Zero G
Michael Foale can be seen exercising in the foreground.
Weightlessness is a phenomenon experienced by people during free-fall. 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 orbit, in deep space (far from a planet, star, or other massive body), in an airplane following a particular parabolic flight path (e.g., the “Vomit Comet”), or in one of several other more unusual situations.
The physics of weightlessness
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. 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
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Apparent weight
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. 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:
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.
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
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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 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 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.
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
Reduced weight in aircraft
Airplanes have been used since 1973 to provide a nearly weightless environment in which to train astronauts, conduct research, and film motion pictures. 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. 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
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, 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.
Other aircraft it has used include the Russian Ilyushin Il-76 MDK and French Caravelle.
Ecuadorian T-39 Condor
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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
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.
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 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 65 g.
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.
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 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 – 3.1 s free fall
Fallturm Bremen University of Bremen in Bremen – 4.74 s free fall
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. 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
The relationship between acceleration and velocity vectors in an orbiting spacecraft
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.
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
If a person were able to survive at the center of a planet, they would experience weightlessness without any acceleration. Symptoms of SAS include nausea and vomiting, vertigo, headaches, lethargy, and overall malaise.
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.
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.