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Artificial gravity is the creation of aninertial force that mimics the effects of agravitational force, usually byrotation.[1] Artificial gravity, orrotational gravity, is thus the appearance of acentrifugal force in arotating frame of reference (the transmission ofcentripetal acceleration vianormal force in the non-rotating frame of reference), as opposed to the force experienced inlinear acceleration, which by theequivalence principle is indistinguishable from gravity.In a more general sense, "artificial gravity" may also refer to the effect of linear acceleration, e.g. by means of arocket engine.[1]
Rotational simulated gravity has been used in simulations to help astronauts train for extreme conditions.[2] Rotational simulated gravity has been proposed as a solution inhuman spaceflight to the adversehealth effects caused by prolonged weightlessness.[3] However, there are no current practical outer space applications of artificial gravity for humans due to concerns about the size and cost of a spacecraft necessary to produce a usefulcentripetal force comparable to the gravitational field strength on Earth (g).[4]Scientists are concerned about the effect of such a system on the inner ear of the occupants. The concern is that using centripetal force to create artificial gravity will cause disturbances in the inner ear leading to nausea and disorientation. The adverse effects may prove intolerable for the occupants.[5]
In the context of a rotating space station, it is the radial force provided by the spacecraft's hull that acts as centripetal force. Thus, the "gravity" force felt by an object is thecentrifugal force perceived in therotating frame of reference as pointing "downwards" towards the hull.
ByNewton's Third Law, the value oflittleg (the perceived "downward" acceleration) is equal in magnitude and opposite in direction to the centripetal acceleration. It was tested with satellites likeBion 3 (1975) andBion 4 (1977); they both hadcentrifuges on board to put some specimens in an artificial gravity environment.
From the perspective of people rotating with the habitat, artificial gravity by rotation behaves similarly to normal gravity but with the following differences, which can be mitigated by increasing the radius of a space station.
TheGemini 11 mission attempted in 1966 to produce artificial gravity by rotating the capsule around theAgena Target Vehicle to which it was attached 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 ofbolas.[10] The resultant force was too small to be felt by either astronaut, but objects were observed moving towards the "floor" of the capsule.[11]
Artificial gravity has been suggested as a solution to various health risks associated with spaceflight.[5] In 1964, theSoviet space program believed that a human could not survive more than 14 days in space for fear that theheart andblood vessels would be unable to adapt to the weightless conditions.[12] This fear was eventually discovered to be unfounded as spaceflights have now lasted up to 437 consecutive days,[13] with missions aboard the International Space Station commonly lasting 6 months. However, the question of human safety in space did launch an investigation into the physical effects of prolonged exposure to weightlessness. In June 1991, the Spacelab Life Sciences 1 on theSpace Shuttle flightSTS-40 flight performed 18 experiments on two men and two women over nine days. In an environment without gravity, it was concluded that the response ofwhite blood cells andmuscle mass decreased. Additionally, within the first 24 hours spent in a weightless environment,blood volume decreased by 10%.[14][4][1] Long periods of weightlessness can cause brain swelling and eyesight problems.[15] Upon return to Earth, the effects of prolonged weightlessness continue to affect the human body as fluids pool back to the lower body, theheart rate rises, a drop inblood pressure occurs, and there is a reduced tolerance forexercise.[14]
Artificial gravity, for its ability tomimic the behavior of gravity on the human body, has been suggested as one of the most encompassing manners of combating the physical effects inherent in weightless environments. Other measures that have been suggested as symptomatic treatments include exercise, diet, andPingvin suits. However, criticism of those methods lies in the fact that they do not fully eliminate health problems and require a variety of solutions to address all issues. Artificial gravity, in contrast, would remove the weightlessness inherent in space travel. By implementing artificial gravity, space travelers would never have to experience weightlessness or the associated side effects.[1] Especially in a modern-day six-month journey toMars, exposure to artificial gravity is suggested in either a continuous or intermittent form to prevent extreme debilitation to the astronauts during travel.[5]
Several proposals have incorporated artificial gravity into their design:
Some of the reasons that artificial gravity remains unused today inspaceflight trace back to the problems inherent inimplementation. One of the realistic methods of creating artificial gravity is the centrifugal effect caused by thecentripetal force of the floor of a rotating structure pushing up on the person. In that model, however, issues arise in the size of the spacecraft. As expressed by John Page and Matthew Francis, the smaller a spacecraft (the shorter the radius of rotation), the more rapid the rotation that is required. As such, to simulate gravity, it would be better to utilize a larger spacecraft that rotates slowly.
The requirements on size about rotation are due to the differing forces on parts of the body at different distances from the axis of rotation. If parts of the body closer to the rotational axis experience a force that is significantly different from parts farther from the axis, then this could have adverse effects. Additionally, questions remain as to what the best way is to initially set the rotating motion in place without disturbing the stability of the whole spacecraft's orbit. At the moment, there is not a ship massive enough to meet the rotation requirements, and the costs associated with building, maintaining, andlaunching such a craft are extensive.[4]
In general, with the small number of negative health effects present in today's typically shorter spaceflights, as well as with the very large cost ofresearch for a technology which is not yet really needed, the present day development of artificial gravity technology has necessarily been stunted and sporadic.[1][14]
As the length of typical space flights increases, the need for artificial gravity for the passengers in such lengthy spaceflights will most certainly also increase, and so will the knowledge and resources available to create such artificial gravity, most likely also increase. In summary, it is probably only a question of time, as to how long it might take before the conditions are suitable for the completion of the development of artificial gravity technology, which will almost certainly be required at some point along with the eventual and inevitable development of an increase in the average length of a spaceflight.[24]
Several science fiction novels, films, and series have featured artificial gravity production.
Linear acceleration is another method of generating artificial gravity, by using the thrust from a spacecraft's engines to create the illusion of being under a gravitational pull. A spacecraft under constant acceleration in a straight line would have the appearance of a gravitational pull in the direction opposite to that of the acceleration, as the thrust from the engines would cause the spacecraft to "push" itself up into the objects and persons inside of the vessel, thus creating the feeling of weight. This is because ofNewton's third law: the weight that one would feel standing in a linearly accelerating spacecraft would not be a true gravitational pull, but simply the reaction of oneself pushing against the craft's hull as it pushes back. Similarly, objects that would otherwise be free-floating within the spacecraft if it were not accelerating would "fall" towards the engines when it started accelerating, as a consequence ofNewton's first law: the floating object would remain at rest, while the spacecraft would accelerate towards it, and appear to an observer within that the object was "falling".
To emulate artificial gravity on Earth, spacecraft using linear acceleration gravity may be built similar to a skyscraper, with its engines as the bottom "floor". If the spacecraft were to accelerate at the rate of 1 g—Earth's gravitational pull—the individuals inside would be pressed into the hull at the same force, and thus be able to walk and behave as if they were on Earth.
This form of artificial gravity is desirable because it could functionally create the illusion of a gravity field that is uniform and unidirectional throughout a spacecraft, without the need for large, spinning rings, whose fields may not be uniform, not unidirectional with respect to the spacecraft, and require constant rotation. This would also have the advantage of relatively high speed: a spaceship accelerating at 1 g, 9.8 m/s2, for the first half of the journey, and then decelerating for the other half, could reachMars within a few days.[26] Similarly, a hypotheticalspace travel using constant acceleration of 1 g for one year would reachrelativistic speeds and allow for a round trip to the nearest star,Proxima Centauri. As such, low-impulse but long-term linear acceleration has been proposed for various interplanetary missions. For example, even heavy (100ton) cargo payloads to Mars could be transported to Mars in27 months and retain approximately 55 percent of theLEO vehicle mass upon arrival into a Mars orbit, providing a low-gravity gradient to the spacecraft during the entire journey.[27]
This form of gravity is not without challenges, however. At present, the only practical engines that could propel a vessel fast enough to reach speeds comparable to Earth's gravitational pull requirechemicalreaction rockets, which expelreaction mass to achieve thrust, and thus the acceleration could only last for as long as a vessel had fuel. The vessel would also need to be constantly accelerating and at a constant speed to maintain the gravitational effect, and thus would not have gravity while stationary, and could experience significant swings ing-forces if the vessel were to accelerate above or below 1 g. Further, for point-to-point journeys, such as Earth-Mars transits, vessels would need to constantly accelerate for half the journey, turn off their engines, perform a 180° flip, reactivate their engines, and then begin decelerating towards the target destination, requiring everything inside the vessel to experience weightlessness and possibly be secured down for the duration of the flip.
A propulsion system with a very highspecific impulse (that is, good efficiency in the use ofreaction mass that must be carried along and used for propulsion on the journey) could accelerate more slowly producing useful levels of artificial gravity for long periods of time. A variety ofelectric 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 areHall effect thrusters andVariable Specific Impulse Magnetoplasma Rockets (VASIMR). Both provide very highspecific 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.[citation needed]
In a number of science fiction plots, acceleration is used to produce artificial gravity forinterstellar spacecraft, propelled by as yettheoretical orhypothetical means.
This effect of linear acceleration is well understood, and is routinely used for 0 g cryogenic fluid management for post-launch (subsequent) in-space firings ofupper stage rockets.[28]
Roller coasters, especiallylaunched roller coasters or those that rely onelectromagnetic propulsion, can provide linear acceleration "gravity", and so can relatively high acceleration vehicles, such assports cars. Linear acceleration can be used to provideair-time on roller coasters and other thrill rides.
In January 2022, China was reported by theSouth China Morning Post to have built a small (60centimetres (24 in)diameter) research facility to simulate lowlunar gravity with the help ofmagnets.[29][30] The facility was reportedly partly inspired by the work ofAndre Geim (who later shared the 2010Nobel Prize in Physics for his research ongraphene) andMichael Berry, who both shared theIg Nobel Prize in Physics in2000 for themagnetic levitation of a frog.[29][30]
In science fiction, artificial gravity (or cancellation of gravity) or "paragravity"[31][32] is sometimes present in spacecraft that are neither rotating nor accelerating. At present, there is no confirmed technique as such that can simulate gravity other than actual rotation 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." In 2006, a research group funded byESA claimed to have created a similar device that demonstrated positive results for the production of gravitomagnetism, although it produced only 0.0001 g.[33]
Developing techniques for manipulating fluids in microgravity, which typically fall into the category known as settled propellant handling. Research for cryogenic upper stages dating back to the Saturn S-IVB and Centaur found that providing a slight acceleration (as little as 10−4 to 10−5 g of acceleration) to the tank can make the propellants assume a desired configuration, which allows many of the main cryogenic fluid handling tasks to be performed in a similar fashion to terrestrial operations. The simplest and most mature settling technique is to apply thrust to the spacecraft, forcing the liquid to settle against one end of the tank.
Interestingly, the facility was partly inspired by previous research conducted by Russian physicist Andrew Geim in which he floated a frog with a magnet. The experiment earned Geim the Ig Nobel Prize in Physics, a satirical award for unusual scientific research. It's cool that a quirky experiment involving floating a frog could lead to something approaching an honest-to-God antigravity chamber.
It is said to be the first of its kind and could play a key role in the country's future lunar missions. The magnetic field supported the landscape and was inspired by experiments to levitate a frog.
