International Space Settlement Design Competition

Priorities for Rotating Settlements

Despite improvements in physicists' ability to understand and measure forces due to gravity, rotation remains the only known reasonable means to provide artificial gravity (or pseudogravity) for settlements in space. Living in a rotating environment can, however, be disorienting.

Simply stated, for a rotating volume to be comfortable for normal people and to provide a practical amount of "gravity", it must be big. Although it is generally accepted than many humans can tolerate a rotation rate of three revolutions per minute (rpm) for long periods of time, it is also recognized that quality of life for a wide range of human ages, activities, and fitness requires a rotation rate of one rpm or less. This means that a community providing the equivalent of one Earth gravity must be at least one mile in diameter.

The issue is that rotation produces not only the outward or centrifugal acceleration simulating gravity inside a rotating structure, but it also induces an effect called the "Coriolis force". Coriolis is not a real force; it is not detectable by a person who stays still in a rotating environment. It does, however, feel very real to a person who is moving or watching something move in a rotating environment. It is due to the fact that we expect things to move in straight lines, according to the laws of Physics, but our senses don't recognize that the "ground" in a rotating settlement is moving in a curve.

In order to visualize the Coriolis effect, imagine a hamster wheel with an inside surface "paved" with paper. Imagine that you draw a straight chalk line on the paper in the rotating wheel, by moving the chalk toward you from the far rim to the near rim. When you take the paper out of the wheel, you will notice that your line is curved. In a rotating space settlement, a ball thrown from one rim toward the other will appear to follow that same type of curved path.

The amount of Coriolis force that is felt or sensed depends on the speed of rotation. The Earth itself is a rotating environment, too, but at a rotation rate of one revolution per day, the Coriolis force is imperceptible. We do, however, know it is there, because it affects the weather and the direction that water swirls as it goes down a drain. In a one-mile diameter settlement rotating to produce the equivalent of one Earth gravity, however, the effect is very noticeable. When an object is dropped, where it hits the "ground" is about 10% off to the side from where we expect it to land. A three-point basketball toss is offset by several inches. A long football pass is offset by a few feet.

To make the situation worse, the direction of the offset appears different, depending on the direction in which the ball is thrown. Imagine again the curved line on the paper in the hamster wheel - if the line were drawn from the other side of the rotating wheel, it would curve in the opposite direction. A spider sitting on the paper in the rotating wheel would see one line curving from left to right, and the other line curving from right to left. Motion parallel to the direction of rotation produces another Coriolis effect: if you run in the direction of the rotation you will feel heavier, and a ball thrown in that direction will go straight but will appear to fall faster than it should. If you run opposite to the direction of rotation, you will feel lighter, and a ball thrown in that direction will also go straight but will appear to loft higher than expected.

Aside from the visual disorientation, there is a physical effect, too: Coriolis force makes people and animals feel sick. This is due to its effect on the fluids in the inner ear, which give us our sense of balance. Turning one's head rapidly or rounding a fast corner on a bicycle in a rotating settlement causes these fluids to move in ways that our brains were not designed to interpret; we feel dizzy, our vision becomes distorted so that stationary objects seem to move, and we feel an effect like seasickness or spacesickness (more appropriately called "Space Adaptation Syndrome"). Elderly people whose sense of balance has deteriorated can become debilitated by these effects.

Although our bodies and brains are extremely adaptable, there are limits to how much adapting we can do. If space settlement residents stayed in the same equivalent gravity all the time, they could eventually adjust to higher rotation rates. But that is not what they will do. Many will work in the micro-gravity manufacturing facilities, laboratories, and docking bays for visiting ships; they will "commute" daily or even several times per day between zero gravity and one gravity. Some will return to the non-rotating sections for zero-gravity recreation and games. They must re-adapt frequently to both zero gravity and rotating environments. In time, nearly everyone who is motivated to live in space can make these adjustments with rotation rates of one rpm. They will feel more comfortable, however, living with lower rotation rates.