Weightlessness is the complete or near-complete absence of the sensation ofweight, i.e., zeroapparent weight. It is also termedzero g-force, orzero-g (named after theg-force)[1] or, incorrectly,zero gravity.
Weight is a measurement of the force on an object at rest in a relatively strong gravitational field (such as on the surface of the Earth). These weight-sensations originate from contact with supporting floors, seats, beds, scales, and the like. A sensation of weight is also produced, even when the gravitational field is zero, when contact forces act upon and overcome a body'sinertia by mechanical, non-gravitational forces- such as in acentrifuge, a rotatingspace station, or within an accelerating vehicle.
When thegravitational field is non-uniform, a body infree fall experiencestidal forces and is not stress-free. Near ablack hole, such tidal effects can be very strong, leading tospaghettification. In the case of the Earth, the effects are minor, especially on objects of relatively small dimensions (such as the human body or a spacecraft) and the overall sensation of weightlessness in these cases is preserved. This condition is known asmicrogravity, and it prevails in orbiting spacecraft.Microgravity environment is more or less synonymous in its effects, with the recognition that gravitational environments are not uniform and g-forces are never exactly zero.
In Newtonian physics the sensation of weightlessness experienced by astronauts is not the result of there being zero gravitational acceleration (as seen from the Earth), but of there being nog-force that an astronaut can feel because of the free-fall condition, and also there being zero difference between the acceleration of the spacecraft and the acceleration of the astronaut. Space journalistJames Oberg explains the phenomenon this way:[2]
The myth that satellites remain in orbit because they have "escaped Earth's gravity" is perpetuated further (and falsely) by almost universal misuse of the word "zero gravity" to describe the free-falling conditions aboard orbiting space vehicles. Of course, this isn't true; gravity still exists in space. It keeps satellites from flying straight off into interstellar emptiness. What's missing is "weight", the resistance of gravitational attraction by an anchored structure or a counterforce. Satellites stay in space because of their tremendous horizontal speed, which allows them—while being unavoidably pulled toward Earth by gravity—to fall "over the horizon." The ground's curved withdrawal along the Earth's round surface offsets the satellites' fall toward the ground. Speed, not position or lack of gravity, keeps satellites in orbit around the Earth.
From the perspective of an observer not moving with the object (i.e. in aninertial reference frame) the force of gravity on an object in free fall is exactly the same as usual.[3] A classic example is an elevator car where the cable has been cut and it plummets toward Earth, accelerating at a rate equal to the 9.81 meters per second per second. In this scenario, the gravitational force is mostly, but not entirely, diminished; anyone in the elevator would experience an absence of the usual gravitational pull, however the force is notexactly zero. Since gravity is a force directed towards the center of the Earth, two balls a horizontal distance apart would be pulled in slightly different directions and would come closer together as the elevator dropped. Also, if they were some vertical distance apart the lower one would experience a higher gravitational force than the upper one since gravity diminishes according to theinverse square law. These two second-order effects are examples of micro gravity.[3]
Airplanes have been used since 1959 to provide a nearly weightless environment in which to train astronauts, conduct research, and film motion pictures. Such aircraft are commonly referred by the nickname "Vomit Comet".
To create a weightless environment, the airplane flies in a 10 km (6 mi)parabolic arc, first climbing, then entering a powered dive. During the arc, the propulsion and steering of the aircraft are controlled to cancel thedrag (air resistance) on the plane out, leaving the plane to behave as if it were free-falling in a vacuum.
Versions of such airplanes have been operated byNASA's Reduced Gravity Research Program since 1973, where the unofficial nickname originated.[4] NASA later adopted the official nickname 'Weightless Wonder' for publication.[5] NASA's current Reduced Gravity Aircraft, "Weightless Wonder VI", aMcDonnell Douglas C-9, is based atEllington Field (KEFD), nearLyndon B. Johnson Space Center.
NASA'sMicrogravity University - Reduced Gravity Flight Opportunities Plan, also known as the Reduced Gravity Student Flight Opportunities Program, allows teams of undergraduates to submit a microgravity experiment proposal. If selected, the teams design and implement their experiment, and students are invited to fly on NASA's Vomit Comet.[citation needed]
TheEuropean Space Agency (ESA) flies parabolic flights on a specially modifiedAirbus A310-300 aircraft[6] to perform research in microgravity. Along with the FrenchCNES and the GermanDLR, they conductcampaigns of three flights over consecutive days, with each flight's about 30 parabolae totalling about 10 minutes of weightlessness. These campaigns are currently operated fromBordeaux - Mérignac Airport byNovespace,[7] a subsidiary ofCNES; the aircraft is flown by test pilots fromDGA Essais en Vol.
As of May 2010[update], the ESA has flown 52 scientific campaigns and also 9 student parabolic flight campaigns.[8] Their first Zero-G flights were in 1984 using a NASA KC-135 aircraft inHouston, Texas. Other aircraft used include theRussianIlyushin Il-76 MDK before founding Novespace, then a FrenchCaravelle and anAirbus A300 Zero-G.[9][10][11]
Novespace created Air Zero G in 2012 to share the experience of weightlessness with 40 public passengers per flight, using the same A310 ZERO-G as for scientific experiences.[12] These flights are sold byAvico, are mainly operated fromBordeaux-Merignac,France, and intend to promote European space research, allowing public passengers to feel weightlessness.Jean-François Clervoy, Chairman of Novespace andESA astronaut, flies with these one-day astronauts on board A310 Zero-G. After the flight, he explains the quest of space and talks about the 3 space travels he did along his career. The aircraft has also been used for cinema purposes, withTom Cruise andAnnabelle Wallis forthe Mummy in 2017.[13]
TheZero Gravity Corporation operates a modifiedBoeing 727 which flies parabolic arcs to create 25–30 seconds of weightlessness.
Ground-based facilities that produce weightless conditions for research purposes are typically referred to asdrop tubes or drop towers.
NASA'sZero Gravity Research Facility, located at theGlenn Research Center inCleveland, Ohio, is a 145 m vertical shaft, largely below the ground, with an integral vacuum drop chamber, in which an experiment vehicle can have a free fall for a duration of 5.18 seconds, falling a distance of 132 m. The experiment vehicle is stopped in approximately 4.5 m ofpellets of expandedpolystyrene, experiencing a peakdeceleration rate of 65g.
Also at NASA Glenn is the 2.2 Second Drop Tower, which has a drop distance of 24.1 m. Experiments are dropped in a drag shield in order to reduce the effects of air drag. The entire package is stopped in a 3.3 m tall air bag, at a peak deceleration rate of approximately 20g. While the Zero Gravity Facility conducts one or two drops per day, the 2.2 Second Drop Tower can conduct up to twelve drops per day.
NASA'sMarshall Space Flight Center hosts another drop tube facility that is 105 m tall and provides a 4.6 s free fall under near-vacuum conditions.[14]
Other drop facilities worldwide include:
Another ground-based approach to simulate weightlessness for biological samples is a "3D-clinostat," also called arandom positioning machine. Unlike a regularclinostat, the random positioning machine rotates in two axes simultaneously and progressively establishes a microgravity-like condition via the principle of gravity-vector-averaging.
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On the International Space Station (ISS), there are small g-forces come from tidal effects, gravity from objects other than the Earth, such as astronauts, the spacecraft, and theSun,air resistance, and astronaut movements that impartmomentum to the space station).[16][17][18] The symbol for microgravity,μg, was used on the insignias ofSpace Shuttle flightsSTS-87 andSTS-107, because these flights were devoted to microgravity research inlow Earth orbit.
Over the years, biomedical research on the implications of space flight has become more prominent in evaluating possible pathophysiological changes in humans.[19]Sub-orbital flights seize the approximated weightlessness, or μg, in the low Earth orbit and represent a promising research model for short-term exposure. Examples of such approaches are theMASER,MAXUS, orTEXUS program run by theSwedish Space Corporation and theEuropean Space Agency.
Orbital motion is a form of free fall.[3] Objects in orbit are not perfectly weightless due to several effects:
If an object were to travel to the center of a spherical planet unimpeded by the planet's materials, it would achieve a state of weightlessness upon arriving at the center of the planet'score. This is because the mass of the surrounding planet is exerting an equal gravitational pull in all directions from the center, canceling out the pull of any one direction, establishing a space with no gravitational pull.[21]
A "stationary" micro-g environment[22] would require travelling far enough into deep space so as to reduce the effect of gravity byattenuation to almost zero. This is simple in conception but requires travelling a very large distance, rendering it highly 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 kilometres 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 kilometres. This is not impossible, but it has only been achieved thus far by fourinterstellar probes: (Voyager 1 and2 of theVoyager program, andPioneer 10 and11 of thePioneer program.) At thespeed of light it would take roughly three and a half hours to reach this micro-gravity environment (a region of space where the acceleration due to gravity is one-millionth of that experienced on the Earth's surface). To reduce the gravity to one-thousandth of that on Earth's surface, however, one needs only to be at a distance of 200,000 km.
| Location | Gravity due to | Total | ||
|---|---|---|---|---|
| Earth | Sun | rest ofMilky 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 |
| 0.1light-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 infree fall, not zero gravity.
Compare thegravitational potential at some of these locations.
Following the advent of space stations that can be inhabited for long periods, exposure to weightlessness has been demonstrated to have some deleterious effects on human health.[23][24] Humans are well-adapted to the physical conditions at the surface of the Earth. In response to an extended period of weightlessness, various physiological systems begin to change and atrophy. Though these changes are usually temporary, long-term health issues can result.
The most common problem experienced by humans in the initial hours of weightlessness is known asspace adaptation syndrome or SAS, commonly referred to as space sickness. Symptoms of SAS includenausea andvomiting,vertigo,headaches,lethargy, and overall malaise.[25] The first case of SAS was reported bycosmonautGherman Titov in 1961. Since then, roughly 45% of all people who have flown in space have suffered from this condition. The duration of space sickness varies, but in no case has it lasted for more than 72 hours, after which the body adjusts to the new environment. NASA jokingly measures SAS using the "Garn scale", named forUnited States SenatorJake Garn, whose SAS duringSTS-51-D was the worst on record. Accordingly, one "Garn" is equivalent to the most severe possible case of SAS.[26]
The most significant adverse effects of long-term weightlessness aremuscle atrophy (seeReduced muscle mass, strength and performance in space for more information) and deterioration of theskeleton, orspaceflight osteopenia.[25] These effects can be minimized through a regimen of exercise,[27] such as cycling for example. Astronauts subject to long periods of weightlessness wear pants with elastic bands attached between waistband and cuffs to compress the leg bones and reduce osteopenia.[28] Other significant effects include fluid redistribution (causing the "moon-face" appearance typical of pictures of astronauts in weightlessness),[28][29] changes in thecardiovascular system as blood pressures and flow velocities change in response to a lack of gravity, a decreased production ofred blood cells, balance disorders, and a weakening of theimmune system.[30] Lesser symptoms include loss of body mass, nasal congestion, sleep disturbance, excessflatulence, and puffiness of the face. These effects begin to reverse quickly upon return to the Earth.
In addition, after longspace flight missions, astronauts may experiencevision changes.[31][32][33][34][35] Such eyesight problems may be a major concern for future deep space flight missions, including acrewed mission to the planetMars.[31][32][33][34][36] Exposure to high levels of radiation may influence the development of atherosclerosis.[37] Clots in the internal jugular vein have recently been detected inflight.[38]
On December 31, 2012, aNASA-supported study reported thathuman spaceflight may harm thebrains ofastronauts and accelerate the onset ofAlzheimer's disease.[39][40][41] In October 2015, theNASA Office of Inspector General issued ahealth hazards report related tohuman spaceflight, including ahuman mission toMars.[42][43]
Space motion sickness (SMS) is thought to be a subtype ofmotion sickness that plagues nearly half of all astronauts who venture into space.[44] SMS, along with facial stuffiness from headward shifts of fluids, headaches, and back pain, is part of a broader complex of symptoms that comprisespace adaptation syndrome (SAS).[45] SMS was first described in 1961 during the second orbit of the fourth crewed spaceflight when the cosmonautGherman Titov aboard theVostok 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.[46] In most cases, however, astronauts will work through the symptoms even with degradation in their performance.[47]
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 severenausea, projectilevomiting,fatigue, malaise (feeling sick), andheadache.[47] 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.[47] Some changes to eye movement behaviors might also occur as a result of SMS.[48] 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 reducedgastrointestinal motility.[49]
Even when the nausea and vomiting resolve, some central nervous system symptoms may persist which may degrade the astronaut's performance.[49] Graybiel and Knepton proposed the term "sopite syndrome" to describe symptoms of lethargy and drowsiness associated with motion sickness in 1976.[50] 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."[51] 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 thevestibular system that occurs when sensory information from the visual system (sight) and theproprioceptive 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.[52] 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 pressure (causing back aches), intracranial pressure (causing headaches), and inner ear fluid pressure (causing vestibular dysfunction).[53]
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."[54] To date, the most common intervention ispromethazine, an injectableantihistamine withantiemetic properties, but sedation can be a problematic side effect.[55] Other common pharmacological options includemetoclopramide, as well as oral and transdermal application ofscopolamine, but drowsiness and sedation are common side effects for these medications as well.[53]
In the space (or microgravity) environment the effects of unloading varies significantly among individuals, with sex differences compounding the variability.[56] Differences in mission duration, and the small sample size of astronauts participating in the same mission also adds to the variability to themusculoskeletal disorders that are seen in space.[57] In addition to muscle loss, microgravity leads to increasedbone resorption, decreasedbone mineral density, and increased fracture risks. Bone resorption leads to increased urinary levels ofcalcium, which can subsequently lead to an increased risk ofnephrolithiasis.[58]
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.[57] The loss of muscle mass occurs because of imbalances 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 theSoyuz-3 andSoyuz-8 missions.[57] 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.[57] A more effective regimen includes resistive exercises or the use of a penguin suit[57] (contains sewn-in elastic bands to maintain a stretch load on antigravity muscles), centrifugation, and vibration.[58] Centrifugation recreates Earth's gravitational force on the space station, in order to preventmuscle atrophy. Centrifugation can be performed with centrifuges or by cycling along the inner wall of the space station.[57] 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.[59] The use ofbeta-2 adrenergic agonists to increase muscle mass, and the use of essential amino acids in conjunction with resistive exercises have been proposed as pharmacologic means of combating muscle atrophy in space.[57]
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.[60] 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 ahydrostatic gradient, some fluid quickly redistributes toward the chest and upper body; sensed as 'overload' of circulating blood volume.[61] 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) andred blood cells are adjusted downward to maintain a normal concentration (relativeanemia).[61] In the absence of gravity, venous blood will rush to theright atrium because the force of gravity is no longer pulling the blood down into the vessels of the legs and abdomen, resulting in increasedstroke volume.[62] 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 causesorthostatic intolerance.[63] Orthostatic intolerance can result in temporary loss of consciousness and posture, due to the lack of pressure and stroke volume.[64] Some animal species have evolved physiological and anatomical features (such as high hydrostatic blood pressure and closer heart place to head) which enable them to counteract orthostatic blood pressure.[65][66] More chronic orthostatic intolerance can result in additional symptoms such as nausea,sleep problems, and other vasomotor symptoms as well.[67]
Many studies on the physiological effects of weightlessness on the cardiovascular system are done in parabolic flights. It is one of the only feasible options to combine with human experiments, making parabolic flights the only way to investigate the true effects of the micro-g environment on a body without traveling into space.[68] Parabolic flight studies have provided a broad range of results regarding changes in the cardiovascular system in a micro-g environment. Parabolic flight studies have increased the understanding of orthostatic intolerance and decreased peripheral blood flow suffered by astronauts returning to Earth. 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 affect the body's ability to send oxygen to the brain without the individual becoming dizzy.[69]Heart rhythm disturbances have also been seen among astronauts, but it is unclear whether this was a result of pre-existing conditions or an effect of the micro-g environment.[70] One current countermeasure includes drinking a salt solution, which increases the viscosity of blood and would subsequently increase blood pressure, which would mitigate post micro-g environment orthostatic intolerance. Another countermeasure includes administration ofmidodrine, which is a selectivealpha-1 adrenergic agonist. Midodrine produces arterial and venous constriction resulting in an increase in blood pressure bybaroreceptor reflexes.[71]
Russian scientists have observed differences between cockroaches conceived in space and their terrestrial counterparts. The space-conceived cockroaches grew more quickly, and also grew up to be faster and tougher.[72]
Chicken eggs that are put in microgravity two days after fertilization appear not to develop properly, whereas eggs put in microgravity more than a week after fertilization develop normally.[73]
A 2006 Space Shuttle experiment found thatSalmonella typhimurium, a bacterium that can cause food poisoning, became more virulent when cultivated in space.[74] On April 29, 2013, scientists in Rensselaer Polytechnic Institute, funded by NASA, reported that, duringspaceflight on the International Space Station,microbes seem to adapt to thespace environment in ways "not observed on Earth" and in ways that "can lead to increases in growth andvirulence".[75]
Under certain test conditions, microbes have been observed to thrive in the near-weightlessness of space[76] and tosurvive in the vacuum of outer space.[77][78]
While not yet a commercial application, there has been interest in growingcrystals in micro-g, as in aspace station or automated artificialsatellite throughLow-gravity process engineering, in an attempt to reduce crystal lattice defects.[79] Such defect-free crystals may prove useful for certain microelectronic applications and also to produce crystals for subsequentX-ray crystallography.
In 2017, an experiment on the ISS was conducted to crystallize themonoclonal antibody therapeuticPembrolizumab, where results showed more uniform and homogenous crystal particles compared to ground controls.[80] Such uniform crystal particles can allow for the formulation of more concentrated, low-volume antibody therapies, something which can make them suitable forsubcutaneous administration, a less invasive approach compared to the current prevalent method ofintravenous administration.[81]
![Protein crystals grown by American scientists on the Russian Space Station Mir in 1995.[82]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aCrystals_grown_in_microgravity.jpg)
The sensation of weightlessness, or zero gravity, happens when the effects of gravity are not felt.
"Jake Garn was sick, was pretty sick. I don't know whether we should tell stories like that. But anyway, Jake Garn, he has made a mark in the Astronaut Corps because he represents the maximum level of space sickness that anyone can ever attain, and so the mark of being totally sick and totally incompetent is one Garn. Most guys will get maybe to a tenth Garn, if that high. And within the Astronaut Corps, he forever will be remembered by that."
One of the nice things about living in space is that exercise is part of your job ... If I don't exercise six days a week for at least a couple of hours a day, my bones will lose significant mass - 1 percent each month ... Our bodies are smart about getting rid of what's not needed, and my body has started to notice that my bones are not needed in zero gravity. Not having to support our weight, we lose muscle as well.
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The dictionary definition ofzero gravity at Wiktionary
Media related toWeightlessness at Wikimedia Commons
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