Search

The exploration and colonization of the outer space represents a foreseeable future for the Humanity. This endeavour involves deepening our knowledge about the formation and evolution of the solar system, of other planetary systems, emergence of life (and its prospects once it exists), the interaction between Earth and Space (particularly with its Sun) and the impact of space conditions (radiation, gravity, etc.) on Earth-borne organisms. Materialization of this exploration and colonization currently drives technological developments in several fronts as optics, electronics and sensors just to mention a few. Other aspects as well law & ethics, psychology, biology, etc cannot be discarded.

The pattern of nucleolar silver staining was determined in the interphase and mitosis of plant cells using two cytological techniques, the selectivity of which was tested densitometrically. For complete accuracy, silver grains were removed and the sections were then observed under uranyl and lead staining. During interphase, the highest density of silver grains occurred in the portions of the dense fibrillar component nearest to the fibrillar centres; the latter were also stained except for the condensed chromatin inclusions typical of the heterogeneous type. During mitosis, not only the nucleolar organiser region (NOR) was stained, but also the prenucleolar material in anaphase and telophase. Two different proteins or sets of proteins are suggested to be responsible for this staining pattern; one of them is associated with NOR chromatin (which is not contained in the mitotic prenucleolar material) and the other with early ribosomal precursors (which have never been found in fibrillar centres). From these two proteins, the “Ag-NOR” reveals either transcriptionally active or potentially active chromatin, characterized by an uncoiled, extended structure, regardless of the state of transcription. This is the structural feature detected by Ag-NOR staining., This work was supported by CSIC Grant No. 41024-10.

18 p.-9 fig., Premise: Light and gravity are fundamental cues for plant development. Our understanding of the effects of light stimuli on plants in space, without gravity, is key to providing conditions for plants to acclimate to the environment. Here we tested the hypothesis that the alterations caused by the absence of gravity in root meristematic cells can be counteracted by light., Methods: Seedlings of wild‐type Arabidopsis thaliana and two mutants of the essential nucleolar protein nucleolin (nuc1, nuc2) were grown in simulated microgravity,either under a white light photoperiod or under continuous darkness. Key variables of cell proliferation (cell cycle regulation), cell growth (ribosome biogenesis),and auxin transport were measured in the root meristem using in situ cellular markers and transcriptomic methods and compared with those of a 1 g control., Results: The incorporation of a photoperiod regime was sufficient to attenuate or suppress the effects caused by gravitational stress at the cellular level in the root meristem. In all cases, values for variables recorded from samples receiving light stimuli in simulated microgravity were closer to values from the controls than values from samples grown in darkness. Differential sensitivities were obtained for the two nucleolin mutants., Conclusions: Light signals may totally or partially replace gravity signals, significantly improving plant growth and development in microgravity. Despite that, molecular alterations are still compatible with the expected acclimation mechanisms, which need to be better understood. The differential sensitivity of nuc1 and nuc2 mutants to gravitational stress points to new strategies to produce more resilient plants to travel with humans in new extraterrestrial endeavors., This work was funded by the Agencia Estatal de Investigación of the Spanish Ministry of Science an Innovation, Grants#ESP2015‐64323‐R and #RTI2018‐099309‐B‐I00 (co‐funded by EU‐ERDF) to F.J.M., and Bonus Recherche from the UPVD to J.S.V. The use of the facilities of microgravity simulation was provided by the ESA‐CORA‐Ground Based Facilities Program, contract Ref. #4000105761 to F.J.M. and R.H. A.M. was recipient of a contract of the Program for Young Researchers Training of the Agencia Estatal de Investigación of the Spanish Ministry of Science an Innovation Ref. #BES‐2013‐063933.

24 p.-9 fig., Gravitropism and phototropism play a primary role in orienting root growth. Tropistic responses of roots mediated by gravity and light have been extensively investigated, and a complex mutual interaction occurs between these two tropisms. To date, most studies have been conducted in 1 g, microgravity, or simulated microgravity, whereas no studies investigated root phototropism in hypergravity. Therefore, we studied the effects of several gravity treatments with those of different light wavelengths on root growth orientation. Here, we report growth and curvature of Brassica oleracea roots under different g levels, from simulated microgravity up to 20 g, and unilateral illumination with different spectral treatments provided by light emitting diodes. Microgravity was simulated with a random positioning machine whereas hypergravity conditions were obtained using the Large Diameter Centrifuge at the laboratories of the European Space Agency in the Netherlands. Four light treatments (white light, blue light, red light, and dark) were used in this study. Overall, roots of seedlings grown in the dark were longer than those developed under unilateral light treatments, regardless of the gravity level. Unilateral blue light or white light stimulated a negative phototropism of roots under all g levels, and root curvature was not affected by either hypergravity or simulated microgravity compared to 1 g. Results also confirmed previous findings on the effect of light intensity on root curvature and highlighted the relevance of blue-light photon flux density in root phototropism. Roots illuminated with red light showed a weak curvature in simulated microgravity but not in hypergravity. Moreover, root curvature under red light was similar to dark-grown roots in all g levels, suggesting a possible involvement of surface-dependent phenomena in root skewing under either red light or dark conditions. Further studies can confirm phototropic responses of B. oleracea in the weightless environment of orbiting spacecraft. Nevertheless, according to our findings, directional lighting represents an effective stimulus to guide root growth in a wide range of gravity conditions., This work was supported by the ESA-HRE CORA contract #4000127705/19/NL/PG/pt. Financial support was also provided by ESA-TEC contract #4000116223/16/NL/KML/hh to Jack J.W.A. van Loon, by Spanish Agencia Estatal de Investigación (Grant RTI2018-099309-B-I00, co-funded by EU- ERDF) to F. Javier Medina, and by NASA (Grant 80NSSC17K0546) to John Z. Kiss.

31 p.-5 fig.-2 tab.-2 fig. supl., Plant cell proliferation is affected by microgravity during spaceflight, but involved molecular mechanisms, key for space agronomy goals, remain unclear. To investigate transcriptomic changes in cell cycle phases caused by simulated microgravity, an Arabidopsis immobilized synchronous suspension culture was incubated in a Random Positioning Machine. After simulation, a transcriptomic analysis was performed with two subpopulations of cells (G2/M and G1 phases enriched) and an asynchronous culture sample. Differential expression was found at cell proliferation, energy/redox and stress responses, plus unknown biological processes gene ontology groups. Overall expression inhibition was a common response to simulated microgravity, but differences peak at the G2/M phase and stress response components change dramatically from G2/M to the G1 subpopulation suggesting a differential adaptation response to simulated microgravity through the cell cycle. Cell cycle adaptation using both known stress mechanisms and unknown function genes may cope with reduced gravity as an evolutionary novel environment., This work was supported by the Spanish “Plan Estatal de Investigación Científica y Técnica y de Innovación” of the Ministry of Economy, Industry and Competitiveness [to F.J.M., Grant numbers AYA2012-33982 and ESP2015-64323-R, co-funded by ERDF], by a pre-doctoral fellowship from CSIC, Spain [to K.Y.K., JAE-PreDoc Program, Ref. JAEPre_2010_01894], the ESA-ELIPS Program [to R.H., ESA GIA Project, contract number 4000105761], and ESA support [to J.v.L., via contract TEC-MMG/2012/263 and 4000107455/12/NL/PA].

38 p.-5 fig.-2 tab.-8 fig. supl., PREMISE:Plants synthesize information from multiple environmental stimuli when determining their direction of growth. Gravity, being ubiquitous on Earth, plays a major role in determining the direction of growth and overall architecture of the plant. Here, we utilized the microgravity environment on board the International Space Station (ISS) to identify genes involved influencing growth and development of phototropically stimulated seedlings of Arabidopsis thaliana., METHODS:Seedlings were grown on the ISS, and RNA was extracted from 7 samples (pools of 10-15 plants) grown in microgravity (μg) or Earth gravity conditions (1-g). Transcriptomic analyses via RNA sequencing (RNA-seq) of differential gene expression was performed using the HISAT2-Stringtie-DESeq2 RNASeq pipeline. Differentially expressed genes were further characterized by using Pathway Analysis and enrichment for Gene Ontology classifications., RESULTS:For 296 genes that were found significantly differentially expressed between plants in microgravity compared to 1-g controls, Pathway Analysis identified eight molecular pathways that were significantly affected by reduced gravity conditions. Specifically, light-associated pathways (e.g., photosynthesis-antenna proteins, photosynthesis, porphyrin, and chlorophyll metabolism) were significantly downregulated in microgravity., CONCLUSIONS:Gene expression in A. thaliana seedlings grown in microgravity was significantly altered compared to that of the 1-g control. Understanding how plants grow in conditions of microgravity not only aids in our understanding of how plants grow and respond to the environment but will also help to efficiently grow plants during long-range space missions., Funding was provided by grants NNX12A065G and 80NSSC17K0546 from NASA (PI = J. Z. Kiss).

20 p.-6 fig., Clinorotation was the first method designed to simulate microgravity on ground and it remains the most common and accessible simulation procedure. However, different experimental settings, namely angular velocity, sample orientation, and distance to the rotation center produce different responses in seedlings. Here, we compare A. thaliana root responses to the two most commonly used velocities, as examples of slow and fast clinorotation, and to vertical and horizontal clinorotation. We investigate their impact on the three stages of gravitropism: statolith sedimentation, asymmetrical auxin distribution, and differential elongation. We also investigate the statocyte ultrastructure by electron microscopy. Horizontal slow clinorotation induces changes in the statocyte ultrastructure related to a stress response and internalization of the PIN-FORMED 2 (PIN2) auxin transporter in the lower endodermis, probably due to enhanced mechano-stimulation. Additionally, fast clinorotation, as predicted, is only suitable within a very limited radius from the clinorotation center and triggers directional root growth according to the direction of the centrifugal force. Our study provides a full morphological picture of the stages of graviresponse in the root tip, and it is a valuable contribution to the field of microgravity simulation by clarifying the limitations of 2D-clinostats and proposing a proper use., This research was funded by Agencia Estatal de Investigación of the Spanish Ministry of Science and Innovation, Grants #ESP2015-64323-R and #RTI2018-099309-B-I00 (co-funded by EU-ERDF), awarded to FJM. The clinostat was obtained by a Grant of the United Nations Zero-Gravity Instrument Project (ZGIP), second and third cycle calls 2014/2018 (UNOOSA), awarded to RH and FJM. AV is a recipient of a contract of the Spanish National Program for Young Researchers Training (BES-2016-077976).

25 p.-4 fig., Any space exploration initiative, such as the human presence in the Moon and Mars, must incorporate plants for life support. To enable space plant culture we need to understand how plants respond to extraterrestrial conditions, adapt to them, and compensate their deleterious effects at multiple levels. Gravity is a major difference between the terrestrial and the extraterrestrial environment. Gravitropism is the process of establishing a growth direction for plant organs according to the gravity vector. Gravity signals are sensed at specialized tissues by the motion of amyloplasts called statoliths and transduced to produce a cellular polarization capable of influencing the transport of auxin. Gravity alterations eventually result in changes in the lateral balance of auxin in the root, producing deviations of the growth direction. Under microgravity, auxin changes affect the root meristem causing increased cell proliferation and decreased cell growth. The nucleolus, the nuclear site of production of ribosomes, is a marker of this unbalance, which could alter plant development. At the molecular level, microgravity induces a reprogramming of gene expression that mostly affects plant defense systems against abiotic stresses, indicating that these categories of genes are involved in the adaptation to extraterrestrial habitats. Nevertheless, no specific genes for plant response to gravitational stress have been identified. Despite this stress, plants survive, developing until the adult stage and reproducing under microgravity conditions. A major research challenge is to identify environmental factors, such as light, which could interact, modulate, or balance the impact of gravity, contributing to the tolerance and survival of plants under spaceflight conditions. Understanding the crosstalk between light and gravity sensing will contribute to the success of the next generation agriculture in human settlements outside the Earth., Work performed in the authors’ laboratory was supported by different grants of the Spanish National Agency for Research of theSpanish Government, e.g. Grants # ESP2015-64323-R and #RTI2018-099309-B-I00 (co-funded by EU-ERDF).

6 p., The authors belong to the ESA Space Omics Topical Team funded by the ESA grant/contract 4000131202/20/NL/PG/pt “Space Omics: Towards an integrated ESA/NASA –omics database for spaceflight and ground facilities experiments” to RH.

Summary Understanding plant adaptive responses to the space environment is a requisite for enabling space farming. Spaceflight produces deleterious effects on plant cells, particularly affecting ribosome biogenesis, a complex stress-sensitive process coordinated with cell division and differentiation, known to be activated by red light. Here, in a series of ground studies, we have used mutants from the two Arabidopsis nucleolin genes (NUC1 and NUC2, nucleolar regulators of ribosome biogenesis) to better understand their role in adaptive response mechanisms to stress on Earth. Thus, we show that nucleolin stress-related gene NUC2 can compensate for the environmental stress provided by darkness in nuc1 plants, whereas nuc2 plants are not able to provide a complete response to red light. These ground control findings, as part of the ESA/NASA Seedling Growth spaceflight experiments, will determine the basis for the identification of genetic backgrounds enabling an adaptive advantage for plants in future space experiments., Graphical Abstract, Highlights • Ribosome synthesis is a target of spaceflight stressor effects on plant development • Nucleolin mutants promote a differential response to light/darkness stress • Red light and NUC2 may counteract the spaceflight alterations in gene expression • Ground controls are important for the interpretation of spaceflight -omics experiments, Plant Biology; Omics; Space Sciences

36 p.-11 fig.-1 tab., Red light is able to compensate for deleterious effects of microgravity on root cell growth and proliferation. Partial gravity combined with red light produces differential signals during the early plant development. Light and gravity are environmental cues used by plants throughout evolution to guide their development. We have investigated the cross-talk between phototropism and gravitropism under altered gravity in space. The focus was on the effects on the meristematic balance between cell growth and proliferation, which is disrupted under microgravity in the dark. In our spaceflight experiments, seedlings of three Arabidopsis thaliana genotypes, namely the wild type and mutants of phytochrome A and B, were grown for 6 days, including red-light photoactivation for the last 2 days. Apart from the microgravity and the 1g on-board control conditions, fractional gravity (nominally 0.1g, 0.3g, and 0.5g) was created with on-board centrifuges. In addition, a simulated microgravity (random positioning machine, RPM) experiment was performed on ground, including both dark-grown and photostimulated samples. Photoactivated samples in spaceflight and RPM experiments showed an increase in the root length consistent with phototropic response to red light, but, as gravity increased, a gradual decrease in this response was observed. Uncoupling of cell growth and proliferation was detected under microgravity in darkness by transcriptomic and microscopic methods, but red-light photoactivation produced a significant reversion. In contrast, the combination of red light and partial gravity produced small but consistent variations in the molecular markers of cell growth and proliferation, suggesting an antagonistic effect between light and gravity signals at the early plant development. Understanding these parameters of plant growth and development in microgravity will be important as bioregenerative life support systems for the colonization of the Moon and Mars., Funding for this study was provided mainly by the Spanish National Plan for Research and Development (MINECO-ERDF co-funding) Grant ESP2015-64323-R to FJM. The access to ISS and RPM facilities was granted by ESA-ELIPS ILSRA-2009-0932 to FJM and GBF Program GIA Project (contract# 4000105761) to RH. This research was supported also by grants (NNX12A0656 and 80NSSC17K0546) from NASA to JZK and the French Space Agency—CNES to ECD and VPL. MAV and AM were recipients of grants of the Spanish National Program for Young Researchers Training (Refs. BES-2010-035741 and BES-2013-063933, respectively).

33 p.-7 fig., Environmental gravity modulates plant growth and development, and these processes are influenced by the balance between cell proliferation and differentiation in meristems. Meristematic cells are characterized by the coordination between cell proliferation and cell growth, that is, by the accurate regulation of cell cycle progression and the optimal production of biomass for the viability of daughter cells after division. Thus, cell growth is correlated with the rate of ribosome biogenesis and protein synthesis. We investigated the effects of simulated microgravity on cellular functions of the root meristem in a sequential study. Seedlings were grown in a clinostat, a device producing simulated microgravity, for periods between 3 and 10days. In a complementary study, seedlings were grown in a Random Positioning Machine (RPM) and sampled sequentially after similar periods of growth. Under these conditions, the cell proliferation rate and the regulation of cell cycle progression showed significant alterations, accompanied by a reduction of cell growth. However, the overall size of the root meristem did not change. Analysis of cell cycle phases by flow cytometry showed changes in their proportion and duration, and the expression of the cyclin B1 gene, a marker of entry in mitosis, was decreased, indicating altered cell cycle regulation. With respect to cell growth, the rate of ribosome biogenesis was reduced under simulated microgravity, as shown by morphological and morphometric nucleolar changes and variations in the levels of the nucleolar protein nucleolin. Furthermore, in a nucleolin mutant characterized by disorganized nucleolar structure, the microgravity treatment intensified disorganization. These results show that, regardless of the simulated microgravity device used, a great disruption of meristematic competence was the first response to the environmental alteration detected at early developmental stages. However, longer periods of exposure to simulated microgravity do not produce an intensification of the cellular damages or a detectable developmental alteration in seedlings analyzed at further stages of their growth. This suggests that the secondary response to the gravity alteration is a process of adaptation, whose mechanism is still unknown, which eventually results in viable adult plants., This work was supported by the Spanish “Plan Estatal de Investigación Científica y Ténica y de Innovación”of the Ministry of Economy and Competitiveness [Grant numbers AYA2012-33982 and ESP2015-64323-R, co-funded by ERDF, and pre-doctoral fellowships to [A.I.M.] and [I.M.](Program FPI)], the French “Centre National d’Etudes Spatiales” (CNES) (G. GauquelinKoch) and the Dutch Space Research Organization (NWO-ALW-SRON) [Grant number MG057].

10 p.-3 fig., Nucleolin is the major nucleolar protein of animal, plant and yeast proliferating cells. It is enriched in the most soluble nuclear or nucleolar protein extract, containing ribonucleoproteins, from which it has been purified. It has a tripartite structure in which each domain accounts for different functions. Despite its multifunctionality, the best characterized role of nucleolin is in the primary cleavage of pre-rRNA, an early step of ribosome biogenesis. In the nucleolus of proliferating cells, nucleolin is mostly located in the dense fibrillar component, following a vectorial pattern, from the periphery of fibrillar centers outwards. This pattern is lost in quiescent cells in which nucleolin is present in low levels. Nucleolin is the most phosphorylated protein of the soluble nuclear extract. It is phosphorylated by casein kinase II and CDKA, and phosphorylation is closely associated with the role of nucleolin in proliferating cells. During mitosis, nucleolin is transported from the mother to the daughter cell nucleolus in the form of processing particles, together with pre-rRNA precursors and other nucleolar proteins. It forms part of prenucleolar bodies and plays a role in nucleologenesis. Recent studies on the nucleolin function, carried out on samples with inactivated nucleolin genes (siRNA downregulated or mutants) have evidenced that nucleolin is absolutely essential for cell proliferation, for the organization of the nucleolus and for transcription and processing of pre-rRNA. In plants, nucleolin controls the auxin responsiveness, thus being involved in the regulation of plant development., Work performed in the authors’ laboratory was supported by different Grants of the Spanish “PlanNacional de Investigación Científica y Tecnológica”,and particularly by the Grant No.ESP2006-13600-C02-02.

11 p.-8 fig.-3 tab., Space experiments provide a unique opportunity to advance our knowledge of how plants respond to the space environment, and specifically to the absence of gravity. The European Modular Cultivation System (EMCS) has been designed as a dedicated facility to improve and standardise plant growth in the International Space Station (ISS). The EMCS is equipped with two centrifuges to perform experiments in microgravity and with variable gravity levels up to 2.0 g. Seven experiments have been performed since the EMCS was operational on the ISS. The objectives of these experiments aimed to elucidate phototropic responses (experiments TROPI-1 and -2), root gravitropic sensing (GRAVI-1), circumnutation (MULTIGEN-1), cell wall dynamics and gravity resistance (Cell wall/Resist wall), proteomic identification of signalling players (GENARA-A) and mechanism of InsP3 signalling (Plant signalling). The role of light in cell proliferation and plant development in the absence of gravity is being analysed in an on-going experiment (Seedling growth). Based on the lessons learned from the acquired experience, three preselected ISS experiments have been merged and implemented as a single project (Plant development) to study early phases of seedling development. A Topical Team initiated by European Space Agency (ESA), involving experienced scientists on Arabidopsis space research experiments, aims at establishing a coordinated, long-term scientific strategy to understand the role of gravity in Arabidopsis growth and development using already existing or planned new hardware., Experimental work reported in this paper and performed in the authors’ laboratories was supported by the French Space Agency (CNES), the Norwegian Research Council, the Spanish National Plan for Research, Development and Innovation (different grants) and the ELIPS Programme of the European Space Agency (ESA). Specifically, the activities related to the ‘Arabidopsis Topical team’ were supported by an ESA grant.

7 p.-1 fig.-2 tab., Genome-wide transcriptional profiling experiments on the International Space Station (ISS) have revealed important alterations in the gene expression of Drosophila. In contrast, simulated microgravity experiments in Earth-based laboratories, where the particular constraints and conditions in orbiting spacecraft are absent, show weaker changes in gene expression. Here we use the "gene expression dynamics inspector" (GEDI) self-organizing maps to investigate the effect of altered gravity on different populations of Drosophila. We employ diamagnetic levitation and different mechanical techniques to generate microgravity and hypergravity and compare the changes in gene expression in these modified environments. In addition to behavioural and reproductive responses detected at the gene expression level, our results also indicate a subtle response of the transcriptome that is finely-tuned to Earth´s gravity., This work was supported by grants from the Spanish Space Program in the “Plan Nacional de Investigacion Cientifica y Desarrollo Tecnologico” [ESP2006-13600-C02-01] and MICINN[AYA2009-07792-E]; the Dutch NWO-ALW-SRON grant [MG-057] and the UK Engineering and Physical Sciences Research Council,Basic Technology grant [GR/S83005/01] and Basic Technology Translation grant [EP/G037647/1]R.J.A.H acknowledges EPSRC for support under a Research Fellowship; grant nos.EP/I004599/1 and EP/J005452/1. R. Herranz was supported by Spanish Ministerio de Educacion y Ciencia International Research Centers fellowships program in ESTEC and by the Consejo Superior de Investigaciones Cientificas CSIC JAE-Doc contract.

4p.-1 fig., Experiments performed in space have evidenced that, in root meristematic cells, the absence of gravity results in the uncoupling of cell growth and cell proliferation, two essential cellular functions that support plant growth and development, which are strictly coordinated under normal ground gravity conditions. In space, cell proliferation appears enhanced whereas cell growth is depleted. Since coordination of cell growth and proliferation is a major feature of meristematic cells, the observed uncoupling is a serious stress condition for these cells producing important alterations in the developmental pattern of the plant. Auxin plays a major role in these processes both by assuring the coupling of cell growth and proliferation under normal conditions and by exerting a decisive influence in the uncoupling under altered gravity conditions. Auxin is a mediator of the transduction of the gravitropic signal and its distribution in the root is altered subsequent to a change in the gravity conditions. This altered distribution may produce changes in the expression of specific growth coordinators leading to the alteration of cell cycle and protein synthesis. Therefore, available data indicate that the effects of altered gravity on cell growth and proliferation are the consequence of the transduction of the gravitropic signal perceived by columella cells, in the root tip., Work performed in the authors’ laboratory was supported by grants of the Spanish National Plan for Research, Development and Innovation, Ref. Nos.ESP2002-12062-E and ESP2006-13600-C02-2.

11p.-2 fig.-6 tab., Growing plants in space for using them in bioregenerative life support systems during long-term human spaceflights needs improvement of our knowledge in how plants can adapt to space growth conditions. In a previous study performed on board the International Space Station (GENARA A experiment STS-132) we evaluate the global changes that microgravity can exert on the membrane proteome of Arabidopsis seedlings. Here we report additional data from this space experiment, taking advantage of the availability in the EMCS of a centrifuge to evaluate the effects of cues other than microgravity on the relative distribution of membrane proteins. Among the 1484 membrane proteins quantified, 227 proteins displayed no abundance differences between µ g and 1 g in space, while their abundances significantly differed between 1 g in space and 1 g on ground. A majority of these proteins (176) were over-represented in space samples and mainly belong to families corresponding to protein synthesis, degradation, transport, lipid metabolism, or ribosomal proteins. In the remaining set of 51 proteins that were under-represented in membranes, aquaporins and chloroplastic proteins are majority. These sets of proteins clearly appear as indicators of plant physiological processes affected in space by stressful factors others than microgravity., The authors would like to thank the National Aeronautics and Space Administration (NASA) who successfully performed the spaceflight experiment; they also thank the astronauts for performing the required tasks on board the ISS. We acknowledge the Norwegian User Support and Operations Center team (NUSOC) for the ground and space preparation of the GENARA-A experiment and we thank the European Aeronautic Defense and Space Company (Astrium EADS) for the design and building of the hardware. We also thank the European Space Agency (ESA) and the Centre National d’Etudes Spatiales(CNES) for their scientific and financial support.