Space travel thesis statement

However, the VIIP syndrome is currently a matter of great concern for long duration missions.

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Many books and review articles have been written on the topic of artificial gravity Stone, ; Lackner and DiZio, ; Young et al. The objective of this paper is to provide a broad overview of recent ground-based and in-flight studies that relate to the effects of artificial gravity as a potential countermeasure. One of the major issues associated with humans in rotating environments is the adverse effect on sensorimotor functions.

The vestibular system is involved in the regulation of other physiological systems, including respiratory, cardiovascular, circadian, and even bone mineralization systems. The physiological responses of these systems to continuous exposure of humans to anything other than Earth gravity and weightlessness are unknown. Research must be undertaken to identify the minimum level, duration, and frequency of the level of artificial gravity exposure that is needed to maintain normal physiological functions.

In addition, the limits for human adaptation to rotation rate, gravity gradient, and Coriolis and cross-coupled accelerations need to be revisited. The rationale for using centrifugation is that the resulting centrifugal force provides an apparent gravity vector during rotation about an eccentric axis. For example, a crewmember standing on the rim of a habitat rotating at about 4 rpm about an axis located at 56 m would experience the sensation of standing upright closely approximating the same experience as on Earth Figure 1.

Figure 1. Artificial gravity. Continuous rotation of a large spacecraft that creates a centrifugal force of 1 G in the habitat would give the static crewmembers the sensation of standing upright as on Earth. In the example of the spacecraft shown in the insert, a 4-rpm rotation rate would generate 1 G in the crew habitat located at 56 m from the axis of rotation. During the early concept phase of human space travel, scientists introduced the idea of creating a substitute for Earth gravity by using centrifugation.

Korolev proposed to connect two Voskhod modules by a m tether and rotate them at 1 rpm to produce 0. Inspired by the pioneering works of Oberth and Noordung , von Braun also proposed a spacecraft having a diameter of 76 m rotating at 3 rpm, the result of which would be a suitable platform for Mars expeditions exposing the occupants to 0. When any linear motion is attempted in any plane that is not parallel to the axis of rotation, a Coriolis force is generated. This is a significant drawback associated with rotating environments.

The Coriolis force combines with the centrifugal force to produce an apparent gravity vector that differs in magnitude or in both magnitude and direction. When the body is moving away from the center of rotation, the force is opposite to the direction of rotation. By contrast, a body motion parallel to the axis of rotation will generate no Coriolis force Crosbie, ; Stone, In addition, any angular displacement of the whole body or body part that is not parallel to the spin axis will create cross-coupled angular accelerations that induce stimulation of all three semicircular canals of the vestibular system.

Such movement in a stationary environment normally stimulates only the semi-circular canals that correspond to the plane of head rotation. The same head movement in a rotating environment also stimulates the canals that lie in the plane of the rotating environment.

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The latter combination of canal stimulation results in illusory sensations of bodily or environmental motion and possibly motion sickness Guedry and Benson, Given that centrifugal force depends on both rotation rate and radius, changes in the artificial gravity level can be achieved either by increasing or decreasing the radius, or by increasing or decreasing the rate of rotation. The radius of the structure will have a direct impact on the cost and complexity of the space vehicle, whereas the rotation rate will mostly influence physiological and psychological responses of the crew on board.

The final design will be the result of a trade-off study between these two options Diamandis, These theoretical limits to rotation rates and radii were based on casual observations of humans walking, climbing, moving objects, and performing nominal head movements in a large-radius centrifuge. These assumptions have largely been taken at face value as correct, but they need to be validated by experimental evidence.

More recent data suggest that the adaptation limits of humans to rotating environment are much greater than these earlier studies had anticipated. For example, it has been observed that subjects in a rotating environment could tolerate a rotation rate up to 10 rpm provided that the exposure is progressive Graybiel et al.

Figure 2. Hypothetical comfort zone bounded by values of artificial gravity level and rotation rate based on theoretical studies in the s see Hall, , for details. According to the model of Stone and Letko the Coriolis and cross-coupled angular accelerations generated at these rotation rates during walking, climbing and handling materials should be the most comfortable for the crewmembers.

However, very little experimental data were actually collected to validate this model. Recent data indicate that the limit of 6 rpm is overly conservative.

Graybiel et al. The nausea-inducing effects of Coriolis and cross-coupled accelerations can also be mitigated by restraining head movement during centrifugation.

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The empirically determined limits for rotation rate and radii proposed in the 60 s for humans to adapt to a rotating environment therefore seem overly conservative. These limits were derived by experimentation under specific limited conditions. Additional experimentation under more extreme conditions may allow extension of these limits. Clearly, further research is warrented. A rotating spacecraft presents serious design, operational, and financial challenges.

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Practically speaking, it is highly likely that humans do not require gravity or fraction of it for 24 h a day 7 days a week to remain healthy. A continuously rotating spacecraft would not be required if intermittent gravity proves to be sufficient. A human rated short-radius centrifuge presents a realistic near-term opportunity for providing intermittent artificial gravity.

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A human-rated centrifuge designed for studying vestibular responses to linear accelerations in orbit flew aboard the Space Shuttle Neurolab mission STS in This experiment was the first and only in-flight evaluation of artificial gravity on astronauts. The results of this experiment suggested that centrifugal force of 0. For those astronauts who rode the centrifuge 20 min every other day during a day space mission, cardiovascular deconditioning was reduced Moore et al.


A human-powered centrifug that couples exercise with artificial gravity is an interesting and novel approach. Exercise was introduced as a countermeasure in the days of the Gemini program, by means of ingenious elastic, pneumatic, mechanical, hydraulic, and electrical devices. Elastic devices only effectively create force, not sustained acceleration.

The assumption is that exercising under such increased inertial forces would decrease the exercise time required to maintain health and fitness in space. If trial results prove positive and the amount of exercise is indeed reduced by centrifugation, such devices are good candidates for long-duration mission countermeasures. On a short-radius centrifuge, the subjects are generally lying supine with their head close to the axis of rotation and their feet directed outwards.

During centrifugation in space, the subject is only exposed to the centrifugal force along their longitudinal body axis, referred to as artificial gravity. However, during centrifugation on Earth, centrifugal force combines with the gravitational force resulting in the so-called gravito-inertial force, which is both larger in magnitude than the centrifugal force itself, and tilted with respect to the longitudinal body axis Figure 3. Figure 3. Constraints for short-radius centrifugation. On Earth, the actual forces exerted on the body during centrifugation are the resultant of the gravitational force in blue and the centrifugal inertial forces in red.

These gravito-inertial forces in green are larger than 1 G and tilted relative to vertical. In space, the centrifugal forces are the only forces generated by centrifugation and aligned with the longitudinal body axis. Note also the gravity gradient, i. The gravity gradient is the variation in artificial gravity level as a function of distance from the center of rotation. The gravity gradient also has an effect on the hydrostatic pressure along the longitudinal body axis.

The hydrostatic pressure influences the circulation of blood to the head and from the lower extremities and therefore affects the functioning of the cardiovascular system. It is not known if the gravity gradient has any critical influences on the cardiovascular and neurovestibular systems.

The Coriolis force is proportional to the linear velocity of the imparted motion, the mass of the moving object, and the rotation rate of the rotating environment. It is important to note that the magnitude of the Coriolis force is not dependent on the radius of the rotating environment. The Coriolis force is therefore equally present in both short- and long-radius centrifuges.

For a given centrifugal force level, the rotation rate of an on-board short-radius centrifuge must be greater than that of a rotating spacecraft, so body motion will result in larger Coriolis force. The ideal solution is of course a rotating spacecraft that provides a constant 1 G acceleration.

In the case of intermittent centrifugation using an onboard short-radius centrifuge, a research program is needed to identify the gravity levels that are necessary to mitigate the deconditioning of physiological systems, determine how these loads should be applied e.

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Artificial gravity should also be integrated with other countermeasures such as exercise, sensorimotor training, and pharmacological prescriptions to optimize crew health. One method for evaluating the effects of different levels or duration of gravity loading on the physiological systems is to test whether intermittent short-radius centrifugation can overcome the deconditioning of bed rest.

In these investigations, the physiological responses measured during bed rest alone are compared with the same physiological responses during bed rest and intermittent centrifugation.

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