Artificial gravity
From Wikipedia, the free encyclopedia
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.
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[edit] Methods
Artificial gravity could be created in several ways:
[edit] Rotation
The spacecraft could rotate so that anything inside will be forced toward the outer wall by centripetal force. Artificial gravity by rotation has the following side effects:
- The Coriolis effect 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 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 proportionately with the 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 momentum 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 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 also complicate spacecraft propulsion and attitude control.
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The engineering challenges of creating a rotating spacecraft are comparatively modest compared 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 a 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 1960's 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.
[edit] 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 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 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 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.
In practice, this idea is far beyond the current state of the art.
[edit] 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 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. 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.
[edit] Tidal forces
In an Earth 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.
[edit] Magnetism
A similar effect to gravity has been created through diamagnetism. It requires magnets with incredibly powerful magnetic fields. At present such devices have been made that were able to levitate at most a small frog, and thus producing a 1 g field to cancel the Earth's; yet it required a magnet and system that weighs thousands of kilograms and is kept superconductive with expensive cryonics and required 6 MW of power[1].
Such extremely strong magnetic fields are far above the permitted levels[specify], 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
[edit] 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. 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.[1]
[edit] Fiction
[edit] Rotational gravity
In the movie 2001: A Space Odyssey a rotating centrifuge in the Discovery spacecraft provides artificial gravity. The movie also 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. A rotating circular set was 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.
Larry Niven's novel Ringworld featured a gigantic habitat encircling a star, which created artificial gravity through rotation. Niven also makes a reference to the Coriolis Effect, when the protagonists see what looks like a giant eye above the horizon. When they get closer, they realise that it is in fact a hurricane, but rotating about an axis parallel to the ground rather than perpendicular to it. Large hurricanes on Earth rotate the way they do due to the Coriolis Effect.
In the computer game Startopia, the main station upon which aliens visit you and buildings are made is of a cylinder-based design and must employ a method of artificial gravity to sustain everyone inside.
In the Gundam universe, gigantic space colonies, called Sides or Colonies, are an important aspect to the plot. They spin to generate artificial gravity.
The anime Cowboy Bebop, the Bebop possesses a ringed area that generates artificial gravity[citation needed], and is often seen being used (with the rest of the ship not rotating[citation needed]).
The book Rendezvous with Rama and the sequels featured an alien construct similar to an O'Neill habitat which was able to generate approximately 1g on the intentionally habitable ground section. The plot employed significant use of the difference in strength of artificial gravity as an object approaches the center of the rotating cylinder.
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.
In the stories based on Sid Meier’s Alpha Centauri, the Unity provided artificial gravity by spinning, though the game made allusions to less conventional technologies developed later on.
In John Varley’s Gaian Trilogy (Titan, Wizard, and Demon), the titular world Gaia, being a torus with a diameter of 1300 kilometers, spins at a rate of one revolution per sixty-one minutes, producing an apparent gravity of one-quarter gee.
In the game Halo: Combat Evolved (by Bungie), the main location of the story is an artificial ringworld that creates artificial gravity by computer-controlled rotational spin. "Halo" (or "Installation 04") is approximately 10,000 km in diameter and is eventually destroyed by the same forces keeping it in operation. A nuclear explosion weakens part of the ringworld, and centrifugal forces tear the ring apart.
[edit] 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. Science-fiction critic Justin B. Rye has proposed the term 'gravity carpet' to describe this technology, but the term has not come into widespread use.
In the Star Trek universe, artificial gravity is achieved by the use of “gravity plating” embedded in a starship's deck.
- In the Star Trek: Enterprise episode "In a Mirror, Darkly", the gravity plating of the USS Defiant is used to fend off a Gorn attack, by greatly increasing the ship's gravity in one section. The Gorn attacker was forced down to the floor and immobilized, where Jonathan Archer easily killed him.
- Benjamin Sisko once built a replica of an ancient Bajoran solar-sailer spacecraft. As these craft were not normally equipped with artificial gravity, Sisko added grav plating to make it easier for him and Jake to pilot the vessel. (DS9 episode "Explorers")
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 a stop to percentages of light speed very quickly.
In Dragonball Z, numerous characters used a spaceship with a built in gravity device to increase the gravity inside the ship to achieve a higher level of physical training.
