Weightlessness
Overview
 
Weightlessness is the condition that exists for an object or person when they experience little or no acceleration except the acceleration that defines their inertial trajectory, or the trajectory of pure free-fall. The physical path of an inertial trajectory depends only on the direction and strength of the sum of the gravitational attractions outside of the inertial reference frame.

The definition and use of 'Weightlessness' are difficult.
Encyclopedia
Weightlessness is the condition that exists for an object or person when they experience little or no acceleration except the acceleration that defines their inertial trajectory, or the trajectory of pure free-fall. The physical path of an inertial trajectory depends only on the direction and strength of the sum of the gravitational attractions outside of the inertial reference frame.

The definition and use of 'Weightlessness' are difficult. Weight
Weight
In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...

 means the force exerted by gravity, weightless means the absence of such forces and weightlessness formally means the condition of zero gravitational force. In common use, however, 'weightlessness' (often with quotation marks) has the meaning given in the preceding paragraph. Astronauts and cosmonauts in the International Space Station are said to experience 'weightlessness', even though, at an altitude of a few hundred km, their weight (the gravitational force acting on them) is only about 10% less than on earth. Their orbit has a large centripetal acceleration towards the earth and their weight is the centripetal force producing it. However, the space station has (almost) exactly the same acceleration towards the earth. Consequently, in the frame of the space station, an unsupported person appears to have no acceleration and so his relative motion in this frame is the same as that of a person without weight in a spacecraft that is not accelerating. Hence the name 'weightlessness'.

If objects are far from a star, planet, moon, or other such massive body, so that they experience very little gravitational interaction with them, they would approach the condition of zero gravity. If they are close to a massive object, but are freely accelerating towards the mass by gravitational acceleration only, they are in free fall
Free fall
Free fall is any motion of a body where gravity is the only force acting upon it, at least initially. These conditions produce an inertial trajectory so long as gravity remains the only force. Since this definition does not specify velocity, it also applies to objects initially moving upward...

 and are weightless. Physically, they both follow Newton's first law of motion which describes linear motion. Such a situation, except for microgravity effects and the inhomogeneity of the gravitational field, cannot be distinguished from weightlessness due to the absence of gravity from a body nearby.

As an example, an accelerated free fall trajectory results in the weightlessness of objects in a falling elevator. The same type of accelerated free fall trajectory causes weightlessness of objects in orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

 about the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

. Such objects are in free fall toward the Earth, as in the falling elevator, but they do not strike the Earth because their forward speed is such that the curved surface of the Earth drops downward and away from the object as fast as the object falls toward the Earth. An astronaut inside an orbiting vehicle has the experience of weightlessness because the action and acceleration due to gravity by itself does not cause a sensation of weight, and all of the other types of forces that do cause such sensations (such as mechanical pushes from the floor or other surfaces that cause g-force
G-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...

 acceleration) are absent.

In all inertial reference frame
Reference frame
Reference frame may refer to:*Frame of reference, in physics*Reference frame , frames of a compressed video that are used to define future frames...

s, while weightlessness is experienced, Newton's first law of motion is obeyed locally within the frame. Inside the frame (for example, inside an orbiting rocket or free-falling elevator), objects in linear motion stay in motion, at the same speed and direction. Objects at rest tend to stay at rest. Objects not in contact with other objects "float" freely. If the inertial trajectory is influenced by gravity, the reference frame will be an accelerated frame as seen from a position outside the gravitational attraction, and (seen from far away) the objects in the frame (elevator, etc.) will appear to be under the influence of a force (the so-called force of gravity). As noted, objects subject solely to gravity do not feel its effects. Weightlessness can thus be realised for short periods of time in an airplane following a specific parabolic flight path. It is simulated poorly, with many differences, in neutral buoyancy
Neutral buoyancy
Neutral buoyancy is a condition in which a physical body's mass equals the mass it displaces in a surrounding medium. This offsets the force of gravity that would otherwise cause the object to sink...

 conditions, such as immersion in a tank of water.

Zero-g is subtly different from zero gravity which literally only refers to the complete absence of gravity, something which is impossible due to the presence of gravity everywhere in the universe. However, gravity causes gravitation gradients, which make themselves apparent to any object of finite size in a gravitational field even in free-fall. These gradients cause very small tidal effects which are impossible to remove by inertial motion, except at a single point in space. All other points near this point feel mechanical stresses from the gradient, as a result of being made to travel along with the inertial motion of the reference point, which is a motion not perfectly inertial for points near it. However, "zero-gravity" is usually used synonymously to mean effective weightlessness, neglecting tidal effects. Microgravity (or µg) is used to refer to situations that are substantially weightless but where g-force
G-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...

 stresses within objects due to tidal effects, as discussed above, are around a millionth of that at the Earth's surface.

The human body is adapted to the gravitational field at the surface of the Earth and a weightless environment can have adverse effects on human health. In the short term, these may include space sickness
Space adaptation syndrome
Space adaptation syndrome or space sickness is a condition experienced by around half of space travelers during adaptation to weightlessness. It is related to motion sickness, as the vestibular system adapts to weightlessness.- Cause and remedy :...

, while in the long term more serious problems such as muscle atrophy
Muscle atrophy
Muscle atrophy, or disuse atrophy, is defined as a decrease in the mass of the muscle; it can be a partial or complete wasting away of muscle. When a muscle atrophies, this leads to muscle weakness, since the ability to exert force is related to mass...

 and bone loss
Spaceflight osteopenia
Spaceflight osteopenia refers to the characteristic bone loss that occurs during spaceflight. Astronauts lose an average of more than 1% bone mass per month spent in space...

 may develop.

Physics
Weightlessness occurs whenever the total force applied to an object is uniformly distributed across the object's mass, or when the object is not acted upon by any force. The conceptually simplest case, apart from the latter, is that where there are no other forces than gravity, while the gravitational field
Field (physics)
In physics, a field is a physical quantity associated with each point of spacetime. A field can be classified as a scalar field, a vector field, a spinor field, or a tensor field according to whether the value of the field at each point is a scalar, a vector, a spinor or, more generally, a tensor,...

 is uniform. This is what approximately applies in the most common cases of approximate weightlessless. Other examples include various types of levitation
Levitation
Levitation is the process by which an object is suspended by a physical force against gravity, in a stable position without solid physical contact...

, if the levitational force is uniformly distributed across the object's mass.

Weightlessness is in contrast with typical human experiences in which a non-uniform force is acting, such as:
  • standing on the ground, sitting in a chair on the ground, etc., where gravity is countered by the reaction force of the ground,
  • flying in a plane, where a reaction force is transmitted from the lift
    Lift (force)
    A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...

     the wings provide (special trajectories
    Trajectory
    A trajectory is the path that a moving object follows through space as a function of time. The object might be a projectile or a satellite, for example. It thus includes the meaning of orbit—the path of a planet, an asteroid or a comet as it travels around a central mass...

     which form an exception are described below),
  • during atmospheric reentry
    Atmospheric reentry
    Atmospheric entry is the movement of human-made or natural objects as they enter the atmosphere of a celestial body from outer space—in the case of Earth from an altitude above the Kármán Line,...

    , or during the use of a parachute
    Parachute
    A parachute is a device used to slow the motion of an object through an atmosphere by creating drag, or in the case of ram-air parachutes, aerodynamic lift. Parachutes are usually made out of light, strong cloth, originally silk, now most commonly nylon...

    , when atmospheric drag decelerates a vehicle,
  • during an orbital maneuver
    Orbital maneuver
    In spaceflight, an orbital maneuver is the use of propulsion systems to change the orbit of a spacecraft.For spacecraft far from Earth—for example those in orbits around the Sun—an orbital maneuver is called a deep-space maneuver .-delta-v:...

     in a spacecraft
    Spacecraft
    A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

    , or during the launch phase, when rocket
    Rocket
    A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...

     engines provide thrust
    Thrust
    Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....

    .


In cases where an object is not weightless, as in the above examples, a force acts non-uniformly on the object in question. Aero-dynamic lift, drag, and thrust are all non-uniform forces (they are applied at a point or surface, rather than acting on the entire mass of an object), and thus do not create the phenomenon of weightlessness. This non-uniform force may also be transmitted to an object at the point of contact with a second object, such as the contact between the surface of the Earth and one's feet, or between a parachute harness and one's body.

Gravity is a field force which may usually be considered to act uniformly on the mass of all objects in the frame of reference. This assumption is valid when the size of the region being considered is small relative to its distance from the center of mass of the gravitational counterpart. The small size of a person relative to the radius of Earth is one such example. In contrast, objects near a black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...

 are subject to a highly non-uniform gravitational field. These non-uniform fields near gravitating bodies produce local tidal force
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...

s inside human bodies and inside spacecraft. Near the Earth, they are responsible for the phenomenon of microgravity.

Weight

The technical definition of weight is the mass of the object, multiplied by the acceleration of the g-force
G-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...

 acting on an object, but in the opposite direction. Thus, humans experience their own body weight as a result of this supporting force, which results in a normal force applied to a person by the surface of a supporting object, on which the person is standing or sitting. In the absence of this force, a person would be in free-fall, and would experience weightlessness. It is the transmission of this reaction force through the human body, and the resultant compression and tension of the body's tissues
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

, that results in the sensation of weight.

Because of the distribution of mass throughout a person's body, the magnitude of the reaction force varies between a person's feet and head. At any horizontal cross-section
Cross section (geometry)
In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...

 of a person's body (as with any column
Column
A column or pillar in architecture and structural engineering is a vertical structural element that transmits, through compression, the weight of the structure above to other structural elements below. For the purpose of wind or earthquake engineering, columns may be designed to resist lateral forces...

), the size of the compressive force being resisted by the tissues below the cross-section is equal to the weight of the portion of the body above the cross-section. (In the arms, the reaction force is equal to the weight of the portion of the arm below the cross-section, and is a tensile, rather than a compressive, force, just as in a hanging rope.)

Sensitivity to forces

In Newton's view, astronauts in Earth orbit are in free fall, since they are in effect falling around the Earth. They are accelerated by gravity toward the Earth, but their inertia in the direction tangential with their path results in a curved path around the planet. In essence, they are always missing the planet in their fall toward it.

One way to view this situation, is to note that gravity by itself does not produce a weight-like force (a g-force
G-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...

) that people can directly sense, since gravity acts upon all parts of the body and the body only senses mechanical stresses (which to a good approximation, gravity does not produce, by itself). Thus, even a person standing on the Earth does not actually feel the pull of "gravity," but actually feels only the push of the ground, acting upward. If this push of the ground is suddenly removed (for example, in a free fall in an elevator), the person experiences weightlessness, because all the forces which have caused the sensation of "weight" have been removed, even though gravitational interactions continue.

Often, the terms zero gravity or reduced gravity are used to mean weightlessness as it is experienced by orbiting spacecraft. The idea of gravitation itself being greatly reduced in this situation is not technically accurate in the physics of Newton, although it is accurate in the physics of Einstein (general relativity).

Spacecrafts are held in orbit by the gravity of the planet which they are orbiting. 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
G-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...

 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
James Oberg
James Edward Oberg is an American space journalist and historian, regarded as an expert on the Russian space program.-Biography:...

 explains the phenomenon this way:

Relativity

To a modern physicist working with Einstein's general theory of relativity, the situation is even more complicated than is suggested above. Einstein's theory suggests that it actually is valid to consider that objects in inertial motion (such as falling in an elevator, or in a parabola in an airplane, or orbiting a planet) can indeed be considered to experience a local loss of the gravitational field responsible for their general motion. Thus, in the point of view (or frame) of the astronaut or orbiting ship, there actually is nearly-zero proper acceleration
Proper acceleration
In relativity theory, proper acceleration is the physical acceleration experienced by an object. It is acceleration relative to a free-fall, or inertial, observer who is momentarily at rest relative to the object being measured...

 (the acceleration felt locally), just as would be the case far out in space, away from any mass. It is thus valid to consider that most of the gravitational field in such situations is actually absent from the point of view of the falling observer, just as the colloquial view suggests (see equivalence principle
Equivalence principle
In the physics of general relativity, the equivalence principle is any of several related concepts dealing with the equivalence of gravitational and inertial mass, and to Albert Einstein's assertion that the gravitational "force" as experienced locally while standing on a massive body is actually...

 for a fuller explanation of this point). However, this loss of gravity for the falling or orbiting observer, in Einstein's theory, is due to the falling motion itself, and (again as in Newton's theory) not due to increased distance from the Earth. However, the gravity nevertheless is considered to be absent. In fact, Einstein's realization that a pure gravitational interaction cannot be felt, if all other forces are removed, was the key insight to leading him to the view that the gravitational "force" can in some ways be viewed as non-existent. Rather, objects tend to follow geodesic paths in curved space-time, and this is "explained" as a force, by "Newtonian" observers who assume that space-time is "flat," and thus do not have a reason for curved paths (i.e., the "falling motion" of an object near a gravitational source).

In the theory of general relativity, the only gravity which remains for the observer following a falling path or "inertial" path near a gravitating body, is that which is due to non-uniformities which remain in the gravitational field, even for the falling observer. This non-uniformity, which is a simple tidal effect in Newtonian dynamics, constitutes the "microgravity" which is felt by all spacially-extended objects falling in any natural gravitational field that originates from a compact mass. The reason for these tidal effects is that such a field will have its origin in a centralized place (the compact mass), and thus will diverge, and vary slightly in strength, according to distance from the mass. It will thus vary across the width of the falling or orbiting object. Thus, the term "microgravity," an overly technical term from the Newtonian view, is a valid and descriptive term in the general relativistic (Einsteinian) view.

Microgravity

The term micro-g environment (also µg, often referred to by the term microgravity) is more or less a synonym of weightlessness and zero-G, but indicates that g-force
G-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...

s are not quite zero, just very small.

Weightless and reduced weight environments

Reduced weight in 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
Vomit Comet
A Reduced Gravity Aircraft is a type of fixed-wing aircraft that briefly provides a nearly weightless environment in which to train astronauts, conduct research and film motion pictures....

".

To create a weightless environment, the airplane flies in a six-mile long parabolic
Parabola
In mathematics, the parabola is a conic section, the intersection of a right circular conical surface and a plane parallel to a generating straight line of that surface...

 arc, first climbing, then entering a powered dive. During the arc, the propulsion and steering of the aircraft are controlled such that the drag
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

 (air resistance) on the plane is canceled out, leaving the plane to behave as it would if it were free-falling in a vacuum. During this period, the plane's occupants experience about 25 seconds of weightlessness, before experiencing about 25 seconds of 2 g acceleration (twice their normal weight) during the pull-out from the parabola. A typical flight lasts around two hours, during which 50 parabolas are flown.

NASA's Reduced Gravity Aircraft

Versions of such airplanes have been operated by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

's Reduced Gravity Research Program since 1973, where the unofficial nickname originated. NASA later adopted the official nickname 'Weightless Wonder' for publication. NASA's current Reduced Gravity Aircraft, "Weightless Wonder VI", a McDonnell Douglas C-9, is based at Ellington Field
Ellington Field
Ellington International Airport is a joint civil-military airport located in the U.S. state of Texas within the city of Houston— southeast of Downtown. Established by the Army Air Service on 21 May 1917, Ellington Field was one of the initial World War I Army Air Service installations when...

 (KEFD), near Lyndon B. Johnson Space Center
Lyndon B. Johnson Space Center
The Lyndon B. Johnson Space Center is the National Aeronautics and Space Administration's center for human spaceflight training, research and flight control. The center consists of a complex of 100 buildings constructed on 1,620 acres in Houston, Texas, USA...

.

NASA's Microgravity University
Microgravity University
Microgravity University, also known as the Reduced Gravity Student Flight Opportunities Program is a program run by NASA which enables undergraduate university students to perform microgravity experiments aboard NASA's DC-9 aircraft at Johnson Space Center in Houston, Texas.Acceptance into the...

 - 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.

European Space Agency A300 Zero-G

The European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...

 flies parabolic flights on a specially-modified Airbus A300 B2 aircraft, in order to perform research in microgravity. ESA flies campaigns of three flights on consecutive days, each flying about 30 parabolas, for a total of about 10 minutes of weightlessness per flight. The ESA campaigns are currently operated from Bordeaux - Mérignac Airport in France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

 by the company Novespace, while the aircraft is operated by DGA Essais en Vol. The first ESA Zero-G flights were in 1984, using a NASA KC-135 aircraft in Houston, Texas. , the ESA has flown 52 campaigns and also 9 student parabolic flight campaigns.
Other aircraft it has used include the Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

n Ilyushin Il-76
Ilyushin Il-76
The Ilyushin Il-76 is a multi-purpose four-engined strategic airlifter designed by Ilyushin design bureau. It was first planned as a commercial freighter in 1967. Intended as a replacement for the Antonov An-12, the Il-76 was designed for delivering heavy machinery to remote, poorly-serviced areas...

 MDK and French Caravelle
Caravelle
Caravelle may be a reference to:* Caravelle, the French marketing name for the typeface Folio* Sud Aviation Caravelle, the short/medium-range jet airliner, produced by Sud Aviation...

.

Ecuadorian T-39 Condor

The Ecuadorian Space Agency jointly operates, with the Ecuadorian Air Force
Ecuadorian Air Force
The Ecuadorian Air Force is the Air arm of the Military of Ecuador and responsible for the protection of the Ecuadorian airspace.-Mission:To develop the military air wing, in order to execute institutional objectives which guarantee sovereignty and contribute towards the nation's security and...

, the Ecuadorian Micro Gravity Flight Program, using a T-39 Sabreliner, modified in-house to fly "cybernetically assisted" parabolas. It has been in operation since May 2008. It is the first Latin American microgravity aircraft. On June 19, 2008, the plane carried seven-year-old Jules Nader as he set the first Guinness World record for the youngest human being to fly in microgravity. Nader worked on a fluid dynamics experiment designed by his brother, Gerard Nader.

Others

The Zero Gravity Corporation
Zero Gravity Corporation
Zero Gravity Corporation is an American company based in Vienna, Virginia, formerly of Fort Lauderdale, Florida, which operates weightless flights from United States airports...

, founded in 1993 by Peter Diamandis, Byron Lichtenberg, and Ray Cronise, operates a modified Boeing 727
Boeing 727
The Boeing 727 is a mid-size, narrow-body, three-engine, T-tailed commercial jet airliner, manufactured by Boeing. The Boeing 727 first flew in 1963, and for over a decade more were built per year than any other jet airliner. When production ended in 1984 a total of 1,832 aircraft had been produced...

 which flies parabolic arcs to create 25-30 seconds of weightlessness. Flights may be purchased for both tourism and research purposes.

Ground-based drop facilities

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
Glenn Research Center
NASA John H. Glenn Research Center at Lewis Field is a NASA center, located within the cities of Brook Park, Cleveland and Fairview Park, Ohio between Cleveland Hopkins International Airport and the Cleveland Metroparks's Rocky River Reservation, and has other subsidiary facilities in Ohio...

 in Cleveland, Ohio
Cleveland, Ohio
Cleveland is a city in the U.S. state of Ohio and is the county seat of Cuyahoga County, the most populous county in the state. The city is located in northeastern Ohio on the southern shore of Lake Erie, approximately west of the Pennsylvania border...

, is a 145-meter 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 meters. The experiment vehicle is stopped in approximately 4.5 meters of pellets
Pelletizing
Pelletizing is the process of compressing or molding a material into the shape of a pellet. A wide range of different materials are pelletized including chemicals, iron ore, animal compound feed, and more.- Pelletizing of iron ore :...

 of expanded polystyrene
Polystyrene
Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...

 and experiences a peak deceleration rate of .

Also at NASA Glenn is the 2.2 Second Drop Tower, which has a drop distance of 24.1 meters. 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 meter tall air bag, at a peak deceleration rate of approximately . 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
Marshall Space Flight Center
The George C. Marshall Space Flight Center is the U.S. government's civilian rocketry and spacecraft propulsion research center. The largest center of NASA, MSFC's first mission was developing the Saturn launch vehicles for the Apollo moon program...

 hosts another drop tube facility that is 105 meters tall and provides a 4.6 second free fall under near-vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...

 conditions.

Humans cannot utilize these gravity shafts, as the deceleration experienced by the drop chamber would likely kill or seriously injure anyone using them; is about the highest deceleration that a fit and healthy human can withstand momentarily without sustaining injury.

Other drop facilities worldwide include:
  • Micro-Gravity Laboratory of Japan (MGLAB) – 4.5 s free fall
  • Experimental drop tube of the metallurgy department of Grenoble
    Grenoble
    Grenoble is a city in southeastern France, at the foot of the French Alps where the river Drac joins the Isère. Located in the Rhône-Alpes region, Grenoble is the capital of the department of Isère...

     – 3.1 s free fall
  • Fallturm Bremen
    Fallturm Bremen
    Fallturm Bremen is a drop tower at the Center of Applied Space Technology and Microgravity at the University of Bremen in Bremen. It has a 123-metre-high drop tube , in which for 4.74 seconds , or for over 9 seconds weightlessness can be produced...

     University of Bremen
    University of Bremen
    The University of Bremen is a university of approximately 23,500 people from 126 countries that are studying, teaching, researching, and working in Bremen, Germany...

     in Bremen – 4.74 s free fall
  • Queensland University of Technology Drop Tower - 2.0 s free fall

Neutral buoyancy

Weightlessness can also be simulated by creating the condition of neutral buoyancy
Neutral buoyancy
Neutral buoyancy is a condition in which a physical body's mass equals the mass it displaces in a surrounding medium. This offsets the force of gravity that would otherwise cause the object to sink...

, in which human subjects and equipment are placed in a water environment and weighted or buoyed until they hover in place. NASA uses neutral buoyancy to prepare for extra-vehicular activity
Extra-vehicular activity
Extra-vehicular activity is work done by an astronaut away from the Earth, and outside of a spacecraft. The term most commonly applies to an EVA made outside a craft orbiting Earth , but also applies to an EVA made on the surface of the Moon...

 (EVA) at its Neutral Buoyancy Laboratory
Neutral Buoyancy Laboratory
The Neutral Buoyancy Laboratory is an astronaut training facility maintained by and located at the Sonny Carter Training Facility on NASA's Johnson Space Center in Houston, Texas. The NBL consists of a large indoor pool of water, the largest in the world, in which astronauts may perform simulated...

. Neutral buoyancy is also used for EVA research at the University of Maryland
University of Maryland, College Park
The University of Maryland, College Park is a top-ranked public research university located in the city of College Park in Prince George's County, Maryland, just outside Washington, D.C...

's Space Systems Laboratory
Space systems laboratory
The Space Systems Laboratory is part of the Aerospace Engineering Department and A. James Clark School of Engineering at the University of Maryland in College Park, Maryland. The Space Systems Laboratory is centered around the Neutral Buoyancy Research Facility, a , water tank that is used to...

, which operates the only neutral buoyancy tank at a college or university.

Neutral buoyancy is not identical to weightlessness. Gravity still acts on all objects in a neutral buoyancy tank; thus, astronauts in neutral buoyancy training still feel their full body weight within their spacesuits, although the weight is well-distributed, similar to force on a human body in a water bed, or when simply floating in water. The suit and astronaut together are under no net force, as for any object that is floating, or supported in water, such as a scuba diver at neutral buoyancy. Water also produces drag, which is not present in vacuum.

Weightlessness in a spacecraft

Long periods of weightlessness occur on spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....

 outside a planet's atmosphere, provided no propulsion is applied and the vehicle is not rotating. Weightlessness does not occur when a spacecraft is firing its engines or when re-entering the atmosphere, even if the resultant acceleration is constant. The thrust provided by the engines acts at the surface of the rocket nozzle rather than acting uniformly on the spacecraft, and is transmitted through the structure of the spacecraft via compressive and tensile forces to the objects or people inside.

Weightlessness in an orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

ing spacecraft is physically identical to free-fall, with the difference that gravitational acceleration causes a net change in the direction, rather than the magnitude, of the spacecraft's velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

. This is because the acceleration vector is perpendicular to the velocity vector.

In typical free-fall, the acceleration of gravity acts along the direction of an object's velocity, linearly increasing its speed
Speed
In kinematics, the speed of an object is the magnitude of its velocity ; it is thus a scalar quantity. The average speed of an object in an interval of time is the distance traveled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as...

 as it falls toward the Earth, or slowing it down if it is moving away from the Earth. In the case of an orbiting spacecraft, which has a velocity vector largely perpendicular to the force of gravity, gravitational acceleration does not produce a net change in the object's speed, but instead acts centripetally
Centripetal force
Centripetal force is a force that makes a body follow a curved path: it is always directed orthogonal to the velocity of the body, toward the instantaneous center of curvature of the path. The mathematical description was derived in 1659 by Dutch physicist Christiaan Huygens...

, to constantly "turn" the spacecraft's velocity as it moves around the Earth. Because the acceleration vector turns along with the velocity vector, they remain perpendicular to each other. Without this change in the direction of its velocity vector, the spacecraft would move in a straight line, leaving the Earth altogether.

Weightlessness at the center of a planet

The net gravitational force due to a spherically symmetrical planet is zero at the center. This is clear because of symmetry, and also from Newton's shell theorem
Shell theorem
In classical mechanics, the shell theorem gives gravitational simplifications that can be applied to objects inside or outside a spherically symmetrical body...

 which states that the net gravitational force due to a spherically symmetric shell, e.g., a hollow ball, is zero anywhere inside the hollow space. Thus the material at the center is weightless.

Human health effects

Following the advent of space station
Space station
A space station is a spacecraft capable of supporting a crew which is designed to remain in space for an extended period of time, and to which other spacecraft can dock. A space station is distinguished from other spacecraft used for human spaceflight by its lack of major propulsion or landing...

s that can be inhabited for long periods of time, exposure to weightlessness has been demonstrated to have some deleterious effects on human health. 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
Space adaptation syndrome
Space adaptation syndrome or space sickness is a condition experienced by around half of space travelers during adaptation to weightlessness. It is related to motion sickness, as the vestibular system adapts to weightlessness.- Cause and remedy :...

 or SAS, commonly referred to as space sickness. Symptoms of SAS include nausea
Nausea
Nausea , is a sensation of unease and discomfort in the upper stomach with an involuntary urge to vomit. It often, but not always, precedes vomiting...

 and vomiting, vertigo
Vertigo (medical)
Vertigo is a type of dizziness, where there is a feeling of motion when one is stationary. The symptoms are due to a dysfunction of the vestibular system in the inner ear...

, headache
Headache
A headache or cephalalgia is pain anywhere in the region of the head or neck. It can be a symptom of a number of different conditions of the head and neck. The brain tissue itself is not sensitive to pain because it lacks pain receptors. Rather, the pain is caused by disturbance of the...

s, lethargy, and overall malaise. The first case of SAS was reported by cosmonaut Gherman Titov
Gherman Titov
Gherman Stepanovich Titov was a Soviet cosmonaut who, on August 6, 1961, became the second human to orbit the Earth aboard Vostok 2, preceded by Yuri Gagarin on Vostok 1...

 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
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...

 jokingly measures SAS using the "Garn scale", named for United States Senator Jake Garn
Jake Garn
Edwin Jacob "Jake" Garn is an American politician, a member of the Republican Party, and served as a U.S. Senator representing Utah from 1974 to 1993...

, whose SAS during STS-51-D
STS-51-D
STS-51-D was the sixteenth flight of NASA's Space Shuttle program, and the fourth flight of Space Shuttle Discovery. The launch of STS-51-D from Kennedy Space Center , Florida, on 12 April 1985 was delayed by 55 minutes, after a boat strayed into the restricted Solid Rocket Booster recovery zone...

 was the worst on record. Accordingly, one "Garn" is equivalent to the most severe possible case of SAS.

The most significant adverse effects of long-term weightlessness are muscle atrophy
Muscle atrophy
Muscle atrophy, or disuse atrophy, is defined as a decrease in the mass of the muscle; it can be a partial or complete wasting away of muscle. When a muscle atrophies, this leads to muscle weakness, since the ability to exert force is related to mass...

 and deterioration of the skeleton
Skeleton
The skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...

, or spaceflight osteopenia
Spaceflight osteopenia
Spaceflight osteopenia refers to the characteristic bone loss that occurs during spaceflight. Astronauts lose an average of more than 1% bone mass per month spent in space...

. These effects can be minimized through a regimen of exercise. 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. Other significant effects include fluid redistribution (causing the "moon-face" appearance typical of pictures of astronauts in weightlessness), a slowing of the cardiovascular system, decreased production of red blood cell
Red blood cell
Red blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system...

s, balance disorders, and a weakening of the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

. Lesser symptoms include loss of body mass, nasal congestion, sleep disturbance, excess flatulence
Flatulence
Flatulence is the expulsion through the rectum of a mixture of gases that are byproducts of the digestion process of mammals and other animals. The medical term for the mixture of gases is flatus, informally known as a fart, or simply gas...

, and puffiness of the face. These effects begin to reverse quickly upon return to the Earth.

Effects on non-human organisms

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.

Fowl eggs which are fertilized in microgravity may not develop properly.

A 2006 Space Shuttle experiment found that Salmonella typhimurium, a bacteria which can cause food poisoning, became more virulent when cultivated in space.

Technical adaptation in zero-gravity

Weightlessness can cause serious problems on technical instruments, especially those consisting of many mobile parts. Physical processes that depend on the weight of a body (like convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

, cooking water or burning candles) act differently without a certain amount of gravity. Cohesion
Cohesion (chemistry)
Cohesion or cohesive attraction or cohesive force is the action or property of like molecules sticking together, being mutually attractive...

 and advection
Advection
Advection, in chemistry, engineering and earth sciences, is a transport mechanism of a substance, or a conserved property, by a fluid, due to the fluid's bulk motion in a particular direction. An example of advection is the transport of pollutants or silt in a river. The motion of the water carries...

 play a bigger role in space. Everyday work like washing or going to the bathroom are not possible without adaptation. To use toilets in space, like the one on the International Space Station
International Space Station
The International Space Station is a habitable, artificial satellite in low Earth orbit. The ISS follows the Salyut, Almaz, Cosmos, Skylab, and Mir space stations, as the 11th space station launched, not including the Genesis I and II prototypes...

, astronauts have to fasten themselves to the seat. A fan creates suction that carries the waste away. Drinking is aided with a straw or from tubes.

See also

  • Artificial gravity
    Artificial gravity
    Artificial gravity is the varying of apparent gravity via artificial means, particularly in space, but also on the Earth...

  • Clinostat
    Clinostat
    A clinostat is a device which uses rotation to negate the effects of gravitational pull on plant growth and development . It has also been used to study the effects of microgravity on cell cultures and animal embryos.-Description:...

  • Human adaptation to space
    Human adaptation to space
    Human physiological adaptation to the conditions of space is a challenge faced in the development of human spaceflight.The fundamental engineering problems of escaping Earth's gravity well and developing systems for in-space propulsion have been examined for well over a century, and millions of...

    • Space medicine
      Space medicine
      Space medicine is the practice of medicine on astronauts in outer space whereas astronautical hygiene is the application of science and technology to the prevention or control of exposure to the hazards that may cause astronaut ill health. Both these sciences work together to ensure that...

    • Space adaptation syndrome
      Space adaptation syndrome
      Space adaptation syndrome or space sickness is a condition experienced by around half of space travelers during adaptation to weightlessness. It is related to motion sickness, as the vestibular system adapts to weightlessness.- Cause and remedy :...

  • Microgravity University
    Microgravity University
    Microgravity University, also known as the Reduced Gravity Student Flight Opportunities Program is a program run by NASA which enables undergraduate university students to perform microgravity experiments aboard NASA's DC-9 aircraft at Johnson Space Center in Houston, Texas.Acceptance into the...

  • Vomit Comet
    Vomit Comet
    A Reduced Gravity Aircraft is a type of fixed-wing aircraft that briefly provides a nearly weightless environment in which to train astronauts, conduct research and film motion pictures....


External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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