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Weightlessness means zero gravity in space.

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Weightlessness is the experience (by people and objects) during free-fall, of having no apparent weight. Although the term 'zero gravity' is often used as a synonym, weightlessness in orbit is not the result of gravity itself being eliminated or even reduced significantly (in fact, the acceleration due to gravity at an altitude of 100 km is only 3% less than at the earth's surface - a person at rest at that altitude would plummet to earth at a familiar rate). Weightlessness (roughly speaking) occurs when a body (e.g. a person) is: falling freely; in orbit; in outer space (far from a planet, star, or other massive body); in an airplane following a particular hyperbolic flight path (e.g. the "Vomit Comet"); or one of several other (even more unusual) frames of reference.

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

More generally, weightlessness occurs when a person (or object) is subject (at most) to the single force of gravity (or is not acted upon by any accelerating force), vs. the far more typical (in human experience) cases in which an equal/opposite force is acting - such as:

  • standing on the ground, sitting in a chair on the ground, etc. (gravity is countered by the reaction force of the ground);.
  • flying in a plane (gravity is countered by the lift the wings provide) - see below for special trajectories which form an exception;.
  • Atmospheric reentry, use of a parachute: atmospheric drag decelerates the vehicle;.
  • during an orbital maneuver in a Spacecraft, or during the launch phase: the rockets provides Thrust.

(The principal difference is that gravity acts directly on a person and/or other bodies, just like on the vehicle's mass - whereas forces like atmospheric drag and thrust act only on the vehicle body itself (and thus only secondarily, through the vehicle, on the person). In the first case the person and the vehicle floor are not 'pushed' towards one another; in the other cases, the force is transmitted through the vehicle's structure to the person and/or contents.)

Overview of weightlessness.

What humans perceive as "weight" is not actually the force of gravity pulling us towards the ground (actually, towards the center of the Earth - although this is the technical definition of "weight"). What we feel as "weight", is actually the normal reaction force of the ground (or whatever surface we are supported by) "pushing" upwards against us to counteract gravity's downward pull - that is: the "apparent weight". (In the remainder of this article, the term 'weight', without 'apparent', is used in this sense.) While this is not always intuitive, imagine the floor dropping out from under you: without it, you'd be falling - and experiencing weightlessness. It's the floor (or ground - whatever), supporting you against gravity's pull - and which keeps you from falling to the center of the Earth - that creates the sensation of "weight".

For example: a wood block in a container in free-fall "experiences" weightlessness. This is because there is no force from the container's bottom on the block, against the pull of gravity, as both the container and the block are being pulled down with the same acceleration. When the container is at rest on the ground, however, the force of gravity pulling downwards on the block is exactly matched (in the opposite direction, and by the same amount) by the support of the bottom of the container.

Because the block is a solid, each horizontal cross section of the block experiences not only the force due to gravity on it, but also the weight of whatever portion of the block is above it. (In the case of an object, or portion thereof, which is not supported from below, but suspended from above, a 'negative pressure', or tension gradient exists. It occurs because each cross section of a hanging object, a rope for instance, must support the weight of every piece below it.) Part of feeling "weight", then, is actually experiencing such a pressure/tension gradient within one's own body parts (e.g.: while standing on one foot, the foot on the ground would feel the pressure of the entire body's weight, whereas the other leg and both arms would feel/be subjected to the tension gradients of their own weight being pulled down against their sockets).

In free-fall, a person or object experiences no measurable (or apparent) weight because all parts of the object are accelerating uniformly (any variations in acceleration due to tidal forces being imperceptible).

Terminology of zero gravity or weightlessness.

Often, the term 'zero gravity' or 'reduced gravity' is used to describe weightlessness, but these are scientifically inaccurate. A spacecraft and its contents are kept in orbit by the gravity of the body it orbits; that they are all subject to roughly the same gravity is the reason for the weightlessness. James Oberg explains:

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.


Weightlessness NASA image.
Candle flame in orbital conditions. NASA image.

The term 'microgravity' is also used because weightlessness in e.g. a spaceship or other container is not perfect. Causes in Earth orbit include:

  • Gravity decreases 1 ppm for every 3m increase in height. Objects which are not points will feel a differential pull on their various parts. (This is actually the tidal force).
  • In a spaceship in orbit the Centripetal force is higher at the upper side. (This is also the tidal force).
  • Objects left alone will "fall" toward the densest part of the spacecraft. When they eventually touch the spacecraft, they will stop moving and feel weight.
  • Though very thin, there is some air at the level of the Space Shuttle's orbit height of 185 to 1,000 km, which causes deceleration due to friction. This is perceived as "weight" in the direction of motion. Above 10,000km, this fades into negligibility compared to solar wind.
  • Left to themselves, different parts of a vehicle either side of its orbital plane are in their own orbital planes. In the frame of reference of the vehicle, this pushes objects inwards towards the orbital plane of the vehicle as a whole.

The microgravity symbol, µg, was used on the insignia of the Space Shuttle flight STS-107, because this flight was devoted to microgravity research (see picture in that article).

Reduced weight.

NASA's KC-135 reduced weight aircraft

NASA's KC-135 Reduced Gravity Aircraft is based at Lyndon B. Johnson Space Center and affectionately called the "vomit comet". It is an airplane that NASA flies in 6 mile long parabolic arcs, first climbing in altitude, then falling, in such a way that the flight path and speed correspond to that of an object without propulsion and not experiencing air friction. This is realized by propulsion and steering such that air friction is compensated and nothing else. The result is that people inside are not pushed towards the bottom or any other side of the plane, i.e. they are temporarily weightless, each time for a period of 25 seconds. Typically one flight lasts about two hours, in which 40 parabolas are flown.

NASA's Microgravity University - Reduced Gravity Flight Opportunities Plan, also known as the Reduced Gravity Student Flight Opportunities Program, allows teams of college undergraduates to submit a microgravity experiment proposal. If selected, the teams design and implement their experiment, and students are invited to fly on the NASA's McDonnell Douglas C-9 (the recent replacement for the KC-135.) The aircraft flies in the pattern described above, so that the experiment has around 20 to 25 seconds to perform its function in microgravity.

Zero Gravity Corporation

The Zero Gravity Corporation operates a modified Boeing 727 which flies parabolic arcs similar to those of NASA's Reduced Gravity Aircraft. Flights may be purchased for both tourism and research purposes.

Weightlessness: European Space Agency A-300 Zero-G.

The European Space Agency flies parabolic flights on a specially-modified Airbus A-300 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, 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.

Ground-based reduced weight facilities

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

Weightlessness: NASA drop facilities.

NASA's Zero-G Research Facility, located at the Glenn Research Center in Cleveland, Ohio, 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 of expanded polystyrene and experiences a peak deceleration rate of 65g.

Also at NASA Glenn is the 2.2 Second Drop Tower which is about 24 meters tall.

NASA's Marshall Space Flight Center hosts another drop tube facility that is 105 meters tall and provides a 4.6 second free fall under near-vacuum conditions.

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 permanent injury.

Weightlessness: Other facilities worldwide.

  • Micro-Gravity Laboratory of Japan (MGLAB) - 4.5 s free fall.
  • Experimental drop tube of the metallurgy department of Grenoble - 3.1 s free fall.
  • Fallturm Bremen University of Bremen in Bremen. - 4.74 s free fall.

Reduced weight in pilot training

People have differing reactions to reduced weight sensations, and these 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 will not be able to complete flight training.

Neutral buoyancy

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 EVAs at its Neutral Buoyancy Laboratory.

Weightlessness in a spaceship.

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

Long periods of weightlessness occur in a spaceship outside a planet's atmosphere, provided no propulsion is applied and the ship is not rotating. This is the case when orbiting the earth (except when rockets fire for orbital maneuvers), but not during atmospheric re-entry. Weightlessness does not occur in a rockets ship that is accelerating by firing its rockets. Even if the rocket accelerates uniformly, the force is applied to the back end of the rocket by the escaping gas and that force is transferred throughout the ship via pressure or tension, precluding weightlessness. Weightlessness in a spaceship or space station is achieved by free-fall. The ship and all things in it are literally falling toward the Earth's surface, but their speed in orbit around the Earth allows for almost perpetual falling.

Weightlessness in the center of a planet.

In the center of a planet a person would feel weightless because the pull of the surrounding mass of the planet would cancel out. More generally, the gravitational force is zero everywhere within a hollow spherically symmetrical planet, by the shell theorem.

Weightlessness: Health effects.

Following the establishment of orbiting stations that can be inhabited for long durations by humans, exposure to weightlessness has been demonstrated to have some deleterious effects to health. Humans are well-adapted to the physical conditions prevailing at the surface of the Earth. When weightless, certain physiological systems begin to alter and temporary and long term health issues can occur.

The most common initial condition experienced by humans after the first couple of hours or so of weightlessness is commonly known as space sickness. The symptoms include general queasiness, nausea, vertigo, headaches, lethargy, vomiting, and an overall malaise. The first case was reported by cosmonaut Gherman Titov in 1961. Since then roughly 45% of all people to experience free floating under zero gravity have also suffered from this condition. The duration of space sickness varies, but in no case has it lasted more than 72 hours. By that time the astronauts have grown accustomed to the new environment. NASA measures SAS using the "Garn scale," named for United States Senator Jake Garn, whose SAS during STS-51-D was so severe to be ranked 13 in this scale.

The most significant adverse effects of long-term weightlessness are muscle atrophy and deterioration of the skeleton; 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 are reversible upon return to 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.

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