Why Would High Rpms be Uncomfortable in Artificial Gravity?

Q:
I've been reading ideas on creating artificial gravity via rotation on spacecrafts/space stations. I understand how the effects of acceleration mimic (or are identical to) the sensation of gravity. I also understand how the strength of the artificial gravity is dependent on both the RPM and the observer's distance from the rotational axis (higher RPM and greater distance -> stronger gravity). What I don't understand is an idea I keep running into online - any rotation greater than 3 RPM is too disorienting for humans in all situations. It seems to be an absolute rule. Do humans have a difficult time handling a certain number of RPM, no matter the distance from the axis? My intuition says, like with most things in physics, it's relative and complicated, and the distance from the axis should play as much of a role in providing or relieving discomfort as the RPM do. I imagine I can spin around in a chair (minimal distance from axis) at 4 RPM without feeling much at all. Maybe my intuition is wrong!
- Will Timbers (age 24)
Englewood, CO, USA
A:

Rotation rates greater than about 2 or 3 rpm (rotations per minute) tend to cause disorientation and discomfort because of the . The strength of the Coriolis "force" is proportional to the rotation rate and doesn't depend on the distance from the axis.

The Coriolis effect isn't really a force, but it behaves like one inside rotating systems. It can be a counterintuitive concept and there are many ways of understanding it, but here's one that makes sense to me:

Consider a rotating platform like a merry-go-round. The whole platform rotates at the same angular speed (you could measure this in rpm). Points on the platform that are closer to the edge travel a longer distance to complete one rotation, so they have to move faster than points closer to the center. If you start at the center of the platform and walk towards the edge, you will step onto parts of the platform that are moving faster and faster at a right angle to your walking path. If you don't compensate for this by also increasing your speed in that direction, you'll be "left behind" and drift in the opposite direction. It will feel like something is pushing you off course.

An astronaut in a rotating spaceship would feel this effect if she moved farther away from or closer to the axis of rotation. Even worse, the Coriolis deflection affects the fluid in the inner ear, which can cause disorientation and nausea. Even if the astronaut just turned her head, moving the spiral shape of the inner ear into and out of line with the plane of the ship's rotation, the changing Coriolis force could cause the inner ear fluid to swirl unexpectedly.

Limiting the rpms of the spaceship makes these effects less noticeable, but requires greater distance from the axis to produce adequate artificial gravity.

Rebecca H.

(published on 11/06/2015)