Microgravity Conditions In Space Research Articles

Demonstration of charge-hold-vent (CHV) and no-vent-fill (NVF) in a simulated propellent storage tank during tank-to-tank cryogen transfer in microgravity

The space exploration from a low earth orbit to a high earth orbit, then to Moon, Mars, and possibly asteroids and moons of other planets is one of the biggest challenges for scientists and engineers for the new millennium. The enabling of in-space cryogenic rocket engines and the Lower-Earth-Orbit (LEO) cryogenic fuel depots for these future manned and robotic space exploration missions begins with the technology development of advanced cryogenic thermal-fluid management systems for the propellant transfer line and storage tank system. One of the key thermal-fluid management operations is the chilldown and filling of the propellant storage tank in space. As a result, highly energy efficient, breakthrough concepts for quenching heat transfer to conserve and minimize the cryogen consumption during chilldown have become the focus of engineering research and development, especially for the deep-space mission to Mars. In this paper, we introduce such thermal transport concepts and demonstrate their feasibilities in space for cryogenic propellant storage tank chilldown and filling in a simulated space microgravity condition on board an aircraft flying a parabolic trajectory. In order to maximize the storage tank chilldown thermal efficiency for the least amount of required cryogen consumption, the breakthrough quenching heat transfer concepts developed include the combination of charge-hold-vent (CHV) and no-vent-hold (NVF). The completed flight experiments successfully demonstrated the feasibility of the concepts and discovered that spray cooling combined with hold and vent is more efficient than the pure spray cooling for storage tank chilldown in microgravity. In microgravity, the data shows that the CHV thermal efficiency can reach 39.5%. The CHV efficiency in microgravity is 6.9% lower than that in terrestrial gravity. We also found that pulsing the spray can increase CHV efficiency by 6.1% in microgravity.

Suppression of bolting in the Arabidopsis <i>hmg1</i> mutant under microgravity conditions in space – Possible involvement of lipid rafts

Suppression of bolting in the Arabidopsis <i>hmg1</i> mutant under microgravity conditions in space – Possible involvement of lipid rafts

Rat bone responses to hindlimb unloading-reloading: Composition, BMD and mechanical properties

IntroductionBone loss in lower limbs under space microgravity condition can persist even after the astronauts return to Earth's gravitational environment. It is crucial to determine the extent and timing of bone recovery after spaceflight, especially for long-term flights. However, there is a lack of comprehensive descriptions of the recovery of various aspects of bone quality, particularly in terms of compositional characteristics that reflect changes in bone quality. Therefore, the objective of this study is to investigate the alterations in bone mineral density, composition characteristics, and mechanical properties of rat lower limbs during unloading (tail suspension) and the subsequent recovery loading period. MethodsRats were subjected to tail suspension for four weeks (TS4) followed by reloading for an additional four weeks. Bone mineral density (BMD) was measured by μ-CT and analyzed. The mechanical properties of the tibia were assessed by a three-point bending test, and the composition characteristics of bone tissue were evaluated using Raman spectroscopy. Regression analysis was performed to identify main component related to mechanical property. ResultsIn the metaphysis, BMD of the tibia significant decreased, particularly in trabecular bone. Upon reloading, only the cortical bone showed recovery. In the diaphysis, cortical bone remained unchanged during unloading but demonstrated delayed bone loss during the reloading period. Simultaneously, the mechanical properties of tibia experienced a substantial reduction after four weeks of unloading, The stiffness and maximum strength did not fully recover, though the elastic modulus and the ultimate strength were restored following the reloading process. Raman spectroscopy characterization found an increase in the carbonate substitution and crystal crystallinity of the diaphysis after 4 weeks of unloading. There was an intriguing alteration in the mineralized collagen ratio, that an increase was observed in the diaphysis while a decrease was observed in the metaphysis. During 4 weeks of reloading, the compositional characteristics of the diaphysis recovered earlier than those of the metaphysis, with the exception of crystallinity which remained unrecovered. Regression analysis indicated that carbonate replacement has a strong correlation to bone mechanical properties during the 4 weeks of unloading and subsequent 4 weeks of reloading. ConclusionsThe data obtained in this study highlight the incomplete recovery of bone mineral density, mechanical properties, and compositional characteristics during reloading in tail-suspended rats. These findings deepen our understanding of microgravity-induced bone loss and provide valuable information to develop interventions not only for astronauts but also for persons on earth during and after prolonged periods of unloading.

Возможный способ защиты оптики космических аппаратов от микрометеорной эрозии

There is a huge potential in Russia for the application of space technology in exploration, especially in the oil and gas sector. Images from space are a valuable source of information for geological predictions. How long can optics last in space and provide high-quality images when exposed to a micrometeor shower? The article proposes a new way to protect optical instruments from micrometeor erosion. The method is based on the use of a self-healing surface of a thin layer of a special liquid applied to the surface to be protected. Liquid resistant to solar and cosmic radiation. The implementation of the method is possible due to the physics of interfacial phenomena under microgravity conditions in space. The technical details of the proposal are being discussed.

Comprehensive analyses of plant hormones in etiolated pea and maize seedlings grown under microgravity conditions in space: Relevance to the International Space Station experiment “Auxin Transport”

Functional relationships between endogenous levels of plant hormones in the growth and development of shoots in etiolated Alaska pea and etiolated Golden Cross Bantam maize seedlings under different gravities were investigated in the “Auxin Transport” experiment aboard the International Space Station (ISS). Comprehensive analyses of 31 species of plant hormones of pea and maize seedlings grown under microgravity (μg) in space and 1 g conditions were conducted. Principal component analysis (PCA) and a multiple regression analysis with the dataset from the plant hormone analysis of the etiolated pea seedlings grown under μg and 1 g conditions in the presence and absence of 2,3,5-triiodobenzoic acid (TIBA) revealed endogenous levels of auxin correlated positively with bending and length of epicotyls. Endogenous cytokinins correlated negatively with them. These results suggest an interaction of auxin and cytokinins in automorphogenesis and growth inhibition of etiolated Alaska pea epicotyls grown under μg conditions in space. Less polar auxin transport with reduced endogenous levels of auxin increased endogenous levels of cytokinins, resulting in changing the growth direction of epicotyls and inhibiting growth. On the other hand, almost no close relationship between endogenous plant hormone levels and growth and development in etiolated maize seedlings grown was observed under μg conditions in space, as per Schulze et al. (1992). However, endogenous levels of IAA in the seedlings grown under μg conditions in space were significantly higher than those grown on Earth, similar to the cases of polar auxin transport already reported.

Effects of microgravity on cryogenic spray quenching of a simulated propellant storage tank with enhancement by surface coating and flow pulsing

In-space cryogenic propulsion will play a vital role in space exploration mission to the Moon and future mission to the Mars. The enabling of in-space cryogenic rocket engines and the Lower-Earth-Orbit (LEO) cryogenic fuel depots for these future manned and robotic space exploration missions begins with the technology development of advanced cryogenic thermal-fluid management systems for the propellant transfer line and storage tank system.One of the key thermal-fluid management operations is the chilldown of the propellant storage tank. As a result, highly energy efficient, breakthrough concepts for quenching heat transfer to conserve and minimize the cryogen consumption during chilldown have become the focus of engineering research and development, especially for the deep-space mission to Mars. In this paper, we introduce such thermal transport concepts and demonstrate their feasibility in space for cryogenic propellant storage tank chilldown in a simulated space microgravity condition on board an aircraft flying a parabolic trajectory. In order to maximize the storage tank chilldown thermal efficiency for the least amount of required cryogen consumption, the breakthrough quenching heat transfer concepts developed include the combination of cryogenic spray quenching cooling, Teflon thin-film coating on the simulated tank surface, and spray flow pulsing. The boiling heat transfer physics that supports the concepts is clearly explained. The completed flight experiments successfully demonstrated the feasibility of the concepts and discovered that spray cooling is the most efficient cooling method for the tank chilldown in microgravity. In microgravity, the data shows that the chilldown thermal efficiency can reach 30% with Teflon coating alone. However, Teflon coating together with flow pulsing was found to substantially enhance the chilldown efficiency further to reach the 50% goal.

Cryogenic spray quenching of simulated propellant tank wall using coating and flow pulsing in microgravity

In-space cryogenic propulsion will play a vital role in NASA’s return to the Moon mission and future mission to Mars. The enabling of in-space cryogenic engines and cryogenic fuel depots for these future manned and robotic space exploration missions begins with the technology development of advanced cryogenic thermal-fluid management systems for the propellant transfer lines and storage system. Before single-phase liquid can flow to the engine or spacecraft receiver tank, the connecting transfer line and storage tank must first be chilled down to cryogenic temperatures. The most direct and simplest method to quench the line and the tank is to use the cold propellant itself that results in the requirement of minimizing propellant consumption during chilldown. In view of the needs stated above, a highly efficient thermal-fluid management technology must be developed to consume the minimum amount of cryogen during chilldown of a transfer line and a storage tank. In this paper, we suggest the use of the cryogenic spray for storage tank chilldown. We have successfully demonstrated its feasibility and high efficiency in a simulated space microgravity condition. In order to maximize the storage tank chilldown efficiency for the least amount of cryogen consumption, the technology adopted included cryogenic spray cooling, Teflon thin-film coating of the simulated tank surface, and spray flow pulsing. The completed flight experiments successfully demonstrated that spray cooling is the most efficient cooling method for the tank chilldown in microgravity. In microgravity, Teflon coating alone can improve the efficiency up to 72% and the efficiency can be improved up to 59% by flow pulsing alone. However, Teflon coating together with flow pulsing was found to substantially enhance the chilldown efficiency in microgravity for up to 113%.

Suppression of Cortical Microtubule Reorientation and Stimulation of Cell Elongation in Arabidopsis Hypocotyls under Microgravity Conditions in Space.

How microgravity in space influences plant cell growth is an important issue for plant cell biology as well as space biology. We investigated the role of cortical microtubules in the stimulation of elongation growth in Arabidopsis (Arabidopsis thaliana) hypocotyls under microgravity conditions with the Resist Tubule space experiment. The epidermal cells in the lower half of the hypocotyls of wild-type Columbia were longer in microgravity than at on-orbit 1 g, which precipitated an increase in the entire hypocotyl length. In the apical region, cortical microtubules adjacent to the outer tangential wall were predominantly transverse to the long axis of the cell, whereas longitudinal microtubules were predominant in the basal region. In the 9th to 12th epidermal cells (1 to 3 mm) from the tip, where the modification of microtubule orientation from transverse to longitudinal directions (reorientation) occurred, cells with transverse microtubules increased, whereas those with longitudinal microtubules decreased in microgravity, and the average angle with respect to the transverse cell axis decreased, indicating that the reorientation was suppressed in microgravity. The expression of tubulin genes was suppressed in microgravity. These results suggest that under microgravity conditions, the expression of genes related to microtubule formation was downregulated, which may cause the suppression of microtubule reorientation from transverse to longitudinal directions, thereby stimulating cell elongation in Arabidopsis hypocotyls.

Simulated Microgravity Increases the Permeability of HUVEC Monolayer through Up-Regulation of Rap1GAP and Decreased Rap2 Activation.

Space microgravity condition has great physiological influence on astronauts’ health. The interaction of endothelial cells, which control vascular permeability and immune responses, is sensitive to mechanical stress. However, whether microgravity has significant effects on the physiological function of the endothelium has not been investigated. In order to address such a question, a clinostat-based culture model with a HUVEC monolayer being inside the culture vessel under the simulated microgravity (SMG) was established. The transmittance of FITC-tagged dextran was used to estimate the change of integrity of the adherens junction of the HUVEC monolayer. Firstly, we found that the permeability of the HUVEC monolayer was largely increased after SMG treatment. To elucidate the mechanism of the increased permeability of the HUVEC monolayer under SMG, the levels of total expression and activated protein levels of Rap1 and Rap2 in HUVEC cells, which regulate the adherens junction of endothelial cells, were detected by WB and GST pull-down after SMG. As the activation of both Rap1 and Rap2 was significantly decreased under SMG, the expression of Rap1GEF1 (C3G) and Rap1GAP in HUVECs, which regulate the activation of them, was further determined. The results indicate that both C3G and Rap1GAP showed a time-dependent increase with the expression of Rap1GAP being dominant at 48 h after SMG. The down-regulation of the expression of junctional proteins, VE-cadherin and β-catenin, in HUVEC cells was also confirmed by WB and immunofluorescence after SMG. To clarify whether up-regulation of Rap1GAP is necessary for the increased permeability of the HUVEC monolayer after SMG, the expression of Rap1GAP was knocked down by Rap1GAP-shRNA, and the change of permeability of the HUVEC monolayer was detected. The results indicate that knock-down of Rap1GAP reduced SMG-induced leaking of the HUVEC monolayer in a time-dependent manner. In total, our results indicate that the Rap1GAP-Rap signal axis was necessary for the increased permeability of the HUVEC monolayer along with the down-regulation of junctional molecules including VE-cadherin and β-catenin.

Suppression of secondary wall formation in the basal supporting region of Arabidopsis inflorescence stems under microgravity conditions in space

Suppression of secondary wall formation in the basal supporting region of Arabidopsis inflorescence stems under microgravity conditions in space

An advance in transfer line chilldown heat transfer of cryogenic propellants in microgravity using microfilm coating for enabling deep space exploration

The extension of human space exploration from a low earth orbit to a high earth orbit, then to Moon, Mars, and possibly asteroids is NASA’s biggest challenge for the new millennium. Integral to this mission is the effective, sufficient, and reliable supply of cryogenic propellant fluids. Therefore, highly energy-efficient thermal-fluid management breakthrough concepts to conserve and minimize the cryogen consumption have become the focus of research and development, especially for the deep space mission to mars. Here we introduce such a concept and demonstrate its feasibility in parabolic flights under a simulated space microgravity condition. We show that by coating the inner surface of a cryogenic propellant transfer pipe with low-thermal conductivity microfilms, the quenching efficiency can be increased up to 176% over that of the traditional bare-surface pipe for the thermal management process of chilling down the transfer pipe. To put this into proper perspective, the much higher efficiency translates into a 65% savings in propellant consumption.

DNA microarray analysis of gene expression of etiolated maize seedlings grown under microgravity conditions in space: Relevance to the International Space Station Experiment “Auxin Transport”

DNA microarray analysis of gene expression of etiolated maize seedlings grown under microgravity conditions in space: Relevance to the International Space Station Experiment “Auxin Transport”

Microarray profile of gene expression in etiolated Pisum sativum seedlings grown under microgravity conditions in space: Relevance to the International Space Station experiment “Auxin Transport”

This paper introduces the use of microarray data technology with Medicago (Medicago truncatula) microarrays to characterize global changes in the transcript abundance of etiolated Alaska pea (Pisum sativum L.) seedlings grown under microgravity (µg) conditions in comparison with those under artificial 1 g conditions on the International Space Station. Of the 44,000 genes of the Medicago microarray platform, more than 25,000 transcripts of pea seedlings were hybridized, suggesting that the microarray platform for Medicago could be useful in the study of gene expression of etiolated pea seedlings grown under µg conditions in space. Gene array data were analyzed according to stringent criteria that restricted the scored genes for specific hybridization values at least twofold. Expression of 1362 and 1558 genes in proximal side (the proximal side) and distal side of the epicotyl to the cotyledons (the distal side), respectively, were highly affected by µg conditions in space. Of the genes analyzed, 407 of 1362 transcripts in the proximal side and 740 of 1558 transcripts in the distal side were expressed at ratios at least twofold. However, in the presence of the auxin transport inhibitor TIBA, 212 of 399 transcripts and 255 of 477 transcripts were expressed at ratios at least twofold as high in the proximal and the distal sides of epicotyls in the seedlings grown under µg conditions, respectively. Based on Venn diagram analysis, 31 transcripts and 24 transcripts were found to commonly increase and decrease, respectively, under µg conditions in space. Venn analysis revealed six auxin-related genes and three water channel AQUAPORIN genes that were responsive to gravity. Among 6 auxin-related genes, the accumulation of transcripts of Auxin-induced protein 5NG4 and Indole-3-acetic acid-amido synthetase GH3.3 tended to increase, and that of Auxin-induced protein, Auxin response factor, SAUR-like auxin-responsive family protein and Auxin response factor tended to decrease under µg conditions, whereas there were no statistic differences between under µg and artificial 1 g conditions. Similarly there were no statistic differences between under µg conditions and artificial 1 g, but the accumulation of NIP3-1 and Plasma membrane intrinsic protein11, and AQUAPORIN1/Tonoplast intrinsic protein tended to increase and decrease, respectively. A possible role of auxin-related genes and AQUAPORIN genes in regulating growth of etiolated pea seedlings grown under µg conditions in space is discussed.

Vegetative and reproductive growth of Arabidopsis under microgravity conditions in space.

We have performed a seed-to-seed experiment in the cell biology experiment facility (CBEF) installed in the Kibo (Japanese Experiment Module) in the International Space Station. The CBEF has a 1 × g compartment on a centrifuge and a microgravity compartment, to investigate the effects of microgravity on the vegetative and reproductive growth of Arabidopsis thaliana (L.) Heynh. Seeds germinated irrespective of gravitational conditions after water supply on board. Thereafter, seedlings developed rosette leaves. The time of bolting was slightly earlier under microgravity than under space 1 × g. Microgravity enhanced the growth rate of peduncles as compared with space 1 × g or ground control. Plants developed flowers, siliques and seeds, completing their entire life cycle during 62-days cultivation. Although the flowering time was not significantly affected under microgravity, the number of flowers in a bolted plant significantly increased under microgravity as compared with space 1 × g or ground control. Microscopic analysis of reproductive organs revealed that the longitudinal length of anthers was significantly shorter under microgravity when compared with space 1 × g, while the length of pistils and filaments was not influenced by the gravitational conditions. Seed mass significantly increased under microgravity when compared with space 1 × g. In addition, seeds produced in space were found not to germinate on the ground. These results indicate that microgravity significantly influenced the reproductive development of Arabidopsis plants even though Earth's gravitational environment is not absolutely necessary for them to complete their life cycle.

Microgravity Affects the Level of Matrix Polysaccharide 1,3:1,4-β-Glucans in Cell Walls of Rice Shoots by Increasing the Expression Level of a Gene Involved in Their Breakdown.

The plant cell wall provides each cell with structural support and mechanical strength, and thus, it plays an important role in supporting the plant body against the gravitational force. We investigated the effects of microgravity on the composition of cell wall polysaccharides and on the expression levels of genes involved in cell wall metabolism using rice shoots cultivated under artificial 1 g and microgravity conditions on the International Space Station. The bulk amount of the cell wall obtained from microgravity-grown shoots was comparable with that from 1 g-grown shoots. However, the analysis of sugar constituents of matrix polysaccharides showed that microgravity specifically reduced the amount of glucose (Glc)-containing polysaccharides such as 1,3:1,4-β-glucans, in shoot cell walls. The expression level of a gene for endo-1,3:1,4-β-glucanase, which hydrolyzes 1,3:1,4-β-glucans, largely increased under microgravity conditions. However, the expression levels of genes involved in the biosynthesis of 1,3:1,4-β-glucans were almost the same under both gravity conditions. On the contrary, microgravity scarcely affected the level and the metabolism of arabinoxylans. These results suggest that a microgravity environment promotes the breakdown of 1,3:1,4-β-glucans, which, in turn, causes the reduced level of these polysaccharides in growing rice shoots. Changes in 1,3:1,4-β-glucan level may be involved in the modification of mechanical properties of cell walls under microgravity conditions in space.

Altered localisation of ZmPIN1a proteins in plasma membranes responsible for enhanced-polar auxin transport in etiolated maize seedlings under microgravity conditions in space.

In the International Space Station experiment 'Auxin Transport', polar auxin transport (PAT) in shoots of etiolated maize (Zea mays L. cv. Golden Cross Bantam) grown under microgravity in space was substantially enhanced compared with those grown on Earth. To clarify the mechanism, the effects of microgravity on expression of ZmPIN1a encoding essential auxin efflux carrier and cellular localisation of its products were investigated. The amounts of ZmPIN1a mRNA in the coleoptiles and the mesocotyls in space-grown seedlings were almost the same as those in 1 g-grown seedlings, but its products were not. Immunohistochemical analysis with anti-ZmPIN1a antibody revealed a majority of ZmPIN1a localised in the basal side of plasma membranes of endodermal cells in the coleoptiles and the mesocotyls, and in the basal and lateral sides of plasma membranes in coleoptile parenchymatous cells, in which it directed towards the radial direction, but not towards the vascular bundle direction. Microgravity dramatically altered ZmPIN1a localisation in plasma membranes in coleoptile parenchymatous cells, shifting mainly towards the vascular bundle direction. These results suggest that mechanism of microgravity-enhanced PAT in maize shoots is more likely to be due to the enhanced ZmPIN1a accumulation and the altered ZmPIN1a localisation in parenchymatous cells of the coleoptiles.

Comprehensive report on the Auxin Transport space experiment: the analysis of gravity response and attitude control mechanisms of plants under microgravity conditions in space on the International Space Station

Comprehensive report on the Auxin Transport space experiment: the analysis of gravity response and attitude control mechanisms of plants under microgravity conditions in space on the International Space Station

Marangoni-Convection-Driven Bubble Behavior and Microstructural Evolution of Sn-3.5Ag/Sn-17Bi-0.5Cu (Wt Pct) Alloy Solidified on Cu Substrate Under Space Microgravity Condition

Marangoni convection significantly affects the solidification structure as it controls the bubble behavior and mass transfer in the melt. Sn-3.5Ag/Sn-17Bi-0.5Cu (wt pct) alloy with different surface tension gradients was fabricated and solidified on a Cu ring substrate under space microgravity condition (SJ-10 satellite) to study the Marangoni convection formation mechanism. The pore and element distributions in the solidified alloy and surface tension gradient in the melt were analyzed. The differences between the microstructures of alloys solidified under microgravity and normal gravity conditions were also investigated. The surface tension gradient induced by Bi concentration difference resulted in the formation of Marangoni convection from the right to left of the melt under the microgravity condition. In the left (Bi-scarce) part of the melt, Marangoni convection induced by the Cu concentration difference flowed from outside to inside. Driven by bubble-agitation convection, Cu mainly migrated from the substrate to the right part of the melt. Therefore, dendrite-like CuxSny was distributed along a gradient. Under the normal gravity condition, significant gravity-induced convection resulted in an even distribution of Bi and Cu, which decreased the contact angle and reduced the surface tension, thus promoting nucleation of the alloy. Therefore, fine dendrite-like CuxSny with larger number density were uniformly distributed in the melt.

Arabidopsis flowering induced by photoperiod under 3-D clinostat rotational simulated microgravity

The transition from vegetative growth to flowering is very important for the plant reproductive success. This process is delayed under microgravity conditions in space, but whether microgravity affects plant flowering at the molecular level is still unknown. Here, we studied the effects of altered gravity on photoperiod-controlled flowering induction by using a random positioning machine (RPM, 3-D clinostat). First, using transgenic plant pro FT:GFP in the Col-0 background and examining the expression of key genes in the photoperiod pathway, such as FT, CONSTANS (CO) and GIGANTEA (GI), we found that 3-D clinostat rotation affected the activity of the FLOWERING LOCUS T (FT) promoter under long-day conditions. Second, using a pro SUC2:FT CDS:GFP construct in the ft-10 mutant background, we observed that the transport of FT from the leaves to the shoot apical meristem was also delayed by simulated microgravity under 3-D clinostation conditions compared with the 1 g stationary control. These results indicate that photoperiod-controlled plant flowering is delayed by 3-D clinostat rotation.

Polar auxin transport is essential to maintain growth and development of etiolated pea and maize seedlings grown under 1 g conditions: Relevance to the international space station experiment

We conducted “Auxin Transport” space experiments in 2016 and 2017 in the Japanese Experiment Module (JEM) on the International Space Station (ISS), with the principal objective being integrated analyses of the growth and development of etiolated pea (Pisum sativum L. cv Alaska) and maize (Zea mays L. cv Golden Cross Bantam) seedlings under true microgravity conditions in space relative to auxin dynamics. Etiolated pea seedlings grown under microgravity conditions in space for 3 days showed automorphogenesis. Epicotyls and roots bent ca. 45° and 20° toward the direction away from the cotyledons, respectively, whereas those grown under artificial 1 g conditions produced by a centrifuge in the Cell Biology Experimental Facility (CBEF) in space showed negative and positive gravitropic response in epicotyls and in roots, respectively. On the other hand, the coleoptiles of 4-day-old etiolated maize seedlings grew almost straight, but the mesocotyls curved and grew toward a random direction under microgravity conditions in space. In contrast, the coleoptiles and mesocotyls of etiolated maize seedlings grown under 1 g conditions on Earth were almost straight and grew upward or toward the direction against the gravity vector. The polar auxin transport activity in etiolated pea epicotyls and in maize shoots was significantly inhibited and enhanced, respectively, under microgravity conditions in space as compared with artificial 1 g conditions in space or 1 g conditions on Earth. An inhibitor of polar auxin transport, 2,3,5-triiodobenzoic acid (TIBA) substantially affected the growth direction and polar auxin transport activity in etiolated pea seedlings grown under both artificial 1 g and microgravity conditions in space. These results strongly suggest that adequate polar auxin transport is essential for gravitropic response in plants. Possible mechanisms enhancing polar auxin transport in etiolated maize seedlings grown under microgravity conditions in space are also proposed.