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Artificial gravity

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Gemini 11 tethered in 1966 the GATV-5006 Agena target vehicle performing various tests including a first artificial gravity test in a microgravity environment.
Proposed Nautilus-X International space station centrifuge demo concept, 2011.

Artificial gravity is the creation of an inertial force that mimics the effects of a gravitational force, usually by rotation.[1] Artificial gravity, or rotational gravity, is thus the appearance of a centrifugal force in a rotating frame of reference (the transmission of centripetal acceleration via normal force in the non-rotating frame of reference), as opposed to the force experienced in linear acceleration, which by the equivalence 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 a rocket 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 in human spaceflight to the adverse health 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 useful centripetal 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]

Centripetal force

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Further information: Absolute rotation and Bucket argument
Artificial gravity space station. 1969 NASA concept. A drawback is that the astronauts would be moving between higher gravity near the ends and lower gravity near the center.

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 the centrifugal force perceived in the rotating frame of reference as pointing "downwards" towards the hull.

By Newton's Third Law, the value of little g (the perceived "downward" acceleration) is equal in magnitude and opposite in direction to the centripetal acceleration. It was tested with satellites like Bion 3 (1975) and Bion 4 (1977); they both had centrifuges on board to put some specimens in an artificial gravity environment.

Differences from normal gravity

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Balls in a rotating spacecraft

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.

  • Centrifugal force varies with distance: Unlike real gravity, the apparent centrifugal force felt by observers in the habitat pushes radially outward from the axis, and the centrifugal force is directly proportional to the distance from the axis of the habitat. With a small radius of rotation, a standing person's head would feel significantly less gravity than their feet.[6] Likewise, passengers who move in a space station experience changes in apparent weight in different parts of the body.[7]
  • The Coriolis effect gives an apparent force that acts on objects that are moving 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, they will feel a force pushing them in or against the direction of spin. These forces act on the semicircular canals of the inner ear and can cause dizziness.[8] Lengthening the period of rotation (lower spin rate) 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, although humans have been shown to adapt to rates as high as 23 rpm.[9]
  • Changes in the rotation axis or rate of a spin would cause a disturbance in the artificial gravity field and stimulate the semicircular canals (refer to above). Any movement of mass within the station, including a movement of people, would shift the axis and could potentially cause a dangerous wobble. Thus, the rotation of a space station would need to be adequately stabilized, and any operations to deliberately change the rotation would need to be done slowly enough to be imperceptible.[8] One possible solution to prevent the station from wobbling would be to use its liquid water supply as ballast which could be pumped between different sections of the station as required.
Speed in rpm for a centrifuge of a given radius to achieve a given g-force

Human spaceflight

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The Gemini 11 mission attempted in 1966 to produce artificial gravity by rotating the capsule around the Agena 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 of bolas.[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]

Health benefits

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Artificial gravity has been suggested for interplanetary journeys to Mars

Artificial gravity has been suggested as a solution to various health risks associated with spaceflight.[12] In 1964, the Soviet space program believed that a human could not survive more than 14 days in space for fear that the heart and blood vessels would be unable to adapt to the weightless conditions.[13] This fear was eventually discovered to be unfounded as spaceflights have now lasted up to 437 consecutive days,[14] 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, a Spacelab Life Sciences 1 flight performed 18 experiments on two men and two women over nine days. In an environment without gravity, it was concluded that the response of white blood cells and muscle mass decreased. Additionally, within the first 24 hours spent in a weightless environment, blood volume decreased by 10%.[15][4][1] Long weightless periods can cause brain swelling and eyesight problems.[16] Upon return to Earth, the effects of prolonged weightlessness continue to affect the human body as fluids pool back to the lower body, the heart rate rises, a drop in blood pressure occurs, and there is a reduced tolerance for exercise.[15]

Artificial gravity, for its ability to mimic 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, and Pingvin 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 to Mars, exposure to artificial gravity is suggested in either a continuous or intermittent form to prevent extreme debilitation to the astronauts during travel.[12]

Proposals

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Rotating Mars spacecraft – 1989 NASA concept.

Several proposals have incorporated artificial gravity into their design:

  • Discovery II: a 2005 vehicle proposal capable of delivering a 172-metric-ton crew to Jupiter's orbit in 118 days. A very small portion of the 1,690-metric-ton craft would incorporate a centrifugal crew station.[17]
  • Multi-Mission Space Exploration Vehicle (MMSEV): a 2011 NASA proposal for a long-duration crewed space transport vehicle; it included 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 in duration. The torus-ring centrifuge would utilize both standard metal-frame and inflatable spacecraft structures and would provide 0.11 to 0.69 g if built with the 40 feet (12 m) diameter option.[18][19]
  • ISS Centrifuge Demo: a 2011 NASA proposal for 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 (9.1 m) with a ring interior cross-section diameter of 30 inches (760 mm). It would provide 0.08 to 0.51 g partial gravity. This test and evaluation centrifuge would have the capability to become a Sleep Module for the ISS crew.[18]
Artist view of TEMPO³ in orbit.
  • Mars Direct: A plan for a crewed 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.[20]
  • The proposed Tempo3 mission rotates two halves of a spacecraft connected by a tether to test the feasibility of simulating gravity on a crewed mission to Mars.[21]
  • The Mars Gravity Biosatellite was a proposed mission meant to study the effect of artificial gravity on mammals. An artificial gravity field of 0.38 g (equivalent to Mars's surface gravity) was to be produced by rotation (32 rpm, radius of ca. 30  cm). Fifteen mice would have orbited Earth (Low Earth orbit) for five weeks and then land alive.[22] However, the program was canceled on 24 June 2009, due to a lack of funding and shifting priorities at NASA.[23]
  • Vast Space is a private company that proposes to build the world's first artificial gravity space station using the rotating spacecraft concept.[24]

Issues with implementation

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Some of the reasons that artificial gravity remains unused today in spaceflight trace back to the problems inherent in implementation. One of the realistic methods of creating artificial gravity is the centrifugal effect caused by the centripetal 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, and launching 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 of research for a technology which is not yet really needed, the present day development of artificial gravity technology has necessarily been stunted and sporadic.[1][15]

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.[25]

In science fiction

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Several science fiction novels, films, and series have featured artificial gravity production.

  • In the movie 2001: A Space Odyssey, a rotating centrifuge in the Discovery spacecraft provides artificial gravity.
  • The 1999 television series Cowboy Bebop, a rotating ring in the Bebop spacecraft creates artificial gravity throughout the spacecraft.
  • In the novel The Martian, the Hermes spacecraft achieves artificial gravity by design; it employs a ringed structure, at whose periphery forces around 40% of Earth's gravity are experienced, similar to Mars' gravity.
    • In the novel Project Hail Mary by the same author, weight on the titular ship Hail Mary is provided initially by engine thrust, as the ship is capable of constant acceleration up to 2g, and is also able to separate, turn the crew compartment inwards, and rotate to produce 1g while in orbit.
  • The movie Interstellar features a spacecraft called the Endurance that can rotate on its central axis to create artificial gravity, controlled by retro thrusters on the ship.
  • The 2021 film Stowaway features the upper stage of a launch vehicle connected by 450-meter long tethers to the ship's main hull, acting as a counterweight for inertia-based artificial gravity.[26]
  • In the television series For All Mankind, the space hotel Polaris, later renamed Phoenix after being purchased and converted into a space vessel by Helios Aerospace for their own Mars mission, features a wheel-like structure controlled by thrusters to create artificial gravity, whilst a central axial hub operates in zero gravity as a docking station.

Linear acceleration

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Further information: Equivalence principle

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 of Newton'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 of Newton'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 reach Mars within a few days.[27] Similarly, a hypothetical space travel using constant acceleration of 1 g for one year would reach relativistic 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 (100 ton) 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.[28]

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 require chemical reaction rockets, which expel reaction 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 in g-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 high specific impulse (that is, good efficiency in the use of reaction 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 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 (VASIMR). 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.[citation needed]

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 well understood, and is routinely used for 0 g cryogenic fluid management for post-launch (subsequent) in-space firings of upper stage rockets.[29]

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.

Simulating lunar gravity

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See also: Gravitation of the Moon § Simulating lunar gravity

In January 2022, China was reported by the South China Morning Post to have built a small (60 centimetres (24 in) diameter) research facility to simulate low lunar gravity with the help of magnets.[30][31] The facility was reportedly partly inspired by the work of Andre Geim (who later shared the 2010 Nobel Prize in Physics for his research on graphene) and Michael Berry, who both shared the Ig Nobel Prize in Physics in 2000 for the magnetic levitation of a frog.[30][31]

Simulating microgravity

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Parabolic flight

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Main article: Reduced-gravity aircraft

Weightless Wonder is the nickname for the NASA aircraft that flies parabolic trajectories. Briefly, it provides a nearly weightless environment to train astronauts, conduct research, and film motion pictures. The parabolic trajectory creates a vertical linear acceleration that 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, referring to motion sickness that aircraft passengers often experience during these parabolic trajectories. Such reduced gravity aircraft are nowadays operated by several organizations worldwide.[citation needed]

Neutral buoyancy

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Main article: Neutral Buoyancy Laboratory

The Neutral Buoyancy Laboratory (NBL) is an astronaut training facility at the Sonny Carter Training Facility at the NASA Johnson Space Center in Houston, Texas.[32] The NBL is a large indoor pool of water, the largest in the world,[33] 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 (ISS).[34]

The principle of neutral buoyancy is used to simulate the weightless environment of space.[32] 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.[32] 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 is 202 feet (62 m) in length, 102 feet (31 m) wide, and 40 feet 6 inches (12.34 m) deep, and contains 6.2 million gallons (23.5 million liters) of water.[34][35] Divers breathe nitrox while working in the tank.[36][37]

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.[38] 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.

Speculative or fictional mechanisms

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Main articles: Gravitational shielding and Anti-gravity

In science fiction, artificial gravity (or cancellation of gravity) or "paragravity"[39][40] 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", 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 0.0001 g.[41] This result has not been replicated.

See also

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References

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  1. ^ a b c d e Young, Laurence; Yajima, Kazuyoshi; Paloski, William, eds. (September 2009). ARTIFICIAL GRAVITY RESEARCH TO ENABLE HUMAN SPACE EXPLORATION (PDF). International Academy of Astronautics. ISBN 978-2-917761-04-5. Archived from the original (PDF) on October 13, 2016. Retrieved February 23, 2022.
  2. ^ Strauss, Samuel (July 2008). "Space medicine at the NASA-JSC, neutral buoyancy laboratory". Aviation, Space, and Environmental Medicine. 79 (7): 732–733. ISSN 0095-6562. LCCN 75641492. OCLC 165744230. PMID 18619137.
  3. ^ Clement, Gilles (February 15, 2015). "Human Research Program Human Health Countermeasures Element: Evidence Report - Artificial Gravity". {{cite journal}}: Cite journal requires |journal= (help)
  4. ^ a b c Feltman, Rachel (3 May 2013). "Why Don't We Have Artificial Gravity?". Popular Mechanics. ISSN 0032-4558. OCLC 671272936. Archived from the original on 1 January 2022. Retrieved 23 February 2022.
  5. ^ Clément, Gilles R.; Bukley, Angelia P.; Paloski, William H. (June 17, 2015). "Artificial gravity as a countermeasure for mitigating physiological deconditioning during long-duration space missions". Frontiers in Systems Neuroscience. 9: 92. doi:10.3389/fnsys.2015.00092. ISSN 1662-5137. PMC 4470275. PMID 26136665.
  6. ^ Fifth Symposium on the Role of the Vestibular Organs in Space Exploration: Held Under the Auspices of the Committee on Hearing, Bioacoustics, and Biomechanics, National Academy of Sciences-National Research Council, and Assisted by the Office of Advanced Research and Technology, National Aeronautics and Space Administration. National Aeronautics and Space Administration. 1973. p. 25.
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  10. ^ Gatland, Kenneth (1976). Manned Spacecraft, Second Revision. New York, NY, USA: MacMillan. pp. 180–182. ISBN 978-0-02-542820-1.
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  17. ^ Craig H. Williams; Leonard A. Dudzinski; Stanley K. Borowski; Albert J. Juhasz (March 2005). "Realizing "2001: A Space Odyssey": Piloted Spherical Torus Nuclear Fusion Propulsion" (PDF). Cleveland, Ohio: NASA. Retrieved September 28, 2011.
  18. ^ a b NAUTILUS – X: Multi-Mission Space Exploration Vehicle Archived March 4, 2011, at the Wayback Machine, Mark L. Holderman, Future in Space Operations (FISO) Colloquium, 2011-01-26. Retrieved 2011-01-31
  19. ^ NASA NAUTILUS-X: multi-mission exploration vehicle includes a centrifuge, which would be tested at ISS Archived February 25, 2011, at the Wayback Machine, RLV and Space Transport News, 2011-01-28. Retrieved 2011-01-31
  20. ^ "NSS Review: The Case for Mars". www.nss.org. Archived from the original on January 11, 2018. Retrieved April 4, 2018.
  21. ^ The Mars Quarterly Archived April 21, 2017, at the Wayback Machine pg15-Tom Hill
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  23. ^ "The Mars Gravity Biosatellite Program Is Closing Down". www.spaceref.com. June 24, 2009. Retrieved April 4, 2018.[permanent dead link]
  24. ^ Werner, Debra (September 15, 2022). "Vast Space to develop artificial-gravity space station". SpaceNews. Retrieved September 17, 2023.
  25. ^ Artificial Gravity, Houston We Have a Podcast NASA.gov. By Gary Jordan and Bill Paloski. March 26, 2021. Retrieved February 11, 2024.
  26. ^ Kiang, Jessica (April 22, 2021). "Review: Anna Kendrick is lost, and found, in space in smart sci-fi 'Stowaway'". Los Angeles Times. Retrieved April 25, 2021.
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  28. ^ VASIMR VX-200 Performance and Near-term SEP Capability for Unmanned Mars Flight Archived March 11, 2011, at the Wayback Machine, Tim Glover, Future in Space Operations (FISO) Colloquium, pp. 22, 25, 2011-01-19. Retrieved 2011-02-01
  29. ^ Jon Goff; et al. (2009). "Realistic Near-Term Propellant Depots" (PDF). American Institute of Aeronautics and Astronautics. Retrieved February 7, 2011. 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.
  30. ^ a b "China building "Artificial Moon" that simulates low gravity with magnets". Futurism.com. Recurrent Ventures. Retrieved January 17, 2022. 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.
  31. ^ a b Chen, Stephen (January 12, 2022). "China has built an artificial moon that simulates low-gravity conditions on Earth". South China Morning Post. Retrieved January 17, 2022. 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.
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  34. ^ a b Strauss, S.; Krog, R.L.; Feiveson, A.H. (May 2005). "Extravehicular mobility unit training and astronaut injuries". Aviat Space Environ Med. 76 (5): 469–74. PMID 15892545. Retrieved August 27, 2008.
  35. ^ "NBL Characteristics". About the NBL. NASA. June 23, 2005. Archived from the original on June 26, 2007.
  36. ^ Fitzpatrick DT, Conkin J (2003). "Improved pulmonary function in working divers breathing nitrox at shallow depths". Undersea and Hyperbaric Medicine. 30 (Supplement): 763–7. PMID 12862332. Archived from the original on August 11, 2011. Retrieved August 27, 2008.{{cite journal}}: CS1 maint: unfit URL (link)
  37. ^ Fitzpatrick DT, Conkin J (July 2003). "Improved pulmonary function in working divers breathing nitrox at shallow depths". Aviat Space Environ Med. 74 (7): 763–7. PMID 12862332. Retrieved August 27, 2008.
  38. ^ Pendergast D, Mollendorf J, Zamparo P, Termin A, Bushnell D, Paschke D (2005). "The influence of drag on human locomotion in water". Undersea and Hyperbaric Medicine. 32 (1): 45–57. PMID 15796314. Archived from the original on July 9, 2009. Retrieved August 27, 2008.{{cite journal}}: CS1 maint: unfit URL (link)
  39. ^ Collision Orbit, 1942 by Jack Williamson
  40. ^ Pale Blue Dot: A Vision of the Human Future in Space by Carl Sagan, Chapter 19
  41. ^ "Toward a new test of general relativity?". Esa.int. Archived from the original on December 28, 2017. Retrieved August 6, 2013.
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