Weightlessness
From Wikipedia, the free encyclopedia
Weightlessness is the experience (by people and objects) during free-fall, 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 from 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:
- 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, landing on a parachute: atmospheric drag decelerates the vehicle
- 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.
Contents |
[edit] 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.
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.
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, a human experiences no weight because all parts of the human object are accelerating uniformly (assuming that there are no tidal forces, which is true to a very good approximation for human-scale objects in earth orbit).
[edit] Microgravity
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.
- 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.
- Left to themselves, different parts of a vehicle either side of its orbital plane are in their own orbital planes, and in the frame of reference of the vehicle this pushes objects inwards towards the orbital plane of the vehicle as a whole.
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.
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).
[edit] Reduced gravity aircraft
[edit] 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 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 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.
[edit] 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.
[edit] 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 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. <ref name="esa.int main page">European Space Agency. A300 Zero-G. ESA Human Spaceflight web site. Retrieved on 2006-11-12.</ref> <ref name="esa.int next campaign">European Space Agency. Next camaign. ESA Human Spaceflight web site. Retrieved on 2006-11-12.</ref> <ref name="esa.int Campaign Organisation">European Space Agency. Campaign Organisation. ESA Human Spaceflight web site. Retrieved on 2006-11-12.</ref>
[edit] Ground-based reduced gravity facilities
Ground-based facilities that produce reduced-gravity conditions for research purposes are typically referred to as drop tubes or drop towers.
[edit] NASA 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; 20g is about the highest deceleration that a fit and healthy human being can withstand momentarily without sustaining permanent injury.
[edit] 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
[edit] 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.
[edit] Weightlessness in a spaceship
Image:Weightless hair.jpg 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 rocket 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.
[edit] 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.
[edit] 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.
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.
[edit] Criticism of the terms "Zero Gravity" and "Microgravity"
It is important to note, as stated at the beginning of this article, that there is plenty of gravity pulling on a spacecraft in orbit around the Earth. Gravity is the very reason why the spacecraft is orbiting. Therefore it is totally inaccurate to say that astronauts are experiencing "zero gravity" or "microgravity". What orbiting astronauts experience is zero-g, a measure of acceleration relative to their spacecraft, which results in weightlessness. But zero-g is not "zero gravity". If there were "zero gravity" or "microgravity", their spacecraft would not be pulled into an orbit around the Earth. It would go in a straight line since bodies will continue moving uniformly straight ahead unless an outside force is applied (inertia).
As a thought experiment, imagine a spacecraft that had the ability to rise up to orbital altitude by going straight up like a helicopter and hover over one spot on the Earth. The astronauts inside would not experience weightlessness. Their ride inside this hovering spacecraft would be similar to riding an elevator up an incredibly tall building and stopping at the top floor. While hovering above Earth's atmosphere, their weight would be very close to what they weigh on the surface of the Earth, even as a space shuttle goes zinging by them. So astronauts in a hovering spacecraft are being pulled by strong gravity just as space shuttle astronauts are pulled by strong gravity. The difference between them is that the orbiting shuttle is freely being pulled toward the center of the Earth. The lack of relative acceleration between the orbiting shuttle and its astronauts inside (who are also being freely pulled toward the center of the Earth) result in them being weightless. But the hovering spacecraft (as with an elevator at the top of an incredibly tall building) is not freely falling. The pull of gravity it is experiencing is being opposed by the hovering force. This force gets transferred to the astronauts within (along with everything else within the spacecraft) resulting in weight. This example illustrates the fact that there is plenty of gravity out in space. If you were to take any object that is orbiting the Earth and stop it dead in its track and then release it, the Earth's gravity would pull it straight down back toward the Earth's surface.
To use confused terms like "zero gravity" and "microgravity" is to mistake the general concept of acceleration for the concept of gravity. "Zero-g" and "micro-g" are perfectly accurate terms referring to the lack of acceleration (in the frame of reference of the spacecraft) that cause weightlessness, even while gravity is strongly pulling the trajectory of that spacecraft into an orbit.
The specific point of confusion is that "g" does not mean "gravity". The designator "g" is an arbitrary scale of acceleration not to be confused with gravity itself. "Zero-g" means zero acceleration, not zero gravity. "1-g" is the acceleration experienced on the surface of the Earth due to gravity, but it is not gravity itself. This scale is widely used because it is easy to relate to from common experience of acceleration due to gravity. But any other scale of acceleration can be used to describe the condition of weightlessness. It could be described using a scale that has nothing to do with Earth's gravity. Similarly, a distance can be measured in feet as well as meters, where a meter has nothing to do with the length of a human foot. For a weightless astronaut to say that they are in zero gravity is the same type of error as saying that an object that has a length of 0.3048 meters is identically one human foot in flesh and blood. "1 foot" is an arbitrary scale for measuring length that was (at some point in history) based on a person's foot, but not to be confused with an actual human foot. "1-g" is an arbitrary scale for measuring acceleration that is based on gravity, but not to be confused with actual gravity. A zero-g environment is also a zero-meters/second^2 environment and a zero-feet/second^2 environment. Any arbitrary scale of acceleration can be used, and none of them have any exclusive relationship to gravity.
Another illustration of this type of mistake is when people erroneously speak of a jet pilot blacking out as a result of "gravity-induced Loss Of Consciousness". The proper term is g-induced Loss Of Consciousness. It is the acceleration produced by their maneuvers that is the culprit for g-LOC. It is clearly "g-induced" and not "gravity-induced", because gravity obviously remained constant at 1-g the whole time for the pilot. Likewise, the purpose of NASA's " Reduced Gravity Aircraft" is not to reduce gravity, but rather to fly in a parabolic arc that brings relative acceleration to zero. "g" is reduced while gravity stays essentially the same. So clearly it is possible to experience zero-g without going into space. Any aircraft can do this by pushing it over into a parabolic arc. Even any car that hits a bump fast enough to leave the ground will experience zero-g for the time that the wheels are not in contact with the road. The easiest way to experience zero-g is to bend your legs and jump off the ground. For the time that you are in the air, you are experiencing weightlessness. The difference with astronauts is that the experience is not momentary because their spacecraft is continually getting pulled toward the Earth. It is possible for non-astronauts to experience longer durations of weightlessness by cliff diving, bungee jumping, freefall parachuting, barreling over a waterfalls or more safely by riding many types of modern amusement park rides that put the occupant in a freefall. What is common for the astronauts as well as these other examples is that it is not gravity that is changing, but rather the acceleration in their falling frame of reference goes to zero-g.
As it stands today, NASA itself is one of the biggest promoters of the erroneous terms "zero gravity" and "microgravity" (along with the similarly erroneous term "reduced gravity"). Astronauts themselves have been quoted as having experienced "no gravity" while in space. Surely they are aware that there was plenty of gravity throughout every orbit they made, with gravity being the very thing that pulled them into an orbit. However, Gravity is also defined as "heaviness or weight." So perhaps the astronauts were simply referring to the fact that they don't "feel" gravity.
[edit] 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
[edit] External links
- Zero-G Research Facility, a NASA facility for ground-based microgravity research.
- Microgravity flights on board the Airbus A300 Zero-G
- NASA Microgravity University, a NASA student program that allows teams of college students to plan a microgravity experiment and fly onboard NASA's C-9, the replacement aircraft for the KC-135
- maniacworld.com "NASA Reduced Gravity Aircraft", videos of the NASA Reduced Gravity Aircraft and of participants in a flight on that aircraft.de:Schwerelosigkeit
fr:Impesanteur ko:무중력 it:Microgravità nl:Gewichtloosheid pl:Nieważkość pt:Microgravidade ru:Невесомость sl:Lebdenje sv:Tyngdlöshet
