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The International Space Station in orbit around Earth, February 2010. The ISS is in a micro-g environment.

imaThe term micro-g environment (also µg, often referred to by the term microgravity) is more or less a synonym for weightlessness and zero-g, but indicates that g-forces are not quite zero, just very small.[1] The symbol for microgravity, µg, was used on the insignias of Space Shuttle flights STS-87 and STS-107, because these flights were devoted to microgravity research in low Earth orbit.

Absence of gravity

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A "stationary" micro-g environment[2] would require travelling far enough into deep space so as to reduce the effect of gravity by attenuation to almost zero. This is the simplest in conception, but requires traveling an enormous distance, rendering it most impractical. For example, to reduce the gravity of the Earth by a factor of one million, one needs to be at a distance of 6 million kilometers from the Earth, but to reduce the gravity of the Sun to this amount one has to be at a distance of 3.7 billion kilometers. (The gravity due to the rest of the Milky Way is already smaller than one millionth of the gravity on Earth, so we do not need to move away further from its center[citation needed]). Thus it is not impossible, but it has only been achieved so far by four interstellar probes (Voyager 1 and 2, part of the Voyager program, Pioneer 10 and 11 part of the Pioneer program) and they did not return to Earth. To reduce the gravity to one thousandth of that on Earth's surface, one needs to be at a distance of 200,000 km.

Location Gravity due to Total
Earth Sun rest of Milky Way
Earth's surface 9.81 m/s2 6 mm/s2 200 pm/s2 = 6 mm/s/yr 9.81 m/s2
Low Earth orbit 9 m/s2 6 mm/s2 200 pm/s2 9 m/s2
200,000 km from Earth 10 mm/s2 6 mm/s2 200 pm/s2 up to 12 mm/s2
6×106 km from Earth 10 μm/s2 6 mm/s2 200 pm/s2 6 mm/s2
3.7×109 km from Earth 29 pm/s2 10 μm/s2 200 pm/s2 10 μm/s2
Voyager 1 (17×109 km from Earth) 1 pm/s2 500 nm/s2 200 pm/s2 500 nm/s2
0.1 light-year from Earth 400 am/s2 200 pm/s2 200 pm/s2 up to 400 pm/s2

At a distance relatively close to Earth (less than 3000 km), gravity is only slightly reduced. As an object orbits a body such as the Earth, gravity is still attracting objects towards the Earth and the object is accelerated downward at almost 1g. Because the objects are typically moving laterally with respect to the surface at such immense speeds, the object will not lose altitude because of the curvature of the Earth. When viewed from an orbiting observer, other close objects in space appear to be floating because everything is being pulled towards Earth at the same speed, but also moving forward as the Earth's surface "falls" away below. All these objects are in free fall, not zero gravity.

Compare the gravitational potential at some of these locations.

Free fall

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What remains is a micro-g environment moving in free fall, i.e. there are no forces other than gravity acting on the people or objects in this environment. To prevent air drag making the free fall less perfect, objects and people can free-fall in a capsule that itself, while not necessarily in free fall, is accelerated as in free fall.[citation needed] This can be done by applying a force to compensate for air drag. Alternatively free fall can be carried out in space, or in a vacuum tower or shaft.

The two cases that can be distinguished are that where the situation is only temporary because after some time the Earth's surface is or would be reached, and the case where the situation can go on indefinitely.

A temporary micro-g environment exists in a drop tube (in a tower or shaft), a sub-orbital spaceflight, e.g. with a sounding rocket, and in an airplane such as used by NASA's Reduced Gravity Research Program, aka the Vomit Comet, and by the Zero Gravity Corporation. A temporary micro-g environment is applied for training of astronauts, for some experiments, for filming movies, and for recreational purposes.

A micro-g environment for an indefinite time, while also possible in a spaceship going to infinity in a parabolic or hyperbolic orbit, is most practical in an Earth orbit. This is the environment commonly experienced in the International Space Station, Space Shuttle, etc. While this scenario is the most suitable for scientific experimentation and commercial exploitation, it is still quite expensive to operate in, mostly due to launch costs.

Tidal and inertial acceleration

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Objects in orbit are not perfectly weightless due to several effects:

  • Effects depending on relative position in the spacecraft:
    • Because the force of gravity decreases with distance, objects with non-zero size will be subjected to a tidal force, or a differential pull, between the ends of the object nearest and furthest from the Earth. (An extreme version of this effect is spaghettification.) At low Earth orbit (LEO) altitudes, the force differential is approximately 0.33 μg/m.
    • In a spacecraft in LEO, the centrifugal force is greater on the side of the spacecraft furthest from the Earth. This is also a tidal force, adding 0.17 μg/m to the first-mentioned effect.[citation needed]
    • "Floating" objects in a spacecraft in LEO are actually in independent orbits around the Earth. If two objects are placed side-by-side (relative to their direction of motion), they will be orbiting the Earth in different orbital planes. Since all orbital planes pass through the center of the earth, any two orbital planes intersect along a line. Therefore, two objects placed side-by-side (at any distance apart) will come together after one quarter of a revolution. If they are placed so they miss each other, they will oscillate past each other, with the same period as the orbit. This corresponds to an inward acceleration of 0.17 μg per meter horizontal distance from the center. If they are placed one ahead of the other in the same orbital plane, they will maintain their separation. If they are placed one above the other (at different radii from the center of the Earth), they will have different potential energies, so the size, eccentricity, and period of their orbits will be different, causing them to move in a complex looping pattern relative to each other.[3]
    • Gravity between the spacecraft and an object within it may make the object slowly "fall" toward a more massive part of it. The acceleration is 0.007 μg for 1000 kg at 1 m distance.
  • Uniform effects (which could be compensated):
    • Though very thin, there is some air at orbital altitudes of 185 to 1,000 km. This atmosphere causes deceleration due to friction. This could be compensated by a small continuous thrust, but in practice the deceleration is only compensated from time to time, so the small g-force of this effect is not eliminated.
    • The effects of the solar wind and radiation pressure are similar, but directed away from the Sun. Unlike the effect of the atmosphere, it does not reduce with altitude.

Commercial applications

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Metal spheres

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In a shot tower (now obsolete), molten metal (such as lead or steel) was dripped through a sieve into free fall. With sufficient height (several hundred feet), the metal would be solid enough to resist impact (usually in a water bath) at the bottom of the tower. While the shot may have been slightly deformed by its passage through the air and by impact at the bottom, this method produced metal spheres of sufficient roundness to be used directly in shotgun shells or to be refined by further processing for applications requiring higher accuracy.

High-quality crystals

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While not yet a commercial application, there has been interest in growing crystals in micro-g, as in a space station or automated artificial satellite, in an attempt to reduce crystal lattice defects.[4] Such defect-free crystals may prove useful for certain microelectronic applications and also to produce crystals for subsequent X-ray crystallography.

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    Comparison of boiling of water under earth's gravity (1 g, left) and microgravity (right). The source of heat is in the lower part of the photograph.
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    A comparison between the combustion of a candle on Earth (left) and in a microgravity environment, such as that found on the ISS (right).
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    Protein crystals grown by American scientists on the Russian Space Station Mir in 1995.[5]
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    Comparison of insulin crystals growth in outer space (left) and on Earth (right).

Health effects of the micro-g environment

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Space Motion Sickness

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Six astronauts who had been in training at the Johnson Space Center for almost a year are getting a sample of a micro-g environment

Space Motion Sickness (SMS) is thought to be a subtype of motion sickness that plagues nearly half of all astronauts who venture into space.[6] SMS, along with facial stuffiness from headward shifts of fluids, headaches, and back pain, is part of a broader complex of symptoms that comprise Space Adaptation Syndrome (SAS).[7]   SMS was first described in 1961 during the second orbit of the fourth manned spaceflight when the Cosmonaut, Gherman Titov aboard the Vostok 2, described feeling disoriented with physical complaints mostly consistent with motion sickness.  It is one of the most studied physiological problems of spaceflight but continues to pose a significant difficulty for many astronauts.  In some instances, it can be so debilitating that astronauts must sit out from their scheduled occupational duties in space – including missing out on a spacewalk they have spent months training to perform.[8] In most cases, however, astronauts will work through the symptoms even with degradation in their performance. [9]

Despite their experiences in some of the most rigorous and demanding physical maneuvers on earth, even the most seasoned astronauts may be affected by SMS, resulting in symptoms of severe nausea, projectile vomiting, fatigue, malaise (feeling sick), and headache.[9] These symptoms may occur so abruptly and without any warning that space travelers may vomit suddenly without time to contain the emesis, resulting in strong odors and liquid within the cabin which may affect other astronauts.[9] Symptoms typically last anywhere from one to three days upon entering weightlessness, but may recur upon reentry to Earth’s gravity or even shortly after landing.  SMS differs from terrestrial motion sickness in that sweating and pallor are typically minimal or absent and gastrointestinal findings usually demonstrate absent bowel sounds indicating reduced gastrointestinal motility.[10]

Even when the nausea and vomiting resolve, some central nervous system symptoms may persist which may degrade the astronaut’s performance.[10] Graybiel and Knepton proposed the term “sopite syndrome” to describe symptoms of lethargy and drowsiness associated with motion sickness in 1976.[11] Since then, their definition has been revised to include “…a symptom complex that develops as a result of exposure to real or apparent motion and is characterized by excessive drowsiness, lassitude, lethargy, mild depression, and reduced ability to focus on an assigned task.”[12] Together, these symptoms may pose a substantial threat (albeit temporary) to the astronaut who must remain attentive to life and death issues at all times.

SMS is most commonly thought to be a disorder of the vestibular system that occurs when sensory information from the visual system (sight) and the proprioceptive system (posture, position of the body) conflicts with misperceived information from the semicircular canals and the otoliths within the inner ear.  This is known as the ‘neural mismatch theory’ and was first suggested in 1975 by Reason and Brand.[13] Alternatively, the fluid shift hypothesis suggests that weightlessness reduces the hydrostatic pressure on the lower body causing fluids to shift toward the head from the rest of the body. These fluid shifts are thought to increase cerebrospinal fluid (causing back aches), intracranial pressure (causing headaches), and inner ear fluid (causing vestibular dysfunction).[14]

Despite a multitude of studies searching for a solution to the problem of SMS, it remains an ongoing problem for space travel.  Most non-pharmacological countermeasures such as training and other physical maneuvers have offered minimal benefit.  Thornton and Bonato noted, “Pre- and inflight adaptive efforts, some of them mandatory and most of them onerous, have been, for the most part, operational failures.”[15] To date, the most common intervention is promethazine, an injectable antihistamine with antiemetic properties, but sedation can be a problematic side effect.[16] Other common pharmacological options include metaclopromide, as well as oral and transdermal application of scopolamine, but drowsiness and sedation are common side effects for these medications as well.[17]

Musculoskeletal Effects

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In the space (or microgravity) environment the effects of unloading varies significantly among individuals, with sex differences compounding the variability.[18] Differences in mission duration, and the small sample size of astronauts participating in the same mission also adds to the variability to the musculoskeletal effects that are seen in space.[19] In addition to muscle loss, microgravity leads to increased bone resorption, decreased bone mineral density, and increased fracture risks.  Bone resorption leads to increased urinary levels of calcium, which can subsequently lead to an increased risk of nephrolithiasis.[20]

In the first two weeks that the muscles are unloaded from carrying the weight of the human frame during space flight, whole muscle atrophy begins. Postural muscles contain more slow fibers, and are more prone to atrophy than non-postural muscle groups.[19] The loss of muscle mass occurs because of inbalances in protein synthesis and breakdown.  The loss of muscle mass is also accompanied by a loss of muscle strength, which was observed after only 2-5 days of spaceflight during the Soyuz-3 and Soyuz-8 missions.[19]  Decreases in the generation of contractile forces and whole muscle power have also been found in response to microgravity.

To counter the effects of microgravity on the musculoskeletal system, aerobic exercise is recommended. This often takes the form of in-flight cycling.[19] A more effective regimen includes resistive exercises or the use of a penguin suit[19] (contains sewn-in elastic bands to maintain a stretch load on antigravity muscles), centrifugation, and vibration.[20] Centrifugation recreates Earth’s gravitational force on the space station, in order to prevent muscle atrophy.  Centrifugation can be performed with centrifuges or by cycling along the inner wall of the space station.[19]  Whole body vibration has been found to reduce bone resorption through mechanisms that are unclear.  Vibration can be delivered using exercise devices that use vertical displacements juxtaposed to a fulcrum, or by using a plate that oscillates on a vertical axis.[21]  The use of beta-2 adrenergic agonists to increase muscle mass, and the use of essential amino acids in conjunction with resistive exercises has been proposed as pharmacologic means of combating muscle atrophy in space.[19]

Cardiovascular Effects

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Astronaut Tracy Dyson talks about studies into cardiovascular health aboard the International Space Station.

Next to the skeletal and muscular system, the cardiovascular system is less strained in weightlessness than on Earth and is de-conditioned during longer periods spent in space.[22] In a regular environment, gravity exerts a downward force, setting up a vertical hydrostatic gradient. When standing, some 'excess' fluid resides in vessels and tissues of the legs. In a micro-g environment, with the loss of a hydrostatic gradient, some fluid quickly redistributes toward the chest and upper body; sensed as 'overload' of circulating blood volume.[23] In the micro-g environment, the newly sensed excess blood volume is adjusted by expelling excess fluid into tissues and cells (12-15% volume reduction) and red blood cells are adjusted downward to maintain a normal concentration (relative anemia).[23] In the absence of gravity, venous blood will rush to the right atrium because the force of gravity is no longer pulling the blood down into the vessels of the legs and abdomen, resulting in increased stroke volume.[24] These fluid shifts become more dangerous upon returning to a regular gravity environment as the body will attempt to adapt to the reintroduction of gravity. The reintroduction of gravity again will pull the fluid downward, but now there would be a deficit in both circulating fluid and red blood cells. The decrease in cardiac filling pressure and stroke volume during the orthostatic stress due to a decreased blood volume is what causes orthostatic intolerance.[25]

Due to the loss of blood to pump, the heart can atrophy in a micro-g environment. A weakened heart can result in low blood volume, low blood pressure and effect the body's ability to send oxygen to the brain without the individual becoming dizzy.[26] Heart rhythm disturbances have also been been seen among astronauts, but it is not clear whether this was due to pre-existing conditions of effects of a micro-g environment.[27]

See also

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References

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  1. ^ "Space myths and misconceptions – space flight". OMNI. 15 (7): 38ff. May 1993.
  2. ^ Depending on distance, "stationary" is meant relative to Earth or the Sun.
  3. ^ Chandler, David (May 1991). "Weightlessness and Microgravity" (PDF). The Physics Teacher. 29 (5): 312–13. doi:10.1119/1.2343327.
  4. ^ "Growing Crystals in Zero-Gravity" News Article by Discovery
  5. ^ Koszelak, S; Leja, C; McPherson, A (1996). "Crystallization of biological macromolecules from flash frozen samples on the Russian Space Station Mir". Biotechnology and Bioengineering. 52 (4): 449–58. doi:10.1002/(SICI)1097-0290(19961120)52:4<449::AID-BIT1>3.0.CO;2-P. PMID 11541085. S2CID 36939988.
  6. ^ Weerts, Aurélie P.; Vanspauwen, Robby; Fransen, Erik; Jorens, Philippe G.; Van de Heyning, Paul H.; Wuyts, Floris L. (2014-06-01). "Space Motion Sickness Countermeasures: A Pharmacological Double-Blind, Placebo-Controlled Study". Aviation, Space, and Environmental Medicine. 85 (6): 638–644. doi:10.3357/asem.3865.2014. PMID 24919385.
  7. ^ "Space Motion Sickness (Space Adaptation)" (PDF). NASA. June 15, 2016. Retrieved November 25, 2017.
  8. ^ "Illness keeps astronaut from spacewalk". ABCNews. February 12, 2008. Retrieved November 25, 2017.
  9. ^ a b c Thornton, William; Bonato, Frederick (2017). The Human Body and Weightlessness | SpringerLink. doi:10.1007/978-3-319-32829-4. ISBN 978-3-319-32828-7.
  10. ^ a b Wotring, V. E. (2012). Space Pharmacology. Boston: Springer. p. 52. ISBN 978-1-4614-3396-5.
  11. ^ Graybiel, A.; Knepton, J. (August 1976). "Sopite syndrome: a sometimes sole manifestation of motion sickness". Aviation, Space, and Environmental Medicine. 47 (8): 873–882. ISSN 0095-6562. PMID 949309.
  12. ^ "Sopite Syndrome: A Revised Definition (PDF Download Available)". ResearchGate. Retrieved 2017-11-27.
  13. ^ T., Reason, J. (1975). Motion sickness. Brand, J. J. London: Academic Press. ISBN 0125840500. OCLC 2073893.{{cite book}}: CS1 maint: multiple names: authors list (link)
  14. ^ Heer, Martina; Paloski, William H. (2006). "Space motion sickness: Incidence, etiology, and countermeasures". Autonomic Neuroscience. 129 (1–2): 77–79. doi:10.1016/j.autneu.2006.07.014. PMID 16935570. S2CID 6520556.
  15. ^ Thornton, William; Bonato, Frederick (2017). The Human Body and Weightlessness | SpringerLink. doi:10.1007/978-3-319-32829-4. ISBN 978-3-319-32828-7.
  16. ^ Space Pharmacology | Virginia E. Wotring | Springer. p. 59.
  17. ^ Heer, Martina; Paloski, William H. (2006). "Space motion sickness: Incidence, etiology, and countermeasures". Autonomic Neuroscience. 129 (1–2): 77–79. doi:10.1016/j.autneu.2006.07.014. PMID 16935570. S2CID 6520556.
  18. ^ Ploutz-Snyder, Lori; Bloomfield, Susan; Smith, Scott M.; Hunter, Sandra K.; Templeton, Kim; Bemben, Debra (2014-11-01). "Effects of Sex and Gender on Adaptation to Space: Musculoskeletal Health". Journal of Women's Health. 23 (11): 963–966. doi:10.1089/jwh.2014.4910. ISSN 1540-9996. PMC 4235589. PMID 25401942.
  19. ^ a b c d e f g Narici, M. V.; Boer, M. D. de (2011-03-01). "Disuse of the musculo-skeletal system in space and on earth". European Journal of Applied Physiology. 111 (3): 403–420. doi:10.1007/s00421-010-1556-x. ISSN 1439-6319. PMID 20617334. S2CID 25185533.
  20. ^ a b Smith, Scott M.; Heer, Martina; Shackelford, Linda C.; Sibonga, Jean D.; Spatz, Jordan; Pietrzyk, Robert A.; Hudson, Edgar K.; Zwart, Sara R. (2015). "Bone metabolism and renal stone risk during International Space Station missions". Bone. 81: 712–720. doi:10.1016/j.bone.2015.10.002. PMID 26456109.
  21. ^ Elmantaser, M.; McMillan, M.; Smith, K.; Khanna, S.; Chantler, D.; Panarelli, M.; Ahmed, S. F. (September 2012). "A comparison of the effect of two types of vibration exercise on the endocrine and musculoskeletal system". Journal of Musculoskeletal & Neuronal Interactions. 12 (3): 144–154. ISSN 1108-7161. PMID 22947546.
  22. ^ Ramsdell, Craig D.; Cohen, Richard J. (2003). Encyclopedia of Space Science and Technology. John Wiley & Sons, Inc. doi:10.1002/0471263869.sst074. ISBN 9780471263869.
  23. ^ a b "Current status and future direction of NASA's Space Life Sciences Program (PDF Download Available)". ResearchGate. Retrieved 2017-11-27.
  24. ^ Aubert, Andre (August 2004). "What Happens to the Human Heart in Space - Parabolic Flights Provide Some Answers" (PDF). ESA. Retrieved November 25, 2017.
  25. ^ Wieling, Wouter; Halliwill, John R; Karemaker, John M (2002-01-01). "Orthostatic intolerance after space flight". The Journal of Physiology. 538 (Pt 1): 1. doi:10.1113/jphysiol.2001.013372. ISSN 0022-3751. PMC 2290012. PMID 11773310.
  26. ^ Bungo, Michael (March 23, 2016). "Cardiac Atrophy and Diastolic Dysfunction During and After Long Duration Spaceflight: Functional Consequences for Orthostatic Intolerance, Exercise Capability and Risk for Cardiac Arrhythmias (Integrated Cardiovascular)". NASA. Retrieved November 25, 2017.
  27. ^ Fritsch-Yelle, Janice M.; Leuenberger, Urs A.; D’Aunno, Dominick S.; Rossum, Alfred C.; Brown, Troy E.; Wood, Margie L.; Josephson, Mark E.; Goldberger, Ary L. (1998). "An Episode of Ventricular Tachycardia During Long-Duration Spaceflight". The American Journal of Cardiology. 81 (11): 1391–1392. doi:10.1016/s0002-9149(98)00179-9. PMID 9631987.
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