Barbary (20 page)

The delicate filaments on each being quivered and twined,
and multitudes of wind-chime voices rang. At first Barbary wondered if she had
hurt their feelings by laughing, and then she believed the beings were laughing
along with her.

“Another ship is approaching,” the musical voice said. “The
beings within appear to be… quite perturbed.”

“They don’t know what’s happened to us,” Heather said. “They
probably think we’ve been swallowed up.”

“As indeed you have.”

“To be eaten, I mean.”

“No. We do not ingest organic molecules. Will you speak with
them?”

“Can we? Please?” Heather said. “My father will be worried.”

“Should we?” Barbary said.

“Of course we should!” Heather said. “What do you mean?”

“Maybe if they worry about us a little more, they won’t be
so mad at us when we go back.”

“If they’re going to be mad, they’re going to be mad,”
Heather said. “I don’t want Yoshi to be worried anymore and I don’t want
anybody out there to do anything that the other beings might think they need to
be shown is futile.”

“Okay,” Barbary said.

“Would you like to speak to them now?”

“Yes, please,” Heather said.

“They will hear you.”

Barbary saw no radio equipment, no change in the chamber to
indicate a transmitter.

“Hi, this is Heather,” Heather said to the air.

“Heather!” Yoshi said. “Are you all right? What about
Barbary?”

“I’m okay.”

“So am I,” Barbary said. “And so is Mick.”

“What’s happening in there?” Jeanne asked.

Barbary looked at Heather, who gazed back at her and smiled.

“We’re with the — the beings in the starship,” Barbary said.
“They’re bringing us home.”

John G. Cramer

The space station doughnut of
2001
and the O’Neill
space-habitat cylinder have become part of the furniture of science fiction, so
much so that we take spin-generated artificial gravity to be interchangeable
with the Earth-normal variety in which we live. But there are differences that
would be quite apparent to anyone living in the spin-generated variety. The
subject of this AV column is an exploration of the differences between the “natural”
gravity of Earth and the “artificial” gravity of a rotating space
station.

My interest in the physics of space station gravity
developed because last year Vonda McIntyre was writing a book with a space
station setting, and she asked my advice. The book,
Barbary
, is about a
teenager who leaves Earth to live in a space station with spin-generated
gravity. I helped Vonda in a very minor way by identifying the physical effects
that the heroine would experience in that environment. What’s it like to
ride an elevator in a space station? How would a ball game look if it were
played there? If you woke up in a strange location, what simple tests would
tell if you were in a rotating space station rather than at rest on the ground?
And so on... I found that there are some interesting side-effects of artificial
gravity, perhaps well known to NASA experts but obscure to the rest of us. And
I was surprised to find that some recent SF hasn’t been too accurate in
describing the space habitat environment.

Looking at the world from a rotating vantage point (be it a
merry-go-round or a space station) is odd and confusing. So let’s start
with a simple concrete example. Suppose that we are on a doughnut space
station, about half the size of the big one in
2001
, providing living
and working space at earth-normal gravity (1 g) for about 150 people. Such a
station might take the form of a “wheel” 15 m wide and 160 m in
diameter, rotating on its axis so that it makes a full rotation every 18
seconds. Because the floor of the space station rotates through its full
circumference in this time, it has a speed (called the
tangential
velocity
because the velocity lies along the tangent of the circle of travel) of 27.9
m/s. A note here on scaling to other sizes: If the station had
4
times
this diameter, the rotation period to give 1 g of artificial gravity would be
twice
as long and the speed of the floor would be
twice
as large.

Let’s do a simple “Mr. Science” experiment
in this space station. Place a phonograph turntable on floor and use it to spin
a cake pan filled with water. Let’s use a cake pan 40 cm in diameter and
spin it at the 78 RPM setting of the turntable. The outer edges of the spinning
cake pan will be moving at a speed of 1.6 m/s with respect to the floor.
Therefore, the edge of the cake pan towards one outside wall of the station is
traveling at an absolute speed of (27.9+1.6)=29.5 m/s, while the opposite edge
of the pan has a speed of (27.9-1.6)=26.3 m/s. The pull of artificial gravity
depends on the square of this tangential speed, so the “fast” edge
experiences an increased pull of 1.12 g, while the pull on the “slow”
edge decreases to 0.89 g. The water in the pan will tend to tilt, climbing
higher on the slow edge and dropping lower on the fast edge. A spinning
gyroscope would tumble in the same way, making the toy top a poor gift for a
space child. And so we see different physical effects in the artificial gravity
of a space station than would be found if the same experiments were performed
in the “natural” gravity of Earth.

This simple experiment has an interesting implication for
the psycho-physiology of human balance. Our equilibrium and our perception of
vertical orientation come from the interaction of the fluid in the semicircular
canals of our inner ears with the nerve fibers there. The vertigo experienced
during and after spinning in an amusement park ride demonstrates what happens
when this mechanism is disturbed. Seasickness is another example. Now suppose
that you stand looking spinward down the long upward-curving hall along the rim
of the space station, and then rapidly turn your head clockwise so that you are
looking at the side wall to your right. Your head has made a rotation similar
to that of the pan on the turntable. The fluid in your semicircular canals will
therefore rise on one side and drop on the other as the water did. The
subjective consequence is that you will “see” the floor tilt to the
left, with the right side wall “rising” and the left side wall “dropping”
momentarily. The amount of perceived floor tilt depends on the ratio of
ear-velocity to floor velocity, but for any but the very largest of space
stations the tilt sensation will be a quite unmistakable. This effect is likely
to be fairly disorienting and may be a source of nausea and vertigo for the “greenhorn”
who has just arrived from “natural” gravity. For the experienced
space station inhabitant, however, the “floor-tilt effect” will
become a useful aid to orientation because it will allows the user to tell
whether he is looking “spinward” (in the direction that the floor
is moving due to the spin) or “anti-spinward” (against the floor
velocity) down the hall.

Head twisting and nodding will also produce other subjective
effects. Facing a wall at right angles to the spin direction and doing a
similar head twist will make the floor seem to tilt up or down. Nodding or
wobbling your head will produce similar effects. Placed in a small closed room,
the experienced space station dweller can establish his orientation with
respect to the spin of the station with a few twists of his head.

The memorable jogging scene of
2001
when astronaut
Frank Poole runs in what we see as a vertical circle brings to mind another
effect. The jogger running spinward down a hall along the rim of the station
increases his tangential velocity, thereby creating a slight increase in the
centrifugal pull he experiences and giving the impression of running uphill.
Running anti-spinward will decrease the pull slightly and create the impression
of running downhill. The change in pull will depend on the ratio of running
speed to floor speed, and the effect would be less in a big station than a
small one.

The mysterious “force” that makes the water tilt
in the pan, moves the fluid in the semicircular canals, and changes the pull on
the runner is called the
Coriolis force
. Like the “centrifugal
force” which makes spin-generated artificial gravity, the Coriolis force
is not a real force of nature, but rather a sort of illusion or pseudo-force
which appears to observers in rotating systems. But if the Coriolis force is an
illusion, its effects are nevertheless quite real. Its actions on air flow on
the Earth’s surface are responsible for the circular weather patterns
visible in satellite weather pictures: the ragged spiral of the hurricane and
the gentle swirl and counter-swirl of high and low pressure areas.

Another Coriolis effect appears when we ride the space
station’s elevator. There are good astronautical engineering reasons for
arranging the station so that arriving shuttles dock at the station hub,
matching velocity and spin with the station before establishing tight
mechanical contact. Arriving passengers exit the shuttle in the zero-gravity
zone of the hub and then ride an elevator to the 1 g zone at the rim where the
living and working areas are located. But what is the elevator ride like? The
elevator must travel 80 m from hub to rim, the rough equivalent of the elevator
in a 25 story building. Let’s assume that the elevator is set to
accelerate to a speed of 5 m/s in a period of 2 seconds, then travel toward the
rim at that speed for 14 seconds, and finally decelerate to zero velocity in
the final 2 seconds of the trip.

With this arrangement, the elevator riders will be pushed
against the ceiling of the car for two second with a force of 0.25 g. During
that 2 second period a pull toward the anti-spinward wall of the car will build
up to a force of 0.22 g. During the 14 second ride this sideways force will
remain constant, but added to it will be a downward force which builds up to 1
g as the centrifugal force of the station’s spin builds. Finally in the
last 2 seconds of the ride the downward force will rise to 1.25 g and the pull
toward the anti-spinward wall will diminish to zero. As the car stops and the
passengers step out the constant 1 g downward pull of the station is all that
remains. And so the passengers have had a very peculiar ride. Their perception
of “down-ness” has migrated from the ceiling to the anti-spinward
wall and finally to the floor, as if the car had rotated 180
^o
during
the trip.

The source of the sideways pull in the elevator is the
Coriolis force. An equivalent view is that the riders in the elevator must
travel from the hub, where they have zero tangential velocity, to the rim,
where they must match the 27.9 m/s tangential velocity of the floor. Clearly
during the elevator ride they must not only be taken “down” along a
radius from the hub to the rim, but they must also be accelerated up to the
speed of their new environment. The sideways push of the elevator wall accomplishes
this. A similar ride in the upward direction from rim to hub would reverse
these forces, and now the sideways pull toward the spinward wall removes the
rim’s tangential speed to match the hub environment.

Finally, let’s consider space station sports. How
would a baseball pitch or a basketball pass be changed in the environment of
the space station? The answer depends on the direction of travel of the ball.
Movement parallel to the station’s axis of rotation, across the long
hallway for example, shows no Coriolis effects. But a ball thrown spinward will
seem to drop, and an anti-spinward pitch will rise due to Coriolis effects.
Similarly a falling object will curve antispinward, a rising object will curve
spinward due to the Coriolis effects, as we saw in the case of the descending
elevator. Athletes after sufficient practice will begin to view these
distortions of trajectory as natural and will automatically include
compensations for them as a part of optimum performance. However, the size of
the compensations needed depends on the tangential velocity of the space
station floor, with higher velocities leading to smaller Coriolis effects. In
an Inter-Orbital Olympics where participants from a variety of stations of
different sizes are assembled for athletic competition there will be a definite
“home-court” advantage. Participants from smaller-diameter space
stations will tend to overcorrect for the Coriolis effects and participants
from larger diameter stations will undercorrect. I wonder how the Inter-Orbital
Olympic Committee will handle that one?

____________________

Reference:

Spin Generated Gravity:
“An Overview of
Artificial Gravity”, R. W. Stone, Jr., NASA Report SP-314 (1973).

____________________

Alternate View Column AV-18

Keywords: centrifugal, Coriolis, force, artificial
gravity, rotation, space station

Published in the February 1987 issue of
Analog Science
Fiction & Fact Magazine;
This column was written and submitted 8/1/86
and is copyrighted © 1986 John G. Cramer. All rights reserved.

No part may be reproduced in any form without the explicit
permission of the author.

Reprinted in
Barbary
with the kind permission of the author.

I’m grateful to
Dr. John G. Cramer
of the University of
Washington in Seattle and “
The Alternate View
” columnist for
Analog Science Fiction and Fact
. He offered expert advice that helped immeasurably in the
creation of the research station
Einstein
and, particularly, in the
descriptions of what it would feel like to live and work in an environment in
which gravity is provided by radial acceleration.

I’m also indebted to the late Gerard K. O’Neill and the
Space Studies Institute
. The society to which Barbary
emigrates grew out of Dr. O’Neill’s proposals for permanent inhabited orbiting
colonies, the mass driver, and other practical ideas for allowing human beings
to live in space.