How do humans perceive gravity?

Gravitational biology


Eberhard Horn

Gravitational biology


Gravitational biology settles with the influence of Gravity (Gravitation; see Infobox 1) on the living organism apart. The beginning of gravitational biology research can be dated to the year 1806, when physiological experiments were carried out on plants using clinostats to test the importance of gravity for the growth of roots and shoots (shoot axis). At the end of the 19th century, clinostat studies on animal cells followed to determine the importance of gravity for the levels of the first cell division (cytokinesis). The use of ballistic flights began in 1948. Monkeys, dogs and mice, along with small animal and plant organisms, became the weightlessness suspended for a period of up to 30 minutes. The Russians succeeded in the first stay of an animal in weightlessness for several days. The bitch “Laika” was exposed to weightlessness for a week on November 3, 1957 on board the Earth satellite Sputnik 2, before she died due to a lack of opportunities to return. With the start of the Gemini and Apollo programs, the Americans used manned space flights for biological experiments that carried out over the Skylab program to the Shuttle transport system (STS) with the addition of the space laboratory developed by Europe Spacelab (see Fig. 1). The Russian manned space flight stood next to unmanned satellites between 1987 and 1999 the space station Me available for biological projects. Space missions usually have international participation, even if the scientific leadership is not incumbent on the Americans or Russians, but on other nations, as is the case with the two German missions D-1 (STS-61A, 1985) and D-2 (STS-55, 1993) or the Japanese Mission Spacelab-J (STS-47, 1992). From 2004 biological research will be carried out on the International space station ISS (see Fig. 2), the construction of which began in 1998.
No real weightlessness (cf. Infobox 2) has an effect on the biological experiments. At the used altitudes of 300 to 500 km there is a residual acceleration of approx. 5Â · 10 due to the proximity to the earth–5G (in which G the acceleration due to gravity or gravitational acceleration at the earth's surface = 9.80665 m / s2 means). In addition, there are transient accelerations due to movements of the team and machines (approx. 10–2 until 10–3G) as well as corrective ignitions of the engines (approx. 10–4 until 10–6G). It was agreed to use the term for this complex gravitational field MicrogravityG) to assign.

Scientific justification

All organisms on our earth experience a force of gravity during their life, which is in the gravitational acceleration of approx. 9.81 m / s2 reflects. This force affects not only the visible organism, but also its components, its organs and cell inclusions up to membrane-bound structures (membrane). It determines the weight of the body (body weight), sedimentation and friction as well as thermal and convection. The biological functional circuits that directly indicate a relation to gravity include the sense of heaviness, the sense of balance (organs of equilibrium) and the sense of posture, movements that oppose gravity in order to maintain stability (biomechanics), as well as the orientation of the plants during their growth (gravitropism, Tropism). A distinction must be made as to whether gravity has a general influence on the organism or whether it is necessary for its individual integrity and stability. The first aspect can be due to exposure to organisms taking place on earth Hypergravity being checked. The necessity of gravity for the integrity of the biological organism and its formation, on the other hand, can only be tested by its exposure to conditions in which gravity is eliminated (Severe withdrawal, severe deprivation). The field of gravitational biology is therefore closely tied to advances in space travel.

Methods of generating or simulating microgravity

In order to gain clues about the importance of gravity for the organism in earth-based experiments simulated microgravity used. It is based on 2 principles. 1) By constant or randomly distributed changes in the position of the organism, the gravity vector - from a statistical point of view - is allowed to act on the organism with equal frequency from all directions (horizontal and random clinostat; see Fig. 3b). In this way, the organism's ability to orientate in relation to the direction of gravity is suspended, but never gravity itself Medium reduced. Real microgravity is caused by a free fall from great heights (Drop tower, balloon drops;see Infobox 2) or during Parabolic flights generated in aircraft or missiles. Here, 4.5 s to 30 min long microgravity periods occur that can be used for short-term biological reactions. Microgravity lasting hours to months is only generated during manned or unmanned orbital flights. Manned space flights offer the advantage of allowing astronauts to intervene in an experiment or to carry out preparative work under microgravity.

Experimental hardware

Experiments are housed in satellites and "spaceships" in special racks (e.g. Anthrorack, Cognilab, Physiolab for medical experiments; Biolabor, Biorack for biological experiments). In addition to the standard equipment, such as syringes, cannulas, measuring probes, etc., experiment-specific apparatus and their electronic control as well as animal and plant rearing (see Fig. 3a) are housed in them. There are also large devices for examinations of the vestibular system (eye movement device, linear slide, centrifuge; see Fig. 3c) and the circulatory system (vacuum bag). For safety reasons, racks for gravitation biological experiments have a glovebox (closed chamber with fold-out gloves) in order to avoid contamination of the room laboratory with test components during preparative work. Biorack, the most successful experimental rack used in 6 missions, was developed by the European Space Agency (ESA). Standard hardware such as the Type I and Type II containers for Biorack are available to which the specific experiments (systems for cell fusion, mini aquariums, growth chambers, etc.) must be adapted (see Fig. 4a, cf. Fig. 4b). Biological racks have a 1 G-Reference centrifuge that can be used to simulate Earth's gravity in orbit. In this way, the specific effect of microgravity on the organism can be separated from effects caused by space radiation.

Basic observations

Cell biology: The main concern of cell biological research (cytology) under space conditions, besides basic research, is the development of methods to recognize the physiological status of the astronaut. The cell biological work has shown that forces caused by gravity are obviously sufficient to cause significant physiological and morphological changes on the cellular level (cell). It is not known how these very weak mechanical forces are converted into chemical reactions.
Different cells have specific organelles that act as statoliths and thus mediate gravity perception (unicellular organisms) Loxodes). This also applies to the root tip cells with their amyloplasts. The compartmentalized mass ratios that enable heavy perception are known from others (Euglena [Euglenophyceae], Paramecium [Paramecia]) and lead to typical ional shifts across the cell membrane and thus to the activation of cilia or flagella. In fact, such cells lose their ability to orient themselves in weightlessness (see Fig. 5). In all other cells that are sensitive to gravity, only hypotheses can be expressed about the mechanisms of gravity perception. This also includes the cytoskeleton (cell skeleton). Fixed “wiring” of the cytoskeleton with the extracellular matrix may play a role. The term Tensegrity (“Tensegrity”), which is made up of “tensional” (= tension) and “integrity” (= wholeness). The basic idea here is that mechanical signals (which also include non-weight-dependent signals) are combined with other environmental signals and converted into a biochemical response via force-dependent changes in the framework geometry and molecular mechanics. When explaining microgravity-related effects on individual cells, it must always be taken into account that the effects may be unspecific - due to the lack of convection currents under weightlessness and the resulting easier compartmentalization of the cell plasma and its organelles.
Such cell types without specific receptors, which showed sensitivity to microgravity, also include the osteoblasts involved in the construction of the skeleton T lymphocytes. Usually they are activated by mitogenic substances (mitogens). They leave their G0- Rest and come over the G1-, S- and G2-Phase to mitosis (color table). This proliferation is reduced by up to 80% under microgravity conditions. This dramatic effect of weightlessness on the immune system was first seen during the first Spacelab mission SL-1 (STS-9, 1983) - in which also interesting findings about the Protein crystallization were obtained under microgravity conditions - and confirmed several times in later missions. The weakening of the immune system is not due to a lack of formation of cytokines such as interleukins, interferons or the tumor necrosis factor. What is known, however, is that lymphocytes exposed to microgravity only absorb 3% of the amount of thymidine (thymidine) that is used by the 1 G-Ground controls will be included.

Skeletal muscle: Even within 5 days of being in microgravity, there is a 5 to 10% atrophy of weight-bearing muscles (muscles, striated muscles) and a decrease in muscle strength (muscle contraction). Simulation experiments using the bed rest technique and animal experiments produced similar results. Atrophy is accompanied by a decrease in protein synthesis and the activity of succinate dehydrogenase within muscle cells, but not by a change in protein breakdown. Reduced neuronal activation as a result of the reduced vestibular tone is also held responsible. At the same time, the contractile and enzymatic properties of the muscles shift from the slow, oxidative to the fast, glycolytic type. The readaptation after the end of the stress-free period lasts for weeks. Movement and strength exercises by spacemen (impeller, expander, etc.) cannot stop these morphological and physiological changes. Therefore, preventive measures are also sought in the pharmacological and physiological area.

Support apparatus: In astronauts and vertebrates (rats, dogs, newts), despite exercise training, gravity deprivation during space flights causes weight-bearing and non-weight-bearing bones osteoporosis off (see Fig. 6A). The extent depends on the length of the microgravity stay and reached 19.8% bone loss in Russian missions after 184 days. The same is achieved through simulation experiments, such as how long bed rest or weight relief is. In addition, there is a deterioration in the mechanical properties of the bones and, as a result, an increased risk of breakage (biomechanics). The demineralization (demineralization) of the bones goes back extremely slowly after the end of a space flight. The net loss of calcium (calcium) from the bone is caused by lower calcium absorption and increased calcium excretion. During development under microgravity, the mineralization of the bones is considerably reduced (see Fig. 6B). In general, osteoblasts change cell morphology and core architecture in weightlessness and reduce their growth. Correspondingly, the activity of biochemical processes that lead to bone formation decreases in them (reduction of type I and type II collagen and mRNA levels of osteocalcin as well as delayed maturation of the protein components).

Cardiovascular system: The cardiovascular system (cardiovascular system) is responsible for the distribution of oxygen, nutrients and breakdown substances in the body. In humans, it is adapted to the upright, bipedal posture (upright gait, bipede, walking). This causes a strong influence of gravity on its stability, which is confirmed by the studies on astronauts under microgravity. The first visible effect is the inflation of the head and neck, which is caused by a fluid displacement directed towards the head as a result of the lack of downward hydrostatic pressure (chubby cheek head). At the same time, the legs lose more than 10% of their volume within the first 24 hours due to a loss of volume in the deep leg veins.
The central venous pressure, measured as the blood enters the right atrium, is usually 4 to 7 mmHg; however, under microgravity it decreases to 2 mmHg (blood pressure). In contrast, the heart increases its stroke volume without changing its beat frequency (heart rate). These values ​​almost normalize again within 7 to 10 days, although a residual deviation remains. The reasons lie in the reduction of the plasma volume (blood volume), an autonomous control of the cardiovascular system modified by the adaptation processes, and the unchanged persistent changes in the distribution of body fluid due to the lack of a stabilizing factor of gravity (French experiments on board von Saljut-7, 1982 , as well as experiments during the German space mission D-2, 1993).
The regulation of blood pressure (blood pressure regulation) also changes under microgravity. A higher stroke volume raises the systolic blood pressure; arterial blood pressure decreases during the first few days. Baroreceptors (pressoreceptors) located in the carotid sinus intervene in the control of the circulation. Baroreceptors are located in two arteries in the body, one in the aortic arch and the other in the carotid arteries. They constantly measure the blood pressure and quickly intervene to regulate the heartbeat rate when the blood pressure changes. Your performance is weakened during a flight. A change in arterial pressure, which causes a beat frequency of a certain altitude on earth, triggers a lower response under weightlessness. Despite these changes, the circulation (blood circulation) is stabilized in itself during a space flight. Irregularities only occur in the case of special loads, such as exiting the spaceship into space (see Fig. 7).
Breathing is characterized by a 15% decrease in tidal volume (minute ventilation), but at the same time by a 15% increase in respiratory rate compared to earth conditions. These changes normalize within a week. The CO2-Concentration (carbon dioxide) at the end of exhalation (alveoli, respiratory mechanics, blood gases, expiration) is normal when the CO2-Level inside the spaceship is normal.
Space travelers experience a significant impairment after the end of their space flight. 70% of them have an inability to keep their blood pressure stable and to stand upright for longer than 5 to 10 minutes (orthostatic intolerance). The extent of this susceptibility is variable and depends on the duration of the flight and inter-individual sensitivities. In addition, there is a low level of resilience and ability to concentrate on attempts to be completed. After flights of up to a month, recovery takes place within a day; this time is longer for longer flights. These observations, which were made after the first space flights, led to the development of exercise programs to be carried out during space flight (bicycle; strength training). The success of such measures is not certain, which is why such exercises are not compulsory in American programs, in contrast to Russian programs. However, the fact that a lack of gravity causes such after-effects led to other technical developments. One way is the one developed by the German Space Agency (DLR) Vacuum chamber (Lower Body Negative Pressure Device, LBNP), which is intended to simulate gravity by artificially generating a negative pressure of 10 to 50 mmHg in the lower half of the body.

Vestibular system
, Sense of balance and spatial orientation: Space flights are suitable for investigating systems in which the sensory input depends on gravity, such as the sense of balance and position (kinesthesia, movement learning), movement (movement) and posture control and the perception of spatial constancy. Except for an increase in the number of synaptic contact points in the macular hair cells and a reduction in GABA-ergic projections of the cerebellum (cerebellum) on the vestibular nuclei (vestibular system), morphologically little changes in the vestibular sensory tract in rats exposed to microgravity. Nevertheless, the macular system can increase its sensitivity under microgravity (physiological activity of the 8th cranial nerve [Statoacusticus] in oyster fish [Opsanus dew;Fish fish], static vestibulo-ocular reflex in perch Oreochromis mossambicus; Perception of position in astronauts).
When moving in weightless space, the stabilizing influence of the above-below differentiation does not apply. In addition, because of the elimination of gravity, the trajectories of falling or flying movements of other objects change. The speed of an object that has been hit and moves parallel to the body axis of a person is constant under weightlessness; on the other hand, on earth this object falls with increasing speed. Changes in the central nervous precalculation when estimating these movements (prediction) are the result. In addition, the accuracy of targeted movements in orbit decreases if they are done at the same speed as on Earth. The analysis of the mechanisms of predictive and goal-oriented performance under weightlessness is necessary for the development of suitable training methods for working in orbit.

Development of sensory, neural and motor systems: Gravity influences early phases during embryonic development such as the development of the dorsoventral polarity (dorsoventral axis, dorsovental pattern formation) directly after fertilization. Under microgravity, however, the cell polarity develops normally. In individual cases, morphological changes remain during development, such as the thickening of the blastocoel roof (Xenopus laevis, African clawed frog) or accelerated cell proliferation (cerebral cortex of mice), without any influence on further development. In other cases, significant changes can occur, such as the formation of simpler patterns of motor end branches on fibers of weight-bearing muscles the longer the space flight lasted, or the increase in the number of statoconia (mechanical senses) in the statocyst (snail Biomphalaria glabrata; SwordtailXiphophorus helleri). However, the microgravity exposure times that were possible during the Spacelab era do not yet allow any statement about the persistence of these changes. Species-specific differences are known in the development of behavior based on gravity (see Fig. 8). During postembryonic development, microgravity causes an increase (fish Oreochromis) or acceptance (Xenopus laevis) the sensitivity of the static vestibulo-ocular reflex; In contrast, it remains unchanged in the compensatory head roll of the crickets, which is analogous to the vestibulo-ocular reflex (see Fig. 9). In contrast, the sensitivity of a neuron that is integrated into this behavior of the cricket decreases during severity withdrawal.
Since the research by T.N. Wiesel and D.H. Hubel is aware of the existence of critical phases in the postembryonic development of sensory, neuronal and motor systems. Critical periods can be identified when the morphology as well as the physiological and perceptual performance of the affected system are tested after the end of the withdrawal period. Exposure to microgravity during space flights lasting several days during the postembryonic development of animals offers the possibility of using the deprivation approach also in the case of gravity. Even if the number of developmental stages examined so far is too small for a final judgment, the existence of critical developmental periods for severity-dependent behaviors, such as the static, vestibulo-ocular reflex, can be assumed (Oreochromis, Xenopus) or the reflex turning from the supine to the prone position (rat) can be assumed.

Adaptation to microgravity and back adaptation to normal earth gravity: The kinetics of adaptation to the conditions of microgravity (μg adaptation) is organ-specific. The vestibular system adapts fastest to a microgravity-specific target value, but not at all the skeletal system and its calcium metabolism (calcium metabolism), so that it moves towards a pathological condition (see Fig. 10). The adaptation does not always follow a saturation curve, but can overshoot the typical microgravity setpoint. If this overdrive is too strong, clinical symptoms appear for a few days, e.g. Space sickness if the vestibular system is oversensitive. The physiological readjustment after a space flight to the gravity of the earth (1g readaptation) leads to complete normalization, with the exception of osteoporotic symptoms, even during postembryonic development. These periods of time are also organ- and species-specific and range from hours to several weeks.

Plants: Studies on plants concern 3 major topics: 1) cultivation under space conditions in order to have enough plants available for experiments and for the nutrition of astronauts (see Infobox 3), 2) development, growth and metabolism as well as 3) the orientation reactions with regard to the vector of gravity (gravitropism in multicellular cells, gravitaxis in single cells).
Development, growth and Metabolism: Bringing plants to development and fertility under weightlessness is hindered by 2 physical factors that cause inadequate nutrition, 1) the lack of convection, which causes the CO required for photosynthesis due to the lack of air currents2-Supply and transpiration is reduced, and 2) the tendency of the water not to be evenly distributed in the room under weightlessness, which causes an uneven distribution of oxygen and thus a deterioration in root ventilation. With successful seed formation under weightlessness, what at Arabidopsis (Schmalwand) and Brassica rapa (Kohl) succeeded, the plants of the F1Generation (filial generation) smaller than those that grew up from seeds obtained on earth. Other microgravity-related impairments included chromosomal abnormalities in the root cells, an inability to regenerate the cap cells of roots and a modified starch / sugar ratio. Ethylene, which accumulates in cultivation chambers under microgravity conditions, is also a hindrance to growth.
Gravitropism: The orientation of a multicellular plant with respect to the vector of gravity is called gravitropism (formerly: geotropism). The trunk grows negatively, the root positively gravitropically. A change in the position of the plant triggers a corrective growth response. Gravitropism is the most intensely studied physiological performance of plants. Earth-based experiments using clinostats led to hypotheses about the mechanisms underlying gravitropism, which, however, could only be proven by the experiments carried out under microgravity conditions. - With the gravitropic orientation several steps take place one after the other. The recognition of the reference direction given by gravity (perception phase) is followed by biochemical or structural changes (transduction and translocation phase), followed by the conversion into the reaction, which manifests itself as an uneven growth in length of cells arranged around the shoot or the root (reaction phase). In some plants such as the green alga Chara (Characeae) or the moss Ceratodon these processes take place in a single cell. In the case of higher plants, the experiments carried out under microgravity strengthen the hypothesis that the sedimentation of amyloplasts in the cells of the root tip is the decisive perception mechanism. This sedimentation does not occur under microgravity (see Fig. 11B). It is still unclear whether sedimenting amyloplasts have to interact with intracellular structures, such as the endoplasmic reticulum, or whether their change in position causes tensions in the cytoskeleton that are transferred to the plasma membrane. A starchless mutant of also has a sensitivity to gravity of the root tips Arabidopsis. It cannot therefore be ruled out that the sedimentation of other cell organelles also contributes to the perception of gravity, albeit with a different weighting. The rhizoid of the green alga Chara, a single cell that anchors the algae in the ground, uses the sedimentation of barium sulphate-containing vesicles (statolites). This sedimentation induces increased growth in the opposite cell wall and thus enables growth directed towards the center of the earth. The increase in growth is based on the fact that vesicles with cell wall material are prevented from incorporating by the statolites and are therefore increasingly discharged on the opposite cell flank. These statolites are held 10–30 μm from the tip of the rhizoid by a network of actin filaments, the tensile forces of which counteract the gravity. Under microgravity there is therefore a further increase in the distance between these statolites and the rhizoid tip (see Fig. 11A). While in the unicellular rhizoid the amyloplasts produce uneven cell growth through simple displacement, in multicellular plants the pressure of the amyloplasts is measured and leads to a polarization of the irritated cells. The primary stimulus perceptions initiate the transduction process, which induces a biochemical signal in the perception cells, which is directed to the site of the reaction (translocation). In higher plants, this signal is probably the plant hormone auxin, because auxin inhibitors (auxin antagonists) block a gravitropic reaction in both the shoot and the root tip. Finally, the reaction phase is characterized by an asymmetrical gradient of auxin with more auxin on the lower side. It is noteworthy that auxin has a growth-promoting effect in the shoot and thus causes the shoot to straighten up, while it inhibits growth in the root tip and thus enables the root tip to be bent downwards. This difference is due to a different sensitivity to auxin in the tissue - auxin concentrations, which still cause growth to increase in the less sensitive shoot, are already over-optimal in the more sensitive root and cause cell growth to be inhibited.

Protozoa: Protozoa (single cells) are eukaryote cells (eucytes) and are particularly suitable for the analysis of intracellular mechanisms in orientation reactions (orientation movements). The orientation reaction to the gravity vector is called Gravitaxis designated. The mechanisms implemented in them reveal two types, the statolith type (Müllerian vesicles in Loxodes) and the plasma density type (Paramecium, Euglena). The distinction arises from investigations according to which Loxodes shows normal gravitaxis in a medium of the same density as the plasma, while Paramecium and Euglena became disoriented. Under microgravity, however, both types lose the ability to gravitax (see Fig. 5). They also have in common the involvement of ion channels in the cell membrane, the low threshold of around 10% of the normal gravity of the earth for triggering the geotaxis and the lack of adaptation phenomena (e.g. due to increased sensitivity, cf. vestibular system of vertebrates) to the microgravity conditions.

Closed, ecological life support systems

Life in space requires the creation of Life support systems (see Infobox 3), which are closed off from the surrounding space. The spaceship itself is such a system, as are the chambers for keeping animals and plants. Because of the shortness and proximity to earth of all previous flights and their experiments, sufficient food was available at the beginning of the flight or, as with the Mir, was supplied by supply ships. Such ecologically open systems are inadequate for interplanetary flights and because of the economic efficiency of near-earth flights lasting more than 2.6 years. Food production must then take place in a completely self-contained, ecological life-support system in which there is a stable equilibrium between producer and consumer. Humans, animals, plants and microorganisms all contribute to this stability. The necessary energy comes from space. The Arizona Biosphere Project (Biosphere 2) however, showed how difficult it is to establish internal stability.
The only closed, ecological life support system that has so far been tested in space is this CEBAS module (Closed Equilibrated Biological Aquatic System), a self-stabilizing freshwater habitat with a volume of 8.6 l developed by the German space agency DLR. Fish (swordtail Xiphophorus helleri), Snails (Biomphalaria glabrata) and microorganisms as consumers as well as plants (horn leaf Ceratophyllum demersum [Hornblattgewächse]) and algae as producers form a community. Water quality, temperature, oxygen concentration and lighting conditions are electronically monitored. Each species fulfills two tasks: it is both an integral part of maintaining the ecological balance and a test organism for basic gravitational research. - The development of large-scale systems of this type must incorporate water, nutrient and gas recycling technologies. In addition, it is essential to keep suitable plants for generations because, in addition to food production, plants ensure water and air purification. The cultivation area that is required to supply an astronaut is about 10 m2 estimated. The methods of growing plants in space are not yet under control, as seeds are not always formed. There are good keeping options for algae; However, their use as food fails due to suitable methods of preparation. In addition to technical problems, contamination by chemical breakdown products (ethylene) is one of the main causes of the low yield. Until 1997, the American space agency NASA carried out projects to develop cultivation methods and life support systems in the CELSS program (Closed Ecological Life Support System Program) using differently sized cultivation chambers (Minitron Plant Life Support System). In 1997, CELSS and the non-biological life support systems development programs became the ALS program (Advanced Life Support Program) to overcome the technical problems.

Future of Gravitational Research - Conquering Space

Upon completion of the successful Apollo Moon and Spacelab programs, the use of the International space station ISS another milestone in the conquest of space. Biological and medical research on the ISS are preparing 2 major projects: 1) the Mars flight and thus the first long-term flight into space and 2) the Space tourism, which aroused increasing interest with the change to the 3rd millennium in the USA, Japan and Europe. It is seen as having multi-billion dollar economic potential, while the flight to Mars is seen as a public affair with an annual cost of $ 2.6 billion. Both large-scale projects require basic research on the effects of short and long-term exposure on organisms. From a biological point of view, a short stay in space in the area of ​​tourism can be more problematic than a long one during an interplanetary flight, since slow adaptation processes are activated but not completed when the tourist returns to Earth. The result is lasting physiological impairment.
A flight to Mars and thus its "conquest" is not very understandable from a biological point of view, since a free life on its surface is impossible due to the lack of an atmosphere. To what extent it can be used economically cannot be said today. There are 2 reasons to dare to venture into space: 1) to answer the question about life forms in space in general (cf. fossil bacteria that are said to have been detected in the Mars meteorite ALH84001 - but this is not undisputed; extraterrestrial life ), and 2) the challenge for humans to cope with this task and conquer Mars and other planets. After successfully completing several flights to Mars, its further conquest will start from a small, manned base station, an artificial ecosystem. Studies on the question of how the only 0.4 G strong gravity affects the organism can be carried out during parabolic flights and using the large centrifuge installed on the ISS.In addition, the analysis of social structures under extreme conditions is becoming increasingly important when preparing for a flight to Mars. In addition to technical problems, the fact that people live together for several months in a confined space (density stress) is seen as a major hazard, and suitable methods must be developed in order to control it in advance of long-term flights. Experiences from expeditions in extreme areas (polar zone, high mountains, deep sea; extreme biotopes) flow into this research. Biological research on the fundamental feasibility of life under space conditions also includes the influences of vacuum, solar UV and general space radiation as well as the effect of extreme temperature jumps. Support is provided by research on model life forms that resist such extreme conditions (cosmic radiation) such as bacteria from hot springs (hydrothermal springs) or anaerobic microbe systems from water-bearing basalt layers, which obtain their life-sustaining energy from geochemical processes. Such organisms can live even if life on the earth's surface were extinguished; they are therefore model candidates for a life on Mars. Geobiophysics.

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