Artificial gravity is the theoretical increase or decrease of apparent gravity via artificial
means, particularly in space, but also on Earth. It can be practically achieved by the
use of different forces, particularly the centripetal force and linear acceleration.
The creation of artificial gravity is considered desirable for long-term space travel or habitation,
for ease of mobility, for in-space fluid management, and to avoid the adverse long-term health
effects of weightlessness. A number of methods for generating artificial
gravity have been proposed for many years, as well as an even larger number of science
fiction approaches using both real and fictitious forces. Practical outer space applications
of artificial gravity for humans have not yet been built and flown, principally due
to the large size of the spacecraft that would be required to allow centripetal acceleration
rotating spacecraft.
Requirement for gravity Without g-force, space adaptation syndrome
occurs in some humans and animals. Many adaptations occur over a few days, but over a long period
of time bone density decreases, and some of this decrease may be permanent. The minimum
g-force required to avoid bone loss is not known—nearly all current experience is with
g-forces of 1 g or 0 g in orbit. There has not been sufficient time spent on the Moon
to determine whether or not lunar gravity is sufficient.
A limited amount of experimentation has been done by Dr. Alfred Smith, of the University
of California, with chickens, since they are bipeds, and mice experiencing high g-force
over long periods in large centrifuges on the Earth.
Rats have been exposed to continuous artificial gravity of 1 g during Russian biosatellite
missions lasting two weeks. The muscle and bone loss in these animals was found to be
less than rats in 0 g. Astronauts were exposed to artificial gravity levels ranging from
0.2 to 1 g for a few minutes during several spaceflight missions, using linear sleds or
rotating chairs. They did not perceive any changes in their spatial orientation when
the g level was lower than 0.5 g at the inner ear level, where the sensory receptors for
gravity perception are located. Methods for generating artificial gravity
Gravity can be simulated in numerous 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. 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. In accordance with the physics
involved, slower rotations or larger rotational radii would reduce or eliminate this problem,
as per Newton's Third Law The Coriolis effect gives an apparent force
that acts on objects that move relative to a rotating reference frame. This apparent
force acts at right angles to the motion and the rotation axis and tends to curve the motion
in the opposite sense to the habitat's spin. If an astronaut inside a rotating artificial
gravity environment moves towards or away from the axis of rotation, he will feel a
force pushing him towards or away from the direction of spin. These forces act on the
inner ear and can cause dizziness, nausea and disorientation. Lengthening the period
of rotation reduces the Coriolis force 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 7 rpm
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.
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 a motor and counterweight
of some kind to spin in the opposite direction. Extra strength is needed in the structure
to keep it from flying apart due to the rotation. However, the amount of structure needed over
and above that to hold a breathable atmosphere is relatively modest for most structures.
If parts of the structure are intentionally not spinning, friction and similar torques
will cause the rates of spin to converge, requiring motors and power to be used to compensate
for the losses due to friction. A traversable interface between parts of the
station spinning relative to each other requires large vacuum-tight axial seals.
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 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. 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 adverse effects on health of weightlessness. Artificial gravity at 0.1g would require a
radius of only 22 m. Likewise, at a radius of 10 m, about 10 rpm would be required to
produce Earth gravity, 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. They were able to generate a small amount of artificial
gravity, about 0.00015 g, by firing their side thrusters to slowly rotate the combined
craft like a slow-motion pair of bolas. The resultant force was too small to be felt by
either astronaut, but objects were observed moving towards the "floor" of the capsule.
The proposed Tempo3 mission rotates two halves of a spacecraft connected by a tether to test
the feasibility of simulating gravity on a manned mission to Mars.
The Mars Gravity Biosatellite was a proposed mission meant to study the effect of artificial
gravity on mammals. An artificial gravity field of 0.38g was to be produced by rotation.
Fifteen mice would have orbited Earth for five weeks and then land alive. However the
program was canceled on June 24, 2009 due to lack of funding and shifting priorities
at NASA. In amusement parks, pendulum rides and centrifuges
provide rotational force. Roller coasters also do, whenever they go over dips, humps,
or loops. When going over a hill, time in which zero or negative gravity is felt is
called air-time, which can be divided into "floater air time" and "ejector air time".
Linear acceleration Linear acceleration, even at a low level,
can provide sufficient g-force to provide useful benefits. Any spacecraft could continuously
accelerate in a straight line, forcing objects inside the spacecraft in the opposite direction
of the direction of acceleration. Most chemical reaction rockets already accelerate
at a sufficient rate to produce several times Earth's g-force but can only maintain these
accelerations for several minutes because of a limited supply of fuel.
A propulsion system with a very high specific impulse could accelerate more slowly producing
useful levels of artificial gravity for long periods of time. A variety of electric propulsion
systems provide examples. Two examples of this long-duration, low-thrust, high-impulse
propulsion that have either been practically used on spacecraft or are planned in for near-term
in-space use are Hall effect thrusters and Variable Specific Impulse Magnetoplasma Rockets.
Both provide very high specific impulse but relatively low thrust, compared to the more
typical chemical reaction rockets. They are thus ideally suited for long-duration firings
which would provide limited amounts of, but long-term, milli-g levels of artificial gravity
in spacecraft. Low-impulse but long-term linear acceleration
has been proposed for various interplanetary missions. For example, even heavy cargo payloads
to Mars could be transported to Mars in 27 months and retain approximately 55 percent
of the LEO vehicle mass upon arrival into a Mars orbit, providing a low-gravity gradient
to the spacecraft during the entire journey. Constant linear acceleration could theoretically
provide relatively short flight times around the solar system. If a propulsion technique
able to support 1g of acceleration continuously were available, a spaceship accelerating at
1g would reach Mars within a few days. 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. This effect of linear acceleration is very
well understood, and is routinely used for 0g cryogenic fluid management for post-launch
in-space firings of upper stage rockets. Roller coasters, especially launched roller
coasters or those that rely on electromagnetic propulsion, can provide linear acceleration
"gravity", and so can relatively high acceleration vehicles, such as sports cars. Linear acceleration
can be used to provide air-time on roller coasters and other thrill rides, such as the
Twilight Zone Tower of Terror. 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 mouse and thus produced a 1 g field to cancel the Earth's. Sufficiently
powerful magnets require either expensive cryogenics to keep them superconductive, or
require several megawatts of power. With such extremely strong magnetic fields,
safety for use with humans is unclear. In addition, it would involve avoiding any ferromagnetic
or paramagnetic materials near the strong magnetic field required for diamagnetism to
be evident. Facilities using diamagnetism may prove workable
for laboratories simulating low gravity conditions here on Earth. The mouse was levitated against
Earth's gravity, creating 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 "paragravity" is sometimes present in spacecraft
that are neither rotating nor accelerating. At present, there is no confirmed technique
that can simulate gravity other than actual 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. String theory predicts that gravity and
electromagnetism unify in hidden dimensions and that extremely short photons can enter
those dimensions. Training for high or low gravitational environments
Centrifuge
High-G training is done by aviators and astronauts who are subject to high levels of acceleration
in large-radius centrifuges. It is designed to prevent a g-induced Loss Of Consciousness,
a situation when g-forces move the blood away from the brain to the extent that consciousness
is lost. Incidents of acceleration-induced loss of consciousness have caused fatal accidents
in aircraft capable of sustaining high-g for considerable periods.
Parabolic flight
Weightless Wonder is the nickname for the NASA aircraft that flies parabolic trajectories
and briefly provides a nearly weightless environment in which to train astronauts, conduct research,
and film motion pictures. The parabolic trajectory creates a vertical linear acceleration which
matches that of gravity, giving zero-g for a short time, usually 20–30 seconds, followed
by approximately 1.8g for a similar period. The nickname Vomit Comet is also used to refer
to motion sickness that is often experienced by the aircraft passengers during these parabolic
trajectories. Such reduced gravity aircraft are nowadays operated by several organizations
world wide. Neutral buoyancy
A Neutral Buoyancy Laboratory is an astronaut training facility, such as the Sonny Carter
Training Facility at the NASA Johnson Space Center in Houston, Texas. The NBL is a large
indoor pool of water, the largest in the world, in which astronauts may perform simulated
EVA tasks in preparation for space missions. The NBL contains full-sized mock-ups of the
Space Shuttle cargo bay, flight payloads, and the International Space Station.
The principle of neutral buoyancy is used to simulate the weightless environment of
space. The suited astronauts are lowered into the pool using an overhead crane and their
weight is adjusted by support divers so that they experience no buoyant force and no rotational
moment about their center of mass. The suits worn in the NBL are down-rated from fully
flight-rated EMU suits like those in use on the space shuttle and International Space
Station. The NBL tank itself is 202 feet in length,
102 feet wide, and 40 feet 6 inches deep, and contains 6.2 million gallons of water.
Divers breathe nitrox while working in the tank.
Neutral buoyancy in a pool is not weightlessness, since the balance organs in the inner ear
still sense the up-down direction of gravity. Also, there is a significant amount of drag
presented by water. Generally, drag effects are minimized by doing tasks slowly in the
water. Another difference between neutral buoyancy simulation in a pool and actual EVA
during spaceflight is that the temperature of the pool and the lighting conditions are
maintained constant. Proposals
There have been a number of proposals that have incorporated artificial gravity into
their design. Discovery II: Was a 2005 vehicle proposal
capable of delivering 172 t to Jupiter's orbit in only 118 days. A very small portion of
the 1,690 t craft would incorporate a centrifuge where the crew would reside.
Multi-Mission Space Exploration Vehicle: this 2011 NASA proposal for a long-duration crewed
space transport vehicle includes a rotational artificial gravity space habitat intended
to promote crew-health for a crew of up to six persons on missions of up to two years
duration. The partial-g torus-ring centrifuge would utilize both standard metal-frame and
inflatable spacecraft structures and would provide 0.11 to 0.69g if built with the 40
feet diameter option. ISS Centrifuge Demo: Also proposed in 2011
as a demonstration project preparatory to the final design of the larger torus centrifuge
space habitat for the Multi-Mission Space Exploration Vehicle. The structure would have
an outside diameter of 30 feet with a 30 inches ring interior cross-section diameter and would
provide 0.08 to 0.51g partial gravity. This test and evaluation centrifuge would have
the capability to become a Sleep Module for ISS crew.
Mars Direct: A plan for a manned Mars mission created by NASA engineers Robert Zubrin and
David Baker in 1990, later expanded upon in Zubrin's 1996 book The Case for Mars. The
"Mars Habitat Unit", which would carry astronauts to Mars to join the previously-launched "Earth
Return Vehicle", would have had artificial gravity generated during flight by tying the
spent upper stage of the booster to the Habitat Unit, and setting them both rotating about
a common axis. Gen1 Enterprise: An experienced engineer who
goes by BTE-Dan created a website outlining in great detail how, with current technology,
we could build the first generation of a full-sized spaceship Enterprise that could carry 1,000
people all over our solar system in reasonable amounts of time. The conceptual design is
complete with a centrifugal gravity wheel to provide spacious comfortable living spaces
for the crew. See also
Media related to Artificial gravity at Wikimedia Commons
Non-inertial reference frame Anti-gravity
Artificial gravity in fiction Coriolis effect
Centrifuge Accommodations Module Fictitious force
Rotating wheel space station Space habitat
Stanford torus References
External links List of peer review papers on artificial gravity
Mars gravity experiment homepage Revolving artificial gravity calculator
Overview of artificial gravity in Sci-Fi and Space Science
NASA's Java simulation of artificial gravity Variable Gravity Research Facility, concept
with tethered rotating satellites, perhaps a Bigelow expandable module and a spent upper
stage as a counterweight.