% title ISSDC Training: Rotating Settlement Priorities

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<H2>Priorities for Rotating Settlements </H2>

<p>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.

<p>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.

<p>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.

<p>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.

<p>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.

<p>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.

<p>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.

<p>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.

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