Introduction

This proposal describes the activity to be executed and the deliverables required by the European Space Agency in relation to a feasibility study in the area of zero and reduced gravity simulations in hypoxic environments. The programme of work involves conducting feasibility trials of hypoxic horizontal and head-up bedrest using human subjects at the Planica facility. In addition, the trials will highlight potential medical and/or technical problems associated with conducting such trials and which would need to be addressed by Lunar habitat simulation studies.

2. Background and Objectives

2.1 Background

Until recently, the focus of the preparations for future space missions has been the interplanetary mission to Mars. As a consequence, there has been a heightened interest in the hazardous effects of prolonged exposure to weightlessness, as would be experienced by astronauts/cosmonauts during their missions to Mars, as well as an interest in the development of countermeasures to minimize or eliminate such negative effects. Physiological responses to weightlessness have been successfully studied by use of the ground-based simulation model in which test subjects are requested to maintain a horizontal, or slightly head-down, body position. The changes in physiological systems during such studies mimic those experienced during a sojourn in space of the same duration.

In 2004, the President of the USA announced a new vision for space explorations, after which the US National Aeronautics and Space Administration (NASA) has refocused its emphasis towards human missions to the Moon and Mars (NASA 2004). With the Aurora program, the European Space Agency (ESA) has formulated similar objectives (ESA 2003, Crawford 2004, Hufenbach & Seibert 2005). As a consequence, some nations with active space programs have now focused their attention on establishing human colonies on the Moon. These colonies would exploit the reduced gravity environment, and mineral resources of the Moon to become financially self-sufficient, and thus lead the progress towards future interplanetary explorations.

The ultimate goal is to conduct human expeditions to Mars, but not until having acquired adequate knowledge about the planet and not until after successfully demonstrating sustained human exploration missions to the Moon (Mendell 2005). There are many reasons to promote a step-by-step approach as regards extraterrestrial human missions, with short-term Lunar missions preceding long-term Lunar missions and eventually Mars missions. Thus, currently experience relevant to a missions operation of the scale and scope of a human expedition to the surface of Mars is lacking. Moreover, reliability and maintainability of hardware and software systems necessary to undertake a 3-year mission to Mars need to be tested under extraterrestrial conditions, and last but not least, any risks concerning health, safety and performance of the crew need to be investigated and adequate countermeasures against unwarranted effects need to be developed before commencing such missions (cf. Crawford 2004). It should also be noted that sustained human expeditions to the Moon are envisioned to, not only provide knowledge needed for future missions to Mars, but also to increase our understanding of several separate scientific areas (Mendell 2005, Bodkin et al. 2006).

2.1.1. Lunar habitats

One problem regarding designing human habitats for use at the surface of the Moon concerns the environmental control and life support system, which, for primarily technical reasons, is desired to have a low operating pressure (Bodkin et al. 2006). A low operating pressure in the Lunar habitat will also reduce the potential risk of decompression sickness during extravehicular activities (Scheuring et al. 2008). To avoid that the reduced environmental gas pressure in the habitat results in detrimental levels of hypoxia amongst crew members, the fraction of oxygen must be increased. The trade-off for restoring the oxygen partial pressure to normoxic levels by increasing the oxygen fraction of the habitat gas mixture is a markedly increased flammability of the gas. It appears that to settle this dilemma a compromise between the elevated oxygen fraction and the reduced environmental gas pressure must be sought; presently, the environmental gas pressures and oxygen fractions that are being discussed range from 52.6 to 57.2 kPa and from 30 to 35 %, respectively. Thus, it is possible that the habitat gas mixture in a Lunar habitat may exhibit a partial pressure of oxygen as low as 15.8 kPa.

2.1.2 Physiological effects of long-term exposure to reduced oxygen and low gravity

Even though it is well known that humans can acclimatize to such levels of hypoxia (for refs. see West et al. 2007) it is as yet unknown how different physiological systems may respond to combined chronic exposure to hypobaric hypoxia and low gravity force field. Furthermore, long-term physiological responses to Lunar gravity (0.16 of Earths gravity) per se are unknown.

Long-term exposure to microgravity brings about mechanical unloading of weight-bearing bones and postural muscles as well as physical inactivity, resulting in bone demineralization, muscle atrophy and reduced muscle strength, collectively referred to as musculoskeletal deconditioning (for refs see Fortney et al. 1996). Microgravity also abolishes hydrostatic pressure gradients acting along blood vessels resulting in redistribution, and subsequently reduction, of the circulating blood volume (cf. Fortney et al. 1996) as well as reduced baroreflex function (Convertino et al 1990) and increased distensibility of dependant blood vessels (Eiken et al. 2008, Kölegård et al 2009). These adaptations, commonly referred to as cardiovascular deconditioning, manifest themselves upon return to Earths gravity force field as reduced aerobic exercise capacity, reduced tolerance to upright posture and to increased gravitoinertial load in the head-to-foot direction (+Gz tolerance; cf. Fortney et al. 1996). Although it might be assumed that chronic exposure to 0.16 G will induce cardiovascular and musculoskeletal deconditioning, which are qualitatively similar to those brought about by microgravity, the extent of such 0.16-G dependant deconditioning remains to be settled. Thus, from the perspective of physiological adaptation, it is desired that relevant ground-based simulations precede the Lunar missions.

2.1.3 Possible ground-based simulation of a Lunar habitat

It is well established that prolonged bedrest in the horizontal or slightly head-down position brings about similar cardiovascular and musculoskeletal adaptive responses as microgravity exposure (Fortney et al. 1996). At least as regards the cardiovascular system it is reasonable to assume that prolonged bedrest in the slightly head-up position may serve as a relevant simulation of a 0.16-G environment. Accordingly, life scientists at NASA and ESA have expressed a wish to be able to undertake prolonged (several weeks) bedrest studies, with the subjects consistently in a hypoxic environment and possibly in a slightly head-up position during the day and in a horizontal position during the night.

Such experiments must either be conducted at a facility located precisely at the desired altitude or in a hypoxic facility capable of housing 10-20 subjects simultaneously at any desired simulated altitude.

2.1.4 Conclusions from the ESA Topical Team Workshop "Simulation of Lunar Habitats"

In November 20089 ESA arranged a Topical team Workshop with the aims of: Discussing possible interaction between hypoxia and bedrest on adaptive physiological responses, and, based on the outcome of these discussions, deciding whether the research potential of such interventions justify establishing a hypoxia and bedrest experimental research program. If so to, recommend a simulation paradigm for future experimental campaigns.

The workshop proceedings (Eiken & Mekjavic, 2009) concluded:

• In view of anticipated long-term Lunar missions, research on hypoxia and unloading/inactivity is needed. Such research is also expected to benefit society in general.
• Due to as yet undefined operational parameters, it is premature to consider a high-fidelity analogue experimental design; the scientific interest is mainly in the interaction of hypoxia and unloading.
• It is recommended that the experimental paradigm is based on interventions with horizontal bedrest and normobaric hypoxia with an inspired oxygen partial pressure of 12.5-15.8 kPa. A research program should include several experimental campaigns with intervention durations varying between two and five weeks. Depending on the duration of the studies these might be designed as “cross-over repeated measures” or "multiple group comparisons".
• The intention is to prepare an FP-7 Space grant proposal, the call for which will be announced during the Spring of 2010 with an anticipated submission deadline during the Fall of 2010. The group will seek support for this from ESA.