Encyclopedia of Bioastronautics

Living Edition
| Editors: Laurence R. Young, Jeffrey P. Sutton

Astronaut Selection – Medical Standards

  • Smith L. JohnstonEmail author
  • Rebecca S. Blue
Living reference work entry
DOI: https://doi.org/10.1007/978-3-319-10152-1_48-1


In this entry, the history of medical screening standards for the selection and retention of professional astronauts is examined. The evolving views on risk and human factors are discussed in their relationship to changing screening and selection criteria, and current selection standards are discussed.

There are many risks inherent to human spaceflight; safeguards are put into place in an attempt to mitigate the risks of each mission operation. One safeguard that has received significant attention and revision over the years of the US space program is the development and implementation of medical screening standards for the selection and retention of astronauts. Medical screening procedures have been designed with the intent of identifying potential crewmembers at the lowest risk possible for in-flight medical conditions that might compromise the successful completion of a spaceflight operation and the objectives within or which might affect the safety of the other crewmembers. For professional astronauts, screening has yet another goal: the retention of crewmembers for the duration of a spaceflight career, in order to return upon the value of the years of training and preparation endowed upon an astronaut before they ever have the opportunity to fly.

The objectives of medical screening include, first and foremost, the selection of healthy crewmembers. To do so, effective screening techniques must identify both overt and asymptomatic disease as well as identifying those individuals at the greatest risk of developing mission-impacting disease over the course of their training or career. Selection standards have not been static over the many years of human spaceflight; on the contrary, the most effective standards have taken into account the rapidly evolving world of medicine, the body of scientific literature that drives appropriate medical decision-making, and advances in technological, medical, and surgical techniques that effectively mitigate or even resolve an issue of risk prior to the point at which a potential astronaut may be screened (NASA 1977, 1997, 2011). The consistent application of medical standards and testing methodology enables the development of a database of information against which standards can be reviewed and revised, and the selection and screening techniques can be improved upon for future selection cycles. This safely allows a larger proportion of the population to participate in space exploration and enables a longitudinal study of astronaut health for the unique occupational exposures astronauts encounter.

Historical Context

Prior to the US Space Shuttle program, there were no NASA-specific guidelines regarding the selection of individuals who would be best qualified for the challenges of spaceflight, simply because there was little experience with human body function in space. The initial Mercury astronauts were selected based upon extremely thorough medical and psychological evaluation and an assessment of response to severe environmental stressors such as acceleration and hypoxia. Any deviation from the highest performance was considered disqualifying. As a result, the Mercury 7 astronauts were extremely fit, psychologically sound, and capable of withstanding any number of physiological stressors. However, such rigorous standards would severely limit NASA’s ability to expand its astronaut corps. It became recognized that a more specific set of minimum selection standards would be required for subsequent operations. These standards would need to be tailored for the human spaceflight environment.

The successful Mercury, Gemini, and Apollo programs had provided the opportunity to witness the human body function in space, providing a basic context for understanding the physiological stressors of the microgravity environment. Even so, it was not until 1977 that NASA documented the first set of medical standards for astronaut selection (NASA 1977). The standards provided were based upon pilot and special operative criteria established by the Air Force, Navy, Department of Defense, and Federal Aviation Administration. Using such standards as a framework, NASA undertook the first Shuttle-era medical selection, utilizing a three-stage process of scrutinizing applicants, in parallel with the evaluation by the Astronaut Selection Board. The first stage involved a highly scrutinized medical questionnaire to screen for disqualifying conditions; the second was an initial medical exam similar to a pilot Second Class medical certificate as performed by the Federal Aviation Administration. The final stage was a comprehensive battery of medical history questionnaires, physical and psychological examinations, and medical testing performed by physicians at the NASA Johnson Space Center.

As comprehensive as these standards appeared to be at first, they continued to evolve over the course of the Space Shuttle program. In 1991, they were revised significantly to include screening for more long-duration missions, including the screening of astronauts that would eventually spend many months aboard the International Space Station (ISS). Evolving requirements reflected the level of health and fitness required to complete mission tasks as well as rigorous pre-mission training in high-performance and acrobatic aircraft, hyper- and hypobaric exposure, hyper- and microgravity environments, and the specialized operations involved in spaceflight. A total of 12 selection cycles were held during the Space Shuttle program; during this same period of time, four major expert panel reviews were held to refine and improve upon selection standards for future cycles.

Current Selection Procedures

With the advent of the ISS program, medical standards were developed by the ISS Multilateral Medical Operations Panel, which included representatives from all ISS partners including NASA, Roscosmos (RSA), the European Space Agency (ESA), the Japanese Space Agency (JAXA), and the Canadian Space Agency (CSA). While the ultimate goal has been to define common standards for all crewmembers involved in ISS operations, the process of developing standards acceptable to all ISS partner agencies has been challenging due to specific cultural, ethnic, and philosophical differences between the agencies and the countries and peoples represented within. As a result, ISS medical standards have been evolving over time with the more realistic goal of meeting common mission objectives with enough internal flexibility to allow individual agencies to utilize equivalent methods for the medical and psychological evaluation of their candidates as well as addressing specific ethnic and cultural differences in disease prevalence.

As mission durations continually expand, now including many months aboard the ISS but potentially soon to include exploration-class missions that may span many years, the consequences of an in-flight incapacitating medical event increase. As a result, greater scrutiny and the need for higher standards and improved risk mitigation have led to increased selection screening methods and surveillance techniques. Current long-duration standards include screening specific to cardiopulmonary disease as well as an assortment of other potentially incapacitating medical conditions. The baseline required screening in place for current ISS selection is presented in the following table (Table 1).
Table 1

Baseline medical screening requirements for International Space Station Program screening

Type of evaluation

Medical screening required


Detailed medical questionnaire including past medical history, family history, occupational and radiation exposures

Detailed physical examination by an aerospace medicine specialist

Specialist consultations including ophthalmologist/optometrist, otolaryngologist, dentist, psychologist/psychiatrist, gastroenterologist, gynecologist


Hematology – complete blood count, thrombophilia evaluation

Biochemistry – renal, liver function, endocrine/thyroid, cardiovascular risk factors, iron indices, human chorionic gonadotropin levels (females), prostate specific antigen levels (males)

Infectious disease screens – HIV, hepatitis A/B/C, varicella titers, H. pylori, tuberculosis, methicillin-resistant Staphylococcus aureus screening

Urinalysis – Routine and microscopic, including 24 h urine collection


Resting electrocardiogram

Maximum effort treadmill exercise stress test

24 h ambulatory electrocardiogram

Pulmonary function testing

Computed tomography coronary artery calcium score


Magnetic resonance imagery of the brain with angiogram

Carotid ultrasound

Thyroid ultrasound

Abdominal ultrasound

Chest radiography

Bone mineral density



The most common causes for disqualification throughout the manned spaceflight programs have been ophthalmological, cardiovascular, otolaryngological, and renal problems. Visual acuity continues to be the most common cause for an applicant to fail to meet selection standards, followed by various cardiovascular disorders including hypertension, dysrhythmias, and vascular abnormalities. The pervasiveness of these common disqualifications stems from both the simple prevalence of the diseases mentioned and each disease’s ability to suddenly, subtly, and severely impact a given mission with its primary presentation or its clinical sequelae. Further, as selection standards have changed, previously disqualifying conditions and histories are now considered acceptable.

One example of such alterations to acceptance criteria is the approval of previously disqualifying refractive surgical procedures such as photorefractive keratectomy (PRK) and laser-assisted in situ keratomileusis (Lasik) procedures for visual acuity correction. Previously, these procedures were universally disqualifying for spaceflight activities and resulted in a high number of application rejections. In 2007, both PRK and Lasik were accepted for all flight activities given no adverse sequelae following the procedure (OCHMO 2007).

Finally, medical standards have evolved to become specific to the type of crewmember being evaluated. Pilots must meet different standards, for example, than do mission specialists or spaceflight participants (“space tourists”). Many of the current NASA selection standards and disqualification criteria remain sequestered in internal documentation; however, to foster the burgeoning commercial spaceflight industry, the NASA and international partner medical standards for commercial spaceflight participant selection for the International Space Station program were published and are now available publically (Bogomolov et al. 2007). Similarly, with the closure of the US Shuttle program, Shuttle selection standards and screening processes were published to provide a historical context for future development of screening and selection processes (Johnston et al. 2014).

Evaluation of Risk

As the ultimate goal of pre-flight selection standards is the mitigation and minimization of risk, a definition of and evidence-based approach to risk is required. The risk assessment techniques utilized by NASA and its international partners is based upon the probability, defined by current medical literature and historical precedence, of an operationally impacting medical event which occurs as a result of a medical condition or its consequences, should such an event occur during the course of a mission. Based upon the review of the literature and specialist opinion, an estimate of the probability of a medical event occurring during flight as a result of any given medical condition is derived. This “probabilistic risk assessment” (PRA) analysis is based upon medical literature, and as such is able to provide a risk percentage as well as confidence intervals on the estimated probability of an event. A risk matrix is utilized by the medical specialists at NASA and its international partners to aid in the decision-making in medical risk scenarios and is presented below (Fig. 1).
Fig. 1

NASA Medical Event Risk Matrix. This matrix is utilized by medical specialists at NASA and its international partners to aid in decision-making in medical risk scenarios. Note that medical events are classified by short- and long-term sequelae and by the likelihood of occurrence. Risk that falls into “green” categorization is generally accepted, “red” is generally disqualified, and risk falling into “yellow” categorization requires extensive evaluation, analysis, weighing of risk:benefit ratios, and agreement among all stakeholders including international partners

The risk matrix allows the plotting of a particular medical event, rated on a scale of least likely to most likely, and least severe consequences to most severe, as assessed by the medical specialists on the decision board. While subject to some degree of interpretation, the use of this matrix by the aerospace medical specialists at NASA and its international partners allows for the evaluation of the risk-benefit ratio involved in a given medical scenario and, as a result, provides context for medical decision-making. This risk matrix has been in use for all of the major medical decision-making processes in recent NASA spaceflight history.

Future Directions

Significant advances have been made in medical screening and the development of standards for manned spaceflight, particularly for career astronauts. As progress in space exploration continues toward long-duration and exploration-class missions, medical standards for the selection of crewmembers for such missions are likely to become more stringent and restrictive, owing to the limitations on medical diagnosis and treatment availability during such long-duration flights and the inability to rapidly and easily return to a definitive care facility on Earth. Such missions will, as a result, present great challenges to the field of aerospace medicine.

There have been multiple advances documented in recent medical literature regarding screening techniques that may prove highly applicable and useful in the astronaut-candidate population. For example, much attention has been given to genomics, particularly the ability to predict future health issues, assess biological aging, predict response to medical therapies, and predict the impact of the space environment upon an astronaut’s future health. There is much speculation regarding how such advances may be integrated in astronaut screening techniques. Even now, NASA’s Human Research Program is conducting an ongoing experiment to evaluate metabolomic and genomic markers, telomeric markers, and alterations of immune response of two identical twin astronauts, one exposed to the spaceflight environment for a full year and the other remaining on Earth (Bailey et al. 2018; Feinberg et al. 2018; Mason 2018). As of yet, the balance between genetic predisposition and environmental exposures and lifestyle changes has not been determined; at the moment, genomics and epigenetics are highly theoretical and have yet to be integrated into the screening process. However, it is not difficult to imagine the incorporation of such techniques in the future; if not for selection, then certainly for career and lifetime medical risk surveillance.

In addition, the advent of commercial spaceflight, particularly suborbital and orbital missions aimed toward tourism and similar activities, will undoubtedly require the expansion of selection techniques to allow for the inclusion of as many paying participants as possible. The burgeoning interest in commercial spaceflight has prompted extensive discussion and significant research regarding medical screening techniques for both crew and passengers of suborbital and orbital commercial vehicles. Guidelines and recommendations have begun to emerge from notable sources, including the Aerospace Medical Association, the Federal Aviation Administration, and NASA-associated interest groups. Generally such recommendations have focused on the provision of a safe, enjoyable spaceflight experience and the avoidance of a sudden, preventable, mission-impacting medical event. Even so, at the time of publication the current standard is that there are no federally mandated standards – individual industry providers are allowed the freedom of screening their own participants and, with little regulation, even their own crew for the developing industry. Whether this changes in the near future is the subject of much debate at this time.

No matter the outcome, historical experience has demonstrated that significant advances in screening techniques and selection criteria can be made through specialist collaboration and a thorough, evidence-based practice of medical decision-making based upon medical literature and up-to-date gold standards of care and prevention. It is certain that longer-duration, interplanetary, and commercial missions will continue to pose great challenges to the field of aerospace medicine, a problem best addressed by practices described here – collaboration, evidenced-based decision-making, and advancement of the medical literature. Continued collection and analysis of data, particularly those that address actual in-flight medical events or “near-miss” events, is vital to the evolution of standards in the future. With the rapid development of new technologies, medical evaluations and techniques, and data acquisition, selection standards and techniques will continue to evolve. The knowledge gained from this field of specialized preventive medicine will undoubtedly benefit the lay population in the advancement of the medical sciences.


  1. Bailey S, McKenna M, Taylor L, George K (2018) Assessing Telomere Length and Telemorase Activity in Twin and Unrelated Astronauts, presented at the HRP Investigator’s Workshop: Gateway to Mars, Galveston, TXGoogle Scholar
  2. Bogomolov VV, Castrucci F, Comtois JM et al (2007) International Space Station medical standards and certification for space flight participants. Aviat Space Environ Med 78(12):1162–1169Google Scholar
  3. Feinberg A, Rizzardi L, Feinberg J, et al. (2018) Comprehensive Analysis of Differential Epigenetic Effects of Space Travel on Monozygotic Twins, presented at the HRP Investigator’s Workshop: Gateway to Mars, Galveston, TXGoogle Scholar
  4. Johnston SL, Blue RS, Jennings RT, Tarver WJ, Gray GW (2014) Astronaut medical selection during the shuttle era: 1981–2011. Aviat Space Environ Med 85:823–827CrossRefGoogle Scholar
  5. Mason C. (2018) The Epigenetic and Transcriptional Dynamics of Long-Term Human Space Travel, presented at the HRP Investigator’s Workshop: Gateway to Mars, Galveston, TXGoogle Scholar
  6. NASA Johnson Space Center (1977) Space and life sciences Directorate. Medical evaluation and standards for astronaut selection: NASA class I pilot astronaut. Report No: JSC-11569. Johnson Space Center, HoustonGoogle Scholar
  7. NASA Johnson Space Center (1997) Space and life sciences Directorate. NASA medical standards selection and annual medical certification: payload specialist – class III. Report No: JSC-25396. Johnson Space Center, HoustonGoogle Scholar
  8. NASA Johnson Space Center (2011) Space and life sciences Directorate. NASA Medical Evaluation Documents (MED) Volume C: Medical standards and certification procedures of spaceflight participants, International space station program, Rev 3.1. Report No: JSC-SSP-50667. Johnson Space Center, HoustonGoogle Scholar
  9. Office of the Chief Health and Medical Officer (OCHMO) (2007) 80771201MED NASA crew members medical standards volume 1 – selection and periodic certification. NASAGoogle Scholar

Copyright information

© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  1. 1.NASA Johnson Space CenterHoustonUSA
  2. 2.Aerospace Medicine and Vestibular Research LaboratoryThe Mayo Clinic, ArizonaScottsdaleUSA

Section editors and affiliations

  • Michael W. Bungo
    • 1
  1. 1.The University of Texas Health Science CenterHoustonUSA