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LRN-01 · Primer · Learn

Space Medicine 101: How the Body Changes in Space

Also: what happens to the body in space · beginner guide · five hazards

RadiationIsolation & ConfinementDistance from EarthGravity FieldsHostile/Closed EnvironmentPreflight & TrainingIn-flightPostflight RecoveryLifetime SurveillanceLEO short-durationLEO long-durationMars transit & surfacePublicStudentClinician

draft AI-assisted Last reviewed: Pending review · Reviewer: Pending review

Bottom line
  • NASA groups spaceflight dangers into five hazards: radiation, isolation and confinement, distance from Earth, gravity fields, and hostile or closed environments [1]. ◆◆◆◆ High · E
  • Most changes are adaptations to weightlessness, such as fluid shifts, motion sickness, and bone and muscle loss. They cause problems on return to gravity [2]. ◆◆◆◆ High · F
  • Space radiation mainly raises long-term cancer risk. NASA caps career dose below 600 mSv [5]. ◆◆◆◆ High · E C
  • Distance changes everything for medical care: Mars messages could take upward of 21–23 minutes one way [10]. ◆◆◆◆ High · E
  • Exercise, nutrition, fluids and careful rehabilitation after landing are the main countermeasures today [13][16]. ◆◆◆◇ Moderate · F E

Proposed certainty (◆ High → ◇ Very low) and evidence-type tags: F flight · A analog · M model/animal · C computational · E expert/operational/standards. AI-proposed, pending reviewer confirmation. How to read the grades.

The big picture

NASA’s Human Research Program (HRP) groups the dangers of spaceflight into five hazards: space radiation, isolation and confinement, distance from Earth, gravity fields, and hostile or closed environments. This site uses the same five, remembered as RIDGE. The hazards do not act alone; they can make each other worse [1].

Most of what happens to a healthy body in space is adaptation, not disease. The body adjusts to weightlessness within days to months. Those same adjustments cause problems when a person comes back to gravity [2].

How long the trip is matters most

A few minutes of weightlessness on a suborbital flight, a few days in orbit, six months on the International Space Station (ISS), and a multi-year Mars mission are very different exposures. Most effects grow with time in space. Some, like motion sickness, peak in the first few days [3].

R · Radiation

What it is. Outside Earth’s protective magnetic field and atmosphere, crews are exposed to galactic cosmic rays (GCR) and to bursts of particles from the Sun called solar particle events (SPE) [4].

What it does. The main concern is a higher lifetime risk of cancer. NASA limits each astronaut’s career radiation dose to less than 600 millisieverts (mSv). The limit is set so the added risk of dying from cancer stays below 3% on average [5].

What helps. Shielding and storm shelters work well against solar particles. Cosmic rays are much harder to block: NASA estimates shielding lowers their dose by only about 7% on average, so keeping missions short is the main lever [4]. Measurements taken on the way to Mars by the Curiosity rover suggest a round trip with current propulsion would deliver about 0.66 sieverts (660 mSv), which is above the career limit [6].

Read more: Space radiation (topic) · Radiation limits & monitoring (clinical) · Radiation & cancer (research)

I · Isolation and confinement

What it is. Small crews live and work in a small space for months, far from family, on tight schedules [1].

What it does. Stress, depression, anxiety and relationship problems can all occur in spaceflight [7]. Sleep is a common problem: on the ISS, use of sleep medicines was about 10 times higher than in adults on Earth [8]. In a 520-day simulated Mars mission on Earth (Mars-500), crewmembers differed widely. Most coped well, but some developed sleep problems or depressive symptoms [9].

What helps. Careful selection and training, private conferences with psychologists and family, and good sleep and lighting schedules [7].

Read more: Behavioral health in isolation (topic) · Behavioral health monitoring (clinical) · Earth analogs (research)

D · Distance from Earth

What it is. The farther a crew travels, the harder it is to get help, supplies or a ride home [1].

What it does. In low Earth orbit (LEO), a seriously ill crewmember can be brought home. On a Mars mission, a one-way radio message could take upward of 21–23 minutes, and there would be periods with no contact at all [10]. Crews would need to handle emergencies largely on their own.

What helps. NASA requires at least two crewmembers per vehicle to be trained as crew medical officers (CMOs) [5]. Medical kits and training are sized to the conditions most likely to occur [11]. Most medical problems in flight so far have been minor, self-limiting or easily treated [12].

Read more: In-flight emergencies & evacuation (clinical) · Mission medical kits (equipment) · Earth-independent operations (research)

G · Gravity fields

This hazard causes most of the changes people notice. Weightlessness affects nearly every body system, and so does the return to gravity [1].

Body systemWhat happens in spaceWhat happens back on Earth
Fluids and heartBody fluids shift toward the head; within about 2 days, blood plasma volume drops by roughly 15% [13]Lightheadedness or fainting on standing (orthostatic intolerance), worse after long flights [14]
Inner ear and balanceSpace motion sickness in most first-time flyers during the first 2–3 days [3]Dizziness and unsteady walking; balance largely recovers in about 2 weeks after long flights [15]
BoneHip and spine lose about 1–1.5% of bone density per month [16]Recovery is slow and can be incomplete a year later [17]
Muscle and fitnessAerobic fitness fell about 17% early in ISS flights despite exercise [18]Fitness was back to preflight by about 30 days after landing in that study [18]
EyesAbout 70% of long-duration crew show at least one sign of spaceflight-associated neuro-ocular syndrome (SANS) [19]Some changes, like flattening of the back of the eye, can last for years [19]
KidneysCalcium from bone and less urine raise kidney-stone risk [20]Stone risk stays higher for about a year after flight [20]
SpineThe spine lengthens by about 6% [2]; back pain is common early in flight [21]Higher rate of disc herniation, especially soon after flight [22]

What helps. Daily exercise, good nutrition and vitamin D, drinking plenty of fluids, salt-water fluid loading and compression garments before landing, and careful rehabilitation after return [13][16][20]. NASA crews are usually advised not to drive during the first week after landing [2].

Read more: Space motion sickness · Orthostatic intolerance · Bone loss · SANS · Postflight recovery (clinical)

E · Hostile and closed environment

What it is. A spacecraft is a sealed bubble. Air, water, temperature, noise and microbes all have to be managed [1].

What it does. Carbon dioxide (CO₂) can build up in the cabin, so NASA limits the 1-hour average to no more than 3 mmHg [23]. During spacewalks (extravehicular activity, EVA), the low pressure inside the suit can release nitrogen bubbles in the body, causing decompression sickness (DCS), the same condition seen in divers [24].

What helps. Air scrubbing and monitoring, and breathing pure oxygen before a spacewalk (prebreathe) to wash nitrogen out of the body [24].

Read more: Decompression sickness & EVA (clinical) · Microbes & infection (research) · Lunar & Mars dust (research)

Common questions

Is it dangerous to be a space tourist?

All spaceflight carries real risk. U.S. law requires commercial operators to tell each spaceflight participant (SFP) in writing about the known and unknown hazards, including the possibility of death or serious injury, and that the government has not certified the vehicle as safe [25]. On the first all-civilian orbital flight (Inspiration4, 3 days), almost all measured changes in the crew returned to baseline after landing [26]. See SFP screening.

Do astronauts get taller?

Yes, while in space. The spine lengthens by about 6% in weightlessness, which can even affect how a spacesuit fits [2]. It goes back after landing.

Does everyone get space sick?

Not everyone, but most first-time flyers do. It usually passes within about 3 days as the brain adapts [3]. See Space motion sickness.

Where to go next

Evidence profile
Overall certainty
◆◆◆◇ Moderate (proposed)
Evidence types
F · Flight dataA · Terrestrial analogE · Expert / operational / standardsC · Computational
Literature through
2026-10-14
Requirements referenced 3
RequirementTitlePlain-language meaning
[V1 4030]Career Space Permissible Exposure LimitCareer effective dose from spaceflight radiation must be less than 600 mSv, for all ages and sexes.
[V1 3006]Crew Medical OfficersAt least two crewmembers per vehicle are trained as crew medical officers.
[V2 6004]Nominal Vehicle/Habitat CO2One-hour average carbon dioxide partial pressure no more than 3 mmHg.

IDs as published in NASA-STD-3001 Vol. 1 Rev C (2023-09-15) and NASA-STD-3001 Vol. 2 Rev F (2026-07-14). NASA-STD-3001 binds NASA programs; it is not a regulation for commercial flights. Paraphrased. Standards quick reference.

References 26
  1. NASA Human Research Program. 5 Hazards of Human Spaceflight. www.nasa.gov/hrp/hazards/ NASA E
  2. NASA OCHMO. Crew Medical Support for Entry, Descent, and Landing. Technical Brief OCHMO-TB-051 (2026). www.nasa.gov/wp-content/uploads/2026/05/ochmo-tb-051-crew-me… NASA E
  3. NASA OCHMO. Space Adaptation Sickness (SAS). Medical Technical Brief OCHMO-MTB-004 (2025). www.nasa.gov/wp-content/uploads/2025/09/ochmo-mtb-004-space-… NASA E
  4. NASA OCHMO. Design for Ionizing Radiation Protection. Technical Brief OCHMO-TB-020. www.nasa.gov/wp-content/uploads/2023/12/ochmo-tb-020-radiati… NASA E
  5. NASA OCHMO. NASA-STD-3001, Volume 1: Crew Health, Revision C (2023). www.nasa.gov/wp-content/uploads/2023/11/nasa-std-3001-vol-1-… Standard E
  6. Zeitlin C, Hassler DM, Cucinotta FA, et al. Measurements of energetic particle radiation in transit to Mars on the Mars Science Laboratory. Science. 2013;340(6136):1080–1084. doi:10.1126/science.1235989 PMID:23723233 Peer-reviewed F
  7. NASA OCHMO. Behavioral Health and Performance. Technical Brief OCHMO-TB-016. www.nasa.gov/wp-content/uploads/2023/12/ochmo-tb-016-behavio… NASA E
  8. Wotring VE. Medication use by U.S. crewmembers on the International Space Station. FASEB J. 2015;29(11):4417–4423. doi:10.1096/fj.14-264838 PMID:26187345 Peer-reviewed F
  9. Basner M, Dinges DF, Mollicone DJ, et al. Psychological and behavioral changes during confinement in a 520-day simulated interplanetary mission to Mars. PLoS One. 2014;9(3):e93298. doi:10.1371/journal.pone.0093298 PMID:24675720 Peer-reviewed A
  10. NASA. Mars Communications Disruption and Delay (fact sheet, 2024). www.nasa.gov/wp-content/uploads/2024/01/mars-communications-… NASA E
  11. NASA OCHMO. Spaceflight Experience and Medical Care. Technical Brief OCHMO-TB-033. www.nasa.gov/wp-content/uploads/2023/12/ochmo-tb-033-spacefl… NASA E
  12. NASA Human Research Program. Evidence Report: Risk of Adverse Health Outcomes and Decrements in Performance due to In-Flight Medical Conditions (2017). humanresearchroadmap.nasa.gov/evidence/reports/Medical.pdf NASA F
  13. NASA OCHMO. Orthostatic Intolerance. Technical Brief OCHMO-TB-019. www.nasa.gov/wp-content/uploads/2023/12/ochmo-tb-019-orthost… NASA E
  14. Lee SMC, Feiveson AH, Stein S, Stenger MB, Platts SH. Orthostatic intolerance after ISS and Space Shuttle missions. Aerosp Med Hum Perform. 2015;86(12 Suppl):A54–A67. doi:10.3357/AMHP.EC08.2015 PMID:26630196 Peer-reviewed F
  15. Mulavara AP, Feiveson AH, Fiedler J, et al. Locomotor function after long-duration space flight: effects and motor learning during recovery. Exp Brain Res. 2010;202(3):649–659. doi:10.1007/s00221-010-2171-0 PMID:20135100 Peer-reviewed F
  16. NASA OCHMO. Bone Loss. Medical Technical Brief OCHMO-MTB-013, Rev D (2026). www.nasa.gov/wp-content/uploads/2026/02/ochmo-mtb-013-bone-l… NASA E
  17. Gabel L, Liphardt AM, Hulme PA, et al. Incomplete recovery of bone strength and trabecular microarchitecture at the distal tibia 1 year after return from long duration spaceflight. Sci Rep. 2022;12:9446. doi:10.1038/s41598-022-13461-1 PMID:35773442 Peer-reviewed F
  18. Moore AD Jr, Downs ME, Lee SMC, Feiveson AH, Knudsen P, Ploutz-Snyder L. Peak exercise oxygen uptake during and following long-duration spaceflight. J Appl Physiol. 2014;117(3):231–238. doi:10.1152/japplphysiol.01251.2013 PMID:24970852 Peer-reviewed F
  19. NASA OCHMO. Spaceflight Associated Neuro-ocular Syndrome (SANS). Medical Technical Brief OCHMO-MTB-001 (2024). www.nasa.gov/wp-content/uploads/2024/02/ochmo-mtb-001-sans.p… NASA E
  20. NASA Human Research Program. Evidence Report: Risk of Renal Stone Formation (2024). humanresearchroadmap.nasa.gov/evidence/other/2024%20Renal%20… NASA F
  21. Kerstman EL, Scheuring RA, Barnes MG, DeKorse TB, Saile LG. Space adaptation back pain: a retrospective study. Aviat Space Environ Med. 2012;83(1):2–7. doi:10.3357/asem.2876.2012 PMID:22272509 Peer-reviewed F
  22. Johnston SL, Campbell MR, Scheuring R, Feiveson AH. Risk of herniated nucleus pulposus among U.S. astronauts. Aviat Space Environ Med. 2010;81(6):566–574. doi:10.3357/ASEM.2427.2010 PMID:20540448 Peer-reviewed F
  23. NASA OCHMO. NASA-STD-3001, Volume 2: Human Factors, Habitability, and Environmental Health, Revision F (2026). standards.nasa.gov/standard/NASA/NASA-STD-3001_VOL_2 Standard E
  24. NASA OCHMO. Decompression Sickness (DCS) Risk Mitigation. Technical Brief OCHMO-TB-037. www.nasa.gov/wp-content/uploads/2023/12/ochmo-tb-037-decompr… NASA E
  25. 14 CFR Part 460 — Human Space Flight Requirements. eCFR, current. www.ecfr.gov/current/title-14/chapter-III/subchapter-C/part-… FAA / regulation E
  26. Jones CW, Overbey EG, Lacombe J, et al. Molecular and physiological changes in the SpaceX Inspiration4 civilian crew. Nature. 2024;632:1155–1164. doi:10.1038/s41586-024-07648-x PMID:38862026 Peer-reviewed F

Machine-checked on 2026-10-14: each URL was fetched, each DOI's title/journal/year matched via Crossref, PubMed IDs matched to DOIs, and claims were checked against the source text (abstract, NASA document or official web page). Human verification against full text is still required before review. NASA documents are U.S. Government works (public domain) and are summarized with attribution; journal articles are summarized, not reproduced.

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