Bones Don't Travel Well

The first book that ever did it to me was The High Frontier, by Gerard K. O'Neill. Princeton physicist, published in 1976, and I read it and never quite came back.¹

What he described was a barrel. An enormous one — miles long, miles around, turning slowly in space, with people living on the inside surface of it. You stand on the inner wall and look up, and what is over your head is not sky. It is the far side of the cylinder, with farms and houses on it, curving up and over you. Sunlight comes in off big mirrors angled in from outside.

I was young enough that it rearranged how I thought about the whole business. Star Bound exists in some part because of that book, and I want to say so plainly before I say anything else in this article. The idea I am about to lay out is his. I am only arguing that fifty years on, it has become the necessary idea rather than the visionary one.

The problem nobody's rocket touches

Everything else in this magazine has been about getting somewhere and staying alive once you are there. Soil, water, machines, materials. Solvable, all of it, and I have taken my swings at each.

This one is different, because it is not about the environment. It is about us.

Put a person in weightlessness and their body starts taking itself apart. Bone goes first and fastest — roughly one to two percent of bone mass a month out of the load-bearing parts, the hips and the spine, which is a rate comparable to serious osteoporosis except it happens to healthy thirty-five-year-olds.² Muscle wastes. The heart, which no longer has to pump uphill, gets smaller. Fluid shifts to the head and pushes on the back of the eyeball, and some astronauts come home with vision that does not fully return.³

We have thirty-odd years of station data on this and the countermeasures are real but partial. Crews exercise a couple of hours a day, every day, strapped to machines, and they still come back needing to relearn how to stand up.⁴

And here is the thing that matters for anything past a visit: all of that data is from missions of months, with a hospital and a gravity well waiting at the end. Nobody has data on years. Nobody has any data at all on children.

That last one ends the conversation for me. You cannot have a colony that is a rotation. A colony is a place where people are born and grow up, and I do not know what a childhood in zero gravity does to a skeleton. Neither does anybody else.

So spin the thing

The fix has been known since before I was born, and it is not exotic. Spin it.

Go around in a circle and the floor has to push you inward to keep you turning. Push feels like weight — the same as being pressed into a car seat on a hard turn, except the turn never ends. Stand on the inside of a turning barrel and the barrel's wall is your floor, and down is straight out.

There is no new physics anywhere in that. It is the oldest physics in the book, and that is precisely why I want to write about it. We do not need a discovery. We need a big structure that turns.

Which brings in the one honest constraint. Spin something small and the difference in force between your head and your feet gets big, and moving your head sets your inner ear arguing with your eyes. People get sick. The published work on this points at large radii and slow rotation — hundreds of meters across, turning a couple of times a minute — rather than a small ring whipping around fast.⁵

Big and slow. Which is exactly what O'Neill drew, and he drew it that way for exactly this reason.

Which is why this connects to the rock

Now put this next to what I have argued in these pages about mining.

If the answer to human physiology is a structure hundreds of meters across, then we are not talking about a spacecraft anymore. We are talking about a building, in space, and the mass of it is enormous.

Lift that out of Earth's gravity well and you will never build it. Not because of technology — because of arithmetic. That is the wall every serious design has run into for fifty years.

Which is why O'Neill was not just drawing barrels. He worked the supply problem too. His plan was to mine the moon and throw the material up with a mass driver, an electromagnetic catapult, because the moon's gravity is shallow enough that you can fling material off it cheaply.⁶ He wrote about other extraterrestrial material as well.

So the argument closes on itself. You need mined material to build a structure big enough to spin. And you need robots to do the mining, and a refinery to process it, and a printer to make the parts. It is all one program. The spinning barrel is not a separate dream from the asteroid mine — the mine is what makes the barrel possible, and the barrel is what makes crews able to stay long enough to run the mine.

Where it actually gets used first

I do not think the first spinning thing is a colony. It is a ship.

The route to the asteroid belt is long. Months and months out, and months and months back, with all of it weightless on current designs, and a crew arriving at the far end of the solar system already degraded and expected to work.

That is the version of this problem we hit first and I think it is the one worth engineering for now. Not a mile-wide cylinder. A ship with a section that turns — a habitat ring, or honestly the simplest version, two modules on a long tether spinning end over end around their common center. Cheap, and the physics is identical.

That is not a fifty-year project. That is a design decision on the first serious long-duration ship anybody builds, and it either gets made or it does not.

Where I will stop

I am going to stay on this side of a line here.

There is a great deal of talk about artificial gravity in the sense of generating a field — making gravity rather than faking it with motion. That is a different subject and I am not going to make claims about it in these pages. When somebody has something demonstrable, this magazine will be glad to cover it.

Spin is not that. Spin is a merry-go-round. It works, it worked in 1976 when O'Neill laid it out, it will work whenever somebody finally builds one at scale, and there is no theoretical argument left to have about it. The only argument is money and mass.

Fifty years on, his answer is still the only one we have that we know works. That is either an indictment of how little we have done since, or a compliment to how well he thought it through.

Probably both.

One more thing, so nobody reads this as the last word. Spin is one idea, and this magazine is going to run many more. It solves the body. It does not tell you how to build something big enough to turn, and it does not rule out somebody smarter coming up with something better. The next piece takes up the first of those. I hope somebody else takes up the second.

Ideas in Star Bound are published open — take them, build them better. Our open proposal.

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A note on where this comes from. I am not a metallurgist, a spacecraft engineer, or a degreed engineer of any kind. But I am a practical one, with more than forty years in heavy industrial construction: welding, millwright work, hydraulics, mining, and thirty-three years of electrical construction and commissioning on the North Slope of Alaska, in a place that punishes anything built wrong. Much of that time I have worked as a field engineer, which means I am the one standing in front of the equipment when the drawing turns out to be wrong, and the fix has to be made on the spot, with what is on hand, and has to work the first time. Anyone who has done that long enough learns that a lot of engineering gets corrected in the field by people with more experience than the people who drew it. That is the eye I bring to these subjects: not what should work on paper, but what actually gets built, and what breaks. The facts I take from other people are footnoted so you can check them and read further. The ideas built on top of those facts are my own, and I would rather see them knocked down by somebody who knows better than left standing because nobody looked.

Sources

1. Space Studies Institute, History — The High Frontier: Human Colonies in Space (1976) won the 1977 Phi Beta Kappa science book award; the technical case first appeared as “The Colonization of Space,” Physics Today, September 1974. ↩
2. NASA, Risk of Spaceflight-Induced Bone Changes — on average one to 1.5 percent of bone density per month in the spine and hips over four-to-six-month missions, with NASA noting it does not yet know whether that rate holds on longer flights. ↩
3. Lee, Mader, Gibson & Tarver, Space Flight-Associated Neuro-ocular Syndrome — optic disc swelling, flattening of the eyeball, and farsighted shifts documented after long ISS flights, with some refractive changes persisting for years. ↩
4. ESA, Exercise — up to two hours a day on the station. In fairness to the countermeasures: a 2023 UT Southwestern study found that with the current regimen astronauts showed no loss of heart mass over six-month flights (Exercise protects astronauts’ hearts during extended space missions), so the heart may be the one item on my list the exercise actually solves. ↩
5. Globus & Hall, Space Settlement Population Rotation Tolerance, NSS Space Settlement Journal, 2017 — the design community’s long-standing limit of one to two rpm implies a radius of roughly 225 to 895 meters for one gravity. The same paper argues, from the adaptation literature, that residents could likely tolerate four to six rpm, which would shrink the structure a great deal. Worth reading before anyone commits to a mile-wide barrel. ↩
6. Kolm, Mass Driver Update, L5 News, September 1980 — O’Neill proposed the mass driver in 1974 and a working model was built at MIT in 1977. ↩

Centripetal force and the head-to-foot gradient in a small rotating structure are textbook mechanics and are not footnoted. The tethered two-module ship is an old idea in the literature and not original to me.