Ziegler, Michael G. MD; Meck, Janice V. PhD
In the middle of the 22nd century, James Kirk, Captain of the Starship Enterprise, returns from time travel to a space station orbiting earth in the year 2001,
Mission Control: “Jim, thanks for traveling to the primitive space station of 2001. Budget cuts to the intragalactic fleet make it important to earn tourism dollars on our starships. I expect a glowing report for our advertising department. So tell me, how was life in space in the year of the first space tourist?”
Captain Kirk slumps in a chair, appearing thin and weak and says, “Well Commander, fortunately I read electronic copies of Psychosomatic Medicine from the year 2001, so I brought along my space sickness bag. I was too sick to eat or drink but still produced tons of urine for 2 days. By the way, have you ever tried to use a toilet in zero gravity without privacy? After 3 days, my stuffy nose and headache were better. I got back my appetite but lost it as soon as I saw the dehydrated stuff they called food. I couldn’t sleep a wink for the first 3 days with my nose stuffy, floating around the space station, and the sun rising every 90 minutes.”
“But Jim, was it still that bad after the first 3 days?”
“Commander, by the time the nausea and headache stopped, the backache and stomachache started. I stopped making much urine, so I tried to drink extra water so my dissolving bones wouldn’t give me calcium kidney stones. After I learned to sleep in weightlessness, I was more alert and could concentrate on the danger from gamma rays and the chance of a meteorite puncture.
After all of that was over, I was comfortable and having a nice time. But boy, coming down is terrible! I really paid the price for all those adaptations that made me so comfortable. I couldn’t stand up without fainting. I’m really happy to return, though. I lost 4 kilos of weight and now all I want is some salty food, a bed, toilet with gravity, and my own sleeping quarters.”
“My goodness, Jim. It sounds like space tourists in 2001 must have been depressed.”
“No Commander, they were all happy as larks, staring out the windows, taking pictures, and doing somersaults in zero gravity. “As for myself, I’ll take the space shields, artificial gravity, and private quarters of the Starship Enterprise anytime.”
The difference between 2001 and the setting for Star Trek is as great as the difference between popular perceptions and the reality of space travel. Although most have heard space travel results in nasal congestion and nausea, most people have not grappled with the psychological and physiological stresses of space. Few are aware, for instance, of the backache, abdominal pain, loss of appetite, insomnia, postural hypotension, and immune alterations associated with space travel. Human adaptation to prolonged space travel appears as daunting a problem as the engineering difficulties involved in travel to Mars. Adapting man to space is a practical goal of the National Aeronautics and Space Administration (NASA). Equally as important is what we learn about the limits of human adaptability from space flight.
Weightlessness provides a dramatic challenge to human physiology. On earth, weightlessness can be modeled partially by head-down bed rest at a −6-degree tilt. This places the ankles at heart level and reproduces some of the fluid shifts that occur in weightlessness. Head-down bed rest, however, does not reproduce the inner ear problems, nausea, lack of gastrointestinal (GI) stimulation, or redistribution of blood flow in the lungs found in weightlessness. In space, the body is no longer compressed by gravity, and astronauts note that arthritis of the knees no longer produces symptoms. However, astronauts grow taller in space, and stretching of the spinal nerve roots can lead to back pain. The article by Styf et al. (1) reveals that a modification of head-down bed rest reproduces the back and abdominal pain that occur in space and seems to increase depressive symptoms.
Weightlessness rapidly alters cardiovascular and hormonal physiology. Astronauts experience a net volume loss of approximately 800 ml/day for the first 2 days. During the first day, ACTH and antidiuretic hormones increase markedly, perhaps as a response to stress and nausea. There is diminished fluid and food intake until the astronauts adapt to space sickness by day 3. By that time, plasma volume has diminished considerably. In one astronaut, electrical activity of the sympathetic nerves was increased when compared with his sympathetic nerve activity while he was recumbent on earth. However, the astronauts’ heart rate and blood pressure decrease in space as does their heart rate and blood pressure variability. This is probably a manifestation of an escape from the effects of gravity and posture. Thus, resting sympathetic nerve activity in space may increase slightly, but overall sympathetic nerve activity decreases because there is no need for the cardiovascular system to cope with gravity.
Fluid losses and the changes in the sympathetic nervous system might be expected to affect blood pressure control on return to earth. Meck et al. (2) find that approximately 20% of the astronauts cannot tolerate upright posture for 10 minutes on return to earth. Earlier studies from her NASA cardiovascular laboratory showed that this was primarily due to diminished peripheral vascular resistance because of failure to activate the sympathetic nervous system. The study in this issue shows that problems with hypotension and sympathetic nervous withdrawal become more severe with long-duration weightlessness, even though there is no additional fall in blood volume.
One explanation of the data from Meck et al. (2) is that the brain “forgets” how to activate the sympathetic nervous system in response to gravity after prolonged exposure to weightlessness. In space, astronauts learn to catch things that do not fall and learn to go around corners without ground contact. They even learn a new definition of what “up” is. On return to earth’s gravity, they have an uncoordinated gait, impaired motor control, and abnormal baroreflexes. Cooke et al. (3) show that after prolonged weightlessness, baroreflex changes can persist for weeks. The concept that people can learn different cardiovascular and sympathetic nerve responses has important implications for psychosomatic medicine.
The greatest stresses from space travel usually occur in the first 3 days and during re-entry and landing. Despite this, astronauts seem uniformly enthusiastic about the first 2 weeks in orbit. When the novelty of space flight wears off, the effects of weightlessness, confined quarters, and a difficult routine might be expected to take their toll. The Russians put potential cosmonauts through extensive psychological testing before selection for space flight. The article by Kanas et al. (4) references a number of psychological problems that have occurred in space. Nevertheless, they find that astronauts (and cosmonauts) are remarkably resilient to the stresses of space.
The 5-month space flight of J. Linenger was one of the most dangerous and stressful on record. Aboard the Mir, a “day” lasted less than 2 hours. In that environment, the body’s internal clock worked well for about 3 months and then seemed to provide a much weaker circadian influence (5). This has implications for sleep, alertness, and immune function in space.
Immune function partially is controlled by circadian rhythms and the sympathetic nervous system. In space, natural clues to time of day and sympathetic nervous stresses are markedly changed, so immune function might also change. This is a matter of some concern because oral antibiotics are absorbed poorly due to the GI changes of weightlessness. The close quarters of the space station are ideal for spreading infectious disease, a matter of such concern that astronauts are allowed only minimal contact with outside persons for the 10 days prior to launch. The paper by Mills et al. (6) shows that there are consistent changes in immune cells on return from weightlessness. This effect may actually be attenuated after longer missions when the initial stress of adaptation to weightlessness has diminished. Stowe et al. (7) show that virus specific T cell immunity diminishes and Epstein-Barr virus reactivates during space flight. The new field of psychoneuroimmunology may have important lessons for man’s adaptation to space.
Most days we give no thought to the effects of gravity or a 24-hour day. Space travel teaches us how many aspects of our existence have evolved to cope with these pervasive influences. Space is not only the final frontier. For many scientists, it is the ultimate experiment. The articles in this issue of Psychosomatic Medicine tell us about outer space, but say even more about the inner workings of man.
1. Styf JR, Hutchinson K, Carlsson S, Hargens AR. Depression, mood state, and back pain during microgravity simulted by bed rest. Psychosom Med 2001; 63: 862–4.
2. Meck JV, Reyes CJ, Perez SA, Goldberger AL, Ziegler MG. Marked exacerbation of orthostatic intolerance after long- vs short-duration spaceflight in veteran astronauts. Psychosom Med 2001; 63: 865–73.
3. Cooke WH, Ames JE, Crossman AA, Cox JF, Kuusela TA, Tahvanainen KU, Moon LB, Drescher J, Baisch FJ, Mano T, Levine BD, Blomquist CG, Eckberg DL. Nine months in space: effects on human autonomic cardiovascular regulation. J Appl Physiol 2000; 89: 1039–45.
4. Kanas NA, Salnitskiy V, Gushin V, Weiss DS, Grund EM, Flynn C, Kozerenko O, Sled A, Marmar CR. Asthenia—does it exist in space? Psychosom Med 2001; 63: 874–80.
5. Monk TH, Kennedy KS, Rose LR, Linenger JM. Decreased human circadian pacemaker influence after 100 days in space: a case study. Psychosom Med 2001; 63: 881–5.
6. Mills PJ, Ziegler MG, Fritsch-Yelle JM, Waters WW, D’Aunno D. Peripheral leukocyte subpopulations and catecholamine levels in astronauts as a function of mission duration. Psychosom Med 2001; 63: 886–90.
7. Stowe RP, Pierson DL, Barrett ADT. Elevated stress hormone levels relate to Epstein-Barr virus reactivation in astronauts. Psychosom Med 2001; 63: 891–5.