Definitions

Artificial gravity

Artificial gravity

Artificial gravity is a simulation of gravity in outer space or free-fall. Artificial gravity is desirable for long-term space travel for ease of mobility and to avoid the adverse health effects of weightlessness.

Methods

Artificial gravity could be created in several ways:

Rotation

A rotating spacecraft will produce the feeling of gravity on its inside hull. The rotation drives any object inside the spacecraft toward the hull, thereby giving the appearance of a gravitational pull directed outward. Often referred to as a centrifugal force, the "pull" is actually a manifestation of the objects inside the spacecraft attempting to travel in a straight line due to inertia. The spacecraft's hull provides the centripetal force required for the objects to travel in a circle (if they continued in a straight line, they would leave the spacecraft's confines). Thus, the gravity felt by the objects is simply the reaction force of the object on the hull reacting to the centripetal force of the hull on the object, in accordance with Newton's Third Law.

From the point of view of people rotating with the habitat, artificial gravity by rotation behaves in some ways similarly to normal gravity but has the following effects:

  • Centrifugal force: Unlike real gravity which pulls towards a center, this pseudo-force that appears in rotating reference frames gives a rotational 'gravity' that pushes away from the axis of rotation. Artificial gravity levels vary proportionately with the distance from the centre of rotation. With a small radius of rotation, the amount of gravity felt at one's head would be significantly different from the amount felt at one's feet. This could make movement and changing body position awkward. Again, slower rotations or larger rotational radii should not lead to such a problem.
  • The Coriolis effect gives an apparent force that acts on objects that move. This force tends to curve the motion in the opposite sense to the habitat's spin. Effects produced by the coriolis effect act on the inner ear and can cause dizziness, nausea and disorientation. Experiments have shown that slower rates of rotation reduce the Coriolis forces and its effects. It is generally believed that at 2 rpm or less no adverse effects from the Coriolis forces will occur, at higher rates some people can become accustomed to it and some do not, but at rates above 7rpm few if any can become accustomed. It is not yet known if very long exposures to high levels of Coriolis forces can increase the likelihood of becoming accustomed. The nausea-inducing effects of Coriolis forces can also be mitigated by restraining movement of the head. Head restraints are perhaps practical for exercising in artificial gravity (an artificial gravity gym), but not for much else.

This form of artificial gravity gives additional system issues:

  • Kinetic energy: Spinning up parts or all of the habitat requires energy. This would require a propulsion system and propellant of some kind to spin up (or spin down) or a motor and counterweight of some kind (possibly in the form of another living area) to spin in the opposite direction.
  • If parts of the structure are intentionally not spinning, friction and similar torques will cause the rates of spin to converge (as well as causing the otherwise-stationary parts to spin), requiring motors and power to be used to compensate for the losses due to friction.
  • Angular inertia can complicate spacecraft propulsion and attitude control particularly when no counterweight is employed.

Calculations

g = frac{R(frac{pi times rpm}{30})^2}{9.81}

or

R = frac{9.81g}{(frac{pi times rpm}{30})^2}

Where:
g = Decimal fraction of Earth gravity
R = Radius from center of rotation in meters
pi = 3.14159
rpm = revolutions per minute

The engineering challenges of creating a rotating spacecraft are comparatively modest to any other proposed approach. Theoretical spacecraft designs using artificial gravity have a great number of variants with intrinsic problems and advantages. To reduce Coriolis forces to livable levels, a rate of spin of 2 rpm or less would be needed. To produce 1g, the radius of rotation would have to be 224 m (735 ft) or greater, which would make for a very large spaceship. To reduce mass, the support along the diameter could consist of nothing but a cable connecting two sections of the spaceship, possibly a habitat module and a counterweight consisting of every other part of the spacecraft. Eugene F. Lally of the Jet Propulsion Laboratory proposed this concept in the early 1960s in a paper titled To Spin or Not to Spin. It is not yet known if exposure to high gravity for short periods of time is as beneficial to health as continuous exposure to normal gravity. It is also not known how effective low levels of gravity would be to countering the health effects of weightlessness. Artificial gravity at 0.1g would require a radius of only 22 m (74 ft). Likewise, at a radius of 10 m, about 10 rpm would be required to produce Earth gravity (at the hips; gravity would be 11% higher at the feet), or 14 rpm to produce 2g. If brief exposure to high gravity can negate the health effects of weightlessness, then a small centrifuge could be used as an exercise area.

The Gemini 11 mission attempted to produce artificial gravity by rotating the capsule around the Agena Target Vehicle which it was attached to by a 36-meter tether. The resultant force was too small to be felt by either astronaut, but objects were observed moving towards the "floor" of the capsule.

The Mars Gravity Biosatellite will study the effect of artificial gravity on mammals. An artificial gravity field of 0.38g (Mars gravity) will be produced by rotation (34 rpm, radius of ca. 30 cm). Fifteen mice will orbit Earth for five weeks and land alive.

Linear Acceleration

The spacecraft could, in theory, continuously accelerate in a straight line, forcing objects inside the spacecraft in the opposite direction of the direction of acceleration. Most rockets already accelerate at a sufficient rate to produce several times Earth's gravity but can only maintain these accelerations for several minutes because of a limited supply of fuel. Theoretically, a propulsion system with a very high specific impulse and high thrust-to-weight ratio could accelerate, producing useful levels of artificial gravity for long periods of time. In addition, constant acceleration would provide relatively short flight times around the solar system. A spaceship accelerating (then decelerating) at 1g would reach Mars in 2–5 days, depending on the point in the synodic period. In a number of science fiction plots, acceleration is used to produce artificial gravity for interstellar spacecraft, propelled by as yet theoretical or hypothetical means.

While this effect of acceleration is very well understood, this concept is far beyond current technological capabilities.

Mass

Another way artificial gravity may be achieved is by installing an ultra-high density core into a spacecraft so that it would generate its own gravitational field and pull everything inside towards it. Technically this is not artificial gravity—it is natural gravity, gravity in its original sense. An extremely large amount of mass would be needed to produce even a tiny amount of noticeable gravity. A large asteroid could exert several thousandths of a g and, by attaching a propulsion system of some kind, would qualify as a space ship, though gravity at such a low level might not have any practical value. In addition, the mass would obviously need to move with the spacecraft; if the spacecraft is to be accelerated significantly, this would greatly increase fuel consumption. Because gravitational force is proportional to the square of the distance from the center of mass, it would be possible to have significant levels of gravity with much less mass than such an asteroid if this mass could be made much denser than current materials. See neutronium and unobtanium. It is generally acknowledged that this method of producing gravity is unlikely to be of any benefit or utility to manned spaceflight inside the solar system.

Tidal forces

In a planetary orbit, a small artificial gravity can be obtained from the tidal force by two spacecraft above each other (or one spacecraft and another mass) connected by a tether. See also tidal stabilization.

Magnetism

A similar effect to gravity has been created through diamagnetism. It requires magnets with extremely powerful magnetic fields. Such devices have been made that were able to levitate at most a small frog and thus produced a 1 g field to cancel the earth's; yet it required a magnet and system that weighed thousands of kilograms, was kept superconductive with expensive cryogenics, and required 6 MW of power.

Such extremely strong magnetic fields are far above the permitted levels, and safety for use with humans is at best unclear. In addition, it would involve avoiding any non-diamagnetic materials near the strong magnetic field required for diamagnetism to be evident. Some other disadvantages of using magnetism on a spaceship are found here: http://www.madsci.org/posts/archives/2005-04/1112370655.Ph.r.html

However, facilities using diamagnetism may prove excellent laboratories for simulating low gravity conditions here on Earth. Note that the frog was levitated against Earth's gravity, simulating a condition similar to microgravity. Lower forces may also be generated to simulate a condition similar to lunar or Martian gravity with small model organisms.

Gravity generator/gravitomagnetism

In science fiction, artificial gravity (or cancellation of gravity) is sometimes present in spacecraft that are neither rotating nor accelerating. At present, there is no confirmed technique that can produce gravity other than sheer mass or acceleration. There have been many claims over the years of such a device. Eugene Podkletnov, a Russian engineer, has claimed since the early 1990s to have made such a device consisting of a spinning superconductor producing a powerful gravitomagnetic field, but there has been no verification or even negative results from third parties. In 2006, a research group funded by ESA claimed to have created a similar device that demonstrated positive results for the production of gravitomagnetism, although it produced only 100 millionths of a g.

See also

References

External links

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