Since the first two-hour excursion into space by Yuri Gagarin in 1961, the lure of manned space travel has proved irresistible to scientists, entrepreneurs, and entertainers alike. Today, as technology becomes more capable of enabling manned travel to Mars and Hollywood’s imagination runs wild with notions of humanity’s spaceflight-steeped future (with recent blockbusters like Star Trek, Prometheus, Star Wars, and even Wall-E), many fallacies about space have emerged. Outer space is often depicted in film as a cold, inhospitable place, where exposure to the perpetual vacuum will make your blood boil and your body burst; alternatively, if neither of those things happen, you’re bound to instantly freeze into a human-popsicle. Meanwhile, many of these same films conveniently ignore the slightly more subtle, yet highly relevant hazards of prolonged spaceflight even in an enclosed vessel at normal atmospheric pressure.

Acute exposure to the vacuum of space: No, you won’t freeze (or explode)

One common misconception is that outer space is cold, but in truth, space itself has no temperature. In thermodynamic terms, temperature is a function of heat energy in a given amount of matter, and space by definition has no mass. Furthermore, heat transfer cannot occur the same way in space, since two of the three methods of heat transfer (conduction and convection) cannot occur without matter.

What does this mean for a person in space without a spacesuit? Because thermal radiation (the heat of the stove that you can feel from a distance, or from the Sun’s rays) becomes the predominant process for heat transfer, one might feel slightly warm if directly exposed to the Sun’s radiation, or slightly cool if shaded from sunlight, where the person’s own body will radiate away heat. Even if you were dropped off in deep space where a thermometer might read 2.7 Kelvin (-455°F, the temperature of the “cosmic microwave background” leftover from the Big Bang that permeates the Universe), you would not instantly freeze because heat transfer cannot occur as rapidly by radiation alone.

The absence of normal atmospheric pressure (the air pressure found at Earth’s surface) is probably of greater concern than temperature to an individual exposed to the vacuum of space [1]. Upon sudden decompression in vacuum, expansion of air in a person’s lungs is likely to cause lung rupture and death unless that air is immediately exhaled. Decompression can also lead to a possibly fatal condition called ebullism, where reduced pressure of the environment lowers the boiling temperature of body fluids and initiates transition of liquid water in the bloodstream and soft tissues into water vapor [2]. At minimum, ebullism will cause tissue swelling and bruising due to the formation of water vapor under the skin; at worst, it can give rise to an embolism, or blood vessel blockage due to gas bubbles in the bloodstream.

Our dependence on a continuous supply of oxygen is the more limiting factor to the amount of time a human could survive in a full vacuum. Contrary to how the lungs are supposed to function at atmospheric pressure, oxygen diffuses out of the bloodstream when the lungs are exposed to a vacuum. This leads to a condition called hypoxia, or oxygen deprivation. Within 15 seconds, deoxygenated blood begins to be delivered to the brain, whereupon unconsciousness results [1]. Data from animal experiments and training accidents suggest that an individual could survive at least another minute in a vacuum while unconscious, but not much longer [3,4].

Long-term effects of space travel

While the effects of space suit malfunction or decompression on the human body are important to recognize, long-term consequences of spaceflight are perhaps more relevant (Figure 1). Many of the immediate physiological impacts of spaceflight are attributed to microgravity, a term that refers to very small gravitational forces. Because life on Earth has evolved to function best under Earth’s gravity, arguably all human organ systems are affected by gravity’s absence. The body is highly adaptive and can acclimatize to a change in gravitational environment, but these physiological adaptations may have pathological consequences or lead to a reduction in fitness that challenges a space-traveler’s ability to function normally upon return to Earth.

Figure 1. Physiological hazards associated with space travel. Exposure to an environment in space with microgravity and ionizing radiation can perturb the cardiovascular, excretory, immune, musculoskeletal, and nervous systems. (Illustration by Mark Springel, edited by Hannah Somhegyi)

On Earth, the cardiovascular system works against gravity to prevent blood from pooling in the legs, thus microgravity results in a dramatic redistribution of fluids from the legs to the upper body within only a few moments of weightlessness [5]. This phenomenon is colloquially known to astronauts as “puffy face” or “bird legs”, referencing the prominent facial swelling and 10-30% decrease in leg circumference. Although fluids return to a somewhat normal distribution within 12 hours, astronauts often complain of nasal stuffiness and eye abnormalities after extended stays in space [6], which are likely symptoms of the increased intracranial pressure, or pressure within the skull. Furthermore, there is a reduction of blood volume, red blood cell quantity, and cardiac output due to lower demands on the cardiovascular system to counteract gravity. This acclimation is physiologically normal and presents no functional limitations in space, but upon return to Earth’s gravity, one of every four astronauts are unable to stand for 10 minutes without experiencing heart palpitations or fainting [5,7].

Because more than half of the muscles of the human body resist gravitational force on Earth, musculoskeletal acclimation to microgravity results in profound muscle atrophy, reaching up to 50% muscle mass loss in some astronauts over the course of long-term missions [5]. The muscular atrophy seen in astronauts closely mirrors that of bedridden patients, and upon return to Earth, some astronauts experience difficulty simply maintaining an upright posture. Diminished burden in space on load-bearing bones, such as the femur, tibia, pelvic girdle, and spine, also causes demineralization of the skeleton and decreased bone density, or osteopenia. Calcium and other bone-incorporated minerals are excreted through urine at elevated levels, thus the microgravity environment puts individuals at risk not only for bone fracture, but for kidney stones as well [8].

The vestibular and sensorimotor systems, our bodies’ sensory networks that contribute to sense of balance and motor coordination, respectively, are also impacted by microgravity. The majority of astronauts experience some level of space motion sickness or disorientation for the first few days in space, and these symptoms generally subside as the body acclimates [5]; however, some astronauts still feel wobbly months after returning to Earth [9]. Furthermore, normal sleep cycles appear to be affected, as astronauts consistently sleep less and experience a more shallow and disturbed sleep in space than on Earth [10]. This may be due to a combination of microgravity or an altered light-dark cycle in space. Many astronauts complain of bright flashes that streak across their vision while trying to sleep, attributed to high-energy cosmic radiation [11].

The Earth’s atmosphere acts as a shield to block many harmful types of space radiation, but humans are dangerously exposed to this radiation in outer space (Figure 2). Ultraviolet (UV) radiation from the sun is largely absorbed by the Earth’s atmosphere and never reaches its surface, but a human unprotected in space would suffer sunburn from UV radiation within seconds. UV rays can be blocked with specially designed fabric in spacesuits and shielding on spacecraft, but higher energy ionizing radiation and cosmic rays—high-energy protons and heavy atomic nuclei from outside our Solar System—can penetrate shielding and astronauts’ bodies alike, potentially having severe health implications [6]. Damaging radiation of this type can cause radiation sickness, mutate DNA, damage brain cells, and contribute to cancer [12]. Several studies also suggest that cosmic radiation increases risk of early-onset cataracts [13], and contributes to astronauts’ increased likelihood of acquiring viral and bacterial infections due to immune system suppression [5].

What does this mean for future space missions?

The prospect of interplanetary missions compounds known health concerns regarding space travel. With our current technology, a manned mission to Mars would take more than two years, and by conservative estimates, simply getting to Mars might take 6 to 8 months. Radiation measurements recorded by NASA’s Curiosity rover during its transit to Mars suggest that with today’s technology, astronauts would be exposed to a minimum of 660 ± 120 millisieverts (a measure of radiation dosage) over the course of a round trip [14]. Because NASA’s career exposure limit for astronauts is only slightly greater at 1000 millisieverts, this recent data is cause for great concern.

Figure 2. Approximate radiation dose in several scenarios on Earth and in space. The radiation exposure associated with a round trip to Mars is extrapolated from recent data from the Mars Space Laboratory (MSL) / Curiosity rover. DOE, Department of Energy; ISS, International Space Station [14]. (Image adapted from NASA/JPL Photojournal: PIA02570 & PIA02004;

The recent radiation data aside, the longest consecutive stay by a human in space is only 438 days [15], and it’s not completely understood how the human body might respond to a trip to Mars and back. The effects of long-term spaceflight may be very nuanced, and this calls for new disciplines that can address the issue of adapting humans to conditions that we were not intended to endure. Frequent exercise, proper nutrition, and pharmacological therapy are three strategies used to combat the deconditioning process, yet some reduction in fitness is inevitable.

One of the fundamental challenges facing scientists who design future space missions is to develop new technologies that can accommodate the physiological limitations of humans traveling in space for indefinite periods of time. Much emphasis on research today is to develop technologies to get to Mars faster, generate artificial gravity, and reduce radiation exposure. While pop culture’s depiction of space travel may largely be fictitious, it may be science fiction that one day enables humans to venture deeper into “the final frontier.”

Mark Springel is a research assistant in the Department of Pathology at Boston Children’s Hospital.


[1] Kanas N, Mansey D. “Basic Issues of Human Adaptation to Space Flight.” Space Psychology and Psychiatry, Dordrecht,: Springer Netherlands, 2008. 15-30. Print.

[2] Czarnik, TR. Ebullism at 1 Million Feet: Surviving Rapid/Explosive Decompression.”

[3] Shayler DJ. Disasters and Accidents in Manned Spaceflight, Springer-Praxis Books in Astronomy and Space Science: Chichester UK, 2000.

[4] Roth EM (1968). Rapid (Explosive) Decompression Emergencies in Pressure-Suited Subjects. NASA CR-1223.NASA Contract Rep NASA CR., Nov: 1-125.

[5] Williams D, Kuipers A, Mukai C, Thirsk R (2009). Acclimation during space flight: effects on human physiology. CMAJ 180(11): 1317-1323.

[6] Setlow RB (2003). The hazards of space travel. Embo Rep, 4(11): 1013-1016.

[7] Mader TH, Gibson CR, Pass AF, Kraimer LA, et al. (2011). Optic disc edema, globe flattening, choroidal folds, and hyperopic shifts observed in astronauts after long-duration space flight. Ophthalmology 118(10): 2058-2069.

[8] Pietrzyk RA, Jones JA, Sams CF, Whitson PA (2007). Renal stone formation among astronauts. Aviat Space Environ Med 78(4 Suppl): A9-13.

[9] Astronaut Says He’s Still Wobbly After Months of Weightlessness. New York Times, February 2, 1998.”

[10] Wide awake in outer space (NASA):

[11] Narici L, Bidoli V, Casolino M, De Pascale MP, et al. (2004). The ALTEA/ALTEINO projects: studying functional effects of microgravity and cosmic radiation. Adv Space Res 33(8): 1352-7.

[12] Townsend LW (2005). Implications of the space radiation environment for human exploration in deep space. Radiat Prot Dosimetry 115(1-4): 44-50.

[13] Chylack LT, Peterson LE, Feiveson AH, Wear ML, et al. (2009). NASA study of cataract in astronauts (NASCA). Report 1: Cross-sectional study of the relationship of exposure to space radiation and risk of lens opacity. Radiat Res 172(1): 10-20.

[14] Zeitlin C, Hassler DM, Cucinotta FA, Ehresmann B (2013). Measurements of energetic particle radiation in transit to Mars on the Mars Science Laboratory. Science 340(6136): 1080-1084.

[15] Staying Put on Earth, Taking a Step to Mars by Michael Schwirtz. New York Times. March 30, 2009.

Additional Resources:

Race to Mars: Known effects of long-term space flights on the human body (Discovery Channel):

Kerr RA (2013). Radiation will make astronaut’s trip to Mars even riskier. Science 340(6136): 1031

Spaceflight bad for astronauts’ vision, study suggests (

Study shows that space travel is harmful to the brain and could accelerate onset of Alzheimer’s (SpaceRef):

Cherry JD, Liu B, Frost FL, Lemere CA, et al. (2012). Galactic cosmic radiation leads to cognitive impairment and increased Aβ plague accumulation in a mouse model of Alzheimer’s disease. PLoS One 7(12): e53275

Buckey JC. Space Physiology, New York: Oxford University Press, 2006. Print.

Clément G. Fundamentals of Space Medicine, Microcosm Press, Dordrecht ; Boston: Kluwer Academic, 2003. Print.

42 thoughts on “The human body in space: Distinguishing fact from fiction

  1. Despite the occssional mansplainers this article is wholly in line with what almost all physicists & astronauts assume (including the russian who WAS exposed in space to the vacuum).
    Thank you, nice article.

  2. Excellent and informative article. Not sure what “the physician” was ranting about. Seems like a well written and referenced article to me. Thank you for the insights.

  3. Hi this is a woman who experiences some of those symptoms at 50 years old. How long does it last?😃

    1. Yeah, space really messes you up. I saw the old NASA films with astronauts in wheelchairs. Takes a lot out of you. Bone loss is primarily it. Need dumbbell spacecraft spinning. NASA’s excuse last I heard was “We don’t have the technology to do that yet.” Nuts.

  4. Correct me if I’m wrong but today’s space craft that are launched from the ground have the bare minimum of shielding. They are essentially tin cans as weight and mass is always a concern for flight control systems and whether you can successfully get the craft off the ground safely or at all.
    I would imagine that to tackle the radiation problem they would most likely have to build/assemble the ship in orbit like they did with ISS to adequately shield it for a round trip to Mars. Launching such a ship from the ground would probably be extremely difficult if not impossible.

    1. You’re definitely right from what I understand. The load of the ship is it’s most limiting factor. The only way to really make an interplanetary vessel would be in orbit. The problem is how many rockets to send up with supplies vs possibly locating asteroids that are rich in the metals we would need and mining/crafting from there. I think to be successful we would have to move at a snails pace for safety reasons. That would require public interest in a long term capacity as well as intense funding and training which is really hard to keep a hold of with how society is.

  5. “Space has no temperature”. Yep, it does. In the solar system it comes in the form of radiant energy from the sun – which, being a form of electromagnetic radiation, can travel across even a perfect vacuum (which, incidentally, not even deep space actually is, its density being equivalent to around 1 hydrogen atom/m^3).

    1. No, The radiant energy keeps going until it finds something to warm up. It doesnt lose any energy on its journey through the imperfect vac of space until it finds mass to try to reach thermal equalibrium with, dust cloud, gas cloud, planet. Those few n far apart hydrogen atoms aren’t generally counted when the temp of imperfect vacuum of space is considered in relation to other object reaching thermal equilibrium with it.
      Oh, i mean it IS from someone who wants to come along and, with a very technical (& largely irrelevant) point mansplain things, but, no, space really has no measurable temp, despite your mansplaining.

      1. I 100% agree with this but at the end of the day how do we really know anything??? So much of what I took for fundamental has turned out to be false and my faith in information itself has been compromised. Don’t you find it strange that trees can live over thousands of years but organic life forms rarely live a couple of hundred? We are living through a simulation of some kind with a creator beyond our own comprehension. Forget what we know and start from there.

        1. Matrix? Yeah, matter=energy. Reality IS dubious, I suppose. But play along, since we’re trapped here anyway. You weren’t here 1,000 years ago, either. We can only depend on boring science to observe perceived truths. Too bad, I guess.
          But it does SEEM to bring about advances that make life better for the next generation. IF they exist.

      2. STFU “Bob”, you ignorant choad.

        You have no idea what you’re babbling about and it’s clear to anyone with a functioning brain. Stop pretending you do and learn more of what you think you know.

  6. As a physician I can assess that this article is complete nonsense:
    1) Double speaking about no space issues (no sky-vacuum issues vs.deadly radiation)
    2) MUS (making sh!t up) doctrine and parroting similar rant.
    3) No science method employed (hypothesis with independent variable, dependent, control, etc.)
    4) Affirming the consequent fallacy: “Man evolved”
    5) Fundy religious belief in a sky-vacuum

    1. Can you please elaborate your answer ? Cause I’m willing to hear you if you have better scientifically correct knowledge about the topic and peer reviewed papers on specific facts discussed here that disprove what the author explain. However, your phrasing only makes you look like a triggered troll who knows absolutely nothing.

      So please, stop with insults, and provide information.

      1. I strongly agree with you. I would love to understand more in depth understanding of what I just read from Mr.Phillip..

        now here is a topic I’d love to find facts behind.
        now hypothetically speaking of course. would a settlement, let’s say on the moon for example or even Mars cause a trigger in human evolution? let’s say over a 750 year span of a baby boom on Mars how would a fetus develop in a more dense atmosphere? would it actually have denser cartilage or would it actually be softer? would the baby take more time to develop a full skull possibly even causing a dip perhaps more flat headed maybe lol?
        and during that 750 year push would the mere bones themselves be so evolutionised( <— ik… sounded good at the time… ) that the mere DNA would forever be altered? for example Mars man meets earth women, well…. you know. would the baby be a different breed type of human? actually better question would the Mars man have troubles breathing on earth for a long period of time. Baah! so many questions! I hope none of this sounds really dumb to you guys…

        1. my deepest apologies Mr Phillip. I had a brain fart i didn’t mean to tag your name, I mean I did but not as the way I put it into words. I strongly agree with you. how that happened I don’t know.

        2. I’m not a professional of anything but I like to read, learn and explore a lot of topics. From what I understand over a long enough time radiation destroys dna. It doesn’t mutate into new exciting things really. People being constantly exposed to it would slowly become sterile or if they were able to become pregnant it would likely miscarry or possibly develop a lot of hindering qualities, as it is being exposed as well during gestation.

          If the fetus was on Mars and under a lead shield it would probably adapt to the environment it was in. I would say that it would probably have less muscle mass for the lighter gravity but you’d still have calcium deficiency I would bet. There are a ton of factors that would change a human on Mars. Being born in microgravity could even cause the spine to weaken over time as there’s no need to reinforce the structure.

          As for having children between Mars and Earth I don’t think there would be a difference for at least millennia upon millennia There are gene pools on the planet that were separated for a very long time and they managed children fine. Assuming again, they’re safe and fertile.

          A person may not even be able to walk on Mars after being in space for so long, let alone returning to Earth without a very long process of slow integration to gravity. Like rising out of the deep sea and slowly adjusting to pressure. You would have to acclimate back in. Otherwise your heart couldn’t sustain the sudden demand of gravity to it’s system again. Blood would pool into the legs and possibly clot. The person would faint and lay prone and possibly have those clots reach a vital area and that’s the end.

          We are in many ways so utterly fragile. Our technology needs to catch up to our fantasy but there is a lot riding against us.

      2. I agree. This self proclaimed so called “physician” has no clue what he/she has no clue on determining what year, date, time, etc….

        Who or whatever wrote this has no clue on not only astrophysics, physics, ot even general math.

        Without crayons cam someone please explain that as a person or him/her/that moves higher that there is less air, and yes it gets colder.

        So why is this person practicing medicine.

    2. 1) this did not seem like double speak to me. The article said what we see in movies is not entirely accurate but there are still dangers. That is not double speak. Also please elaborate on “sky-vacuum”. Rephrasing something to make it sound stupider is lazy, and does nothing for clarity. I don’t know what you are trying to imply.
      2) Where specifically? Also, what is meant by “parroting similar rant”? who is parroting and similar to what?
      3) I didn’t think this article sold itself as being a study backed by the scientific method. Given we aren’t about to start shooting people into space, to see what kills them, it seems that this article does an ok job at sharing our best guesses around these things based on anecdotal evidence and observations (which is all we really have)
      4)I assume you are referring to the argument that since man evolved with gravity, his organs will probably all perform sub-optimally without it. I agree this is not a solid argument but it seems decent as a good starting point for a hypothesis.
      5) what? Please be more clear. Again, your attempts to be dismissive/insulting are robbing your statements of clarity and credibility. Are you saying that the idea that space is (close to) a vacuum is a fundamental religious belief? Which religion? Please elaborate as this is an interesting claim.

      1. yeah I’m pretty sure this is just some guy that went to a cheep college trying to go pro and stopped half way and fell face first into a flat earth theory and it dumbed him down

    3. Finally I’m reading something that makes sense….. Every piece of information that comes out of NASA may as well have come out of a child’s fairy tale

    4. Uhm, no. This article is in kine with both hypothesized effects AND what hapoened to the Russian. Do your research. YOUR criticism shows no scientific methodology or rationalis pro doctorum equatii.

  7. “More than half of the muscles of the human body resist gravitational force on Earth, musculoskeletal acclimation to microgravity results in profound muscle atrophy, reaching up to 50% muscle mass loss in some astronauts over the course of long-term missions”
    is it true? I never estimate that can be happen…

  8. I wonder if, once we have individuals staying in space for more than 438 days at a time if we will see an subsequent increase in radiation levels. I am almost certain that manned space missions will continue to grow as we advance technologically and outward into space. I am excited, but still wary of the potential consequences. Nice article!

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