Effect of spaceflight on the human body

Human adaptation to space is a challenging field in the development of more practical human spaceflight.

The fundamental engineering problems of escaping Earth’s gravity well and developing systems for in-space propulsion have been picked at for well over a century and millions of man-hours of research have been spent on them. However there has now been a fundamental shift from research into the mechanics of space flight to the issue of how humans will actually survive and work in space for long periods of time. This question requires input from the whole gamut of physical and biological sciences and has now become the greatest challenge to deep space exploration. A fundamental step in confronting this challenge is understanding the effects long-term space travel has on the human body. These effects are the subject of this article.

Importance of subject

The sum of mankind’s experience has resulted in the accumulation of 58 person-years in space and a much better understanding of how the human body adapts. However, in the future, industrialisation of space and exploration of inner and outer planets will require humans to endure longer and longer periods in space. A recent report has highlighted the severe limits to the quality and quantity of current data that make it difficult for scientists to extrapolate all the effects of living in space long term. The majority of the data comes from missions of short duration and so some of the long-term physiological effects of living in space have never been adequately investigated. A round trip to Mars with current technology is estimated to involve at least 18 months in transit alone. How the human body reacts to such time periods in space is a vital part of the preparation for such journeys. On board medical facilities need to be able to cope with any type of trauma or emergency as well as contain a huge variety of diagnostic and interventional instruments in order to keep a crew healthy over a long period of time. These will be the only facilities available to cope with not only trauma, but also the adaptive responses of the human body in space.

Public perception of issues

Public perception of living in space is still more skewed towards the TV sci-fi of Star Trek and Star Wars where gravity is artificially produced and journeys to distant stars take less than a day. The harsh reality of living in space is much more difficult and much less romantic than people believe. The well being of humans in space can be separated into two areas, their physical well being and their psychological well-being.

Unprotected Effects

The environment of space is highly dangerous, without appropiate protection. The greatest threat is from the lack of pressure in the vacuum environment, while temperature and radiation effects also have a small influence.

Contrary to imagery in the public media (such as in Total Recall), a short term exposure to space of up to 30 seconds is unlikely to cause permanent physical damage. Thanks to the containing tension of the skin, the body will not explode, though some slight swelling may occur. Nor would blood boil, as the body automatically regulates blood pressure - though exposed fluids such as saliva may indeed evaporate. While space is typically very cold, due to the lack of a medium to allow convection, loss of heat is by radiation only, and so very slow. Therefore, there is no danger of immediately freezing.

Some physical damage may result if the victim attempted to hold his/her breath on introduction to the low pressure environment. In that case, a ruptured lung may result from the imbalance in pressure. Damage may also be done to ear drums, and the gastric system. Without the protection of the atmosphere, solar radiation, particularly ultraviolet rays may cause severe sunburn in a few seconds. After 10 seconds, decompression sickness may also result.

However, the primary threat is of asphyxiation. In the low pressure environment, normal gas exchange would instead cause the rapid deoxygenation of the bloodstream. After up to 15 seconds, the deoxygenated blood would reach the brain, and loss of consciousness would result. Death would gradually follow after two minutes of exposure - though the limits are uncertain. If actions are taken quickly, and normal pressure restored within around 90 seconds, the victim may well make a full recovery.

As well as experimentation with dogs and monkeys, a few cases of loss of pressurisation have occured in the past, especially in experimentation on spaceflight projects. One such case is discussed in a NASA technical report: Rapid (Explosive) Decompression Emergencies in Pressure-Suited Subjects:

"At NASA's Manned Spacecraft Center (now renamed Johnson Space Center) we had a test subject accidentally exposed to a near vacuum (less than 1 psi) in an incident involving a leaking space suit in a vacuum chamber back in '65. He remained concious for about 14 seconds, which is about the time it takes for O2 deprived blood to go from the lungs to the brain. The suit probably did not reach a hard vacuum, and we began repressurizing the chamber within 15 seconds. The subject regained conciousness at around 15,000 feet equivalent altitude. The subject later reported that he could feel and hear the air leaking out, and his last conscious memory was of the water on his tongue beginning to boil."

There has been one recorded incident of death from decompression in spaceflight, Soyuz 11 decompression accident, in 1971.

See also: Spacesuit

Protected effects

Despite modern technology, some hazards still prove impossible to remove. The most important factor affecting human’s physical well being in space is weightlessness, more accurately defined as microgravity environments. Living in this type of environment impacts on 3 types of human tissue:

• gravity receptors
• fluids
• weight bearing structures

Gravity Receptors

Living on earth we constantly feel the constant gravitational pull and although we are not conscious of it our bodies constantly react automatically to maintain posture and locomotion in a downward pulling world. In microgravity environments, these constant signals the body is so used to are transformed. The otolith organs in the middle ear sensitive to linear accelerations no longer perceive a downwards bias, muscles are no longer required to contract to maintain posture and pressure receptors in the feet and ankles no longer signal the direction of down. These changes can immediately result in visual-orientation illusions where the astronaut feels he has flipped 180 degrees. Over time however the brain adapts and although these illusions can still occur most astronauts begin to see down as where the feet are. People returning to earth initially have great difficulty maintaining their balance but recover the ability very quickly underlying the remarkable ability of the human body to adapt to weightless conditions. Over half of astronauts also experience symptoms of motion sickness for the first three days of travel due to the conflict between what the body expects and what the body actually perceives.

See also: Space adaptation syndrome

Fluids

The second effect of weightlessness takes place in human fluids. The body is made up of 60% water, much of it intra-vascular and inter-cellular. Within a few moments of entering a microgravity environment, fluid is immediately re-distributed to the upper body resulting in bulging neck veins, puffy face and sinus and nasal congestion which can last throughout the duration of the trip and is very much like a cold. In space the autonomic reactions of the body to maintain blood pressure are not required and fluid is distributed more widely around the whole body. This results in a decrease in plasma (water in the blood stream) volume of around 20%. These fluid shifts initiate a cascade of adaptive systemic effects that can be dangerous upon return to earth. Orthostatic intolerance results in astronauts returning to earth being unable to stand unassisted for more than 10 minutes at a time without fainting. This is due in part to changes in the autonomic regulation of blood pressure and the loss of plasma volume and becomes worse the longer the time spent in space although as yet all individuals have returned to normal within a few weeks of landing.

Weight bearing structures

The third and most worrying effect of weightlessness involves bones and muscles. Without the effects of gravity skeletal muscle is no longer required to maintain our posture and the muscle groups used in moving around in a weightless environment are very different to those required in terrestrial locomotion. Consequently some muscles atrophy rapidly. The types of fibre in muscles also change. Slow twitch endurance fibres used to maintain posture are replaced by fast twitch rapidly contracting fibres that are insufficient for any heavy labour. Bone metabolism also changes. Normally bone is laid down in the direction of mechanical stress, however in a microgravity environment there is very little mechanical stress. This results in a loss of bone tissue approximately 1.5% per month especially from the lower vertebrae, hip and femur. Elevated blood calcium levels from the lost bone result in dangerous calcification of soft tissues and potential kidney stone formation. This is disastrous when a million miles away from the nearest medical facility. It is still unknown whether bone recovers completely. Loss of bone and muscle make it very difficult for humans to move and even breathe under the weight of Earth’s pull upon their return. The longer the flight the more loss will occur until it will become impossible for the individual to survive the pull of Earth’ gravity for any extended period of time.

Effects of radiation

Weightlessness is not the only factor to effect the human body in space. Without the protection of the atmosphere astronauts are exposed to high levels of radiation through a steady flux of cosmic particles. A year in even low-earth orbit results in a dose of radiation 10 times that of the annual dose on earth resulting in a high risk of astronauts developing cancer. High levels of radiation can create “chromosomal aberrations” in blood lymphocytes. These cells are heavily involved in immunity and so any damage may contribute to the lowered immunity experienced by astronauts. Over time immunodeficiency results in the rapid spread of infection between crewmembers, especially in such confined areas. Radiation has also recently been linked to a higher incidence of cataracts in astronauts. Protective shielding and protective drugs may lower the risks to astronauts to an acceptable level, but data is scarce and longer-term exposure will inevitably result in greater risks.

Other physical effects

Other physical discomforts such as back and abdominal pain are commonly experienced with no clear aetiology. These may be part of the aesthenia syndrome reported by cosmonauts living in space over an extended period of time but seen as anecdotal by astronauts. Fatigue listlessness and psychosomatic worries are also part of the syndrome. Data is inconclusive, however the syndrome does appear to exist as a manifestation of all the internal and external stress crews in space must face. The amount and quality of sleep experienced in space is poor due to highly variable light dark cycles on flight decks and poor illumination during daytime hours in space craft. Even the habit of looking out of the window before retiring can send the wrong messages to the brain resulting in poor sleep patterns. These disturbances in circadian rhythm have profound effects on the neurobehavioural responses of crew and aggravate the psychological stresses they already experience.

Psychological effects

The psychological effects of living in space have not been clearly analyzed but analogues on Earth do exist, such as arctic research stations and submarines. The enormous stress on crew coupled with the body adapting to other environmental changes can result in anxiety, insomnia and depression. According to current data however astronauts and cosmonauts seem extremely resilient to psychological stresses. Interpersonal issues can have an enormous influence on a human’s well being and yet little research has been undertaken to examine crew selection issues in relation to this. Interestingly the Mars Arctic Research Station and Mars Desert Research Station have examined the influence of different crew selections when living in a completely isolated environment and may provide vital data for future experiences.

Future prospects

At the moment only rigorously tested humans have experienced the wonders and dangers of space. When the time comes for off-world colonisation families will be exposed to these dangers and the effects on the elderly and on the very young are completely unknown. Factors such as nutritional requirements and physical environments which have not been examined here will become ever more important. Overall there is too little data on all the effects of living in space which makes mitigating all the risks during a long haul space trek all the more difficult. Test beds such as the ISS need to be utilised fully to research these risks and more humans need to live and work in space for our knowledge to grow. It is a testament to the human capacity for endevour that many would still gladly volunteer for such assignments.

The space environment is still full of many unknowns, and there may well be hazards of which we are currently not aware. Meanwhile, future technologies such as artificial centrifugal gravity and more complex bioregenerative life support systems may go some way towards neutralising the remaining hazards, but as it is today space travel is a difficult and dangerous business and only our natural adaptive abilities will allow us to succeed.

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