Imagine that you're inside a vehicle — or other machine — spinning around so fast that the force presses your body against the wall or seat. As you spin faster and faster that pressure forcing you against the wall increases (and conversely it decreases as the spin slows down). The weight feels just like the force of gravity that keeps your body grounded to the earth.
If you're like most people, your most dramatic experience with this type of spinning force is probably from an amusement park ride — specifically a classic Rotor Ride that has produced a great deal of joy (and yes vomit) since the middle of the 19th century.
But a handful of people, including astronauts and military pilots, experience the same phenomenon in a human-rated centrifuge, a machine that spins to produce these high "G forces," also called acceleration. They experience this G-force aboard high-performance aircraft during high speed turns, and during launches into space and when spacecrafts rapidly slow as they reenter Earth's atmosphere.
What Is Artificial Gravity?
In a very real sense, this type of rotation produces gravity — artificial gravity to be precise. It provides weight to your body — weight that your bones and muscles cannot distinguish from the weight that Earth, or another planet, provides on account of its sheer mass.
Consequently, for decades, science fiction writers have envisioned rotating spaceships that create artificial gravity for astronauts during the longest phases of space missions. These phases are when they are not extra-heavy due to the ship accelerating to build up speed, or decelerating in the atmosphere, but weightless due to the craft coasting, negating the effects of gravity.
Two examples of such artificial gravity in science fiction are the 2015 film "The Martian" and the 1968 epic "2001: A Space Odyssey." "The Martian" features an interplanetary craft, the Hermes, with a large, wheel-shaped section that rotates on its journey between Earth and Mars. As the camera zooms in, you notice that "up" for astronauts inside the Hermes is always toward the center of the wheel, while "down," the "floor," is the rim. Space Station V in "2001: A Space Odyssey" is a spinning station that generates artificial gravity equal to that of the moon's gravity.
Apart from mere comfort, there are good reasons why we need artificial gravity on long distance space missions. For one, in weightlessness our bodies changes in ways that could be harmful when astronauts arrive at their destinations — such as Mars — or return to Earth. Bones lose mineral content (they soften, becoming vulnerable to fracture); muscles atrophy (they shrink and weaken); fluids shift toward the head and also are excreted from the body, causing changes in the cardiovascular system and lungs; the nervous system is thrown out of whack; and in recent years space medicine researchers have found what could be permanent eye damage in some astronauts. Add to that research suggesting that gravity may be required for humans to have a normal pregnancy in space and it almost seems like a no-brainer that any spacecraft carrying humans around the solar system either should rotate, or have some part of the ship that does.
Researching Artificial Gravity
Are NASA and others researching this possibility?
The answer is yes. Since the 1960s, NASA scientists have been considering the prospect of artificial gravity by way of rotation. However, the effort, funding and overall enthusiasm has waxed and waned through the decades. There was a surge in research in the 1960s when NASA was working on sending man to the moon (the budget for NASA at that time was nearly 5 percent that of the entire federal government —10 times what it is today).
While NASA has not emphasized research on artificial gravity over the past half-century, scientists both inside and outside of the space agency are studying a range of situations. Mice spinning in a small centrifuge aboard the International Space Station survived with no problem and Earth-bound humans are learning how to adapt in spinning rooms. There's one at the Ashton Graybiel Spatial Orientation Laboratory at Brandeis University and the DLR Institute of Aerospace Medicine in Cologne, Germany, is home to the DLR Short-Arm Centrifuge, Module 1. It's the only one of its kind in the world researching the effects of altered gravity, especially as it pertains to health risks that occur in microgravity.
Why Don't We Have Rotating Spaceships?
But if the need for artificial gravity is so clear, why bother with research in space, or on Earth? Why don't engineers simply get to work designing spinning ships, like the Hermes?
The answer is that artificial gravity requires a trade-off, because all that spinning creates problems. As on the Rotor Ride, moving your head while you're spinning that fast causes nausea. Spinning also impacts the fluid in your inner ear and any other body parts that you move while you're in a rotating environment.
And that nausea, disorientation and movement problems worsen the faster you rotate (the number of revolutions per minute [RPMs]). But the amount of artificial gravity that can be produced depends both on the RPMs and the size of whatever is rotating.
To experience a given amount of gravity — for example one-half the usual amount that you feel on Earth — the length of the radius of rotation (the distance from you standing on the floor to the center of whatever is spinning) determines how fast you need to spin. Build a wheel-shaped craft with a radius of 738 feet (225 meters) and you'll produce full Earth gravity (known as 1G) rotating at just 1 RPM. That's slow enough that scientists are very sure that nobody would get nauseous or disoriented.
Other than the floor being a little bit curved, things aboard such a craft would feel pretty normal. But building and flying such an enormous structure in space would entail numerous engineering challenges.
This means that NASA and any other space agencies or organizations likely to send people around the solar system in the future must settle for a lower amount of gravity, a faster rotation (more RPMs) — or both. Since there is no laboratory on the moon where the surface gravity is about 16 percent that of Earth's surface, making it a great place to research the effects of low gravity, as opposed to weightlessness, there simply isn't not enough data to know how much gravity humans may need for long-term space missions or space colonies. Such data is needed, as is data on how much rotation humans can reasonably tolerate, and that's the rationale for ongoing artificial gravity research.