Of greater concern than routine adaptations to spaceflight are changes to the body that may not be noticed immediately but appear over time and are noticed anecdotally or through examination of longitudinal study data. In a report published in 2004 after the first ten years of such data collection, three trends were noted: cataracts, cancer, and thyroid dysfunction.[9] However, there was no significant difference in occurrence among astronauts as compared to matched non-astronaut populations. The thyroid issue was readily resolved and corrected when it was traced to the high iodine content of the space shuttle’s drinking water. Cataracts and cancer tend to be associated with radiation exposure and aging and have not been attributed to microgravity. The cancers diagnosed at that time included malignant melanoma, leukemia, lymphoma, Hodgkin’s disease, and kidney and gall bladder carcinomas. There were four instances of prostate cancer and one instance of breast cancer in a total of fourteen cancer cases among more than three hundred astronauts.

  At that time, the primary cause of death among astronauts since the beginning of spaceflight was accidents and injuries, with cancer and cardiovascular disease accounting for multiple, but fewer, deaths. The available evidence then did not suggest that these diagnoses were outside the range of normal compared to the general population, nor was there evidence of a different propensity for these medical conditions between men and women (except prostate and breast cancer). In the twenty years since the original study, there have been other accidental deaths, surpassed by an increasing number of natural deaths by disease and aging, but the basic conclusions about astronauts’ health have not been challenged by a similar review of NASA’s longitudinal health data. Research has not indicated a correlation between normal astronaut mortality and spaceflight.[10]

  Radiation exposure is the most daunting risk of spaceflight because of its known correlation with various types of cancer on Earth. Astronauts are exposed to ionizing radiation and cosmic rays when they are beyond the protective atmosphere, and radiation in the space environment penetrates spacecraft and spacesuits. Astronauts wear dosimeter badges while in space, and their cumulative radiation doses over time are carefully tracked. The level of prior exposure may become a factor in crew selection for a particular mission, along with mission duration and whether the mission requires spacewalks, to avoid exceeding permissible exposure limits. An astronaut may meet the radiation exposure lifetime limit by flying three or four space station missions. The possible long-term effects of radiation exposure may include cataracts, early onset dementia, and increased risk of cancer and circulatory diseases. There is no indication yet that astronauts or cosmonauts have a higher-than-normal mortality rate from radiation exposure, but conservative limits on exposure are accepted as ethically responsible protection.[11]

  Certain body tissues are highly sensitive to radiation. Formation of cataracts seems to be associated with aviation and spaceflight. For women, cancers of the thyroid, breast, ovaries, uterus, and lungs are of concern with radiation exposure because they are common cancer sites among Western women in general. Any such cancers might occur many years later with aging, so it is important that astronauts be informed and reasonably protected from this risk. Until recently, female astronauts were restricted to a lower lifetime level of cumulative radiation exposure, which in effect meant that they could not fly as often or as long as males.

  In 2021, following recommendations by the National Academies Institute of Medicine, NASA proposed to revise its radiation exposure standard to a common dose-based limit rather than an age- and sex-based calculation.[12] This revision erased the more conservative dose standard for females and set a single new standard for both sexes, regardless of age, while keeping all astronauts below a 3 percent increase in probability of death by radiation exposure sometime during their lives. This universal standard eliminated an astronaut’s sex as a factor in making mission assignments to the space station or beyond. NASA implemented this policy change in 2022. Both scientific data and a commitment to equal access to space, as well as closer alignment with the standard used by international partner space agencies, informed NASA’s decision to adopt this new protective standard for maximum lifetime radiation dose for spaceflight in low Earth orbit. As research continues, the standard may be adjusted again for longer, more distant expeditions to the Moon and Mars, where astronauts will be exposed to a much harsher radiation environment beyond Earth’s protective magnetic field.

  Much that is currently known and unknown about the body’s responses to both microgravity and radiation exposure in space will be research priorities in preparation for future exploration missions beyond Earth. Longer missions may entail greater risks. Exposure times to radiation and microgravity sometimes will be logged in years instead of weeks and months, and the body may respond differently at much longer durations. Current knowledge is based on spaceflight experience in low Earth orbit and modeling based on experiments with animals, simulated space environments, and analog data from survivors of nuclear accidents, Hiroshima, and Nagasaki. Risks related to flying on missions of very long duration beyond low Earth orbit on the Moon and Mars, and any additional physical and psychological stresses that may entail, will require study and possibly further protection.

  Researchers are also interested in possible interactions between exposure to microgravity and exposure to radiation, which may become more pronounced on long expeditions in deep space. Astronaut Scott Kelly’s yearlong stay on the International Space Station in 2015–2016 revealed several concerning changes in his genes and chromosomes, eyes and optic nerves, carotid arteries, and immune response, as well as slight cognitive declines in periodic testing. Only some of these changes reverted to normal after his return from space.[13] His experience is helping to set the agenda for new and ongoing research to ensure the health and safety of long-duration expeditionary crews. Younger members of today’s astronaut corps and those yet to be selected for long expeditions may face increased risks to their health. Advances in shielding technologies, countermeasures, and medicines may afford future crews additional protections not yet available.

  Differences in the Effects of Spaceflight on Men and Women?

  Since women joined the astronaut corps, researchers have wondered whether there are any differences between males and females in short- or long-term health risks from exposure to microgravity and space radiation. That question could not be addressed for some time, because there were too few female astronauts for adequate data or detection of trends. It is still a challenging research question, because not as many women as men have spent six to twelve months in space. However, evidence is gradually becoming available for assessment of some, but not all, health risks through the lens of male/female differences. Since the 1980s, several studies focused on possible sex-related differences in the body’s response to spaceflight have reached similar conclusions that differences are minor, usually with the caveat that there is insufficient data to identify or predict really significant differences.

  One of the earliest papers to present a medical perspective on women in space was published in 1984 when only three women—Valentina Tereshkova, Svetlana Savitskaya, and Sally Ride—had flown in space.[14] Its author was a NASA flight surgeon trained in psychiatry, Patricia Santy, who was interested in physiological and psychological considerations for women in space. She reviewed the then-known physical adaptations experienced by astronauts and posed some speculative medical and social issues that might uniquely affect women. Anthropometric differences in body size, reach, and strength would need to be accounted for in spacesuit and equipment design. Management of menses would need to be addressed. No data yet existed about gynecological and hormonal function in space, topics ripe for investigation. She noted some psychological and social concerns about stereotypes of masculinity and femininity that might affect crew cohesion, particularly with women in leadership and command roles. None of these were reasons to restrict women in space but were suggestive areas for research. Santy surmised from earthbound studies that slight but insignificant differences in physiological adaptation to spaceflight might manifest.

  In general, women astronauts were not keen to seek out differences that might be used to restrict their participation, preferring to treat gender as a nonissue unless it somehow affected performance.[15] One true gender issue was accommodation for urination while strapped in their seats during launch and entry or while on a spacewalk, when a toilet wasn’t accessible. Male pilots and astronauts used a urine collection device that was a condom or cuff attached to a small plastic drain tube attached to a collection bag, all worn under the flight suit. Relief was effortless unless there was a leak. That device obviously would not work for women, but it took time and false starts before a solution was reached. NASA developed and the women astronauts evaluated a few unsatisfactory options.[16] No one wanted to risk irritation or infection by using an invasive catheter. An alternative was a close-fitted fabric undergarment like a girdle or biking shorts lined with absorbent padding that became a gel when wet, but it was uncomfortable and expensive to make. Another was a small elongated funnel with an attached drain tube to a collection bag; the funnel was to be worn snug against the woman’s body inside her underwear. It leaked, overflowed, and was uncomfortable. The final solution was inspired by disposable baby diapers. The adult version for incontinence was soft, comfortable, highly absorbent, disposable, and inexpensive, and several pull-on or adjustable styles were commercially available. Relief was effortless and not messy. The women liked this option so well that the men eventually adopted it too for launch, reentry, and extravehicular activity. To use the toilets on the shuttle and space station, everyone had a personal funnel, with a slightly different shape for males and females, for urination into a drain hose.

  By 2000, when women had been flying on missions for almost twenty years, interest in possible gender-related differences related to spaceflight was growing to ensure that appropriate countermeasures and health care could be provided.[17] NASA enlisted the National Space Biological Research Institute to conduct a study on gender-related issues in spaceflight research and health care. The institute convened a two-day workshop in 1999, chaired by physician-astronaut Rhea Seddon.[18] Participants included another physician-astronaut, Ellen Baker, and at least ten researchers from medical schools and health agencies who were invited to engage in a review of the knowledge base and the status of research. The group reported that as yet there were only a few instances of measurable gender differences, such as postflight orthostatic intolerance (dizziness or fainting from insufficient blood flow to the head), but that there was not enough data to identify or predict significant differences. They recommended more research involving more women, aiming for a better understanding of differences, and then development of feasible countermeasures if necessary to protect women astronauts’ health and safety.

  This workshop report also identified several unique health care concerns for female astronauts, mainly their ability to prevent or plan pregnancy and to rely on highly effective contraception. These needs were crucial to their flight availability and family planning. There had been no effort yet to investigate any microgravity effects on hormonal activity, the efficacy of birth control methods, or the efficacy of contraceptive medications used for other purposes such as maintaining bone density or reducing the risk of certain gynecological issues—issues that should be studied. Finally, the workshop noted the possibility of performance differences between men and women due to technology, such as spacesuit design, and task procedures. The report clearly stated, “It should be the goal that all people selected to be astronauts be able to perform all tasks associated with the astronaut job regardless of size or gender.” The unstated point was that some equipment was indeed hampering the performance of some women astronauts, and such design issues should be remedied.

  Ostensibly meant to identify priorities for research into gender-related issues, the workshop had another purpose. At the time, NASA was toying with the idea of an all-female shuttle mission.[19] There was a flurry of NASA and press commentary on the possibility of an “unmanned” crew. The NASA administrator then, Daniel Goldin, thought it would be inspirational. The life sciences chief thought it would be valuable for gathering more data. Both agreed that such a mission needed a strong science justification and hoped the workshop would provide that. However, the group’s report concluded, “None of these recommendations require that all-female crews be flown.” In addition, most of the women astronauts wanted no part in a mission that could be viewed as a publicity stunt; it seemed too political, and new research findings from one brief mission seemed unlikely. A scientific program for a series of missions to study specific women’s health issues might have gained more interest and legitimacy. Some reports in the media reflected the same concerns.[20] Although all-male crews were common, the women didn’t want to be assigned to a sex-segregated mission; they wanted to be assigned on the basis of their abilities, not their anatomy. Seeing no good rationale for proceeding, NASA dropped the idea, and an intentional all-woman mission has not yet occurred.

  In 2001 NASA authors published an overview of known and potential gender-related differences in physiological responses to spaceflight.[21] At the time, the only data-supported difference was postflight orthostatic intolerance, a transient condition that women were significantly likelier than men to experience upon returning to Earth’s gravity, but it was not a clinical problem. Some male astronauts experienced cardiac dysrhythmias in flight, but there was no comparative data yet from in-flight cardiovascular monitoring of female astronauts. No gender-related difference in bone density by mineral loss was reported; regular exercise in space was credited for that result. Nor did any evidence indicate gender-specific risks for formation of renal stones during spaceflight. There was no indication that spaceflight altered the immune system differently for males and females, or evidence of any significant gender differences in neurosensory-motor functions. Gender-related differences in drug metabolism had been recognized on Earth but not yet studied in spaceflight; such differences could be important for medical treatment in space, a reason for exploring this issue more thoroughly. Although the average woman has less muscle mass than the average man, in the physically fit astronaut population strength training reduces these general differences, especially in microgravity, which largely negates them. At the time, there was not much data to evaluate women’s deconditioning and changes in strength after spaceflight, so no comparisons with men could be made. This report’s conclusion was that “individual differences in physiological responses within genders are usually as large as, or larger than, differences between genders.”[22]

  With both the space shuttle and space station in operation after 2000, individual astronauts were flying more often and staying longer in space, the number of women veterans of spaceflight was increasing, and more data was becoming available for study, but researchers still could not address the question of sex-based differences with certainty. In 2011 the National Academy of Sciences stressed the need to understand the influences of sex and gender on human responses to spaceflight. In response, NASA and the Space Biomedical Research Institute undertook another review of relevant research and priorities, establishing six workgroups to summarize the current body of data and publishing a set of reports in 2014 as The Impact of Sex and Gender on Adaptation to Space.[23] Each workgroup reported on a particular area of health—cardiovascular, immunologic, sensorimotor, musculoskeletal, neurosensory, reproductive, and behavioral—and made recommendations for new research to fill knowledge gaps.

  Some differences were noted between women and men, but their causes and significance remained uncertain. For example, male astronauts are more likely to experience visual impairment and hearing loss in space than female astronauts, and females have a more robust immune response in space (as on Earth) than males. Women have a greater loss of blood plasma volume than men. Some men and women develop kidney stones after, but not during, spaceflight. Female astronauts report more urinary tract infections in space than their male colleagues do, a difference possibly related to the design of the waste management system. These conditions are not major health problems, and they may not differ appreciably from incidence in the population at large. Measurable or statistical differences do not necessarily mean clinical significance.

  While the summary report argued that “it is imperative to examine and understand the influences that sex and gender have on physiological and psychological changes that occur during spaceflight,” it also acknowledged the difficulty of drawing conclusions from a still small sample size of female astronauts.[24] Nevertheless, this review indicated that progress had occurred in the effort to understand how spaceflight affects males and females. Recommendations to reduce the disparity in data included assigning more women to space missions, encouraging more female and male astronauts to participate in research studies, including sex and gender factors in experiment designs, and pursuing more detailed research into reported differences.

  The motive for investigating and understanding differences between the sexes is to ensure astronauts’ safety and appropriate health care as they encounter the risks of spaceflight. As the understanding of space physiology continues to deepen, the consensus holds that while individuals vary, there are few if any significant differences in the ways that male and female bodies react to spaceflight. The authors of the most recent edition of Principles of Clinical Medicine for Space Flight present no definitive findings of significant differences between male and female physiological adaptations.[25] The human body in general seems to have a common and predictable range of responses to microgravity and radiation, and individuals vary within that range regardless of their sex. No alarming preponderance of males or females has been noted for any of the conditions studied. Minor differences do not affect ability to do the job and do not portend different health consequences. The first generation of women astronauts may have been wise not to worry about sex-related differences unless they became a problem; no such problems in health or performance have arisen.