The term micro-g environment (also µg, often referred to by the term
microgravity) is more or less a synonym for weightlessness and zero-g,
but indicates that g-forces are not quite zero—just very small.
The symbol for microgravity, µg, was used on the insignias of Space
STS-87 and STS-107, because these flights were devoted
to microgravity research in low
1 Absence of gravity
2 Free fall
3 Tidal and inertial acceleration
4 Commercial applications
4.1 Metal spheres
4.2 High-quality crystals
5 Health effects of the micro-g environment
5.1 Space Motion Sickness
5.2 Musculoskeletal Effects
5.3 Cardiovascular Effects
6 Impacts to Worker Safety
7 See also
9 External links
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
attenuation 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
kilometers from the Earth, but to reduce the gravity of the
this amount one has to be at a distance of 3.7 billion kilometers.
(The gravity due to the rest of the
Milky Way is already smaller than
one millionth of the gravity on Earth, so we do not need to move away
further from its center). Thus it is not impossible,
but it has only been achieved so far by four interstellar probes
Voyager 1 and 2, part of the Voyager program,
Pioneer 10 and 11 part
of the Pioneer program) and they did not return to Earth. To reduce
the gravity to one thousandth of that on Earth's surface, one needs to
be at a distance of 200,000 km.
Gravity due to
rest of Milky Way
200 pm/s2 = 6 mm/s/yr
200,000 km from Earth
up to 12 mm/s2
7009600000000000000♠6×106 km from Earth
7012370000000000000♠3.7×109 km from Earth
Voyager 1 (7013170000000000000♠17×109 km from Earth)
0.1 light-year from Earth
up to 400 pm/s2
At a distance relatively close to
Earth (less than 3000 km),
gravity is only slightly reduced. As an object orbits a body such as
the Earth, gravity is still attracting objects towards the
the object is accelerated downward at almost 1g. Because the objects
are typically moving laterally with respect to the surface at such
immense speeds, the object will not lose altitude because of the
curvature of the Earth. When viewed from an orbiting observer, other
close objects in space appear to be floating because everything is
being pulled towards
Earth at the same speed, but also moving forward
as the Earth's surface "falls" away below. All these objects are in
free fall, not zero gravity.
Compare the gravitational potential at some of these locations.
What remains is a micro-g environment moving in free fall, 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 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.
Two cases can be distinguished: Temporary micro-g, where after some
time the Earth's surface is or would be reached, and indefinite
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 recreational purposes.
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.
Tidal and inertial acceleration
Objects in orbit are not perfectly weightless due to several effects:
Effects depending on relative position in the spacecraft:
Because the force of gravity decreases with distance, objects with
non-zero size will be subjected to a tidal force, or a differential
pull, between the ends of the object nearest and furthest from the
Earth. (An extreme version of this effect is spaghettification.) In a
spacecraft in LEO, the centrifugal force is also greater on the side
of the spacecraft furthest from the Earth. At a low
Earth orbit (LEO)
400 km altitude, the overall differential in g-force is
approximately 0.384 μg/m. 
"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 0.128 μg per meter horizontal distance from
the center at 400 km altitude. 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.
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 wind and radiation pressure are similar, but
directed away from the Sun. Unlike the effect of the atmosphere, it
does not reduce with altitude.
In a shot tower (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
While not yet a commercial application, there has been interest in
growing crystals 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.
Comparison of boiling of water under earth's gravity (1 g, left) and
microgravity (right). The source of heat is in the lower part of the
A comparison between the combustion of a candle on
Earth (left) and in
a microgravity environment, such as that found on the
Protein crystals grown by American scientists on the Russian Space
Mir in 1995.
Comparison of insulin crystals growth in outer space (left) and on
Health effects of the micro-g environment
Space Motion Sickness
Six astronauts who had been in training at the Johnson Space Center
for almost a year are getting a sample of a micro-g environment
Space Motion Sickness (SMS) is thought to be a subtype of motion
sickness that plagues nearly half of all astronauts who venture into
space. SMS, along with facial stuffiness from headward shifts of
fluids, headaches, and back pain, is part of a broader complex of
symptoms that comprise Space Adaptation Syndrome (SAS). SMS was
first described in 1961 during the second orbit of the fourth manned
spaceflight when the Cosmonaut,
Gherman Titov aboard the Vostok 2,
described feeling disoriented with physical complaints mostly
consistent with motion sickness. It is one of the most studied
physiological problems of spaceflight but continues to pose a
significant difficulty for many astronauts. In some instances, it can
be so debilitating that astronauts must sit out from their scheduled
occupational duties in space – including missing out on a spacewalk
they have spent months training to perform. In most cases, however,
astronauts will work through the symptoms even with degradation in
Despite their experiences in some of the most rigorous and demanding
physical maneuvers on earth, even the most seasoned astronauts may be
affected by SMS, resulting in symptoms of severe nausea, projectile
vomiting, fatigue, malaise (feeling sick), and headache. These
symptoms may occur so abruptly and without any warning that space
travelers may vomit suddenly without time to contain the emesis,
resulting in strong odors and liquid within the cabin which may affect
other astronauts. Symptoms typically last anywhere from one to
three days upon entering weightlessness, but may recur upon reentry to
Earth’s gravity or even shortly after landing. SMS differs from
terrestrial motion sickness in that sweating and pallor are typically
minimal or absent and gastrointestinal findings usually demonstrate
absent bowel sounds indicating reduced gastrointestinal motility.
Even when the nausea and vomiting resolve, some central nervous system
symptoms may persist which may degrade the astronaut’s
performance. Graybiel and Knepton proposed the term “sopite
syndrome” to describe symptoms of lethargy and drowsiness associated
with motion sickness in 1976. Since then, their definition has
been revised to include “…a symptom complex that develops as a
result of exposure to real or apparent motion and is characterized by
excessive drowsiness, lassitude, lethargy, mild depression, and
reduced ability to focus on an assigned task.” Together, these
symptoms may pose a substantial threat (albeit temporary) to the
astronaut who must remain attentive to life and death issues at all
SMS is most commonly thought to be a disorder of the vestibular system
that occurs when sensory information from the visual system (sight)
and the proprioceptive system (posture, position of the body)
conflicts with misperceived information from the semicircular canals
and the otoliths within the inner ear. This is known as the ‘neural
mismatch theory’ and was first suggested in 1975 by Reason and
Brand. Alternatively, the fluid shift hypothesis suggests that
weightlessness reduces the hydrostatic pressure on the lower body
causing fluids to shift toward the head from the rest of the body.
These fluid shifts are thought to increase cerebrospinal fluid
pressure (causing back aches), intracranial pressure (causing
headaches), and inner ear fluid pressure (causing vestibular
Despite a multitude of studies searching for a solution to the problem
of SMS, it remains an ongoing problem for space travel. Most
non-pharmacological countermeasures such as training and other
physical maneuvers have offered minimal benefit. Thornton and Bonato
noted, “Pre- and inflight adaptive efforts, some of them mandatory
and most of them onerous, have been, for the most part, operational
failures.” To date, the most common intervention is
promethazine, an injectable antihistamine with antiemetic properties,
but sedation can be a problematic side effect. Other common
pharmacological options include metaclopromide, as well as oral and
transdermal application of scopolamine, but drowsiness and sedation
are common side effects for these medications as well.
In the space (or microgravity) environment the effects of unloading
varies significantly among individuals, with sex differences
compounding the variability. Differences in mission duration, and
the small sample size of astronauts participating in the same mission
also adds to the variability to the musculoskeletal disorders that are
seen in space. In addition to muscle loss, microgravity leads to
increased bone resorption, decreased bone mineral density, and
increased fracture risks.
Bone resorption leads to increased urinary
levels of calcium, which can subsequently lead to an increased risk of
In the first two weeks that the muscles are unloaded from carrying the
weight of the human frame during space flight, whole muscle atrophy
begins. Postural muscles contain more slow fibers, and are more prone
to atrophy than non-postural muscle groups. The loss of muscle
mass occurs because of imbalances in protein synthesis and breakdown.
The loss of muscle mass is also accompanied by a loss of muscle
strength, which was observed after only 2–5 days of spaceflight
during the Soyuz-3 and Soyuz-8 missions. Decreases in the
generation of contractile forces and whole muscle power have also been
found in response to microgravity.
To counter the effects of microgravity on the musculoskeletal system,
aerobic exercise is recommended. This often takes the form of
in-flight cycling. A more effective regimen includes resistive
exercises or the use of a penguin suit (contains sewn-in elastic
bands to maintain a stretch load on antigravity muscles),
centrifugation, and vibration. Centrifugation recreates Earth’s
gravitational force on the space station, in order to prevent muscle
atrophy. Centrifugation can be performed with centrifuges or by
cycling along the inner wall of the space station. Whole body
vibration has been found to reduce bone resorption through mechanisms
that are unclear. Vibration can be delivered using exercise devices
that use vertical displacements juxtaposed to a fulcrum, or by using a
plate that oscillates on a vertical axis. The use of beta-2
adrenergic agonists to increase muscle mass, and the use of essential
amino acids in conjunction with resistive exercises have been proposed
as pharmacologic means of combating muscle atrophy in space.
Microgravity can also lead to height increases; in 2018 a Japanese
astronout reported growing 2 centimetres in just three weeks of
micro-gravity on board the International Space Station.
Astronaut Tracy Dyson talks about studies into cardiovascular health
aboard the International Space Station.
Next to the skeletal and muscular system, the cardiovascular system is
less strained in weightlessness than on
Earth and is de-conditioned
during longer periods spent in space. In a regular environment,
gravity exerts a downward force, setting up a vertical hydrostatic
gradient. When standing, some 'excess' fluid resides in vessels and
tissues of the legs. In a micro-g environment, with the loss of a
hydrostatic gradient, some fluid quickly redistributes toward the
chest and upper body; sensed as 'overload' of circulating blood
volume. In the micro-g environment, the newly sensed excess blood
volume is adjusted by expelling excess fluid into tissues and cells
(12-15% volume reduction) and red blood cells are adjusted downward to
maintain a normal concentration (relative anemia). In the absence
of gravity, venous blood will rush to the right atrium because the
force of gravity is no longer pulling the blood down into the vessels
of the legs and abdomen, resulting in increased stroke volume.
These fluid shifts become more dangerous upon returning to a regular
gravity environment as the body will attempt to adapt to the
reintroduction of gravity. The reintroduction of gravity again will
pull the fluid downward, but now there would be a deficit in both
circulating fluid and red blood cells. The decrease in cardiac filling
pressure and stroke volume during the orthostatic stress due to a
decreased blood volume is what causes orthostatic intolerance.
Orthostatic intolerance can result in temporary loss of consciousness
and posture, due to the lack of pressure and stroke volume. More
chronic orthostatic intolerance can result in additional symptoms such
as nausea, sleep problems, and other vasomotor symptoms as well.
Many studies on the physiological effects of weightlessness on the
cardiovascular system are done in parabolic flights. It is one of the
only feasible options to combine with human experiments, making
parabolic flights the only way to investigate the true effects of the
micro-g environment on a body without traveling into space.
Parabolic flight studies have provided a broad range of results
regarding changes in the cardiovascular system in a micro-g
environment. Parabolic flight studies have increased the understanding
of orthostatic intolerance and decreased peripheral blood flow
suffered by Astronauts returning to Earth. Due to the loss of blood to
pump, the heart can atrophy in a micro-g environment. A weakened heart
can result in low blood volume, low blood pressure and effect the
body's ability to send oxygen to the brain without the individual
becoming dizzy. Heart rhythm disturbances have also been seen
among astronauts, but it is not clear whether this was due to
pre-existing conditions of effects of a micro-g environment. One
current countermeasure includes drinking a salt solution, which
increases the viscosity of blood and would subsequently increase blood
pressure which would mitigate post micro-g environment orthostatic
intolerance. Another countermeasure includes administration of
Midodrine, which is a selective alpha-1 adrenergic agonist. Midodrine
produces arterial and venous constriction resulting in an increase in
blood pressure by baroreceptor reflexes.
Impacts to Worker Safety
Space Motion Sickness can lead to degraded astronaut performance.
SMS threatens operational requirements, reduces situational awareness,
and threatens the safety of those exposed to micro-g environments.
Lost muscle mass leads to difficulty with movement, especially when
astronauts return to earth. This can pose a safety issue if the need
for emergency egress were to arise. Loss of muscle power makes it
extremely difficult, if not impossible, for astronauts to climb
through emergency egress hatches or create unconventional exit spaces
in the case of a crash upon landing. Additionally, bone resorption and
inadequate hydration in space can lead to the formation of kidney
stones, and subsequent sudden incapacitation due to pain. If this
were to occur during critical phases of flight, a capsule crash
leading to worker injury and/or death could result. Short-term and
long-term health effects have been seen in the cardiovascular system
from exposure to the micro-g environment that would limit those
exposed after they return to
Earth or a regular gravity environment.
Steps need to be taken to ensure proper precautions are taken into
consideration when dealing a micro-g environment for worker
Orthostatic intolerance can lead to temporary loss of
consciousness due to the lack of pressure and stroke volume. This loss
of consciousness inhibits and endangers those affected and can lead to
μFluids@Home — a distributed computing project that models the
behavior of liquid rocket propellants in micro-g
European Low Gravity Research Association
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