The next small step

Abstract

The microgravity experienced in space missions has serious effects on human physiology. How to get a crew to Mars in an optimal state for landing and exploration remains a matter of some debate Astrodynamic considerations and existing propulsion technology limit the speed with which a crew can be delivered to and returned from the surface of Mars. A typical, energy efficient mission profile might involve six months of outward bound journey, up to a year and a half of exploration on the planet surface, and a return flight lasting another six months.1 All told this comes to nearly one thousand days, more than twice the length of any single mission in the history of human space flight and an order of magnitude longer than routine International Space Station operations.Several hazards await the crews of Mars missions, including radiation exposure and the psychological stress of spending 30 months in a confined habitat, further from Earth than any human in history, with death no more than a hull's thickness away. This article focuses on the effects of prolonged weightlessness on the human body and our current understanding of the effects of microgravity on physiology. Physiology of microgravityProlonged exposure to microgravity seems to affect almost all physiological systems. Disturbances of haemopoiesis, immunosuppression, and endocrine changes have all been observed. [2][3][4] The effects of microgravity that are of key importance to human space operations are those on the musculoskeletal, neurovestibular, and cardiovascular systems. Effects on the musculoskeletal systemThat demineralisation of bone should occur in the face of the unloading associated with weightlessness is predictable from Wolffe's law. The rate and extent of this process is considerable, with losses of 1-2% of bone mass per month in flight.5 If unabated over the duration of a mission to Mars, this bone demineralisation, with its resultant hypercalcaemia and hypercalcuria, would leave crews at substantially increased risk of pathological fractures and renal calculus formation.The osteoporosis associated with space flight has been well documented. 6 The bone loss seems to be site specific, predominantly in the load bearing regions of the legs and lumbar spine.5 Study data variously implicate reduced bone formation resulting from osteoblastic dysfunction and excessive osteoclastic resorption. Both processes are probably involved, but their relative importance and how they are orchestrated remain unclear.In the absence of gravitational load, skeletal muscle also atrophies. Reductions in muscle volume and in peak force and velocity of contraction have been observed. The quality and quantity of muscle also change, with phenotypic shifts in muscle fibre type evident from biopsy samples. 9 These changes seem to occur in muscle groups associated with load bearing functions. In these groups the intrinsic mechanical and metabolic properties of slow twitch muscle fibres, associated with high oxidative capacity and low fatigueability, seem to alter to resemb...

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