Microgravity environment
Encyclopedia
The term micro-g environment (also µg, often referred to by the term microgravity) is more or less a synonym of weightlessness
and zero-G, but indicates that g-force
s are not quite zero, just very small. The symbol for microgravity, µg, was used on the insignia of Space Shuttle
flight STS-87
because this flight was devoted to microgravity research.
to almost zero. This is the simplest in conception, but requires traveling an enormous distance, rendering it most impractical. For example, to reduce the gravity of the Earth by a factor of one million one needs to be at a distance of 6 million km from the Earth, but to reduce the gravity of the Sun to this amount one has to be at a distance of 3700 million km. (The gravity due to the rest of the Milky Way
is already smaller than one millionth of the gravity on Earth, so we need not move away further from its center.) Thus it is not impossible, but it has only been achieved by a few spaceships such as Voyager 1
, which do not return to Earth. To reduce the gravity to one thousandth of that on Earth one needs to be at a distance of 200,000 km.
From stationarity the gravity from "the rest of the Milky Way" would cause a free fall, covering a distance of 100 pm in one second, 360 nm in one minute, 1.3 mm in one hour, 70 cm in one day, 37 m in one week, 100 km in one year, and 10,000 km in 10 years (at a speed at that last location of 6 cm/s).
Compare the gravitational potential at some of these locations.
, i.e. there are no forces other than gravity acting on the people or objects in this environment. To prevent air drag making the free fall less perfect, objects and people can free-fall in a capsule that itself, while not necessarily itself in free fall, is accelerated as in free fall. This can be done by applying a force to compensate for air drag. Alternatively free fall can be carried out in space, or in a vacuum tower or shaft.
The two cases that can be distinguished are that where the situation is only temporary because after some time the Earth's surface is or would be reached, and the case where the situation can go on indefinitely.
A temporary micro-g environment exists in a drop tube (in a tower or shaft), a sub-orbital spaceflight
, e.g. with a sounding rocket
, and in an airplane such as used by NASA
's Reduced Gravity Research Program, aka the Vomit Comet
, and by the Zero Gravity Corporation
. A temporary micro-g environment is applied for training of astronauts, for some experiments, for filming movies, and for fun.
A micro-g environment for an indefinite time, while also possible in a spaceship going to infinity in a parabolic or hyperbolic orbit, is most practical in an Earth orbit. This is the environment commonly experienced in the International Space Station
, Space Shuttle
, etc. While this scenario is the most suitable for scientific experimentation and commercial exploitation, it is still quite expensive to operate in, mostly due to launch costs.
Objects in orbit are not perfectly weightless due to several effects:
(now obsolete), molten metal (such as lead
or steel
), was dripped through a sieve into free fall. With sufficient height (several hundred feet), the metal would be solid enough to resist impact (usually in a water bath) at the bottom of the tower. While the shot may have been slightly deformed by its passage through the air and by impact at the bottom, this method produced metal spheres of sufficient roundness to be used directly in shotgun
shells or to be refined by further processing for applications requiring higher accuracy.
s in micro-g, as in a space station
or automated artificial satellite
, in an attempt to reduce crystal lattice defects. Such defect-free crystals may prove useful for certain microelectronic applications and also to produce crystals for subsequent X-ray crystallography
.
Weightlessness
Weightlessness is the condition that exists for an object or person when they experience little or no acceleration except the acceleration that defines their inertial trajectory, or the trajectory of pure free-fall...
and zero-G, but indicates that g-force
G-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...
s are not quite zero, just very small. The symbol for microgravity, µg, was used on the insignia of Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
flight STS-87
STS-87
STS-87 was a Space Shuttle mission launched from KSC pad 39-B on 19 November 1997. It was the 88th flight of the Space Shuttle, and the 24th flight of Columbia...
because this flight was devoted to microgravity research.
Absence of gravity
A "stationary" micro-g environment would require travelling far enough into deep space so as to reduce the effect of gravity by attenuationAttenuation
In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated by water.In electrical engineering and telecommunications, attenuation affects the...
to almost zero. This is the simplest in conception, but requires traveling an enormous distance, rendering it most impractical. For example, to reduce the gravity of the Earth by a factor of one million one needs to be at a distance of 6 million km from the Earth, but to reduce the gravity of the Sun to this amount one has to be at a distance of 3700 million km. (The gravity due to the rest of the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
is already smaller than one millionth of the gravity on Earth, so we need not move away further from its center.) Thus it is not impossible, but it has only been achieved by a few spaceships such as Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...
, which do not return to Earth. To reduce the gravity to one thousandth of that on Earth one needs to be at a distance of 200,000 km.
| Location | Gravity due to the Earth Earth Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets... | Gravity due to the Sun Sun The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields... | Gravity due to the rest of Milky Way Milky Way The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky... | Total |
|---|---|---|---|---|
| Earth's surface | 9.81 m/s2 | 6 mm/s2 | 200 pm/s2 = 6 mm/s/yr | 9.81 m/s2 |
| Low Earth orbit Low Earth orbit A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km... |
9 m/s2 | 6 mm/s2 | 200 pm/s2 | 9 m/s2 |
| 200,000 km from Earth | 10 mm/s2 | 6 mm/s2 | 200 pm/s2 | up to 12 mm/s2 |
| 6 million km from Earth | 10 μm/s2 | 6 mm/s2 | 200 pm/s2 | 6 mm/s2 |
| 3700 million km from Earth | 29 pm/s2 | 10 μm/s2 | 200 pm/s2 | 10 μm/s2 |
| Voyager 1 Voyager 1 The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of... (17,000 million km from Earth) |
1 pm/s2 | 500 nm/s2 | 200 pm/s2 | 500 nm/s2 |
| 0.1 light-year Light-year A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres... from Earth |
400 am/s2 | 200 pm/s2 | 200 pm/s2 | up to 400 pm/s2 |
From stationarity the gravity from "the rest of the Milky Way" would cause a free fall, covering a distance of 100 pm in one second, 360 nm in one minute, 1.3 mm in one hour, 70 cm in one day, 37 m in one week, 100 km in one year, and 10,000 km in 10 years (at a speed at that last location of 6 cm/s).
Compare the gravitational potential at some of these locations.
Free fall
What remains is a micro-g environment moving in free fallFree fall
Free fall is any motion of a body where gravity is the only force acting upon it, at least initially. These conditions produce an inertial trajectory so long as gravity remains the only force. Since this definition does not specify velocity, it also applies to objects initially moving upward...
, i.e. there are no forces other than gravity acting on the people or objects in this environment. To prevent air drag making the free fall less perfect, objects and people can free-fall in a capsule that itself, while not necessarily itself in free fall, is accelerated as in free fall. This can be done by applying a force to compensate for air drag. Alternatively free fall can be carried out in space, or in a vacuum tower or shaft.
The two cases that can be distinguished are that where the situation is only temporary because after some time the Earth's surface is or would be reached, and the case where the situation can go on indefinitely.
A temporary micro-g environment exists in a drop tube (in a tower or shaft), a sub-orbital spaceflight
Sub-orbital spaceflight
A sub-orbital space flight is a spaceflight in which the spacecraft reaches space, but its trajectory intersects the atmosphere or surface of the gravitating body from which it was launched, so that it does not complete one orbital revolution....
, e.g. with a sounding rocket
Sounding rocket
A sounding rocket, sometimes called a research rocket, is an instrument-carrying rocket designed to take measurements and perform scientific experiments during its sub-orbital flight. The origin of the term comes from nautical vocabulary, where to sound is to throw a weighted line from a ship into...
, and in an airplane such as used by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's Reduced Gravity Research Program, aka the Vomit Comet
Vomit Comet
A Reduced Gravity Aircraft is a type of fixed-wing aircraft that briefly provides a nearly weightless environment in which to train astronauts, conduct research and film motion pictures....
, and by the Zero Gravity Corporation
Zero Gravity Corporation
Zero Gravity Corporation is an American company based in Vienna, Virginia, formerly of Fort Lauderdale, Florida, which operates weightless flights from United States airports...
. A temporary micro-g environment is applied for training of astronauts, for some experiments, for filming movies, and for fun.
A micro-g environment for an indefinite time, while also possible in a spaceship going to infinity in a parabolic or hyperbolic orbit, is most practical in an Earth orbit. This is the environment commonly experienced in the International Space Station
International Space Station
The International Space Station is a habitable, artificial satellite in low Earth orbit. The ISS follows the Salyut, Almaz, Cosmos, Skylab, and Mir space stations, as the 11th space station launched, not including the Genesis I and II prototypes...
, Space Shuttle
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
, etc. While this scenario is the most suitable for scientific experimentation and commercial exploitation, it is still quite expensive to operate in, mostly due to launch costs.
Objects in orbit are not perfectly weightless due to several effects:
- Effects depending on relative position in the spacecraft:
- In Low Earth orbitLow Earth orbitA low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
(LEO), the force of gravity decreases upward by 0.33 μg/m. Objects which have a non-zero size will be subjected to a tidal forceTidal forceThe tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...
, or a differential pull, between the high and low ends of the object. (An extreme version of this effect is spaghettificationSpaghettificationIn astrophysics, spaghettification is the vertical stretching and horizontal compression of objects into long thin shapes in a very strong gravitational field, and is caused by extreme tidal forces...
.) - In a spacecraft in LEO, the centrifugal forceCentrifugal forceCentrifugal force can generally be any force directed outward relative to some origin. More particularly, in classical mechanics, the centrifugal force is an outward force which arises when describing the motion of objects in a rotating reference frame...
is greater on the side of the spacecraft furthest from the Earth. This is also a tidal force, adding 0.17 μg/m to the first-mentioned effect. - "Floating" objects in a spacecraft in LEO are actually in independent orbits around the Earth. If two objects are placed side-by-side (relative to their direction of motion) they will be orbiting the Earth in different orbital planes. Since all orbital planes pass through the center of the earth, any two orbital planes intersect along a line. Therefore two objects placed side-by-side (at any distance apart) will come together after one quarter of a revolution. If they are placed so they miss each other, they will oscillate past each other, with the same period as the orbit. This corresponds to an inward acceleration of 0.17 μg per meter horizontal distance from the center. If they are placed one ahead of the other in the same orbital plane, they will maintain their separation. If they are placed one above the other (at different radii from the center of the earth) they will have different potential energies, so the size, eccentricity, and period of their orbits will be different, causing them to move in a complex looping pattern relative to each other.
- Gravity between the spacecraft and an object within it may make the object slowly "fall" toward a more massive part of it. The acceleration is 0.007 μg for 1000 kg at 1 m distance.
- In Low Earth orbit
- Uniform effects (which could be compensated):
- Though very thin, there is some air at orbital altitudes of 185 to 1,000 km. This atmosphere causes deceleration due to friction. This could be compensated by a small continuous thrust, but in practice the deceleration is only compensated from time to time, so the small g-force of this effect is not eliminated.
- The effects of the solar windSolar windThe solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
and radiation pressureRadiation pressureRadiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
are similar, but directed away from the Sun. Unlike the effect of the atmosphere it does not reduce with altitude.
Metal spheres
In a shot towerShot tower
thumb|The Shot Tower, Bristol, EnglandA shot tower is a tower designed for the production of shot balls by freefall of molten lead, which is then caught in a water basin. The shot is used for projectiles in firearms.-Process:...
(now obsolete), molten metal (such as lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
or steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
), was dripped through a sieve into free fall. With sufficient height (several hundred feet), the metal would be solid enough to resist impact (usually in a water bath) at the bottom of the tower. While the shot may have been slightly deformed by its passage through the air and by impact at the bottom, this method produced metal spheres of sufficient roundness to be used directly in shotgun
Shotgun
A shotgun is a firearm that is usually designed to be fired from the shoulder, which uses the energy of a fixed shell to fire a number of small spherical pellets called shot, or a solid projectile called a slug...
shells or to be refined by further processing for applications requiring higher accuracy.
High quality crystals
While not yet a commercial application, there has been much interest in growing crystalCrystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s in micro-g, as in a space station
Space station
A space station is a spacecraft capable of supporting a crew which is designed to remain in space for an extended period of time, and to which other spacecraft can dock. A space station is distinguished from other spacecraft used for human spaceflight by its lack of major propulsion or landing...
or automated artificial satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
, in an attempt to reduce crystal lattice defects. Such defect-free crystals may prove useful for certain microelectronic applications and also to produce crystals for subsequent X-ray crystallography
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
.
See also
- μFluids@Home is a distributed computingDistributed computingDistributed computing is a field of computer science that studies distributed systems. A distributed system consists of multiple autonomous computers that communicate through a computer network. The computers interact with each other in order to achieve a common goal...
project that models the behavior of liquid rocket propellantsLiquid rocket propellantsThe highest specific impulse chemical rockets use liquid propellants. This type of propellant has a long history going back to the first rockets and is still in use, for example in the Space Shuttle and Ariane 5.-Early development:...
in micro-g. - WeightlessnessWeightlessnessWeightlessness is the condition that exists for an object or person when they experience little or no acceleration except the acceleration that defines their inertial trajectory, or the trajectory of pure free-fall...
External links
- Overview of microgravity applications and methods
- Criticism of the terms "Zero Gravity" and "Microgravity", a persuasion to use terminology that reflects accurate physics (Sci.space post).
- Space Biology Research at AU-KBC Research Centre