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MIT Technology Review

Can constant acceleration be used to produce artificial gravity in space?

Your space questions, answered.

January 10, 2020
Tracy Caldwell Dyson ISSTracy Caldwell Dyson ISS
Tracy Caldwell Dyson ISSNASA

Every week, the readers of our space newsletter, The Airlock, send in their questions for space reporter Neel V. Patel to answer. This week: how to produce artificial Earth-like gravity through acceleration in space. 

Your piece a few weeks ago about how gravity always just “works” in sci-fi films got me wondering: Remember that poor guy back in the ’50s who rode a rocket sled to see how many “g’s” a human could endure? The film from these tests showed how his face was squished and contorted as he took greater and greater loads. It was obvious that humans can’t go from zero to relativistic speeds instantly without becoming a bio-paste. So, assuming a person—after achieving orbit––wanted to comfortably accelerate at 1 g up to the speed of light, how long would that take?

In the meantime, would they enjoy a normal gravity-like existence as they cruise along, with the back wall of the spacecraft acting as the floor? Maybe then space travelers could eat a bowl of cereal for breakfast as nature intended. —Clayton

For those who aren’t familiar, Clayton is referring to Eli Beeding Jr., an Air Force captain who in 1958 rode a rocket-powered sled backwards as part of a military program to learn more about the effects of g-forces on the body. The accelerometer on his chest clocked in at 82.6 g as the sled accelerated to about 34 miles per hour in 0.1 seconds. Beeding blacked out but luckily didn’t suffer anything more than some bruises to his back. 

G-forces can wreak havoc on the human body. (The TV series The Expanse illustrates pretty well how torturous this process can be.) When g-forces are experienced vertically, from head to toe (as astronauts would experience them on the way to space), blood moves out from the head toward the extremities, causing you to lose vision or even consciousness. The physical force itself puts an immense pressure on your skin, muscles, and bones, and can be devastating over a prolonged amount of time.

The physical feeling of g-forces acting on your body are not an effect of speed, but of acceleration. When you’re driving in a car or flying on a commercial airline, you feel the g-force push on your body only when the vehicle is accelerating. Once you reach a steady cruising speed, things begin to feel normal. 

If you’re accelerating at a fast enough rate to produce a constant 1 g, then sure, you’ll be able to create artificial, Earth-like gravity. From Earth’s frame of reference, if you’re accelerating at a constant rate of 1 g, then you’d reach near the speed of light in about a year, having covered about 0.5 light-years in distance. (Remember, if you’re a passenger inside the spacecraft, time will pass differently, as discussed in a previous issue of The Airlock.) While ramping up to near light speed, you’d theoretically be able to to eat your breakfast of champions as you would on Earth.

On the flip side, you have to be prepared to decelerate properly as you get closer to your destination. This could take months or even years—it sort of depends on what is tolerable to the travelers inside the spacecraft. If you’re trying to emulate a 1 g environment again, then you basically have to plan for a year’s worth of deceleration en route to your destination. 

And the truth is, if you’re able to accelerate fast enough to reach near light speed in about a year, then it’s reasonable to think you could probably get to that speed more quickly by accepting even higher g-forces. That's a tantalizing prospect, but again—humans aren’t really designed to handle all that. Bruce Thompson of NASA Quest once told Gizmodo: “Imagine traveling to Mars, accelerating all the way at 3 gravities. You would weigh three times your normal weight for the duration of the trip and would barely be able to move [...] Tissues break down, capillaries break down, and the heart has to do many times its proper work. You could not count on being in good shape when you arrived.” Certainly not as nature intended.