.8 M/S² (yes, really).
Spin-derived gravity
The last way that characters in The Expanse experience gravity is the most exotic and hard to intuitively imagine, though there are some real-world examples to rely on. Characters living on the asteroids Ceres and Eros, or on Tycho Station or onboard The Behemoth/Medina Station are all experiencing spin-gravity. On Tycho and Medina, this is pretty straight-forward: there’s a ring (or a drum, in the case of Medina) that spins at such a rate that anyone standing on the inside wall (their feet pointing outwards, their heads pointing toward the center of the ring/drum) will feel the centrifugal force of the spin pulling them outward, away from the center of spin (technically, what we think of as centrifugal force is really just the inertial force being countered by the centripetal force of the outer wall of the station pushing the passengers/crew toward the center of spin, but it’s more intuitive to think of it as an outward-pushing force). This is the same effect that you might experience at a fair or amusement park on a ride usually called the “Round-Up.” Patrons stand on a circular platform against a wall and as the ride spins faster the force pushing them against the outside wall becomes greater than the force of Earth’s gravity and the floor can drop away leaving the riders suspended by the spin-forces. The same thing is happening on Tycho and Medina, but on a much larger scale. The result is that characters can walk around and experience something very similar to a gravitational field.
On the asteroids like Ceres and Eros, the mass of the asteroids is so small that the gravity provided by their mass is negligible at best, and too low for humans to spend long periods of time there without experiencing the significant health consequences associated with prolonged exposure to microgravity. But in The Expanse, clever engineers figured out a way to “spin-up” these asteroids (using many hundreds of Epstein fusion drives firing constantly for decades). A number of underground habitats were established near the surface of the rocky bodies along their equators, and once the asteroids were spinning fast enough, Belter colonists could inhabit them like they were inside a giant space station with a spinning ring. So in any scenes on Ceres or Eros, “down” is toward the surface of the asteroids and “up” is toward the core, or center-of-spin.
Watch this establishing scene of Ceres from S1 E1 and pay attention. The camera shows us how gravity behaves inside the asteroid, but don’t blink, or you might miss it (the critical shot begins at about :20):
And here is a little illustration I just made that is in no way a rip off and is completely original in every way, shut up:
While this is a nifty sci-fi idea, it turns out there are some significant physics problems associated with the idea of spinning-up something as massive as Ceres. In particular, the amount of thrust required to get it up the the required rotational speed in something like a decade is fantastically huge, even for engineers with access to powerful Epstein Drives. Perhaps more worryingly, even if you had enough energy to do the task, the asteroid would fling itself apart if it began spinning fast enough to create the spin-gravity suggested in The Expanse. But hey, James S. A. Corey is the pen name of a pair of writers, not a pair of engineers. It’s still a pretty cool idea.
Unlike acceleration-derived gravity, one can distinguish spin gravity from mass-derived gravity. In particular, the farther inward toward the center of spin you go, the smaller the gravitational effect — this includes moving away from the equator and toward the poles of the sphere. Gravity will be strongest at the equator. At the exact center of these bodies and at the poles, the gravitational effect (from spin) is zero. So if you were trapped in a room and didn’t know if the gravity you were experiencing was due to acceleration or spin, you could theoretically measure the force of gravity at your feet and at your head, calculate the difference, and then infer what the cause of gravity was. And if it was caused by spin, you could further calculate the radius of the spinning object and the speed of its spin (this is actually used as a plot device in early chapters of Andy Weir’s excellent book, Project Hail Mary).
In the TV show, there are occasional nods to the effects of spin gravity where you might see liquid pouring from a bottle at an angle due to the coriolis effect, for example. In the season two scenes where Miller goes to Eros to plant a series of nukes to blow it up, he and Diogo need to remain tethered while on the surface lest they be flung out into space by the rapidly spinning asteroid. The Behemoth’s spinning drum and subsequent gravity was a plot point in season three as it was the only way to generate enough gravity to effectively treat the many wounded people caught in the “slow zone.”
Mag Boots
I suppose there is one other way that The Expanse handles gravity, but it’s the least interesting. Space suits are fitted with magnetized boots that one can easily click on or off. If the boots are near magnetic deck plating, they adhere the wearer by their feet to the deck and allow for some awkward “walking” around in microgravity environments. It makes sense that suits would be equipped in this way, but the show makes too much use of them, in my humble opinion. It depicts mag-boot use as standard operating procedure for most anyone in any microgravity situation inside a spaceship. I understand why — simulating the look of zero-G is hard to do on TV and expensive to pull off well. So it’s convenient if everyone has short or severely pulled-back haircuts and if they all wear mag boots all the time so the actors can stand on the deck and talk even when they are ostensibly in a zero-G context. But the truth is anyone who is used to living and working in space would quickly become comfortable navigating in microgravity without having their feet locked to the floor all the time. This is one small way in which the show doesn’t live up to the vision of life in space offered by the books, but it’s a pretty forgivable one.
If you made it all the way to the end of this post, congratulations, you are a nerd. Please note I am not a physicist, so if I got anything wrong, I do apologize. Also, go buy Randall Munroe’s new book!
