P atients on earth with illness can be described as people who live in a normal earth environment but who have abnormal physiology. In contrast, astronauts are people with normal physiology who live in an abnormal environment. It is this abnormal environment in space that, for the most part, causes unique alterations in astronauts' physiology that require the attention of clinicians and scientists. In this review, we build on the first article 1 in this series and provide an overview of the many complex physiologic changes that take place in short-and long-duration space flight, most often in response to microgravity.The goal of sending people farther into space and extending the duration of missions from months to years will challenge the current capabilities of space medicine. The knowledge and experience in bioastronautics, associated with almost 50 years of human space flight, will be critical in developing countermeasures and clinical interventions to enable people to participate in these missions and return safely to earth. EvidenceTo complement our first-hand experiences from space (collectively over 2000 hours), we reviewed technical and special publications from the National Aeronautics and Space Administration (NASA) and peer-reviewed medical literature. Most of the literature in this field is made up of case series and descriptive studies. In this article, unreferenced statements reflect our opinions as physician-astronauts who have observed first-hand the physiologic acclimation to microgravity. Our clinical experiences as crew medical officers have also been incorporated where applicable. AcclimationAcute changes in normal physiology in response to abnormal environments are labelled acclimation for short-term exposure (hours to days) or acclimatization for longer-term exposure (days to months). In this review, we use the term acclimation to describe the physiologic and psychological responses to the space-flight environment. Table 1 provides a timeline of these responses from launch to the period after landing.Microgravity has the largest effect of the space-flight environment on human physiology; all organ systems are affected to some degree. Isolation and confinement can also have important effects on the psychological well-being of astronauts. Table 2 outlines the key effects of the space-flight environment on humans and the countermeasures that are taken to address them. Shift in body fluidsAcclimation of the cardiovascular system to weightlessness is complex and not completely understood. Control mechanisms involving the autonomic nervous system, cardiac functions and peripheral vasculature all play a role. 20,21 However, the primary cause of these acclimations can be attributed to a redistribution of body fluids toward the head. 22 The supine prelaunch position with the lower limbs raised above the thoracoabdominal coronal plane initiates a fluid shift, which continues during orbit, with blood and other fluids moving from the lower limbs to the torso and head. During space flight, the volume i...

H uman space exploration is dependent on robust spacecraft design and sophisticated life-support technologies, both of which are critical for working in the hostile space environment. This article focuses on the specific challenges of the space environment. In an upcoming issue, a Dispatch from Space provides a personal look at space travel, and 2 other articles address the acclimation necessary for people to travel and live in space and the technological advances that can be applied to health care on earth.The early space program progressed from suborbital missions lasting minutes to orbital flights lasting days, demonstrating that people can both survive and work in space. Almost 50 years have elapsed since those initial flights, with remarkable progress in extending the duration of missions and the complexity of the objectives. The International Space Station circles the earth at an altitude of more than 300 km in an environment characterized by high vacuum, microgravity, extremes of temperature, meteoroids, space debris, ionospheric plasma, and ultraviolet and ionizing radiation. The development of new technologies to send people farther in space and keep them there longer is critical to the future of human space exploration.There are different definitions for the boundary to space. National Aeronautics and Space Administration (NASA) uses flight above 80 km to designate individuals as astronauts, while the Fédération Aéronautique Internationale uses the 100 km Karman line as the internationally accepted boundary to space. Beyond this altitude, aerodynamic flight is not possible, and spacecraft must travel faster than orbital velocity to manoeuvre and remain in orbit. EvidenceOnly about 350 people have flown in space over the last 4 decades, making it difficult to develop higher levels of clinical evidence to assess the efficacy of interventions in space medicine. Case series and descriptive studies represent the majority of the published literature in the field.1 A review of technical and special publications from NASA and peerreviewed literature was undertaken to complement our experiences. Each of us has experienced space first-hand, and, as a group, we have logged 2000 hours in space. One of the authors (D.W.) holds the Canadian record for spacewalking, with over 17 hours spent working outside of the space station. Environmental characteristicsResearch programs into bioastronautics and longitudinal studies of astronaut health have amassed considerable data that can help us to understand the environmental character- Robert Key points• Because of the harsh environment in space, astronauts are at risk for both short-and long-term health risks.• The 2 major challenges associated with spaceflight are radiation effects and the physiologic consequences of a microgravity environment.• Many of the immediate risks (decompression, thermal injury, arcing injuries) are mitigated by the design of the spacecraft and spacesuits.• The biologic effects of long-term exposure to space radiation are unclear but may include ...

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The objective of this study was to investigate depth perception in astronauts during and after spaceflight by studying their sensitivity to reversible perspective figures in which two-dimensional images could elicit two possible depth representations. Other ambiguous figures that did not give rise to a perception of illusory depth were used as controls. Six astronauts and 14 subjects were tested in the laboratory during three sessions for evaluating the variability of their responses in normal gravity. The six astronauts were then tested during four sessions while on board the International Space Station for 5–6 months. They were finally tested immediately after return to Earth and up to one week later. The reaction time decreased throughout the sessions, thus indicating a learning effect. However, the time to first percept reversal and the number of reversals were not different in orbit and after the flight compared to before the flight. On Earth, when watching depth-ambiguous perspective figures, all subjects reported seeing one three-dimensional interpretation more often than the other, i.e. a ratio of about 70–30%. In weightlessness this asymmetry gradually disappeared and after 3 months in orbit both interpretations were seen for the same duration. These results indicate that the perception of “illusory” depth is altered in astronauts during spaceflight. This increased depth ambiguity is attributed to the lack of the gravitational reference and the eye-ground elevation for interpreting perspective depth cues.

As part of the international Matroshka-R and Radi-N experiments, bubble detectors have been used on board the ISS in order to characterise the neutron dose and the energy spectrum of neutrons. Experiments using bubble dosemeters inside a tissue-equivalent phantom were performed during the ISS-16, ISS-18 and ISS-19 expeditions. During the ISS-20 and ISS-21 missions, the bubble dosemeters were supplemented by a bubble-detector spectrometer, a set of six detectors that was used to determine the neutron energy spectrum at various locations inside the ISS. The temperature-compensated spectrometer set used is the first to be developed specifically for space applications and its development is described in this paper. Results of the dose measurements indicate that the dose received at two different depths inside the phantom is not significantly different, suggesting that bubble detectors worn by a person provide an accurate reading of the dose received inside the body. The energy spectra measured using the spectrometer are in good agreement with previous measurements and do not show a strong dependence on the precise location inside the station. To aid the understanding of the bubble-detector response to charged particles in the space environment, calculations have been performed using a Monte-Carlo code, together with data collected on the ISS. These calculations indicate that charged particles contribute <2% to the bubble count on the ISS, and can therefore be considered as negligible for bubble-detector measurements in space.