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File:Foale ZeroG.jpg

Astronauts on the International Space Station display an example of weightlessness. Michael Foale can be seen exercising in the foreground.

Weightlessness is a phenomenon experienced by people during free-fall and is a condition associated with a number of cognitive and psychological effects. Although the term zero gravity is often used as a synonym, weightlessness in orbit is not the result of the force of gravity being eliminated or even significantly reduced (in fact, the force of the Earth's gravity at an altitude of 100 km is only 3% less than at the Earth’s surface). Weightlessness typically occurs when an object or person is falling freely, in spaceflight, in deep space (far from a planet, star, or other massive body), in an airplane following a particular parabolic flight path or in one of several other more unusual situations.

The physics of weightlessness[edit | edit source]

Weightlessness occurs whenever all forces applied to a person or object are uniformly distributed across the object's mass (as in a uniform gravitational field), or when the object is not acted upon by any force. This 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 the wings provide (special trajectories which form an exception are described below)
  • during atmospheric reentry, or during the use of a parachute, when atmospheric drag decelerates a vehicle
  • during an orbital maneuver in a spacecraft, or during the launch phase, when rocket engines provide thrust

In cases where an object is not weightless, as in the above examples, a force acts non-uniformly on the person or object in question. Aerodynamic 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 prevent 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 can usually be considered to act uniformly on the mass of all people and 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 attractor. The small size of a person relative to the radius of Earth is one such example. In contrast, objects near a black hole are subject to a highly non-uniform gravitational field.

Terminology[edit | edit source]

Apparent weight[edit | edit source]

While the technical definition of weight is the size of the force of gravity acting on an object, humans experience their own body weight as a result of what is called apparent weight, or the reaction force applied to a person by the surface on which the person is standing or sitting. In the absence of this reaction 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, 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 of a person's body (as with any column), 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.)

Zero gravity[edit | edit source]

Often, the terms zero gravity or reduced gravity are used to mean weightlessness as it is experienced by orbiting spacecraft, but this is not technically accurate. Spacecraft are held in orbit by the gravity of the planet which they are orbiting. The sensation of weightlessness experienced by astronauts is not the result of there being zero gravitational acceleration, but of there being zero difference between the acceleration of the spacecraft and the acceleration of the astronaut. Space journalist James Oberg explains the phenomenon thusly:[1]

The myth that satellites remain in orbit because they have "escaped Earth's gravity" is perpetuated further (and falsely) by almost universal use of the zingy but physically nonsensical phrase "zero gravity" (and its techweenie cousin, "microgravity") 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 up, and the failure to understand this fundamental concept means that many other things people "know" just ain't so.

Microgravity[edit | edit source]


Candle flame in orbital conditions.

The term microgravity is used to describe environments where the force of gravity is present but has a negligible effect. Objects in orbit are not perfectly weightless due to several effects:

  • In Low Earth orbit, the force of gravity decreases by one part per million for every three meter increase in altitude. Objects which have a non-zero size will be subjected to a tidal force, or a differential pull, between the high and low ends of the object.
  • In a spacecraft in orbit, the centrifugal force is greater on the side of the spacecraft furthest from the Earth. This is also a tidal force.
  • Objects within a spacecraft will slowly "fall" toward the densest part of the spacecraft. When they eventually come to rest on the wall of the spacecraft, they will have weight.
  • Though very thin, there is some air at the level of the Space Shuttle's orbital altitude of 185 to 1,000 km. This atmosphere causes deceleration due to friction. This has the effect of giving objects a small "weight" oriented in the direction of motion. Above 10,000 km, this effect becomes negligible compared to the effect of the solar wind.
  • "Floating" objects in the Space Shuttle are actually in independent orbits around the earth. If two objects are placed side-by-side (relative to their direction of motion) they will be orbiting the earth in different orbital planes. Since all orbital planes pass through the center of the earth, any two orbital planes intersect along a line. Therefore two objects placed side-by-side (at any distance apart) will come together after one quarter of a revolution. If they are placed so they miss each other, they will oscillate past each other twice per orbit. If they are placed one ahead of the other in the same orbital plane, they will maintain their separation. If they are placed one above the other (at different radii from the center of the earth) they will have different potential energies, so the size, eccentricity, and period of their orbits will be different, causing them to move in a complex looping pattern relative to each other.[2]

The symbol for microgravity, µg, was used on the insignia of Space Shuttle flight STS-107, because this flight was devoted to microgravity research.

Weightless and reduced weight environments[edit | edit source]

Reduced weight in aircraft[edit | edit source]

Airplanes have been used since 1973 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 six-mile long parabolic arc, first climbing, then entering a powered dive. During the arc, the propulsion and steering of the aircraft are controlled such that the drag (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 40 parabolas are flown.

Reduced weight in pilot training[edit | edit source]

People have differing reactions to reduced weight sensations, and these reactions can compromise flight safety if an aircraft pilot is not trained to respond properly, particularly in an emergency. Normally in flight training, flight instructors will gradually introduce reduced weight maneuvers, while carefully monitoring the student pilot. Most students become accustomed to the sensation and are able to perform satisfactorily with some training. Students who are not able to overcome their anxiety are not able to complete flight training.[3]

Neutral buoyancy[edit | edit source]

Weightlessness can also be simulated with the use of neutral buoyancy, 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 (EVA) at its Neutral Buoyancy Laboratory. Neutral buoyancy is also used for EVA research at the University of Maryland's Space Systems Laboratory, which operates the only neutral buoyancy tank at a college or university.

Neutral buoyancy is not identical to weightlessness, but it is similar in that the astronaut is free to move all parts of their body. This is because the net buoyancy force that supports the astronaut is distributed widely over their body. Drag is also a significant factor when moving in a neutral buoyancy environment, whereas astronauts on EVA do not experience any drag.

Weightlessness in a spacecraft[edit | edit source]

File:Centripetal force.PNG

The relationship between acceleration and velocity vectors in an orbiting spacecraft

File:Weightless hair.jpg

Astronaut Marsha Ivins demonstrates the effect of weightlessness on long hair during STS-98

Long periods of weightlessness occur on spacecraft 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 orbiting 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. This is because the acceleration 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 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, 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.

Health effects of weightlessness[edit | edit source]

Further information: Space medicine

Following the advent of space stations 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 or SAS, commonly referred to as space sickness. Symptoms of SAS include nausea and vomiting, vertigo, headaches, lethargy, and overall malaise. 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 as the worst on record. Accordingly, one "Garn" is equivalent to the most severe possible case of SAS.[4]

Cognitive effects[edit | edit source]

Physical effects[edit | edit source]

The most significant adverse effects of long-term weightlessness are muscle atrophy and deterioration of the skeleton, or spaceflight osteopenia. These effects can be minimized through a regimen of exercise. Other significant effects include fluid redistribution, a slowing of the cardiovascular system, decreased production of red blood cells, balance disorders, and a weakening of the immune system. 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.

Many of the conditions caused by exposure to weightlessness are similar to those resulting from aging. Scientists believe that studies of the detrimental effects of weightlessness could have medical benefits, such as a possible treatment for osteoporosis and improved medical care for the bed-ridden and elderly.

See also[edit | edit source]

References[edit | edit source]

  1. Oberg, James (May 1993). Space myths and misconceptions. Omni 15 (7).
  2. "Weightlessness and Microgravity", David Chandler, The Physics Teacher, May 1991, pp. 312-13
  3. Reduced G Familiarization from Gliding New Zealand.
  4., pg 35, Johnson Space Center Oral History Project, interview with Dr. Robert Stevenson:

    "Jake Garn was sick, was pretty sick. I don't know whether we should tell stories like that. But anyway, Jake Garn, he has made a mark in the Astronaut Corps because he represents the maximum level of space sickness that anyone can ever attain, and so the mark of being totally sick and totally incompetent is one Garn. Most guys will get maybe to a tenth Garn, if that high. And within the Astronaut Corps, he forever will be remembered by that."

Further reading[edit | edit source]

Books[edit | edit source]

  • Berry, C. A. (1973). Weightlessness. Oxford, England: Nasa Scientific & Technical Informa.
  • Dominoni, A. (2003). Conditions of microgravity and the body's "second skin". Mahwah, NJ: Lawrence Erlbaum Associates Publishers.
  • Young, L. R., & Shelhamer, M. (1990). Weightlessness enhances the relative contribution of visually-induced self-motion. Hillsdale, NJ, England: Lawrence Erlbaum Associates, Inc.

Papers[edit | edit source]

Aamodt, S. (1998). Neurolab launches the decade of the brain into space: Nature Neuroscience Vol 1(1) May 1998, 10-12.

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  • Arrott, A. P., Young, L. R., & Merfeld, D. M. (1990). Perception of linear acceleration in weightlessness: Aviation, Space, and Environmental Medicine Vol 61(4) Apr 1990, 319-326.
  • Baranski, S., & et al. (1983). Electrogustometric investigations during manned space flight: Aviation, Space, and Environmental Medicine Vol 54(1) Jan 1983, 1-5.
  • Benson, A. J., Guedry, F. E., Parker, D. E., & Reschke, M. F. (1997). Microgravity vestibular investigations: Perception of self-orientation and self-motion: Journal of Vestibular Research: Equilibrium & Orientation Vol 7(6) Nov-Dec 1997, 453-457.
  • Beregovoi, G. T., Krylova, N. V., Soloveva, I. B., & Shibanov, G. P. (1974). Assessing the effectiveness of human performance in space flight: Voprosy Psychologii No 4 Jul-Aug 1974, 3-9.
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