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.


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[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.

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