The Problem With Spinning Spacecraft

While it would be awesome for people to be able to live in space, a “weightless” environment poses some serious challenges. Humans function best on Earth’s surface, where they are affected by a constant gravitational force. Without it, there are well-known consequences of long-term exposure to microgravity, including bone mass loss and muscle atrophy.

So if we want to live in space, we’ll need to create an artificial gravity environment. We only have one method for doing that: Build a vehicle that moves with a constant acceleration. The most common concept is to create a spacecraft that spins. But that’s not quite as easy as it sounds. Here’s why.

Feeling Weight

First, let’s go over the basics of gravity and what it means to feel your weight.

Gravity is an attraction between objects that have mass. Since both your body and the Earth have mass, there is an attractive force that pulls you toward the Earth and keeps you on the ground. Even though this force is constantly acting on you, you don’t feel it, because the Earth simultaneously pulls on all parts of your body, making the effect undetectable.

I know what you are thinking: “I’m sitting here on this chair, and I can absolutely feel my weight.” Actually, what you are feeling isn’t gravity. It’s the force of the chair (and the ground) pushing up on you. We call this upward-pushing force your “apparent weight.”

We can get a good feel for the concept of apparent weight by taking a quick trip in an elevator. The elevator starts at rest. But when you push a button, it begins moving upwards. That means it must have an upward acceleration—at least for a very short amount of time, until the elevator gets up to its traveling speed. During this upward acceleration, you feel a bit heavier. Then once the elevator gets near its programmed floor, it has to slow down. That means it accelerates in the downward direction. During this time, you feel lighter.

But, of course, your actual weight never fluctuated. Your real weight is a measure of how much force gravity exerts on your body, and it is the result of the interaction between your mass (m) as well as the mass of the Earth and your distance from its center. On Earth, gravity exerts a force of 9.8 newtons per kilogram. (Mass and weight are different things, so on a planet with different gravity, your weight would be different, even though your mass would be the same.)

Riding an elevator changes none of these factors. What it does change is your apparent weight. It’s a little strange, but this effect is super useful for a spacecraft.

Linear Acceleration

Let’s say you are in space where there’s no gravity—or even in low Earth orbit, where there is microgravity, which is the name we give to a “weightless” environment. What if your spacecraft had a giant elevator that constantly accelerated upwards? If the acceleration of the elevator had the same value as the gravitational field on the surface of the Earth, your weight would feel exactly like it does now.

Of course, a spaceship with an infinite elevator is impractical. It would be easier to just make the whole vehicle accelerate. That would absolutely create artificial gravity. In fact, this is the primary method used on ships in the science fiction series The Expanse.