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Weightlessness
Weightlessness
Weightlessness
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Astronauts on the International Space Station experience only microgravity and thus display an example of weightlessness. Michael Foale can be seen exercising in the foreground.

Weightlessness is the complete or near-complete absence of the sensation of weight, i.e., zero apparent weight. It is also termed zero g-force, or zero-g (named after the g-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's inertia by mechanical, non-gravitational forces- such as in a centrifuge, a rotating space station, or within an accelerating vehicle.

When the gravitational field is non-uniform, a body in free fall experiences tidal forces and is not stress-free. Near a black hole, such tidal effects can be very strong, leading to spaghettification. 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 as microgravity, 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.

Weightlessness in Newtonian mechanics

[edit]
In the left half, the spring is far away from any gravity source. In the right half, it is in a uniform gravitation field.
  1. Zero gravity and weightless
  2. Zero gravity but not weightless (spring is rocket propelled)
  3. Spring is in free fall and weightless
  4. Spring rests on a plinth and has both weight1 and weight2

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 no g-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 journalist James 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 an inertial 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 not exactly 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 the inverse square law. These two second-order effects are examples of micro gravity.[3]

Weightless and reduced weight environments

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Zero gravity flight maneuver

Reduced weight in aircraft

[edit]
Main article: Reduced-gravity aircraft

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 the drag (air resistance) on the plane out, leaving the plane to behave as if it were free-falling in a vacuum.

NASA's KC-135A plane ascending for a zero gravity maneuver

NASA's Reduced Gravity Aircraft

[edit]

Versions of such airplanes have been operated by NASA'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", a McDonnell Douglas C-9, is based at Ellington Field (KEFD), near Lyndon B. Johnson Space Center.

NASA's Microgravity 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]

European Space Agency A310 Zero-G

[edit]

The European Space Agency (ESA) flies parabolic flights on a specially modified Airbus A310-300 aircraft[6] to perform research in microgravity. Along with the French CNES and the German DLR, they conduct campaigns of three flights over consecutive days, with each flight's about 30 parabolae totalling about 10 minutes of weightlessness. These campaigns are currently operated from Bordeaux - Mérignac Airport by Novespace,[7] a subsidiary of CNES; the aircraft is flown by test pilots from DGA Essais en Vol.

As of May 2010, 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 in Houston, Texas. Other aircraft used include the Russian Ilyushin Il-76 MDK before founding Novespace, then a French Caravelle and an Airbus A300 Zero-G.[9][10][11]

Commercial flights for public passengers

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Inside a Russian Ilyushin 76MDK of the Gagarin Cosmonaut Training Center

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 by Avico, are mainly operated from Bordeaux-Merignac, France, and intend to promote European space research, allowing public passengers to feel weightlessness. Jean-François Clervoy, Chairman of Novespace and ESA 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, with Tom Cruise and Annabelle Wallis for the Mummy in 2017.[13]

The Zero Gravity Corporation operates a modified Boeing 727 which flies parabolic arcs to create 25–30 seconds of weightlessness.

Ground-based drop facilities

[edit]
Zero-gravity testing at the NASA Zero Gravity Research Facility

Ground-based facilities that produce weightless conditions for research purposes are typically referred to as drop tubes or drop towers.

NASA's Zero Gravity Research Facility, located at the Glenn Research Center in Cleveland, 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 of pellets of expanded polystyrene, experiencing a peak deceleration rate of 65 g.

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 20 g. 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's Marshall 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:

Random Positioning Machines

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Another ground-based approach to simulate weightlessness for biological samples is a "3D-clinostat," also called a random positioning machine. Unlike a regular clinostat, the random positioning machine rotates in two axes simultaneously and progressively establishes a microgravity-like condition via the principle of gravity-vector-averaging.

Neutral buoyancy

[edit]

Orbits

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The relationship between acceleration and velocity vectors in an orbiting spacecraft
US astronaut Marsha Ivins demonstrates the effect of weightlessness on long hair during STS-98
The International Space Station in orbit around Earth, February 2010. The ISS is in a micro-g environment.

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 the Sun, air resistance, and astronaut movements that impart momentum to the space station).[16][17][18] 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.

Sub-Orbital flights

[edit]

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 the MASER, MAXUS, or TEXUS program run by the Swedish Space Corporation and the European Space Agency.

Orbital Motion

[edit]

Orbital motion is a form of free fall.[3] 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.) In a spacecraft in low Earth orbit (LEO), the centrifugal force is also greater on the side of the spacecraft furthest from the Earth. At a 400 km LEO altitude, the overall differential in g-force is approximately 0.384 μg/m.[20][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 extremely thin, there is some air at orbital altitudes of 185 to 1,000 km. This atmosphere causes minuscule 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 tiny 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.
  • Other Effects:
    • Routine crew activity: Due to the conservation of momentum, any crew member aboard a spacecraft pushing off a wall causes the spacecraft to move in the opposite direction.
    • Structural Vibration: Stress enacted on the hull of the spacecraft results in the spacecraft bending, causing apparent acceleration.

Weightlessness at the center of a planet

[edit]

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's core. 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]

Absence of gravity

[edit]

A "stationary" micro-g environment[22] would require travelling far enough into deep space so as to reduce the effect of gravity by attenuation 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 four interstellar probes: (Voyager 1 and 2 of the Voyager program, and Pioneer 10 and 11 of the Pioneer program.) At the speed 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 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.

Health effects

[edit]
Main articles: Effect of spaceflight on the human body and Space medicine
Astronaut Clayton Anderson as a large drop of water floats in front of him on the Discovery. Cohesion plays a bigger role in space.

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 as space adaptation syndrome or SAS, commonly referred to as space sickness. Symptoms of SAS include nausea and vomiting, vertigo, headaches, lethargy, and overall malaise.[25] The first case of SAS was reported by cosmonaut Gherman 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 for United States Senator Jake Garn, whose SAS during STS-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 are muscle atrophy (see Reduced muscle mass, strength and performance in space for more information) and deterioration of the skeleton, or spaceflight 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 the cardiovascular system as blood pressures and flow velocities change in response to a lack of gravity, a decreased production of red blood cells, balance disorders, and a weakening of the immune system.[30] Lesser symptoms include loss of body mass, nasal congestion, sleep disturbance, excess flatulence, and puffiness of the face. These effects begin to reverse quickly upon return to the Earth.

In addition, after long space flight missions, astronauts may experience vision changes.[31][32][33][34][35] Such eyesight problems may be a major concern for future deep space flight missions, including a crewed mission to the planet Mars.[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, a NASA-supported study reported that human spaceflight may harm the brains of astronauts and accelerate the onset of Alzheimer's disease.[39][40][41] In October 2015, the NASA Office of Inspector General issued a health hazards report related to human spaceflight, including a human mission to Mars.[42][43]

Space motion sickness

[edit]
See also: Space adaptation syndrome
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.[44] 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).[45] SMS was first described in 1961 during the second orbit of the fourth crewed 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.[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 severe nausea, projectile vomiting, fatigue, malaise (feeling sick), and headache.[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 reduced gastrointestinal 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 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.[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 is promethazine, an injectable antihistamine with antiemetic properties, but sedation can be a problematic side effect.[55] Other common pharmacological options include metoclopramide, as well as oral and transdermal application of scopolamine, but drowsiness and sedation are common side effects for these medications as well.[53]

Musculoskeletal effects

[edit]

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 the musculoskeletal disorders that are seen in space.[57] 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.[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 the Soyuz-3 and Soyuz-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 prevent muscle 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 of beta-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]

Cardiovascular effects

[edit]
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.[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 a hydrostatic 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) and red blood cells are adjusted downward to maintain a normal concentration (relative anemia).[61] 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.[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 causes orthostatic 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 of midodrine, which is a selective alpha-1 adrenergic agonist. Midodrine produces arterial and venous constriction resulting in an increase in blood pressure by baroreceptor reflexes.[71]

Effects on non-human organisms

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Main articles: Effect of spaceflight on the human body, Infection, Medical treatment during spaceflight, and Space medicine

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 that Salmonella 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, during spaceflight on the International Space Station, microbes seem to adapt to the space environment in ways "not observed on Earth" and in ways that "can lead to increases in growth and virulence".[75]

Under certain test conditions, microbes have been observed to thrive in the near-weightlessness of space[76] and to survive in the vacuum of outer space.[77][78]

Commercial applications

[edit]
See also: Commercial use of space, Scientific research on the International Space Station, and ESA Scientific Research on the International Space Station
Candle flame on Earth (left) versus in orbital conditions (right)

High-quality crystals

[edit]

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 through Low-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 subsequent X-ray crystallography.

In 2017, an experiment on the ISS was conducted to crystallize the monoclonal antibody therapeutic Pembrolizumab, 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 for subcutaneous administration, a less invasive approach compared to the current prevalent method of intravenous administration.[81]

  • 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.
    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.
  • A comparison between the combustion of a candle on Earth (left) and in a microgravity environment, such as that found on the ISS (right).
    A comparison between the combustion of a candle on Earth (left) and in a microgravity environment, such as that found on the ISS (right).
  • Protein crystals grown by American scientists on the Russian Space Station Mir in 1995.[82]
    Protein crystals grown by American scientists on the Russian Space Station Mir in 1995.[82]
  • Comparison of insulin crystals growth in outer space (left) and on Earth (right).
    Comparison of insulin crystals growth in outer space (left) and on Earth (right).
  • Liquids may also behave differently than on Earth, as demonstrated in this video

See also

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Weightlessness

View on Grokipedia
from Grokipedia
Weightlessness, also known as microgravity, is the condition in which objects and people experience an apparent absence of weight due to continuous free fall, where gravitational forces are effectively canceled by orbital motion or equivalent acceleration, resulting in a sensation of floating despite the presence of gravity. This state occurs when an object, such as a spacecraft in low Earth orbit, falls toward Earth at the same rate as its surroundings, typically at speeds around 17,500 miles per hour (28,000 kilometers per hour), creating a balanced trajectory where no net force from gravity is felt relative to the environment. Although an object's mass remains unchanged, its apparent weight registers as zero on a scale, distinguishing weightlessness from true zero gravity, as Earth's gravitational pull is still, for example, about 90% as strong at typical low Earth orbit altitudes of 200–250 miles (320–400 kilometers). In space environments like the , weightlessness enables unique scientific research by allowing phenomena such as fluid behavior, combustion, and material formation to be observed without the interference of 's full . Physiologically, prolonged exposure leads to effects including , bone density loss at rates up to 1% per month, and cardiovascular changes, necessitating countermeasures like exercise to mitigate deconditioning in astronauts. On Earth, brief periods of weightlessness can be simulated through parabolic aircraft flights, drop towers, or roller coasters, providing valuable testing grounds for space-related studies.

Definition and Fundamental Physics

Newtonian Perspective

In Newtonian mechanics, weight is defined as the normal force exerted by a surface on an object in contact with it, which counteracts the gravitational force to prevent free fall. This normal force is what we perceive as the sensation of weight in everyday situations, such as standing on the ground where the surface pushes upward with a force equal to mgmg, with mm being the object's mass and gg the local gravitational acceleration (approximately 9.8 m/s² on Earth's surface). Weightlessness arises when an object experiences no net contact forces, such as the normal force, and is solely under the influence of gravity, resulting in uniform acceleration at gg. In this state of free fall, the object follows a parabolic trajectory dictated by gravity alone, with no additional forces altering its motion relative to the gravitational field. The apparent weight can be expressed by the equation W=mg−maW = mg - ma, where aa is the acceleration of the reference frame containing the object; during free fall, a=ga = g, yielding W=0W = 0. This condition occurs not only in vertical drops but also in scenarios where the acceleration matches gravity's pull, leading to a lack of perceived weight. Isaac Newton's law of universal gravitation, formulated in 1687, provides the foundational framework for understanding weightlessness by describing gravity as a force proportional to the product of masses and inversely proportional to the square of the distance between them: F=Gm1m2r2F = G \frac{m_1 m_2}{r^2}, where GG is the . This law elucidates as a form of perpetual , where satellites or planets continuously "fall" toward the central body but maintain a stable due to their tangential velocity balancing the . Newton's insights, building on earlier work by Galileo on falling bodies, established that all objects accelerate uniformly under gravity regardless of mass, enabling the conceptual link between simple and complex orbital paths. It is important to distinguish weightlessness from zero gravity: gravity is never truly absent in these scenarios, as the gravitational force persists, but its effects on perceived weight are nullified by the matching acceleration of the free-falling frame. In Newtonian terms, this equivalence highlights that weightlessness is a consequence of inertial motion under , not the elimination of the itself.

Equivalence Principle and Free Fall

The , formulated by , posits that the inertial mass and gravitational mass of any object are identical, ensuring that all bodies accelerate identically under regardless of their composition or mass. This equivalence implies that, in a local region, the physical effects of a uniform are indistinguishable from those experienced in an accelerated reference frame, such as a accelerating upward at the same rate as the . Consequently, weightlessness arises in because the only force acting is , which imparts the same acceleration to all objects, eliminating as measured by contact forces like . Free fall represents a trajectory where gravitational attraction is the sole influence, resulting in weightlessness for all objects within the falling frame, independent of their mass or density. In this state, observers and objects inside a freely falling enclosure, such as an elevator, perceive no gravitational effects, as their acceleration matches that of the enclosure itself. Einstein illustrated this through a thought experiment: an observer in a sealed elevator cannot distinguish whether they are in deep space far from any gravitational sources or in free fall within a gravitational field, as both scenarios yield identical local physics with no detectable weight. This indistinguishability holds because the principle equates the experience of gravity with uniform acceleration, rendering weightlessness a direct consequence of such equivalence. The forms the foundational insight for Einstein's general , which reinterprets not as a force but as the curvature of induced by and . In this framework, weightlessness in corresponds to motion along geodesics in curved , where the absence of tidal forces—differences in gravitational pull across an extended object—mimics a zero- environment. Mathematically, within a sufficiently small region, the can be approximated as uniform, allowing the acceleration of to precisely replicate the conditions of an inertial frame devoid of , thus upholding the principle's predictions.

Simulation Methods

Parabolic Aircraft Flights

Parabolic flights simulate weightlessness by flying a modified in a series of parabolic arcs, creating brief periods of microgravity through controlled . The typically climbs at a steep of approximately 45 degrees for about 20 seconds, subjecting passengers to around 1.8 g of , before reducing and pushing over the top of the parabola at an altitude of roughly 8 kilometers. This initiates a lasting 20 to 25 seconds, during which the interior experiences near-weightlessness as the plane and its contents fall together under alone, followed by a 20-second pull-out phase returning to 1 g. The historical development of these flights traces back to NASA's KC-135 program in the 1960s, when the modified military tanker, nicknamed the "Vomit Comet," began conducting parabolic maneuvers over the to train astronauts and test equipment for the Apollo missions. The KC-135 followed a standardized flight profile involving 30 to 40 parabolas per sortie, with each arc featuring a pull-up to 1.8 g, a zero-g phase of about 25 seconds, and a pull-out to 1.8 g, allowing repeated exposure to varying g-forces for physiological and engineering studies. This program continued until 2004, when the aging KC-135 was retired due to maintenance costs and fatigue. NASA transitioned to the C-9B Skytrain II in 2005, which operated similar profiles but with enhanced instrumentation for reduced-gravity research until its retirement in 2014, after which NASA contracted commercial providers for parabolic services, including ongoing campaigns as of 2025. The (ESA) advanced its own program with the introduction of the Zero-G in 2015, operated by Novespace from Bordeaux-Mérignac Airport in , marking a shift to a more modern, fuel-efficient platform for microgravity campaigns. As of 2025, the A310 remains active, supporting ESA's research with flights executing 31 parabolas per mission, each providing 20 seconds of weightlessness amid 1.8 g transitions. Commercially, the Zero-G Corporation offered public parabolic flights from 2004 until August 2025 using a modified aircraft, known as G-Force One, enabling civilians, researchers, and trainees to experience weightlessness without government affiliation. These flights accommodated up to 35 passengers, who received pre-flight briefings, flight suits, and anti-nausea aids like Dramamine, while adhering to strict FAA Part 121 standards equivalent to commercial airlines, including redundant systems and medical oversight for a spotless safety record over more than 900 missions. Passengers reported sensations of floating, somersaulting, and brief disorientation during the parabolas, with post-flight celebrations featuring photos and memorabilia to commemorate the experience. Operations were paused in August 2025 due to certification issues, with resumption unclear as of November 2025. Each parabolic flight typically delivers 20 to 30 seconds of high-quality microgravity per arc, enabling up to 30 to 40 parabolas over a 60- to 90-minute mission, though durations can vary slightly by aircraft and atmospheric conditions. For instance, the Zero-G achieved about 30 seconds per parabola across 15 arcs, totaling around 7.5 minutes of cumulative weightlessness, while the ESA A310 targets 20 seconds of zero g within its 31 parabolas. However, the quality of microgravity is not perfect, as atmospheric , wind gusts, or pilot corrections can introduce residual accelerations up to 0.05 g, potentially disrupting sensitive experiments and reducing the effective free-fall purity compared to orbital conditions.

Drop Towers and Ground Facilities

Drop towers simulate weightlessness by releasing experimental capsules from significant heights within evacuated shafts, allowing them to undergo with minimal air resistance and achieving microgravity levels as low as 10^{-6} g. This principle leverages the equivalence of and inertial motion in a , providing short periods of pure weightlessness for scientific investigations. Prominent facilities include NASA's Zero Gravity Research Facility at the Glenn Research Center, operational since 1966, which offers 5.18 seconds of microgravity in a 510-foot (155 m) underground vacuum chamber. The ZARM Drop Tower at the University of Bremen in Germany, established in 1990, provides up to 9.3 seconds of microgravity through catapult-assisted drops from a 146-meter tower, following an initial standard drop duration of 4.74 seconds. Japan's Japan Microgravity Center (JAMIC) drop tower, with a 710-meter depth, delivered up to 10 seconds of microgravity for materials and combustion research until its closure in 2003. These systems employ drop capsules, typically up to 1 meter in diameter, that house experiments and are released into the vacuum shaft; upon reaching the bottom, deceleration occurs via energy-absorbing materials such as granules or , limiting impact forces to around g to protect payloads. Drop towers are advantageous for their cost-effectiveness in testing non-biological samples, high repeatability with multiple runs per day, and recent enhancements like catapult mechanisms that extend microgravity durations beyond traditional free falls. However, their primary limitations include very brief microgravity periods ranging from 2 to 10 seconds, which restrict complex dynamic studies, and the inability to accommodate human subjects due to constraints.

Orbital and Suborbital Spaceflight

In , weightlessness is achieved during circular orbits when the centripetal acceleration required for the spacecraft's curved path exactly balances the toward Earth's center, resulting in a state of continuous . This balance is described by GMr2=[v](/page/Velocity)2r\frac{GM}{r^2} = \frac{[v](/page/Velocity)^2}{r}, where GG is the , MM is Earth's mass, rr is the orbital radius, and vv is the orbital . Suborbital spaceflights provide brief periods of weightlessness through ballistic trajectories that arc above the at approximately 100 km altitude before falling back to Earth. Blue Origin's rocket, operational since its first crewed flight in 2021, delivers about three minutes of high-quality microgravity during its 11-minute total journey. Similarly, Virgin Galactic's , with spaceflights beginning in 2018 and commercial operations from 2021, offers four to five minutes of weightlessness per flight, peaking near 100 km. In contrast, sustains weightlessness for extended durations by maintaining a stable around . The (ISS), operational since 1998, provides microgravity environments lasting six months or more per expedition, with residual accelerations typically below 10−6g10^{-6} g during quiescent periods, enabling long-term scientific research. Historical milestones include Yuri Gagarin's pioneering one- flight on in 1961, marking the first human experience of orbital weightlessness for 108 minutes. Emerging private initiatives, such as Axiom Space's planned station module launching to dock with the ISS in 2027, aim to extend commercial orbital access beyond the ISS's retirement. Suborbital flights primarily support short-duration tourism and targeted research experiments, limited by their parabolic paths, while orbital platforms like the ISS facilitate in-depth studies requiring prolonged microgravity exposure.

Laboratory-Based Techniques

Neutral buoyancy techniques simulate weightlessness by submerging objects or astronauts in water tanks, where the buoyant force counters gravitational pull to create an apparent zero-gravity environment. This method relies on , adjusting the density of submerged items through weights or foam to achieve , allowing free movement as in microgravity. Developed for , it provides a controlled, Earth-based analog for extravehicular activities (EVAs). NASA's Neutral Buoyancy Laboratory (NBL), located at the in , exemplifies this approach with its massive indoor pool measuring 202 feet long, 102 feet wide, and 40 feet deep, holding 6.2 million gallons of chlorinated water maintained at 84–86°F. Established in 1997 as part of the Sonny Carter Training Facility, the NBL evolved from earlier facilities: training began in 1966 for Gemini missions using external pools, followed by NASA's Water Immersion Facility in 1967—a 25-foot diameter, 16-foot deep tank—and the Weightless Environment Training Facility in 1980. Today, the NBL supports EVA training for assembly and future exploration missions by deploying full-scale mockups of and habitats underwater. Rotational devices, such as clinostats and Random Positioning Machines (RPMs), offer another laboratory method to mimic weightlessness by continuously reorienting samples relative to the gravity vector, effectively averaging its direction over time to simulate microgravity. Clinostats, first introduced in by for plant studies, rotate samples around a single axis to counteract . Since the , RPMs—developed in the by Dutch Space—have advanced this concept with two independently motorized frames for three-dimensional rotation, randomizing orientation at speeds exceeding biological response times (e.g., 60 degrees per second) while minimizing centrifugal forces. These devices are particularly suited for biological research, enabling long-duration experiments on cell cultures without the intermittency of free-fall methods. Two-dimensional (2D) clinostats, which rotate flat or layered samples around one horizontal axis, simulate microgravity by averaging the 1g gravity vector for thin specimens like cell monolayers, reducing directional cues and promoting isotropic growth. In contrast, 3D RPMs extend this to complex, volumetric organisms or tissues by rotating around two axes, providing a more uniform averaging of gravity and minimizing artifacts from single-axis motion. The 3D approach is preferred for three-dimensional models, such as organoids or small animals, as it better approximates the random orientation in true microgravity. These techniques find primary applications in pre-flight at facilities like the NBL, where simulations prepare crews for spacewalk procedures, and in biological experiments using RPMs to study cellular responses, such as and tissue development under simulated weightlessness. For instance, RPMs facilitate 3D cell culturing to investigate microgravity's effects on mammalian cells, serving as ground-based analogs for hardware validation. However, limitations persist: introduces residual drag from water and potential Coriolis forces during motion, while clinostats and RPMs generate shear stresses and incomplete cancellation, leading to discrepancies in biological outcomes compared to actual orbital microgravity. Recent advances in the integrate rotational simulators with for partial gravity studies. The European Space Agency's (ESA) Large Diameter (LDC) at ESTEC, with its 8-meter arms generating 1–20g hypergravity, combines with RPMs to explore fractional g environments (e.g., lunar or Martian levels) by modulating rotation speeds. In 2024, ESA's Academy Experiments Programme utilized the LDC and RPM for projects like Team SelenarFungi's two-week study on lettuce cultivation with mycorrhizal fungi in regolith simulants, assessing plant growth under altered gravity to inform future space agriculture.

Human Physiological Impacts

Acute Effects

Upon entry into weightlessness, astronauts commonly experience space motion sickness (SMS), a condition affecting approximately 70% of individuals during the first 1-3 days of spaceflight. Symptoms include nausea, vomiting, headache, pallor, cold sweating, and malaise, primarily resulting from sensory conflict between the vestibular, visual, and proprioceptive systems in the absence of gravity. This conflict arises as the brain receives mismatched signals, leading to disorientation and gastrointestinal distress. Incidence is higher among women, potentially due to greater susceptibility to motion sickness in general. A prominent acute response is the headward fluid shift, where blood and other bodily fluids redistribute toward the upper body within hours of microgravity exposure. This cephalad movement, driven by the loss of hydrostatic gradients, causes facial puffiness (), nasal , and a noticeable reduction in leg volume, often described as "puffy face and chicken legs." The shift contributes to headaches and can exacerbate symptoms of . Vestibular disturbances further compound initial adaptation, as the otolith organs in the , which detect linear including , no longer register a consistent gravitational vector. This leads to and illusions, such as perceived tilting or inversion, impairing balance and coordination during the early phase of flight. These effects typically peak shortly after launch and subside as the adapts over days. Early manifestations of spaceflight-associated neuro-ocular syndrome (SANS) also emerge acutely, linked to elevated intracranial pressure from the fluid shifts. Signs include globe flattening of the eye, detectable via imaging, which can alter visual acuity and contribute to hyperopic shifts. To mitigate SMS, pre-flight administration of scopolamine, an anticholinergic agent, is often employed to reduce nausea and vomiting severity.

Chronic Effects

Prolonged exposure to weightlessness induces profound adaptations in the , primarily through the elimination of mechanical loading that normally stimulates and muscle maintenance. decreases at a rate of 1-2% per month in regions, with trabecular —such as in the vertebrae and —being most vulnerable due to heightened resorption and reduced formation driven by absent gravitational forces. experiences rapid , with losses reaching up to 20% in muscles like those in the legs and back within the first two weeks, exacerbated by diminished neural and protein synthesis in microgravity. The cardiovascular system undergoes from sustained , beginning with a 10-15% reduction in plasma volume during the initial weeks as fluids shift cephalad, leading to decreased overall and adaptation. This contributes to upon re-entry to , where astronauts struggle to maintain arterial in upright postures, often requiring supportive measures for mobility. Over longer durations, the heart muscle remodels with and reduced mass, reflecting the lower workload in microgravity and potential vascular stiffness changes. Immunological function is compromised in chronic weightlessness, with T-cell proliferation and activation suppressed, impairing adaptive immunity and production. Latent viruses, including Epstein-Barr and varicella-zoster, frequently reactivate due to this dysregulation and , elevating the risk of opportunistic infections during and after missions. Visual impairments arise from spaceflight-associated neuro-ocular syndrome (SANS), affecting 20-30% of long-duration astronauts and characterized by optic disc edema from alterations and fluid shifts. Studies from missions in the 1990s and ongoing research document significant bone loss in the lumbar spine after six months, underscoring the cumulative impact on skeletal integrity.

Countermeasures and Adaptations

Astronauts aboard the (ISS) follow rigorous exercise regimens as a primary against the musculoskeletal induced by weightlessness. The Advanced Resistive Exercise Device (ARED), installed in 2008, utilizes a and system to simulate gravitational loading through up to 30 different resistance exercises, mimicking to target muscle and bone preservation. These protocols typically require approximately 2.5 hours of daily exercise, combining 60 minutes of resistance training six days per week with 30 minutes of aerobic activity four to seven days per week, using devices like ARED alongside treadmills and cycle ergometers. Pharmacological interventions complement exercise to address specific physiological challenges in weightlessness. Bisphosphonates, such as alendronate, have been tested since the early in clinical trials to mitigate bone loss by suppressing activity and reducing resorption rates during . For instance, the Flight Experiment on the ISS demonstrated that alendronate, when combined with resistive exercise, enhanced bone mass preservation compared to exercise alone by further inhibiting breakdown. Additionally, anti-nausea medications like (administered intramuscularly at 25-50 mg) and (0.4 mg orally) are used to counteract , which affects up to 70% of crew members shortly after launch. Nutritional strategies play a crucial role in supporting health amid demineralization risks. High-calcium diets, providing 1,000-1,200 mg daily, paired with supplementation of 800 IU per day, help maintain calcium balance and prevent elevations in serum calcium levels during missions, as evidenced by studies on ISS crews. These interventions counteract the reduced synthesis from limited exposure in space, ensuring adequate absorption for skeletal integrity. Emerging technologies offer promising avenues for more comprehensive mitigation. Lower body negative pressure (LBNP) devices simulate gravitational fluid shifts by applying negative pressure to the lower extremities, thereby countering cephalad fluid redistribution and generating ground-reaction forces to support cardiovascular and musculoskeletal health. via , proposed for future deep-space missions like those to Mars, involves rotating habitats to produce centripetal equivalent to partial , potentially integrating with exercise to prevent more effectively than current methods. The effectiveness of these countermeasures varies, with exercise protocols significantly attenuating muscle atrophy—reducing losses by up to 50% in key muscle groups compared to early missions without such regimens—though full recovery of bone density often requires several months to years post-flight. In the , 2025 updates from ongoing standard measures investigations, including Artemis II preparations, emphasize integrated exercise and pharmacological approaches to inform countermeasures for lunar and Mars missions, with and biomarker data highlighting persistent needs for optimization against prolonged weightlessness.

Biological Effects on Non-Humans

Cellular and Molecular Responses

In non-human organisms, exposure to weightlessness induces significant alterations in , particularly the upregulation of stress-related genes. Studies conducted using Biological Research in Canisters (BRIC) experiments on the (ISS) have demonstrated that microgravity triggers changes in genes associated with hypoxia, heat shock responses, and cell wall remodeling in plant seedlings, such as . In microbial systems, similar BRIC investigations reveal upregulation of stress genes involved in remodeling, enabling adaptation to the absence of gravitational cues and preventing structural collapse in and . These transcriptional shifts highlight microgravity's role as a potent environmental stressor, promoting survival mechanisms at the molecular level. At the cellular signaling level, weightlessness disrupts mechanotransduction pathways, which rely on gravitational forces to transmit mechanical signals into biochemical responses. In bacteria like , simulated microgravity alters the abundance of signal transducer proteins, leading to impaired stress sensing and modified metabolic pathways. Yeast cells, such as , exhibit similar disruptions under low-shear modeled microgravity, with changes in genomic expression affecting mechanosensitive pathways that regulate cell wall integrity and protein synthesis. These alterations result in reduced protein synthesis efficiency and altered cellular morphology, as mechanotransduction fails to integrate gravitational input for proper cytoskeletal organization. Weightlessness also compromises DNA repair mechanisms, increasing mutation rates in microbial populations. NASA studies from the 2000s on cultures exposed to conditions showed elevated compared to ground controls, attributed to hindered pathways under microgravity. More recent simulated microgravity experiments confirm that prolonged exposure leads to the accumulation of unique mutations in , particularly under nutrient-limited conditions, enhancing adaptive evolution but risking genomic instability. Microgravity exacerbates by elevating levels of (ROS) in cells, as the lack of gravitational orientation disrupts normal signaling. In non-human cell models, including and microbial systems, simulated and real microgravity conditions increase ROS production, leading to mitochondrial dysfunction and without the regulatory cues provided by Earth's . This oxidative imbalance activates compensatory pathways but can overwhelm cellular defenses, contributing to broader molecular damage. In model organisms, weightlessness induces shifts in apoptosis regulation at the cellular level. In , spaceflight and simulated microgravity alter apoptotic miRNA and mRNA expression profiles, promoting excessive in response to environmental stressors like altered . Similarly, fruit fly () cells exposed to microgravity show dysregulated during development, with changes linked to enhanced in neural and muscular tissues. Recent 2025 studies on myogenic cells under combined simulated microgravity and further indicate telomere lengthening, potentially via activation of alternative lengthening pathways, which may influence and longevity mechanisms.

Organismal and Ecosystem Studies

Studies of plant growth in microgravity have revealed significant alterations in developmental patterns, particularly in model organisms like . Experiments conducted during NASA's Advanced Plant Experiments (APEX) missions on the (ISS) since the 2010s demonstrate enhanced root elongation and accelerated cell proliferation in Arabidopsis seedlings under microgravity conditions, contrasting with the reduced gravitropic responses that typically orient roots downward on . These changes arise from disrupted gravity-sensing mechanisms, leading to disoriented root skewing and altered cell wall composition, which could inform strategies for cultivating crops in space habitats. Such findings highlight the potential for microgravity to accelerate certain growth phases, though they pose challenges for stable systems reliant on reliable plant orientation and nutrient uptake. Animal responses to weightlessness often manifest as behavioral and physiological disorientation, as observed in aquatic vertebrates. In investigations using medaka fish (Oryzias latipes) aboard the ISS via the Aquatic Habitat facility, microgravity induced looping swim patterns and impaired schooling behavior, attributed to fluid shifts and vestibular disturbances that mimic dizziness in higher organisms. Similarly, frog embryo development, exemplified by Xenopus laevis in spaceflight experiments, exhibits anomalies such as extra cell layers during the blastula stage and delayed , though many embryos develop into outwardly normal tadpoles capable of regulating some gravitational cues. These disruptions underscore how microgravity interferes with gravity-dependent orientation during early , potentially affecting locomotion and habitat adaptation in space. Insect colonies and microbial communities display disorganized structures and heightened pathogenicity in weightless environments. Ant colonies of Tetramorium caespitum, tested on the ISS in 2014, showed altered collective search patterns and reduced surface adhesion, leading to frequent detachment and inefficient foraging despite maintained teamwork, indicating partial disorganization of social hierarchies. Concurrently, microbial ecosystems like Salmonella enterica serovar Typhimurium form denser biofilms and exhibit increased virulence in microgravity, as evidenced by NASA's OS-526 experiment, where spaceflight cultures displayed upregulated adhesion genes and enhanced lethality in mouse models compared to ground controls. These shifts suggest that weightlessness promotes biofilm stability at the expense of balanced community dynamics, raising concerns for contamination control in enclosed space habitats. Multi-generational studies on mammals demonstrate successful under prolonged microgravity exposure. On the ISS in the 2020s, experiments showed viable initial development from space-exposed , with F1 and F2 of space-flown females exhibiting health issues including metabolic and behavioral deficits. As of 2025, experiments have successfully produced healthy from stem cells exposed to six months of ISS conditions, indicating resilience in reproductive cells to space environments. These effects, linked to disrupted endocrine signaling and instability, indicate cumulative impacts on reproductive success over multiple cycles. Simulations of ecosystems in closed-loop bioreactors further illustrate imbalances induced by microgravity. The Closed Equilibrated Biological Aquatic System (CEBAS) on STS-90 demonstrated a stable closed in microgravity, with effective cycling but challenges in gas management leading to differences in oxygen distribution and system dynamics compared to 1g controls. In broader bioregenerative life support systems (BLSS), microbial and plant interactions in space analogs reveal non-optimal fluxes, where upstream processes cause dynamic imbalances in cultivation, emphasizing the need for gravity-mimicking countermeasures to sustain closed ecosystems.

Applications and Research Outcomes

Materials Science Advancements

Weightlessness provides a unique environment for by eliminating buoyancy-driven and , allowing for more controlled growth and processing of materials that are distorted by on . This absence of gravitational effects enables the production of higher-quality crystals and alloys with fewer defects, uniform compositions, and enhanced properties, which have applications in , , and . In crystal growth, microgravity facilitates the formation of defect-free protein crystals by minimizing convective flows that disrupt and growth on . NASA's Protein (PCG) experiments, conducted on missions such as , demonstrated this advantage, producing crystals up to 10 times larger than ground controls through techniques like vapor diffusion and batch methods. These larger, more ordered crystals improved resolution, aiding research. Similarly, inorganic crystals benefit, as the lack of allows for slower, more uniform incorporation of atoms into the lattice. For semiconductors and alloys, the Bridgman technique in microgravity yields highly uniform crystals by suppressing melt convection, resulting in consistent dopant distribution and reduced inclusions. Experiments on (GaAs) crystals, including those supported by the (ESA) in the , produced materials with superior structural quality and fewer defects compared to Earth-grown counterparts. These improvements enhance electrical resistivity and performance in optic applications, such as lasers and detectors. A meta-analysis of over 160 microgravity experiments confirms that 83% showed greater compositional uniformity in semiconductors like GaAs. Combustion studies in weightlessness reveal behaviors unattainable on , such as perfectly spherical flames around fuel droplets due to the absence of buoyancy-induced flow. The Flame Extinguishment Experiment (FLEX) on the (ISS) in the 2010s investigated droplet combustion with fuels like and , observing reduced soot production in certain conditions— flames produced minimal , appearing as dim blue spheres. These findings inform models for cleaner combustion on , enabling designs with lower emissions and improved efficiency for engines and industrial processes. Commercial outcomes from these advancements include in-orbit manufacturing technologies, exemplified by Made In Space (now part of ), which deployed the first 3D printer to the ISS in 2014. This system successfully produced functional parts like tools from plastic filaments, demonstrating that microgravity does not hinder additive manufacturing and can even enhance material properties such as tensile strength. The global in-space manufacturing market, encompassing space-derived materials, reached USD 6.3 billion in 2025, driven by applications in , alloys, and beyond. In mid-2025, returned the first space-manufactured pharmaceutical crystals to , advancing in-orbit drug production capabilities. Despite these advances, challenges persist in scaling production due to high launch costs, historically around $20,000 per , which limit the volume of raw materials sent to and make large-scale economically unfeasible. Even with recent reductions to about $2,700 per via reusable launchers, the expense continues to constrain experimentation and commercialization to small batches.

Biomedical and Pharmaceutical Research

Microgravity environments, such as those on the (ISS), have enabled significant advances in drug crystallization by producing higher-quality protein crystals that are larger, more ordered, and uniform compared to those grown on . This is particularly beneficial for pharmaceutical applications, including the development of treatments for diseases like cancer. In the , Merck conducted trials on the ISS to grow crystals of (Keytruda), resulting in improved structural resolution that aids in designing more effective formulations with enhanced stability and injectability for cancer therapies. These efforts build on earlier shuttle missions but were scaled up via ISS access, demonstrating microgravity's role in overcoming terrestrial limitations like that disrupt crystal formation. Insulin crystals have also been grown in microgravity, showing potential for improved diabetes treatments through better-ordered structures. In , microgravity facilitates the growth of scaffold-free 3D organoids that more accurately replicate human tissue architecture, providing novel models for studying diseases like cancer . For instance, in the 2020s, researchers from Encapsulate Bio launched colorectal cancer patient-derived organoids to the ISS aboard CRS-30 in 2024, where microgravity promoted into tumor-like clusters without gravitational distortion. These organoids exhibited distinct patterns indicative of early metastatic behavior, such as altered responses in APC-mutated cells, offering insights into mechanisms not observable in ground-based 2D or scaffolded models. Ongoing trials planned for 2025 aim to expand this to multi-site studies, potentially accelerating personalized therapies by testing drug efficacy in these realistic microgravity-grown structures. Rodent studies in microgravity have proven invaluable for modeling human , as the rapid bone loss observed mirrors postmenopausal or spaceflight-induced demineralization, allowing efficient testing of therapeutic interventions. In experiments using mouse-derived primary exposed to simulated microgravity via rotary systems, treatment with parathyroid hormone-related protein (PTHrP 1-36) analogs reversed microgravity-induced and restored anabolic signaling pathways, as evidenced by analysis of changes. These findings, from 's OSD-107 study released in 2016 but informing ongoing ISS rodent missions, highlight PTH analogs' potential to counteract by modulating viability and mineralization, providing a translational model for Earth-based therapies. Microgravity research has also enhanced production by promoting more uniform structures, which can improve and manufacturing consistency. In the , NASA-initiated studies, beginning around with bacterial like , explored growth in microgravity to reduce aggregation and achieve better particle uniformity in cell cultures. This approach, later validated in ISS experiments like Pasteur's 2020 influenza virus cultivation in MDCK cells, demonstrated higher yields of consistent , potentially leading to more effective seasonal flu with reduced variability in . NASA's ongoing ISS research has tested CRISPR-Cas9 in microgravity, with 2021 demonstrations (Genes in Space-5) confirming efficient in cells to study mechanisms under space radiation conditions. Collaborations with pharmaceutical companies, such as Merck's continued ISS protein studies, underscore industry investment in space-based biomedical tools to fast-track therapies for degenerative diseases.

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