Artificial gravity

From Freepedia

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.

Contents

Methods

Artificial gravity could be created in several ways:

Rotation

The spacecraft could rotate so that anything inside will be forced toward the outside by centrifugal force. Artificial gravity by rotation has the following side effects:

  • Coriolis forces produced by rotation could 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 a 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.
  • Gravity gradients: Artificial gravity levels vary depending on distance from the center 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.
  • Angular movement: As noted high angular velocities produce high levels of Coriolis forces, angular moment (the amount of energy to spin) would require a propulsion system of some kind to spin up (or spin down). Also if parts of the spaceship are intentionally not spinning, friction and torque will cause the rate of spin to decrease (as well as cause the otherwise-stationary parts to spin). Fly wheels and thrusters would be needed to keep the appropriate sections of a spacecraft spinning or not. Angular inertia can also complicate space craft propulsion and attitude control.

Image:Stanford-torus-by-donald-e-davis-med.jpg

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 a spaceship, possibly a habitat module and a counterweight consisting of every other part of the spacecraft. 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 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.

Acceleration

The spacecraft could 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 rate to produce several times earth's gravity, but can only maintain these 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 a long periods of time. In addition, constant acceleration would provide relatively short flight times around the solar system. A spaceship accelerating (then deccelerating) at 1g would reach Mars in 2-5 days, depending on the relative distance. 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.

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 gravity. An extremely large amount of mass would be needed to produce even a tiny amount of gravity. A large asteroid could produce 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.

Tidal forces

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

Magnetism

In science fiction artificial gravity (or cancellation of gravity) is sometimes present in spacecraft that are neither rotating nor accelerating. It is not possible with current technology to create artificial gravity of this type, although a similar effect can be created through diamagnetism, but it would involve avoiding any non-diamagnetic materials near the strong magnetic field required for diamagnetism to be evident. It would also require magnets with incredibly powerful magnetic fields. At present such devices have been made that could levitate at most a frog, and thus produce up to 1g; yet requiring a magnet and system that weighs over 1000 kg and is keep superconductive with expensive cryogenics.

Fiction

Image:2001 movie poster.jpg

Rotational Gravity

In the movie 2001: A Space Odyssey a rotating spaceship provides artificial gravity. The movie features a rotating space station. The people would be walking inside the circle; their feet toward the exterior and their head toward the center, the floor and ceiling would curve upwards. Rotating circular set were used in at least one instance to make this effect, with the actors always at the bottom, as they walked the set would be turned to keep the actors at the bottom and prevent them from falling over as they walked up the curved floor.

In the television series Babylon 5, the Earth Alliance made extensive use of rotational gravity in its space stations and some larger military vessels, as well as civilian cruise ships. It has been suggested that the cruise ships would alter their rate of spin gradually en route to match the destination, helping to acclimate the passengers to the new gravity they would find upon arrival.

Field generators

In many science fiction stories, there are artificial gravity generators that create a gravitational field based on a mass that does not exist. It helps the story by creating a more Earth-like spaceship, and in the case of a movie or television program, it helps the production because it is a lot cheaper than the special effects needed to simulate weightlessness.

In Gene Roddenberry's Andromeda, set thousands of years in the future, gravity field generators not only provide gravity for the people inside the ship, but also reduce inertial mass of ships such as the Andromeda Ascendant to just under a kilogram. This greatly increases the efficiency of their Magneto-Plasma Dynamic Drive, allowing them to go from stopping to percentages of light speed very quickly.

It is noteworthy that in the stories based on Sid Meier's Alpha Centauri, the Unity had no gravity field generator, and provided more conventional artificial gravity by spinning.

External link

Retrieved from "http://en.freepedia.org/Artificial_gravity.html"


Views
Personal tools
In other languages
Similar Links