Why Medical Fitness is Crucial for Astronauts

For space agencies, validating the medical health of candidates is considered to be the most important aspect of selection.  The medical exams are a large proportion of the overall selection process — and they can feel intensely personal or even invasive.  But they are essential because in space, where human physiology is pushed to the absolute limit, any medical issue can jeopardize the individual astronaut, the mission and the entire crew. The medical assessment aims to identify potential health risks that could affect an astronaut during a space mission.  There are no exceptions and no waivers — if you are considered medically unfit you will be excluded from the process.

Can You Influence Your Medical Assessment Results?

While there are multiple stages in the astronaut selection process where candidates can prepare and improve, the medical stage is largely seen as a factor of luck. While most details about the medical tests have been confidential in the past, there's more transparency now (1). However, the bottom line remains: while you can prepare yourself by understanding the medical expectations and requirements, it's difficult to change your medical fitness.

For those curious about medical checks, aviation medical licenses can offer some insight. These are globally available, and you don’t need to be a pilot to try them.  Anyone can book an assessments, undergo the tests and get a medical certificate (if all tests are passed).  In the paste, NASA have used FAA Class 2 or 3 and ESA have used EASA Class 2 as part of the initial screening process (2, 3, 4, 5).  However, it’s important to bear in mind that these are just a glimpse of what is involved — astronaut medical checks are far more exhaustive.

How Many Make the Medical Cut?

A typical failure rate for medical selection is between 15 to 40%, although less than that may pass that stage of selection.  For example, ESA had a stage dedicated solely to medical assessments where about 80 applicants entered the stage, but only 40 progressed (6).  However, since some agencies, like NASA and the Canadian Space Agency, combine medical assessments with other tests, it's hard to pinpoint exactly how many applicants fail purely on medical grounds.

When Do Medical Assessments Occur?

The timing varies between space agencies. Given the high costs and in-depth nature of these medical evaluations, they often take place in the later stages of the selection process.  Many agencies also feel ethically obliged to only put very highly qualified candidates through the rigorous and highly personal medical exams.  Here's a quick snapshot:

  • ESA: Start with a basic medical check when applying, followed by a comprehensive evaluation in Stage 3.
  • NASA: Medical evaluations begin in Stage 1, with a complete screening in Stage 3.
  • Canadian Space Agency (CSA): The process starts with an initial medical check in Stage 1, then additional tests in Stages 2 and 4.
  • ROSCOSMOS: Applicants submit medical documentation when applying, with a medical panel review in Stage 2.

More details

For a full breakdown of all the tests involved for NASA applicants, check out the article on NASA’s medical standards.  For more information on the stages of selection for the major space agencies, check out Stages in Selection: How Top Space Agencies Pick Their Astronauts


  1. NASA Astronaut Medical Standards
  2. NASA request for medical
  3. Federal Aviation Administration (FAA) medical standards
  4. ESA Astronaut handbook
  5. EASA Class 2 Medical Standards
  6. ESA astronaut selection stages (2008-09)