Artificial gravity
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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
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.
In order for an object inside the rotating spacecraft to feel the pull of artificial gravity, the object must be in motion relative to the center of rotation of the spacecraft. In the absence of air friction, an object that is not moving would remain in place (even when not at the center of motion) and would not "fall" toward the hull. This illustrates the fact that artificial gravity is a result of the object's circular motion and its reaction against the similarly moving spacecraft hull; artificial gravity is not an attraction between the object and the hull.
From the point of view of people rotating with the habitat, artificial gravity by rotation works similarly to normal gravity but has the following side effects:
- 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.
- 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.
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.
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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.
[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.
While this effect of acceleration is very well understood, this concept's practical uses are far beyond current technological capability.
[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 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.
[edit] 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.
[edit] Magnetism
A similar effect to gravity has been created through diamagnetism. It required 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[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
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.
[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 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. The movie also features a rotating space station.
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. A number of early Known Space and Man-Kzin Wars stories also make use of rotational gravity, prior to the adoption of "gravity polarizer" technology which generates artificial gravity fields.
In the Gundam universe, gigantic space habitats similar to O'Neill cylinders, called Colonies, are an important aspect to the plot. They spin to generate artificial gravity.
In the anime Cowboy Bebop, the Bebop possesses a ringed area that generates artificial gravity and is often seen being used (with the rest of the ship not rotating).
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 g.
In Iain M. Banks's The Culture novels, Orbitals are made ten million kilometres in circumference so that they spin with a rate that gives a natural day/night cycle while the center is in orbit around a star.[2]
In the game Halo: Combat Evolved, the main location of the story is an artificial ringworld that creates artificial gravity by computer-controlled rotational spin (inspired by the aforementioned Larry Niven's novel Ringworld[citation needed]). "Halo" (or "Installation 04") is approximately 10,000 km in diameter and is eventually destroyed by the same forces keeping it in operation. A fusion 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 The "Mirror" 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 gravity 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 the anime Dragon Ball Z, gravity simulation plays a key part in various character's training regime. It is also used to demonstrate the character's increasing strength. For example, when Goku first arrives on King Kai's planet, he is nearly crushed by the gravity, which is ten times that of Earth's. By the end of his visit, nearly a year later, he is able to move at great speed under such conditions. This method of training gradually appears more and more in the universe, and the gravity gets stronger as well. Ten times Earth's gravity goes from a seemingly indomitable level of opposition to nothing, and several hundred times Earth's gravity becomes the standard. Vegeta even had a Gravity Room built into his house.
In the Wii game Super Mario Galaxy, many structures in the game have their own gravity, causing Mario to go upside-down or sideways as he runs or jumps across the surface. In some areas, there are switches than can change the direction of gravity.
