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the absence of any observable effects of gravitationgravitation,
the attractive force existing between any two particles of matter. The Law of Universal Gravitation

Since the gravitational force is experienced by all matter in the universe, from the largest galaxies down to the smallest particles, it is often called
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. This condition is experienced by an observer when he and his immediate surroundings are allowed to move freely in the local gravitational fieldfield,
in physics, region throughout which a force may be exerted; examples are the gravitational, electric, and magnetic fields that surround, respectively, masses, electric charges, and magnets. The field concept was developed by M.
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. All bodies in the weightless environment experience the same acceleration. The more massive bodies (see massmass,
in physics, the quantity of matter in a body regardless of its volume or of any forces acting on it. The term should not be confused with weight, which is the measure of the force of gravity (see gravitation) acting on a body.
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) in the surroundings experience a stronger gravitational force, but they also have more inertiainertia
, in physics, the resistance of a body to any alteration in its state of motion, i.e., the resistance of a body at rest to being set in motion or of a body in motion to any change of speed or change in direction of motion. Inertia is a property common to all matter.
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, or resistance to acceleration. As seen by a stationary outside observer, they appear to move together without any constraint. To the observer being accelerated, objects appear to float freely in space and to move with uniform speed in a straight line when given a push. Three examples of situations where weightlessness is encountered are: (1) an elevator falling freely in a vacuum; (2) a space capsule orbiting the earth; (3) a spacecraft drifting in outer space with its engines off. For the effects of weightlessness on the body, see space medicinespace medicine,
study of the medical and biological effects of space travel on living organisms. The principal aim is to discover how well and for how long humans can withstand the extreme conditions encountered in space, as well as how well they can readapt to the earth's
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The condition associated with free fall, i.e. the motion of an unpropelled body in a gravitational field. The acceleration of a person in a freely-falling object, such as a spacecraft, is equal to that of the freely-falling object. The person thus experiences no sensation of weight and floats freely. Many crew members on the US space shuttles and the USSR and US space stations have suffered a combination of symptoms, akin to those of motion sickness, as they adapt to conditions of weightlessness. It usually takes 1–3 days to overcome the problem (known as space adaptation syndrome or space sickness), duties being performed more slowly than scheduled. This can be serious on short-term missions. Other effects observed are a drop in activity of white blood cells, which fight disease, and a reduction in red blood cell count. The adaptation to conditions of normal Earth gravity following periods of weightlessness have been investigated by both the USSR and the USA after spaceflights of ever-increasing duration. No long-term effects have been observed to date. See also microgravity.



(or zero gravity), the state of a physical body in which the external forces acting on the body or the movements executed by the body do not cause the particles of the body to exert pressure on one another. If a body lies in the earth’s gravitational field on a horizontal plane, the body is acted on by the force of gravity and the reaction of the plane in the opposite direction, as a result of which the particles exert pressure on one another. The human body perceives such pressure as the sensation of weight.

The situation is similar in an elevator descending with an acceleration a ≠ g, where g is the free-fall acceleration. When a = g, however, the body (all of its particles) and the elevator perform a free fall and do not exert pressure on each other. This phenomenon is weightlessness. Gravity acts on all the particles of a body under weightlessness, but there are no external forces applied to the surface of the body, such as support reactions, that could cause the particles to exert pressure on each other.

A similar phenomenon is observed for bodies placed on an artificial earth satellite or spacecraft. Such bodies and all their particles, having received the same initial velocity as the satellite, will move under the action of gravitational forces along their own orbits with identical accelerations as free bodies without exerting pressure on each other; that is, they are in a weightless state. Just as in the case of a body in a elevator, a gravitational force acts upon these bodies, but there are no external forces applied to the surfaces of the bodies that could cause the bodies or their particles to exert pressure on one another.

In general, a body under the action of external forces will be in a weightless state if (1) the acting external forces are only mass (gravitational) forces; (2) the field of these mass forces is locally homogeneous (that is, the forces of the field impart accelerations identical in magnitude and direction to all the particles of the body in each position of the body); and (3) the initial velocities of all the particles of the body are identical in magnitude and direction (that is, the body has only translational motion). Thus, any body that is small in comparison with the earth’s radius and that performs free translational motion in the earth’s gravitational field will be weightless in the absence of other external forces. The result will be similar for motion in the gravitational field of any other heavenly body.

Owing to the marked differences between weightlessness and the terrestrial conditions under which the instruments and assemblies of artificial earth satellites, spacecraft, and booster rockets are constructed and adjusted, weightlessness is one of the most important problems of space exploration. It is a particularly important problem for systems having containers partly filled with liquid, such as propulsion systems, with liquid-propel-lant rocket engines designed to be frequently switched on during space flight. Under weightlessness, a liquid can occupy any position in a container and thus disrupt the normal functioning of the system, for example, the delivery of fuel components from fuel tanks. Therefore, certain requirements must be satisfied to start liquid-propellant propulsion systems under weightless conditions: the liquid and gaseous phases in fuel tanks must be kept separated by elastic diaphragms (for example, on the Mariner interplanetary probe), part of the liquid must be held in the intake device by a strainer system (on the Agena rocket), and brief overloads (artificial “gravity”) must be created by means of auxiliary rocket engines before the main propulsion system is switched on. Special methods must also be used to separate the liquid and gaseous phases under weightless conditions in several units of the life-support system and in fuel cells of the power supply system, for example, collection of the condensate by a system of porous wicks and separation of the liquid phase by a centrifuge. Spacecraft mechanisms, such as those for unfolding solar batteries and antennas and for docking, are designed to operate under weightlessness.

Certain technological processes that are difficult or impossible to perform under terrestrial conditions can be performed under weightlessness, such as production of composite materials with a homogeneous structure throughout the volume and production of bodies with a precise spherical shape from molten material using surface tension. The first attempt to weld various materials under weightlessness and in a vacuum was carried out during the flight of the Soviet spacecraft Soyuz 6 in 1969. Several technological experiments (on welding and the flow and crystallization of molten materials) were performed on the American orbital station Skylab in 1973.

Weightlessness must be given particular consideration in the flight of manned spacecraft, since the living conditions of man under weightlessness markedly differ from the usual terrestrial conditions, altering some vital functions. For example, weightlessness causes the central nervous system and receptors of many analyzer systems (vestibular apparatus, the muscles and joints, and the blood vessels) to function under unusual conditions. Weightlessness is therefore regarded as a specific integrated stimulus acting on man and animals throughout orbital flight. The response to the stimulus is adaptive physiological processes, with the extent of the manifestation varying with the duration of weightlessness and, to a much lesser degree, the individual characteristics of the organism.

Weightlessness induces disturbances in the vestibule of the ear in some cosmonauts. A sensation of fullness in the head owing to the increased flow of blood to the head persists for a long time. However, adaptation to weightlessness usually takes place without serious complications. Man remains fit and able to successfully perform a variety of tasks, including those requiring precise coordination or the expenditure of a considerable amount of energy. Motor activity during weightlessness requires a much smaller expenditure of energy than similar activity under gravity. If protective measures are not taken during an extended flight, man experiences the following changes in the initial hours and days after landing (period of readaptation to terrestrial conditions): (1) inability to remain upright while standing still or moving and a sensation of heaviness in parts of the body (the surrounding objects are felt to be unusually heavy, and there is an inability to control muscular effort); (2) hemodynamic disorders during work of moderate or high intensity, with presyncopal and syncopal states possible after changing from a horizontal position to a vertical position (orthostatic test); (3) metabolic disorders, especially of the water-salt metabolism, which result in tissue dehydration, decreased volume of circulating blood, and reduced content of some elements in the tissues, specifically, potassium and calcium; (4) impairment of the oxygen metabolism during physical exertion; (5) decrease in immunobiological resistance; and (6) vestibular and autonomic disorders. All these changes are reversible. Normal function can be rapidly restored by physical therapy and drugs. The disagreeable effects of weightlessness during spaceflight can be prevented or limited through exercise, electrostimulation of muscles, application of negative pressure to the lower half of the body, or the use of pharmacological and other agents. In a flight lasting about two months (the second crew on Skylab in 1973), a pronounced prophylactic effect was achieved mainly by having members of the crew perform an exercise regime. Highly intense work that raised the pulse to 150–170 beats a minute was performed on a bicycle ergometer for an hour a day. Functional restoration of the crew’s circulatory and respiratory systems took place within five days of landing. Metabolic changes and statokinetic and vestibular disturbances were less pronounced.

The creation of artificial “gravity” aboard a spacecraft would probably be an effective means of preventing the disturbances caused by weightlessness. This can be done, for example, by setting up a space station in the form of a large rotating wheel and putting workplaces along the “rim” of the wheel. As the rim rotates, all objects on it will be pressed toward the outer lateral surface, which will function as a “floor”; the reaction of the floor applied to the surfaces of the bodies will create artificial gravity. The creation of even slight artificial gravity on spacecraft will help prevent the unfavorable effects of weightlessness on animals and man.

Some of the theoretical and practical problems of space medicine are solved by the extensive use of laboratory methods of simulating weightlessness, including restriction of muscular activity, removal of the usual support along the long axis of the body, and reduction of the hydrostatic pressure of the blood. This can be accomplished by having an individual remain horizontal or bent over (head below the legs), confined to bed for a prolonged continuous period, or (3) immersed in a liquid for several hours or days.


Kakurin, L. I., and B. S. Katkovskii. “Nekotorye fiziologicheskie aspekty dlitel’noi nevesomosti.” In Itogi nauki: Seriia Biologiia, fasc. 8. Moscow, 1966.
Mediko-biologicheskie issledovaniia v nevesomosti. Moscow, 1968.
Fiziologiia v kosmose. Moscow, 1972. (Translated from English.)



A condition in which no acceleration, whether of gravity or other force, can be detected by an observer within the system in question. Also known as zero gravity.