The physiology of survival: Space

摘要

Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did do. So, throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover. On 14 January, 2004, the United States announced an ambitious ‘Vision for Space Exploration’, promising that humankind would establish a permanent presence beyond Earth. What sounded then like the rhetoric of science fiction has, within two decades, become the blueprint for a new era of exploration – science fact. Artemis is preparing to return humans to the Moon and, ultimately, to carry them further still – to Mars. A mission to the Red Planet – lying anywhere between ∼55 and 400 million km from Earth, depending on orbital alignment – would extend over a thousand days, subjecting crews to physiological and psychological stresses of unprecedented duration and severity, so daunting that many still consider the endeavour beyond human limits. Space is, after all, unforgiving. Strip away the romance of rockets, the poetry of the pale blue dot, the cinematic allure of interstellar travel, and what remains is a vast vacuum filled with radiation, mind-bending temperature swings and a gravitational void that slowly unravels biology, dismantling the very systems that keep us upright on Earth. The human body, exquisitely tuned to one atmosphere, one gravity and a narrow ecological niche, is ill-suited to this alien environment. Surviving beyond Earth is not about grit alone – it is about physiology pushed to its absolute limits. And yet, it is precisely this hostility that presents physiologists with their most thrilling challenge in the greatest living laboratory ever conceived: to understand, predict and ultimately defend life against the cosmos itself. Indeed, confronting adversity has long been a catalyst for progress, driving science and ingenuity in directions that comfort and safety would never have inspired. Physiology has taught us that environmental stress and human adaptation are inseparable. Altitude research has illuminated the limits of vascular oxygen transport, diving medicine has revealed the limits of pressure, and polar expeditions have revealed how human spirit and physiology entwine at the edge of endurance – the journeys across frozen frontiers were not only cartographic but existential, tracing the fragile line between survival and surrender. Space is now revealing how far human biology can bend without buckling. Where exploration was once defined by how far a ship could sail beyond the horizon, it is now measured by how far physiology can extend life in the most extreme and inhospitable environments. The question is no longer whether humans can survive in space, but how biology can be harnessed to make that survival possible, sustainable and perhaps even beneficial. Central to this endeavour is recognising the cumulative totality of exposures astronauts face – the space exposome (Figure 1). More than 30 hazards are now catalogued by NASA's Human Research Program (NASA, 2023), ranging from cosmic radiation and altered gravity to circadian disruption, immune dysregulation, nutritional deficiency, and the psychological demands of isolation and confinement. Each hazard is life-threatening, yet none exists in isolation – stressors interact and synergise, the whole becoming more than the sum of its parts, yielding a complex environment that challenges our physiology in novel and unpredictable ways (Bailey, 2025) (Figure 1). The spacesuit is an astronaut's lifeline – a self-contained biosphere and integrated life-support system meticulously engineered to counter the lethal physics and unforgiving physiology of space. It provides pressure to prevent ebullism – the boiling of body fluids, including saliva and tears, in a vacuum – and provides precious oxygen that would otherwise lead to anoxia-induced unconsciousness within 6–10 s (Bailey, 2019), while preserving a workable degree of mobi