ARTIFICIAL GRAVITY
Definition
Artificial gravity (AG) is not gravity at all. It is not a field force or a ‘‘force at a
distance.’’ Neither does its strength obey the inverse square law of attraction that
determines the orbital motion of planets. However, in terms of its action on any
mass, it is indistinguishable from ‘‘real gravity.’’ Instead of gravitational pull, it
exerts a centrifugal force, proportional to the mass that is being accelerated
centripetally in a rotating device. Although the effect of AG on an extended body
differs from that of true gravity, the effects on any given mass are equivalent.
Thus AG is simply the imposition of acceleration on a body to recover the forces
that are eliminated by the free fall of orbital flight. (Of course, real gravity is not
eliminated in orbit. The pull toward Earth in Earth orbit and toward the Sun in
interplanetary orbit is balanced by the ‘‘free fall’’ acceleration of the spacecraft
and its contents toward Earth or the Sun. To an observer or instrument onboard
the spacecraft, it feels as though the pull of gravity were removed.)
Provision of AG
In principle, AG could be provided by various means. A continuously thrusting
rocket that accelerated a spacecraft halfway to Mars would generate AG equal to
the acceleration level. Intermittent impulsive AG would be imposed on an astronaut
who jumps back and forth between two opposing trampolines or even
between two stationary walls in a spacecraft. However, the term artificial gravity
is generally reserved for a rotating spacecraft or a centrifuge within the spacecraft.
Every stationary object within the centrifuge is forced away from the axis
of rotation toward the outer ‘‘floor’’ by a force proportional to the mass of the
object, its distance from the center of rotation, and the square of the angular
velocity of the device.
Why AG May Be Necessary
Probably the most serious health threat to humans during interplanetary flight
comes from radiation exposure en route and on some extraterrestrial surface.
Beyond that, prolonged exposure to weightlessness itself can result in deconditioning
many of the body’s systems. For space voyages of several years, such as
those envisioned for exploration of Mars, the human requires some sort of
‘‘countermeasure’’ to reduce or eliminate this deconditioning. Intensive and sustained
exercise on a treadmill, bicycle, or rowing machine was used on the U.S.
and Russian spacecraft to minimize the problems of weightlessness. The procedure
is uncomfortable and excessively time-consuming for most astronauts.
Furthermore, its effectiveness is not proven for all users. Other kinds of countermeasures,
including diet, fluid loading before reentry, lower body negative
pressure, or wearing a ‘‘penguin suit’’ to force joint extension against a resistive
force are either marginally effective or present an inconvenience or hazard.
The physiological effects of weightlessness are generally adaptive to
space flight and present a hazard only upon return to Earth or landing on another
planet (1). However, they may present hazards in flight in the event of a
bone fracture, a vigorous muscle contraction, or alterations in the heart’s
rhythm.
Aside from the severe danger of space radiation, the principal physiological
risk of long flight is deterioration of the skeleton. Bones are living tissue, constantly
being strengthened by calcium extracted from the blood and destroyed by
returning calcium to the blood. Bone maintenance requires a compressive load
along the axis of the bone and some high-force impulsive loading. In the absence
of these loads that are normally provided by gravity and walking, the major
bones that support body weight begin to deteriorate, and a net loss of body
calcium occurs, independent of the amount taken in with food or supplements.
The long bones in the legs and the vertebrae in the spine lose crucial size and
strength during prolonged bed rest. Similarly, they lose strength in spaceflight.
Calcium is lost at a rate of about 1/2% per month, and the losses are reflected in
the density and size of weight-bearing bones. For a spaceflight of two years, a
25% decrease in bone size might occur (unless the process reaches a plateau),
thus increasing the risk of fracture and severely hampering the bone’s ability
to mend.
Muscles involved in weight bearing, as well as bones, begin to weaken with
disuse in weightlessness. The major muscle groups in the legs and back that
normally support weight lose mass and are also ‘‘reprogrammed,’’ so that fibers
previously devoted to slow steady tension are used for brief bursts instead. The
shifting of fluid from the legs and lower trunk to the head and chest that produces
the first symptoms of head-fullness discomfort on orbit initiates an early
loss of body fluid, including blood plasma. The relative excess of red blood cells is
countered by stopping their production in the bone marrow and additionally by
destroying young red blood cells. The cardiovascular regulating system that acts
to maintain adequate blood pressure when we stand up, is no longer needed in
space and shows signs of deterioration. Neither the fluid loss and resulting
‘‘space anemia,’’ nor the loss of cardiovascular regulation and tone normally
cause any difficulty in orbit. During reentry and back on Earth, however, the
renewed exposure to gravity can cause weakness and fainting.
The balance system that keeps humans from falling depends on the detection
of gravity by the otolith organs in the inner ear. Because the only stimulus to
the organs in weightlessness is linear acceleration, considerable reinterpretation
of vestibular signals takes place. A consequence of this process is the common
occurrence of space sickness early in flight and postural disturbances and vertigo
after return.
The immune system that fights infection may also be compromised by
space flight, although it is unclear whether weightlessness alone is the major
factor.
In addition, a variety of human factor problems arise in weightlessness,
including the constant need for handholds or footholds for stabilization and the
possibility of disorientation within a spacecraft. However, these problems are
often balanced by the ease of moving heavy objects, the use of three-dimensional
space, and the shear pleasure of floating in weightlessness.
The notion of creating a substitute for gravity through centrifugation was introduced
early in the conception of human space travel. Tsiolkovsky, the influential
Russian space visionary, discussed the idea in 1911, and his concepts were
picked up 50 years later by Korolev, who designed a flexible tether system for the
Voskhod manned missions . It was, however, never built. A detailed engineering
proposal for an AG station was introduced by Noodhung in 1927, a full 50
years before the first satellite was launched. When Von Braun described his
vision of space exploration in 1953, he included a large rotating torus to deal with
weightlessness .
The popularization of AG, however, is attributable to the science fiction
community. The large rotating torus in Clarke and Kubrick’s 2001: A Space
Odyssey presented an idealized version of life in space, free of health problems
and the negative effects usually associated with transiting from the rotating to
the stationary parts of the station. By 1965, preliminary tests on a short-radius
centrifuge first showed that subjects who were deconditioned by bed rest could be
protected against cardiovascular deconditioning by periodic centrifugation .
Experience with AG in space has been quite limited. Rats were centrifuged
continuously at 1 g for several days and showed no deconditioning. Human experiments,
however, have not been conducted to date. Early attempts to test AG
by tethering a Gemini spacecraft to an Agena rocket were inconclusive and
nearly led to disaster when the thruster nozzle stuck on Gemini 8, sending the
pair of space vehicles into an uncontrollable spin. The 2.5-m-radius centrifuge on
the International Space Station should afford the opportunity to examine the
adequacy of various levels of AG in protecting rodents during spaceflight.
