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:
- 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.
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, 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.
The term microgravity is also used because weightlessness in e.g. a spaceship or other container is not perfect. Causes 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.
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.
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.
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.
NASA's Zero-Gravity Research Facility
NASA's Zero-Gravity Research Labrotory is 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 5-second free fall, and then is decelerated at ca. 32g by pellets of expanded polystyrene. Alternatively, the vehicle starts at the bottom and is projected upward by a high-pressure pneumatic accelerator, after which it falls down again. Then the total duration of the weightlessness is 10 seconds.
Nearby there is also the 2.2 Second Drop Tower.
These are not for people, just for packages: the deceleration, and in the upward mode, also the acceleration, are too high, and the vehicle is too small.
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.
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.
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.
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.
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.
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.
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.