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The physics of weightlessness Edit

Weightlessnes 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:

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. 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 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

Apparent weight Edit

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 normal 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

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. 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 zero difference between the acceleration of the spacecraft and the acceleration of the astronaut. Space journalist James Oberg explains the phenomenon this way:[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.

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 acceleration, 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 for a fuller explanation of this point). However, this loss of gravity, in Einstein's theory, is for a different reason that is popularly supposed: the loss of gravity in orbit, or in a falling elevator, is due to the falling motion itself, and not due to increased distance from the Earth. However, the gravity nevertheless is considered to be absent. In the theory of general relativity, the only gravity which remains for the observer following a falling path or "inertial" path, is that which is due to non-uniformities in the gravitational field. This non-uniformity, which is a tidal effect, constitutes part of the "microgravity" which is felt by all spacially-extended objects falling in any natural gravitational field originating from a mass, since such a field will have its origin in a centralized place (the compact mass), and thus will vary slightly in strength, according to distance from the mass.

Microgravity Edit

Main article: Micro-g environment

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. (This flight was also notable as being the final mission of the Space Shuttle Columbia, which broke up during re-entry.)

Weightless and reduced weight environments Edit

Reduced weight in aircraft Edit

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 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.

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.[3] NASA later adopted the official nickname 'Weightless Wonder' for publication.[4] NASA's current Reduced Gravity Aircraft, a McDonnell Douglas C-9, is based at 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.

European Space Agency A300 Zero-G Edit

The European Space Agency flies parabolic flights on a specially-modified Airbus A300 aircraft, in order to research microgravity. The ESA flies campaigns of three flights on consecutive days, each flight 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 by the company Novespace,[5] while the aircraft is operated by the Centre d'essais en Vol (CEV - French Test Flight Centre). The first ESA Zero-G flights were in 1984, using a NASA KC-135 aircraft in Houston, Texas. As of March 2006, the ESA has flown 43 campaigns. Other aircraft it has used include the Russian Ilyushin Il-76 MDK and French Caravelle.[6][7][8]

Ecuadorian T-39 Condor Edit


The Ecuadorian Space Agency jointly operates, with the Ecuadorian Air Force, 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. [9] 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.[10]

Others Edit

The Zero Gravity Corporation, founded in 1993 by Peter Diamandis, Byron Lichtenberg, and Ray Cronise, operates a modified Boeing 727 which flies parabolic arcs like those of NASA's Reduced Gravity Aircraft. Flights may be purchased for both tourism and research purposes.

In Austria, a company called Paul's Parabelflug offers parabolic flights, but they are prohibited from offering zero-g flights, and now offer only Martian and lunar gravity flights.


A company in Hungary briefly offered parabolic flights, but went out of business after only a few flights.


A Swedish company, Xero, planned to fly parabolic flights with the mammoth Ilyushin Il-76, but the person in charge of the project left the company, and the project was cancelled.


Reduced weight in pilot training Edit

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.[11]

Ground-based drop facilities Edit

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


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 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.


Humans cannot utilize these gravity shafts, as the deceleration experienced by the drop chamber would likely kill or seriously injure anyone using them; 20 g is about the highest deceleration that a fit and healthy human being can withstand momentarily without sustaining injury.

Other drop facilities worldwide include:

Neutral buoyancy Edit

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.

It is important to note that 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, though the suit and astronaut together are under no net force. 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

File:Centripetal force.PNG
File:Weightless hair.jpg

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 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 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.

Weightlessness at the center of a planet Edit

If a person were able to survive at the center of a planet, they would experience weightlessness without any acceleration. This is because the force of gravity exerted by the surrounding planet would be the same in all directions, and would effectively cancel out, for a net force of zero.

More generally, the net gravitational force is zero everywhere within a hollow, spherically symmetrical planet. This is known as the shell theorem.

Health effects of weightlessness Edit

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 during STS-51-D was the worst on record. Accordingly, one "Garn" is equivalent to the most severe possible case of SAS.[13]

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. 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.[14] Other significant effects include fluid redistribution (causing the "moon-face" appearance typical of pictures of astronauts in weightlessness)[15][14], 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.


Effects on non-human organisms Edit

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.[16]

Fowl eggs which are fertilized in microgravity may not develop properly.[17]

See also Edit

References Edit

  1. Oberg, James (May 1993). "Space myths and misconceptions". Omni 15 (7), Retrieved on 2 May 2007. 
  2. "Weightlessness and Microgravity", David Chandler, The Physics Teacher, May 1991, pp. 312-13
  3. Reduced Gravity Research Program
  4. NASA "Weightless Wonder"
  5. Novespace: microgravity, airborne missions
  6. European Space Agency. "A300 Zero-G". ESA Human Spaceflight web site. Retrieved on 2006-11-12.
  7. European Space Agency. "Next camaign". ESA Human Spaceflight web site. Retrieved on 2006-11-12.
  8. European Space Agency. "Campaign Organisation". ESA Human Spaceflight web site. Retrieved on 2006-11-12.
  9. EXA and FAE Develops First Zero-G Plane in Latin America
  10. Youngest person to experience microgravity
  11. Reduced G Familiarization from Gliding New Zealand.
  12. Marshall Space Flight Center Drop Tube Facility
  13., pg 35, Johnson Space Center Oral History Project, interview with Dr. Robert Stevenson:
  14. 14.0 14.1 "Health Fitness", Space Future
  15. "The Pleasure of Spaceflight", Toyohiro Akiyama, Journal of Space Technology and Science, Vol.9 No.1 spring 1993, pp.21-23
  16. "Mutant super-cockroaches from space". New Scientist (January 21, 2008).
  17. "Egg Experiment in Space Prompts Questions". New York Times (1989-03-31).

External links Edit

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