The gravity shaped the animal and vegetable worlds during millions of years. If gravity did not exist we would not need such a complex cardiovascular system, in particular with protection mechanisms allowing to maintain a normal circulation during upright posture. The bone tissue would be useless, just as muscular tissue, since the role of these systems is to fight against gravity. Same remark for the nervous system.

Gravity is essential every day to preserve these functions. This role is already obvious after fifteen days of microgravity when appearing deteriorations of the cardiovascular system, osseous, muscular and nervous system, to name only the main ones, can be seen. Even at the cellular level, microgravity involves modifications of the genes expression and deteriorations of the answer and morphology of the cells.

We spend most of our life to fight against gravity. Thus, for a better knowledge of a system, the scientific approach consists in studying the consequences of its exclusion. This amply justifies the human or animal studies carried out during the space flights. The same applies to vegetable kingdom (gravitropism).

The objectives of in progress and future projects are to look further into the knowledge of the dependant mechanisms of gravity which rule the human development, but also to study the reversibility of the harmful effects of gravity on the body and the human behavior.

The results will enable to evaluate the possibilities of seeing a sedentary man on the Moon and Mars, able to work and to reproduce.

Gravitational vegetable biology

During the development of the organs and constituting tissues, the plants fixed in an environment should be able to integrate and specifically answer to the external signals, whether they are of biotic or abiotic origin.

Among the perceived stimuli, the "gravity" factor is a force impossible to circumvent in the terrestrial environment. It plays an essential role in the orientation of the growth of the plants (gravitropism), offering the unique advantage to the roots to penetrate into the ground to draw the biogenic salts and to the air organs to grow vertically to photosynthesize their sugars. For trees, the acquisition of the stand up posture is due to the formation of the wood, which is characterized by a differentiation process (lignification).

The study of the perception of gravity and its consequences has a fundamental importance in the comprehension of the molecular and cellular mechanisms of the growth and the development of the plants (gravitropism and formation of wood). The thorough study of these mechanisms requires a comparison between two distinct situations: application or not application of the gravity factor. On ground, if the application of the gravity factor is natural (1 G) and can be even amplified (experiments in hypergravity in centrifuge), its prolonged suppression is impossible. Only certain effects of the microgravity can be simulated by the use of clinostats. The space experiments are thus impossible to circumvent in order to place the plants under microgravity conditions.

Physiology and biology of the development

All the complementary aspects of the primary sensor to the behavioral expression, which take part in the integration of gravitating information during the development were approached within the framework of a multi-field approach under the aegis of the CNES. Among the obtained results, the most outstanding indicate that the early exposure to the hypergravity induces a modification of the gravity sensor, indicating for the first time that a primary receiver can be modified by the information that it receives.

It was shown on Amphibians (Xenopus laevis and Cynops pyrrhogaster tadpoles) that the microgravity modifies significantly the morphology of the sensor itself. In addition, it was shown on the rat that the exposure to the hypergravity during the development modifies the physiological properties of the vestibular sensory cells. The primary stimulus is thus able to influence, directly on the level of the peripheral sensory receiver, the development of the molecular mechanisms which will be necessary to the detection of sensory information.

The providing of space tools (access to the ISS, and installation of the European modules Columbus and Habitat mouse) to the scientific community enables the study of the reproduction, gestation and development in flight, as well as aspects associated with the return in normal gravity. The exposures to the microgravity on several generations can be studied and, in particular, their possible effects on the drift of the genic expression. The existence of critical periods in the development of the functions associated with perception of gravity can be definitively checked.

Bone tissues and space

The conditions of the space missions involve an osseous loss at carrying territories level, in the human as well as in the rodent. This osseous loss is fast in the trabecular envelope, metabolically very active, then it also occurs in the cortical envelope.

Recovery on ground is not effective when the follow-up on Earth is equivalent to the duration of the space mission. In the animal, it takes at least twice more time on ground than in flight to see a recovery of the trabecular osseous mass. This time seems even longer for the cortical envelope. In the human, after six months of reambulation on ground, time equivalent to the duration of the mission (Mir station), one is in statu quo situation.

A tomographic device used on ground before/after flight is able to analyze this microarchitecture on the trabecular and cortical levels in carrying and less carrying bones. The current countermeasures use programs of physical exercise which showed their inefficiency. Medicamentous strategies and food supplementations will be coupled in the future with the physical exercise programs.

The muscle

During space flights or in simulated microgravity condition the skeletal muscles of the organism undergo a particularly severe atrophy for the slow type postural muscles (such as the soleus in the lower limbs). This atrophy results in losses of associated mass and force, on the one hand, with a disappearance of a fraction of muscular proteins involved in structural cohesion, elasticity, the contractile process or the muscular metabolism and, on the other hand, with a transformation of part of the remaining proteins.

The transformation of various proteins of the muscles highlights a genic adaptation of the expression of these proteins. It is thus essential to go towards the comprehension of the origin and the mechanisms underlying these transformations, using genomic and of the proteomic. Works have begun on a model of rat and must be developed for:

characterizing known or unknown gene populations,
determining which type of modifications undergo certain proteins that can be, by their functional repercussions, the early markers of the atrophy and to open the possibility of countermeasures, in particular pharmacological ones.

Nervous system and perception of motricity

During daily activities the nervous system must unceasingly manage a multitude of effects induced by gravity on the human body. First of all, the primary effects exerted on the body. Gravity dictates the laws of the movement of our body and our limbs. For example, in order to avoid the falls, the hold of the upright station requires a constant intervention of the neuromuscular system and the realization of a simple movement of the arm requires a different muscular control according to whether the movement is exerted upwards or downwards. The dynamic interactions between our body and the surrounding physical world are clearly configured according to constraints' imposed by gravity. Moreover, the fluids circulation within the body is strongly influenced by the constant pressure exerted by gravity, thus the orthostatic regulation corresponds to one of the major functions of the autonomous nervous system.

The cerebral functions have also developed during the evolution by taking into account the gravity constraints imposed on cognition. For example, the world in which we live is first of all two-dimensional (with several layers) in particular for the creatures on Earth such as the human being. The neuronal processes which enable us to navigate in this world can be thus specialized for the representation of 2D space configurations. On Earth, we also expect to see the objects laid out in a particular way: the objects being on a table will usually be laid out in stable vertical or horizontal position; the objects accelerate from top to bottom in free fall; just as we meet our fellows in upright position. The success which rises from these predictive performances can be crucial to survival, such as during the escape from a predator or the avoidance of a falling stone. The cognitive, motor and autonomous functions depend on the capacity of the nervous system to detect and anticipate the effects of gravity on the body.

Gravity can be perceived via a variety of channels. The inner ear otoliths measure linear accelerations due to gravity. We can feel the action of thorough gravity on our members and our bowels. Sat on a chair the gravity forces are perceived through the contact areas of our body with the seat and are transmitted in the form of quite specific cutaneous information. We can even "see" to some extent gravity simply by looking at the horizon, the vertical orientation of the buildings and the trees and the direction of the falling rain. On Earth, it is very difficult to separate the influence from the various sensory methods on the neuro-regulating functions, because the handling of each one can induce parallel effects on the others. In microgravity, we can to some extent remove the anchoring reference point provided by the gravitational vector and examine the effects of the remaining sensory counterparts independently.

A research program using the space flights and the parabolic flights is thus being conducted to study the interaction between gravity and the nervous system on the human being. The ground devices (centrifuge, bedrest, dry immersion) supplement the means available for the scientists in this domain. It is necessary to note the importance of these experiments for the future projects.

A better understanding of the adaptive mechanisms of the human nervous system to the new environment without gravity, as well as its readjustment with the usual environment (Earth) or particular one (Mars, the Moon) should help us to make a success of the future missions of exploration by man beyond the Earth.

Cardiac and vascular peripheral regulation

The adaptation of the cardiovascular system after a few days under microgravity results in a new hemodynamic balance which remains stable in flight (7 - 180 J). The blood pressure and the rate of heartbeat at rest are not modified to a significant degree, as well as the cerebral arterial hemodynamic. The monitoring of the ECG highlights a modification of the sympathetic/parasympathetic balance. Plasmatic and cardiac cavities volume decrease (8 - 12%). The consequences of these two last observations are observable only during the return in normal gravity.

At the peripheral level, vascular resistances decrease in the lower limbs. These important modifications do not limit the activity of the astronaut, do not present a danger to their health and are reversible quickly after the flight. It is not necessarily the same for the myocardic atrophy (-8 to -15%), which constitutes a handicap with the effort during the return on Earth. In the same way, the distension of the jugular vein reveals a stagnation of blood at the cerebral level which, in the long term, could induce tissue and functional modifications. A whole of physical countermeasures, which make it possible to reduce these two last modifications induced by the microgravity (leg cuff, exercise, lower body negative pressure: LBNP), have been validated and their physiological effects quantified.

Several parameters enable to quantify the cardiovascular deconditioning induced by the microgravity and responsible for the post flight orthostatic intolerance: blood pressure, heart rate, cerebral flow, femoral flows. After a space flight or a bedrest, 60 % of the subjects present an orthostatic intolerance, which results in the fall of the cerebral perfusion and the arterial pressure.

Space radiology

The radiative environment of space is very particular and requires well targeted experiments in radiobiology for a better understanding of the radiological risks incurred by the man in space. These last years the research supported by the CNES was initially focused on an improvement of physical and biological dosimetry. By using tissue equivalent proportional counters as Nausicaa (CNES-IPSN collaboration) and conventional biological dosimetry, i.e. the measurement of the chromosomal aberrations, a good similarity between these two methods was highlighted. This fundamental work underlines without any doubt the genotoxicity of the radiations met in space.

Nutrition and microgravity

The cost of the physical activity in microgravity is higher, probably because of a reduction of the mechanical efficiency. The direct nutritional consequence is that the energy overcost of the protocols of prescribed exercises such as countermeasures against the deconditioning syndromes can be significant. For example, 2.5 hours of exercises corresponding to 1.7 MJ/h represent at least 49% of the daily needs. These data show the importance to measure the energy cost of the various physical exercises applied in flight.

The priority axes of research in space nutrition can be set according to two modes, one purely operational, the other one fundamental:

The operational aspect consists in determining the requirements in macro and micronutriments for the astronauts during long missions on board the ISS but also for exploratory missions to the Moon and Mars.
The fundamental aspect aims at characterizing the role of the energy deficit systematically observed in flight in the different deconditioning: muscular, osseous and cardiovascular in microgravity.

From the experimental point of view, it is advisable to work in partnership with the food industries to develop food packed with expiry dates higher than three years for long duration missions, thus the necessity to work out new conservation techniques. NASA already developed a system for one year missions. The challenge is to ensure a significant variety in the menus, to maintain at the same time the morale and the performances of the astronauts.

All the studies to be planned in flight must be tested on ground during simulations (bedrest) or in similar environments such as containment in polar base (Concordia). The advantage of such studies on ground is to allow a mechanistic approach of the problems which is not possible in flights.

Containment and psychological constraints

The factors of risk on the psychological level for a flight to Mars in a 1.000 days scenario with a mixed and multinational crew from 4 to 6 people are related to the stress factors of the situation. Insulation represents "physical and social separation from the rest of the world". The participants live out of their daily framework and are separated from their families. There is a disappearance of the reference marks and usual sources of gratification, only the professional gratifications persist.

This involves a reduction of the exchanges and social roles. The absence of emotional and sexual relations, can cause, in addition to frustrations, a fear of being abandonned, an anguish in front of the readjustment afterwards.

The reactions of the individuals to these stress factors are of several orders:

physical disorders as sleep disorders, tiredness, cephalgias, lumbagos, digestive disorders, and various pains;
subjective disorders, with a deterioration of the perception of time (especially compression), hallucinations, an impression of unreality;
mood disorders, homesickness, highlighted by time spent to look at the Earth through the windows and by the need for communication with the outside world, an increase in the anxiety and light depressive signs, sometimes an alternation of apathy and euphoric excitation, irritability;
performance disorders, with sometimes aberrant behaviors.

However, the problems generally appear more frequently in the interpersonal relations field. At the collective level, tensions, conflicts, hostility between the members of a crew are often evoked. These difficulties often come from the heterogeneity of the crews (different cultures, nationalities, mother tongues).