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

Weightlessness is the experience (by people and objects) during freefall, of having no apparent weight. This condition is also known as microgravity (see below). Weightlessness in common spacecraft is not due to an increased distance to the earth; the acceleration due to gravity at an altitude of 100 km is only 3% less than at the surface of the earth. Weightlessness means a zero g-force or zero apparent weight; acceleration is only due to gravity, as opposed to the cases where other forces are acting, including: Astronauts on the International Space Station. ... Astronauts on the International Space Station. ... Michael Foale C. Michael Foale PhD (born 6 January 1957) is a British-born astronaut with dual UK-US citizenship; he is a veteran of four space shuttle missions and extended stays on both Mir and the International Space Station. ... Freefall or free fall in the strict sense is the condition of acceleration which is due only to gravity. ... The apparent weight of an object is the value a spring weighing scale would measure. ... Acceleration is the time rate of change of velocity, and at any point on a v-t graph, it is given by the gradient of the tangent to that point In physics, acceleration (symbol: a) is defined as the rate of change (or time derivative) of velocity. ... ... g (also gee, g-force or g-load) is a non-SI unit of acceleration defined as exactly 9. ... The apparent weight of an object is the value a spring weighing scale would measure. ...

• standing on the ground, sitting in a chair on the ground, etc. (gravity is cancelled by the reaction force of the ground)
• flying in a plane (gravity is cancelled by the lift the wings provide) - see below for special trajectories which form an exception
• atmospheric reentry, landing on a parachute: gravity is opposed by atmospheric drag
• during an orbital maneuver in a spacecraft: the rocket provides thrust

The difference is that gravity acts directly on a person and other masses, just like on the vehicle, while forces like atmospheric drag and thrust first act on the vehicle, and through the vehicle on the person. In the first case the person and the vehicle floor are not pushed toward each other, while in the other cases they are. Lift consists of the sum of all the fluid dynamic forces on a body perpendicular to the direction of the external flow around that body. ... A trajectory is an imagined trace of positions followed by an object moving through space. ... // Terminology, definitions and jargon Over the decades since the 1950s, a rich technical jargon has grown around the engineering of vehicles designed to enter planetary atmospheres. ... Landing is the last part of a flight, where a flying animal or aircraft returns to the ground. ... The Apollo 15 capsule landed safely despite a parachute failure. ... Atmospheric drag is a form of drag, which is the force that opposes an object moving through a liquid or gas. ... An orbital maneuver is a change from one orbit to another, accomplished by applying thrust. ... Ariane 5 lifts off with the Rosetta space probe on March 2, 2004. ... A Redstone rocket, part of the Mercury program A rocket is a vehicle, missile or aircraft which obtains thrust by the reaction to the ejection of fast moving exhaust gas from within a rocket engine. ... Thrust is a reaction force described quantitatively by Newtons Second and Third Law. ...

Overview

What humans experience as weight is not actually the force due to gravity (even though that 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 in contact with) pushing upwards against us to counteract the force due to gravity, that is the apparent weight. In physics, a force is an external cause responsible for any change of a physical system. ... Newtons first and second laws, in Latin, from the original 1687 edition of the Principia Mathematica. ... The apparent weight of an object is the value a spring weighing scale would measure. ...

For example, a wood block in a container in free-fall experiences weightlessness. This is because there is no reaction to the wood block's weight from the container, as it is being pulled down with the same acceleration. The acceleration of the container equals the acceleration of the block, which equals the acceleration caused by gravity. When the container is at rest on the ground, however, the force on each piece of the block is not uniform. Because the block is not accelerating, there is also a force upward that arises 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. Part of feeling weight, then, is actually experiencing a pressure gradient within one's own body. Free Fall opens with one of the most stunning first paragraphs I have ever, or am ever likely to, read. ...

There is another aspect of the feeling of weight that a pressure gradient does not account for, an example of which is the way that our arms are pulled downward with respect to our body. This effect comes from the fact that something hanging is not supported directly via a pressure from the ground. In fact the effect is almost the exact opposite of a pressure gradient, it is a tension gradient. It occurs because each cross section of a hanging object, a rope for instance, must support the weight of every piece below it.

Hence, in short, weightlessness has nothing to do with whether we are under the influence of a gravitational force, but has to do with whether there are force gradients across our body. In free-fall, all parts of an object accelerate uniformly (assuming that there are no tidal forces, which is true to a very good approximation for human-scale objects in earth orbit)), and thus a human would experience no weight. Comet Shoemaker-Levy 9 after breaking up under the influence of Jupiters tidal forces. ...

Microgravity

Candle flame in microgravity conditions. NASA image.

The term microgravity is also used because weightlessness in e.g. a spaceship or other container is not perfect. Causes include: Image File history File links Description: This image shows the appearance of a candle flame in microgravity conditions. ... Image File history File links Description: This image shows the appearance of a candle flame in microgravity conditions. ... A lit candle. ... NASA Logo Listen to this article · (info) This audio file was created from the revision dated 2005-09-01, and does not reflect subsequent edits to the article. ... Main article: Weightlessness A microgravity environment is one where gravity has little or no measurable effect. ...

• Gravity decreases 1 ppm for every 3m increase in height.
• In a spaceship in orbit the required centripetal force is higher at the upper side.
• Though very thin, there is some air at the level of the orbit, which causes deceleration due to friction.

The "weight" caused by the first two items (the tidal force) is directed vertically away from the spacecraft, i.e. vertically away from Earth in the portion which is farther from Earth (or the body it is in orbit around) than the center of gravity of the spacecraft and vertically toward Earth for the rest. For the last item it is forward. Parts per million (ppm) is a measure of concentration that is used where low levels of concentration are significant. ... Centripetal force is the force pulling of an object toward the center of a circular path as the object goes around the circle. ... Comet Shoemaker-Levy 9 after breaking up under the influence of Jupiters tidal forces. ...

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). The Space Shuttle Columbia seconds after engine ignition, 1981 (NASA). ... The STS-107 crewmembers strike a ‘flying’ pose for their traditional in-flight crew portrait in the SPACEHAB aboard the Space Shuttle Columbia. ...

NASA's KC-135 Reduced Gravity 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 realised 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 Logo Listen to this article · (info) This audio file was created from the revision dated 2005-09-01, and does not reflect subsequent edits to the article. ... The Boeing KC-135 Stratotanker is an aerial refueling tanker aircraft, first manufactured in 1956 and expected to remain in service into the 2020s. ... Weightlessness inside the Vomit Comet The Vomit Comet was the nickname given to the aircraft used by NASAs Reduced Gravity Research Program. ... An aerial view of the complete Johnson Space Center facility in Houston, Texas in 1989. ... Vomiting (or emesis) is the forceful expulsion of the contents of ones stomach through the mouth. ... Comet Hale-Bopp A comet is a small body in the solar system that orbits the sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail — both due primarily to the effects of solar radiation upon the comets nucleus, which itself is a minor planet... A parabola A parabola (from the Greek: παραβολή) is a conic section generated by the intersection of a cone, and a plane tangent to the cone or parallel to some plane tangent to the cone. ... Propulsion method may refer to a number of different articles: For a list of space transport methods, see spacecraft propulsion. ... In physics, friction is the non-conservative resistive force that occurs when two surfaces travel along each other when forced together. ...

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. Zero Gravity Corporation (also known as ZERO-G) is a Fort Lauderdale-based company which operates weightless flights. ... Sun Country Airlines B727-200 The Boeing 727 is a large commercial jet airliner carrying as many as 189 passengers. ...

NASA's Zero-G Research Facility

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. The Glenn Research Center is located in Cleveland, Ohio between Cleveland Hopkins International Airport and the Metropark. ... ... Styrofoam redirects here. ...

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

These are not for people, just for experiment packages for the deceleration levels are too high.

Weightlessness in a spaceship

Weightlessness for a more extended period of time occurs in a spaceship outside the earth's atmosphere, as long as no propulsion is applied, and that it is not rotating about its axis; orbiting the earth this is the case except when rockets are on for orbital maneuvers, and until atmospheric re-entry. Ariane 5 lifts off with the Rosetta probe on 2nd of March, 2004. ... An orbital maneuver is a change from one orbit to another, accomplished by applying thrust. ... Atmospheric entry is the transition from the vacuum of space to the atmosphere of any planet or other celestial body. ...

A rocket ship that is accelerating by firing its rockets is a very different matter. Even if the rocket is accelerating uniformly, the force is applied to the back end of the rocket by the gas escaping out the back. This force must be transferred to each part of the ship through either pressure or tension, and thus weightlessness is not experienced. A Redstone rocket, part of the Mercury program A rocket is a vehicle, missile or aircraft which obtains thrust by the reaction to the ejection of fast moving exhaust gas from within a rocket engine. ...

Weightlessness in the centre of a planet

In the centre 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. The phrase hollow earth refers to the esoteric idea that the planet Earth has a hollow interior, almost always associated with the idea that it has a habitable inner surface. ... In Newtonian physics, the shell theorem states that the gravity due to a uniform spherical shell is zero on an object inside the shell, and acts on an object outside the shell as if the entire mass of the shell were at its center. ...

Health effects

Following the establishment of orbiting stations that can be inhabited for long durations by humans, exposure to microgravity has been demonstrated to have some deleterious effects to health. Humans have evolved for life on the surface of the Earth. When the constraint of gravity is removed, certain physiological systems begin to function to the detriment of the overall system. Earth, also known as Terra, and Tellus mostly in the 19th century, is the third-closest planet to the Sun. ...

The most common initial condition experienced by humans in weightless conditions 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. Space adaptation syndrome, or space sickness, is what astronauts go through during adaptation to zero gravity. ... Nausea (Greek Ναυτεία) is the sensation of unease and discomfort in the stomach with an urge to vomit. ... Vertigo can refer to a number of things: Vertigo is a form of dizziness often associated with balance disorder. ... A headache (medically known as cephalgia) is a condition of mild to severe pain in the head; sometimes upper back or neck pain may also be interpreted as a headache. ... Fatigue is a feeling of excessive tiredness or lethargy, with a desire to rest, perhaps to sleep. ... Vomiting (or emesis) is the forceful expulsion of the contents of ones stomach through the mouth. ... U.S. Space Shuttle astronaut Bruce McCandless II using a manned maneuvering unit. ... Gherman Titov Gherman Stepanovich Titov (Russian: Герман Степанович Титов; September 11, 1935, Verkhnee Zhilino – September 20, 2000, Moscow) was a Soviet cosmonaut and the second person to orbit the Earth. ... 1961 (MCMLXI) was a common year starting on Sunday (link will take you to calendar). ...

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 weight loss, nasal congestion, sleep disturbance, excess flatulence, and puffiness of the face. These effects are reversible upon return to Earth. A top-down view of skeletal muscle Muscle is a contractile form of tissue. ... Atrophy is the partial or complete wasting away of a part of the body. ... In biology, the skeleton or skeletal system is the biological system providing support in living organisms. ... The circulatory system or cardiovascular system is the organ system which circulates blood around the body of most animals. ... Human red blood cells Red blood cells are the most common type of blood cell and are the vertebrate bodys principal means of delivering oxygen from the lungs or gills to body tissues via the blood. ... The immune system is the system of specialised cells and organs that protect an organism from outside biological influences. ... Flatulence is a mixture of gases that are produced by symbiotic bacteria and yeasts living in the gastrointestinal tract of mammals, and aerosolized particles of feces, and it is released under pressure through the anus with a characteristic sound and unsavory odor. ... Earth, also known as Terra, and Tellus mostly in the 19th century, is the third-closest planet to the Sun. ...

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. Osteoporosis is a disease of bone in which bone mineral density (BMD) is reduced and bone microarchitecture is disrupted. ...

See also

• Artificial gravity
• Human adaptation to space
• μFluids@Home—a distributed computing project for the computer simulation of two-phase fluid behavior in microgravity and microfluidics problems

Artificial gravity is a simulation of gravity in outer space or free-fall. ... Human adaptation to space is a challenging field in the development of more practical human spaceflight. ... μFluids@Home is a computer simulation of two-phase fluid behavior in microgravity and microfluidics problems. ... In computer science, distributed computing studies the coordinated use of physically distributed computers. ... A subset of the phases of matter, fluids include liquids, gases, plasmas and, to some extent, plastic solids. ... Microfluidics is a multidisciplinary field comprising physics, chemistry, engineering and biotechnology that studies the behavior of fluids at the microscale and mesoscale, that is, fluids at volumes thousands of times smaller than a common droplet. ...

External links

• Zero-G Research Facility, a NASA facility for ground-based microgravity research