Human Spaceflight

M. Colleen Gino

Introduction

Over the past 40 years, humans have risen above the Earth’s atmosphere, walked on the moon, performed complex mechanical tasks in the vacuum of space and spent uninterrupted months on orbiting spacecraft. We have grown accustomed to seeing humans in space, to the point that we may take for granted the health and safety of our spacefarers. There is little doubt that the extreme success of our space program is responsible for misleading the public into believing that space travel has few associated risks. Nothing could be further from the truth. From lift-off to touch-down, astronauts are exposed to a number of hazardous situations that have the potential to cause irreparable harm. The purpose of this paper is to explore the risks associated with human space flight. The most common health problems encountered in both short-duration and long-duration flights will be discussed, along with the methods of prevention for these problems.

Getting There

Imagine yourself strapped tightly into a chair located in a small metal cabin that is perched atop a rocket full of explosive chemicals, waiting for the explosives to be ignited after which you will be accelerated to such high speeds that you will be unable to move for five to ten minutes. This is not a description of the newest ride at an amusement park, but a description of a Space Shuttle launch. The first set of challenges faced by humans entering into space are encountered during the acceleration and de-acceleration periods during lift-off and re-entry.

Acceleration and de-acceleration are expressed in terms of "G", with 1-G describing the effects of gravity we experience under normal conditions while at rest on the surface of the Earth. Therefore, when accelerating to 2-G you would feel the force of gravity doubled. As G-forces increase due to acceleration, you begin to feel heavier and experience difficulty in moving your arms and legs. As G-forces continue to increase, you may experience difficulties in moving your hands and feet, and are not able to lift your head to look around. At 5-G’s, your ten pound head would weight 50 pounds, a load that your neck muscles are not accustomed to handling.

There are other effects of increasing G-forces as well. Blood circulation becomes impaired as the heart must work harder to pump blood through the body. This impaired circulation can lead to a dimming of consciousness as the heart can no longer pump blood to the brain. You will start to experience tunnel vision, then colors will fade away and everything will appear white, then fade to black. You have just experienced a gravity induced loss of consciousness, or blackout. In general, anything more than 4 or 5 G’s will cause the average person who has not been trained to deal with these effects to black out. Extremely high G-levels can lead to more serious complications such as compacted bladders, bursting red blood cells, subdural hematomas, the inability to breathe and the cessation of circulation.

Shuttle astronauts are exposed to a very tolerable 3-G’s. Effects are mitigated by a variety of means, including the arrangement of seating relative to direction of acceleration and de-acceleration. Seats are arranged such that the entire body faces the direction of acceleration or de-acceleration, so forces are spread across the body rather than concentrated in one area. In addition, the seats are constructed of special composite materials designed to lessen the stresses on the body, and are contoured such that they provide extra support to areas of the body that are more susceptible to injury, such as the neck and the lower back. Astronauts are strapped in to these specially designed chairs so they remain seated correctly (1).

While exposure to 3-G’s poses no problems for astronauts during take-off, even this moderate level of G-forces can be problematic during landing, when astronauts with an already weakened cardiovascular system from the effects of microgravity are exposed to 15 minutes or more of 2 to 3-G forces. Space Shuttle astronauts wear G-suits during re-entry to prevent blackouts. The partial pressure suit is inflated on the abdomen and legs, so as to stop blood from rushing to the lower part of the body when returning from zero gravity.

Being There

Before humans had been sent into space, many scientists made predictions about the effect microgravity might have on the human body, predictions which often were in contradiction with each other. Heart specialists said that the heart would race uncontrollably, or it would stop beating altogether. Psychologists predicted that astronauts would either experience euphoria or become profoundly depressed. It was thought that their bones would become soft, their eyeballs would change shape such that their vision would be impaired, and their ability to think clearly would be affected. Doctors were not sure that food could be swallowed in space, and if swallowed, whether digestion would occur. We now know these fears to be foundless, but there are many real physical problems associated with microgravity.

One common problem, experienced by more than half of the Space Shuttle astronauts, is Space Adaptation Sickness (SAS). There are a number of symptoms associated with SAS including disorientation, pallor, malaise, loss of motivation, irritability, drowsiness, stomach awareness, and infrequent but sudden vomiting. Symptoms of SAS typically peak sometime during the first two days of the flight, then dissipate completely by the third or fourth day. The effects can range from mild to severe, with most people experiencing mild symptoms. In a study of 57 cases of SAS, 46% were considered mild, 35% moderate, and only 11% severe (2). Although the symptoms are not debilitating in most cases, any mission-critical activities such as EVA’s are not scheduled until several days into the flight, after the time when SAS symptoms have usually disappeared.

Some researchers believe that SAS may be related to spacecraft size, with the likelihood of experiencing SAS being less in smaller spacecraft. This conclusion is based on the fact that none of the astronauts in early space missions, which were conducted in extremely small capsules, reported suffering from the symptoms associated with SAS, but 60-70% of the Space Shuttle astronauts experience these symptoms on their first Shuttle flights (3).

There is no way to predict who will and won’t be susceptible to SAS. In general, women are less affected than men, and experienced crew members are less likely to suffer from SAS than the less experienced members.

Fluid Shift

In the presence of gravity on Earth, blood pools in the lower part of the body. After a day or so in the microgravity of space, however, fluids begin to shift toward the head. Faces become visibly puffy as fluids pool in the facial tissues, and some astronauts have complained of feeling "stuffy headed". Since fluid pooling in the head is not a usual condition, the body perceives this as excess fluid and begins to eliminate it. This shift of fluid and consequent elimination stabilizes in about five days. In this time, blood plasma is decreased by approximately 12%, and total body water is decreased by about 2 – 3%.

This normal response to microgravity causes no difficulties while in space, but the body must re-adapt to a normal gravity environment upon return to Earth. Fluid shifts downward, resulting in low blood pressure, weakened pulse, dizziness and possible fainting. There are several measures astronauts can take to facilitate the re-adaptation to 1-G, with vigorous exercise during the mission being but one measure. Another aid is the use of a lower body negative-pressure device, a sack-like device that is pumped up once on the lower body in order to help redistribute fluids in the body, similar to the way they would be in normal gravity environments. Finally, astronauts typically increase their body fluids just before returning to Earth by drinking extra fluids and using salt tablets to aid in the retention of fluid.

Muscles

On Earth, our muscles are constantly working – whether walking, lifting, even sitting or standing upright – during all of these normal activities our muscles are working against gravity to some degree. In a micro gravity environment, there is no appreciable force of gravity for the muscles to work against, so muscle tone is lost. This deconditioning is evident in as few as five days and is progressive. During longer flights, muscles can shrink in size and lose their strength. As the muscles continue to atrophy, they become less resistant to fatigue, and one may experience uncontrolled muscle twitches and a loss of fine motor control. There is a danger of the weakened ligaments and muscles tearing under stresses that would not be a problem under normal circumstances.

As an example, after a 211 day tour of duty on Mir, cosmonauts Berezovoy and Lebedev returned to Earth so debilitated that they had to be carried off their spacecraft, and could barely walk for a week. They required extensive rehabilitation to return to their pre-flight levels of fitness (4). However, the loss of muscle strength and control can be kept to much lower levels by exercising regularly during the mission. A good example of the success of exercise is Shannon Lucid, who after 188 days on board Mir, was able to walk off the spacecraft unassisted, and who experienced a lesser degree of muscle debilitation than she would have had she not exercised regularly during the mission.

But exercise alone is not enough. It is believed that astronauts may lose as much as 25% of their muscle mass during extended missions. Scientists are therefore working to gain a better understanding of how the muscle breakdown actually occurs, and are developing more sophisticated methods to increase muscle mass through the use of hormones and even gene therapy (5).

Bone Mass Loss

It has been found that due to microgravity astronauts experience a progressive loss of calcium and bone mass over time. Changes in bone density is due in part to the reduced work load on the skeletal structure, and also to the loss of calcium that occurs. This loss of calcium leads to a loss of bone mass, which weakens bones and leaves them more susceptible to fractures and breaks. Moreover, when the calcium leaves the blood stream, it is processed by the kidneys to be eliminated in urine. This increase in calcium in the kidneys can contribute to the formation of kidney stones.

In a Skylab IV study of the calcium levels of the astronauts on an 84-day tour of duty. Loss of calcium through urination increased daily until it peaked then leveled off at about 30 days. However, the loss of calcium through defecation increased throughout the 84 days of the mission, at which time the average calcium loss had increased to about 300mg per day (6).

The bone mass loss is approximately 1 – 1 ½% per month, so is negligible on short-duration flights. In addition, the loss of bone density is reversible to some extent once the person is back on the Earth. However, since the levels are generally not restored to their pre-flight levels, this could preclude an astronaut from returning to space for a long-duration mission. For example, a number of cosmonauts who have spent extended periods in space have begun to show signs of osteoporosis (7). It is estimated that during a two to three year mission, the time necessary to reach Mars, the crew members could lose 20% or more of their bone mass (8). In addition, recent studies point to the possibility of tooth loss as well. While the bone density could be built up again upon return to Earth, the tooth loss is permanent.

Current methods of prevention include doses of vitamins D and K, various pharmaceutical agents, exposure to UV light and resistive exercises.

Immune System Changes

Evidence has shown that the immune system is most likely weakened to some extent for a number reasons. First of all, the rapid loss of plasma volume and red blood mass during the redistribution of bodily fluids due to microgravity can lead to a reduction in the red blood cell count. Wounds take longer to heal, and people can be more susceptible to illnesses and infections, including the activation of viruses already present in their bodies. The weakening of the immune system can also be associated with lack of sleep, stress, isolation, and exposure to space radiation. Because astronauts must live and work closely together in small quarters, the likelihood of disease or infection spreading throughout the crew is increased in view of their weakened immune systems.

Prevention methods can be straightforward, and sound similar to those that we undertake here on Earth: get plenty of rest, eat well and exercise regularly. Drugs can be administered to restore immunity, and special monitoring systems or "bacterial probes" can be spread throughout the spacecraft to monitor the purity of the air and water.

Radiation

There are two primary sources of radiation that astronauts can be exposed to. The first form of radiation is cosmic rays, energetic particles that originate outside our galaxy. Basically they are atomic nuclei that have been accelerated to speeds up to 80% of the speed of light. The second form of radiation consisists of energetic particles emitted by the Sun, particularly during solar flares. Low levels of radiation do not pose a significant risk. But the effects of radiation are cumulative, so exposure becomes increasingly dangerous as the length of the stay increases. Therefore, the risks due to radiation increase arithmetically on long-duration flights. In addition, crew members of long-duration interplanetary missions that travel through the Van Allen radiation belts around the Earth will be exposed to a higher level of radiation than those remaining in Earth orbit, such as Shuttle or International Space Station astronauts.

Some of the effects of space radiation seem more interesting than hazardous. For example, the Apollo astronauts in transit to the Moon described seeing unidentified flashes of light. Even the Mir astronauts noticed this, particularly when Mir passed through the "South Atlantic Anomaly." It is thought that this phenomenon is caused by the high-energy particles striking the retina, sending false signals to the brain that light-flashes occurred.

Most effects of radiation are dangerous, and can be responsible for the malfunction and death of bone, blood and other cells. Cataracts can form in the eyes, and both benign and malignant tumors can form throughout the body. Genetic code alteration can occur, causing infertility and sterility, or birth defects and still births. Lymph tissue and bone marrow are particularly sensitive to radiation.

Nutritional Health

In the earliest days of the space program, meals were nothing more than edible paste in a tube, or gelatin covered bite-sized pieces of food. Later in the program, meals were freeze dried or dehydrated food stuffs, that would be injected with water before eating. In those early days, food was designed to meet minimum nutritional needs, but was generally bland and boring. Astronauts often described mealtime as being a necessary but annoying interruption to a busy day. The situation has improved, and current Shuttle astronauts can choose from a menu of over 70 food items and 20 beverages. The situation will improve further for astronauts stationed on the International Space Station, as they will have deliveries of fresh food to augment their standard fare.

There are concerns with longer-duration space flights, however. It has been found that in general, astronauts don’t eat as much as they should, a problem which can lead to weight loss and other nutritional concerns. In particular, low levels of vitamin D can pose a threat, as vitamin D is essential for bone health. In a normal Earth environment, the body produces vitamin D when exposed to ultraviolet (UV) light. However, the spacecraft’s shielding does not allow the inhabitants to be exposed to UV light, so the body is unable to produce vitamin D. This can become quite problematic for those who are already experiencing bone density loss due to microgravity. Prevention measures include doses of vitamin D and UV light treatments.

Staying There

Most of what is known about the effects of microgravity on the human body has been learned from space missions. However, as longer duration space missions are undertaken, we can expect to encounter new difficulties and challenges that we are not prepared for. The normal tour of duty for astronauts on the International Space Station will be from three to six months. While there have been a number of astronauts and cosmonauts who have remained in space for longer periods than this, the after effects of such a long mission may not have been adequately studied. A better understanding of the effects of long duration missions must be achieved before we can seriously consider long duration missions.

Long-duration space flights that entail leaving the Earth’s orbit, such as a mission to Mars, are even more dangerous. First, humans are likely to encounter some yet unforeseen and unknown problems or dangers. There will be no studied response for dangers that have not yet been conceptualized. In such an eventuality, the problem will be compounded by the lack of real-time communication with Earth. Even at Mars, a message would take about ten minutes to get to Earth, with the response taking another ten minutes to return. Even a matter of minutes could make the difference between the success or failure of dealing with an emergency situation.

The three main areas that are of concern to long-duration space flight are radiation hazards, bone density loss, and behavioral adaptation. The danger of radiation and bone loss have been discussed previously, so this section will focus on selected elements concerning behavioral adaptation.

Behavioral Adaptation

The ability to adapt is dependent upon both the person and the environment. In other words, the ease and success of your adaptation depends both on who you are and where you are. You could be considered and easy-going, well-balanced person, capable of adapting to many challenging situations. But the basic conditions encountered in space are not the same as those encountered on Earth.

First of all, space is dangerous. You could be blown up, irradiated, frozen, suffocated, accelerated or shaken to death. You are no doubt aware of these hazards at all times. Second, you are isolated. While you may constantly be in close proximity to other crew members (which can introduce another set of problems), you are isolated in both time and space from those you love – your spouse, children, family, pets. Third, you experience extreme confinement. There is no where to go to be alone, no long walks on the beach; there is no room for privacy. Last, you have precious few amenities. No fresh fruit or vegetables, limited forms of entertainment. Taken alone or in small doses, none of these conditions seem like they would cause insurmountable problems. But add them all up, then multiply them by many months. It then becomes easier to understand how the most well-tempered and well-adjusted person could react quite out of proportion to even the most trivial situations encountered in space.

Interior Space

A number of other factors dealing with interior space come into play on long-duration flights. An environment that is adequate for short-duration flights should not necessarily be considered adequate for long-duration flights. For example, individuals require more personal space when they are confined to that space for longer periods of time. People typically become more sensitive to smells, sounds, lighting, and temperature, as the length of the stay increases. Sleep difficulties can increase, leading to chronic fatigue. The lack of variety of food can lead to loss of appetite, which could lead to weight loss and malnutrition. Hygiene issues take on new importance as people are not able to wash or change clothes as frequently as they would like. Many of these issues are compounded when dealing with crew members from different nations. Differences in language and culturally related social behaviors can add yet another level of complexity to an already complex situation.

Confinement and Isolation

The typical problems associated with months of confinement and isolation includes fatigue, insomnia, headaches, digestive problems and a decline in motivation. Even more difficult to deal with are the social tensions produced in such situations. There have been reports of strained crew relations, heightened friction between crew members, and an increase in social conflict.

These problems can be magnified when individuals from different cultures are involved. It has been shown that interpersonal and psychosocial issues become heightened when dealing with differences in nationality, religion, social and cultural values, and religious beliefs. For example, in a survey of nine astronauts who participated in multi-cultural missions (9), forty-two incidents related to cultural differences were reported. Nine occurred in pre-flight activities, twenty-six in-light and seven post-flight. While the majority of the incidents were believed to have low to medium impact on operations, five of the in-flight incidents were rated as having a high impact. Such psychosocial problems continue to be a challenge to which no solution has yet been discovered.

Conclusion

Space is an extreme environment, with many associated risks and dangers. Humans in space must be protected from many hazards, from space radiation to the debilitating effects of microgravity responsible for progressive loss of bone mass. A myriad of psychological stressors exist as well, many which have not yet been addressed adequately. However, it is part of our basic human nature to explore new environments and push boundaries beyond their current limit. We have been extremely successful at dealing with the problems involved with sending humans into space for short to medium periods of time. While there are marked differences between the preparedness that is necessary for short-duration Earth orbiting missions and long-duration missions to planets and beyond, humans will no doubt rise to the challenge and expand our horizons to include deep space.

References:

(1), (3), (4) Harrison, Albert, Spacefaring: The Human Dimension, 2001, University of California Press

(2) Davis, J.F. et al, Space Motion Sickness during 24 Flights of the Space Shuttle, Aviation, Space and Environmental Medicine, 1988

(5), (8) Swinburne Astronomy Online, Studies in Space Exploration CD, 2002

(6) Rambault, P.C. et al, A Study of Metabolic Balance in Crewmembers of Skylab IV, Acta Astronautica, 1979