Prior to the first flights of Salyut and Skylab, the effects of long-term exposure to a weightless environment were a matter of speculation. Aerospace engineers and space medicine teams from both the Soviet Union and the United States understood that unless humans could adequately adapt to weightlessness, hopes for more sophisticated space stations and long-duration spaceflights would never be realized. To this end, many of the experiments conducted aboard space stations involved determining, testing, measuring, and assessing changes in the conditions of the astronauts and cosmonauts themselves. In a very real sense, the researchers were the subjects of their own experiments. As one might expect, the early flights of Salyut and Skylab established the first thresholds for tolerance of weightlessness; Mir and the ISS have since tested those limits.
The principal concern among space physicians regarding the functioning of the heart in weightlessness is the issue of blood and fluid distribution in the body. Physicians explain that under normal conditions, blood and other body fluids tend to pool in the legs. To counter this effect of gravity, veins in human legs have evolved valves that open and close to assist blood circulation back up to the heart. In orbit, however, blood pressure equalizes and fluids tend to reverse what they do on Earth and pool toward the head.
Space physicians are always aware of the importance of a calm environment on long space station deployments. They try to accommodate the psychological needs of the crew, especially during their personal time between dinner and bedtime. For those two to three hours, crew members are free to do whatever they wish. One of the favorite activities on the ISS, in addition to e-mailing family and friends, is playing musical instruments.
Astronaut Carl Walz once lived on the ISS for 196 days. Before he went up in 2001, Walz recalls in an interview with Karen Miller in her online article "Space Station Music," people asked him what kind of things he would be interested in taking along. Walz said, "Well, a keyboard would be nice. And they said, we'll look into that." He got his request.
A surprising number of astronauts play instruments. There was once even an astronaut rock-and-roll band. And a surprising variety of musical instruments have found their way into space; in addition to the keyboard, there has been a flute, a guitar, a saxophone, and even an Australian aboriginal wind instrument known as a didgeridoo. Astronaut Ellen Ochoa, a classical musician, brought her flute.
In Miller's article, Ochoa recalls, "When I played the flute in space, I had my feet in foot loops." In a weightless environment, even the small force of the air blowing out of the flute would be enough to move Ochoa around the shuttle cabin. In fact, even with her feet hooked into the loops, she could feel that force pushing her back and forth as she played. Still, she adds, "Music makes it seem less like a space ship, and more like a home."
This shift of blood and other fluids toward the head precipitates many problems. The brain interprets its increased blood supply as an increase in total fluid volume rather than simply a redistribution. In response to this misperception, the brain signals the kidneys and other organs to decrease the volume of blood and other fluids by pulling water out through increased urination. The decrease in fluid volumes is not in itself a problem, but the process in turn triggers losses of minerals such as calcium, which leads to loss of critical bone minerals.
Additional tests performed aboard both Skylab and the ISS indicate that an astronaut's blood volume decreases by 10 percent. Although it appears that fluid volume may stabilize at some reduced level, crew members must consume more water, a resource in short supply, to prevent dehydration.
One solution for maintaining normal blood and fluid distribution while in weightlessness is to wear pressure suits such as those worn during launch and reentry. American and Russian crews on the ISS have experimented with the regular periodic wearing of lower-body pressure suits in order to push fluids into the lower extremities. Although this has had some limited success in stabilizing blood volume, astronauts complain about the discomfort of the suits, which they say inhibits their work.
In addition to the effects of weightlessness on fluid distribution, space physicians noticed changes to the cardiovascular system among the crews of all space stations. Initially detected during Skylab and Salyut missions, these changes included a lowering of the diastolic blood pressure—that is, the pressure during the heart's relaxation phase—and a tendency for fainting among space crews. On Salyut and Skylab, the volume of blood actually pumped by the heart was generally elevated during flight. Given the documented 10 percent drop in actual blood volume, this meant the heart was working harder than it did on Earth. This occurred in spite of a progressive decrease in cardiac size.
Other more precise measurements on the ISS using echocardiography confirmed these earlier findings and provided additional information. Echocardiography, in which ultrasound is used to make images of the heart chambers, valves, and surrounding structures, yielded remarkable results. Researchers discovered that the volume of the right ventricle, the chamber that pumps blood to the lungs, decreased by 35 percent during the first day of flight. Meanwhile, the left ventricle, the chamber that pumps blood to the rest of the body, increased in size by 20 percent during the first day, then decreased to 85 percent of its preflight volume during the second day. The volume of pumped blood varies dramatically, and the heart rate increases by 20 percent. As a result, space physicians realize that cardiac output increases substantially during the first day, then decreases to preflight level.
In addition to the use of echocardiography to evaluate the cardiovascular system, in-flight sampling of blood and urine affords researchers the ability to study the chemical and gas composition of the blood as well as the functioning of the kidneys, which filter waste from the blood. These tests reveal a decrease in the red blood cell count in returning astronauts, what is known as spaceflight induced anemia. Research also indicates changes in cellular morphology—that is, the shape of the cells. The normal shape of red blood cells is that of a disk slightly concave on both sides. This shape provides more surface area for the cell to absorb oxygen. When this shape changes to become slightly twisted or flattened, it causes a dramatic reduction in red blood cell absorption of oxygen as well as nutrients. Research also indicates that upon return to Earth, blood and fluid
Just as prolonged weightlessness affects the cardiovascular system, so too does it affect the skeletal system. Normally bone mass is deposited where it is needed and is reduced where it is not. Because the mechanical and pressure demands on bone are greatly reduced in a weightless environment, bone soon begins to dissolve and the resulting calcium, nitrogen, and phosphorus is absorbed and finally removed from the body by the kidneys.
Skylab astronauts lost an average of 8 percent of their bone mass in three months. Soviet cosmonauts, who usually remained in orbit for six months, averaged 15 percent loss, although one cosmonaut lost 20 percent while another lost only 8 percent. Such bone atrophy does not, however, affect the entire skeleton. Evidence on Skylab and the ISS suggests that non-weight-bearing bones such as the skull and fingers are not affected. In the legs and spine, however, which do bear weight on Earth, bone mass declines, as calcium is lost from both the cortical (outer) and trabecular (inner) bone tissue. Diminished bone mass becomes a problem when the astronaut returns to Earth. Also, since the blood carries excess calcium to the kidneys for elimination, the risk of kidney stones, which are made up of calcium, increases.
Space physicians attempt to control bone loss by requiring daily exercise. Aboard a space station, running on a treadmill offers the best workout possible for maintaining bone strength. The downside for astronauts is that an elasticized harness must be used to simulate gravity by pulling the user against the running surface. Astronauts find that such an arrangement is so uncomfortable that they are forced to take breaks every five or ten minutes.
Whether lost bone is regained once astronauts return to Earth's gravity is not entirely certain. Medical experts fear that the body's calcium balance might be restored before the bones have replaced all the lost minerals, resulting in permanent damage. Although cortical bone may regenerate, space physicians fear that loss of trabecular bone may be irreversible. According to Dr. Jay Shapiro, team leader for bone studies at the National Space Biomedical Research Institute, "The magnitude of this effect has led NASA to consider bone loss an inherent risk of extended space flights." 21
The experience of space travelers so far suggests that this risk is real. For example, when the Soviet cosmonaut Yuri Romanenko returned to Earth from Mir after completing his 326-day mission in 1987 (a record at that time), his bones were so brittle and weak that he had to be carried to a hospital. There, he was gradually allowed to increase weight on his skeletal structure over a period of several weeks because of fears that he might otherwise break many of the bones in his lower extremities if he were allowed to walk too soon.
In a weightless environment, muscles, like bones, atrophy from lack of use. Within the orbiting space stations, astronauts are able to move around by softly pushing against walls with a finger or toe and are able to move large loads without breaking a sweat. In 1982 Soviet cosmonauts returned from a 211-day mission on Salyut in obviously debilitated conditions. According to W. David Compton and Charles D. Benson in their book Living and Working in Space: A History of Skylab , "Although they had exercised daily, their muscles were so flabby that they were barely able to walk for a week, and for several weeks afterwards required intensive rehabilitation." 22
Human muscle is of three types: smooth, cardiac, and skeletal. It is the effects of weightlessness on skeletal muscles, those that make movement of the whole body possible, that most concern space medicine specialists.
The bulk of skeletal muscles affected by gravity are located in the lower body. These are constantly under stress in order to keep the body upright. Other muscles also work against gravity—for example, those in the upper arms, shoulders, and back that are used for lifting and moving objects. These muscles, while used constantly on Earth, are hardly used in orbit, where even heavy objects float. When these muscles are not used, they atrophy. Muscle atrophy of 5 to 10 percent can occur by just eight days into a flight. Although muscle atrophy does eventually taper off over time, by the time astronauts have fully adapted to weightlessness, a large portion of muscle mass has been lost.
Experience has shown that all those returning to Earth following extended stays in space have difficulty standing or maintaining their posture. Astronauts also have coordination and walking problems until they are able to retrain their muscles to work against gravity. American astronaut John Blaha, who served on Mir, told fellow astronaut and author Jerry M. Linenger that when he returned from space, he had to be carried off the shuttle on a stretcher. Blaha went on to say that his muscles were so weak that "there was no way I could move. I felt like I weighed a thousand pounds. I could not even lift my arm, let alone stand up and walk. No way." 23
Russian and American space physicians are just as concerned with the psychological effects of long-term stays on space stations as they are about the physical effects. Although the psychology of working in weightlessness is not a major concern, the psychological effects on space station crews of remaining in confined quarters for hundreds of days, far from friends and families, is a serious concern to NASA and other government agencies that deal with the ISS.
Stress has been a by-product of the isolation and close quarters common to all space stations. When psychological problems are discussed, the "twenty-four-hour mutiny" that occurred aboard Skylab is frequently brought up. For one twenty-four-hour period, astronaut Gerald Carr, Ed Gibson, and Bill Pogue refused to do any work, choosing instead to relax, look out the window, and rest. This unexpected rebellion by men acustomed to following orders is seen as evidence that long-duration spaceflights place great stress on astronauts, causing them to act in ways unimaginable on Earth.
Although all space station astronauts have experienced intense stress, some of the most noticeable forms of unsettling behavior have been seen on Mir. In 1996, for example, American astronaut John Blaha,
In the days of sailing ships, healthy young sailors would occasionally throw themselves from the boat and drown, overcome by a fascination with the deep, seemingly endless sea. This often-reported syndrome, labeled "the call of the abyss," seems to have a modern-day equivalent in spaceflight. Just as psychologists describe some people who are compelled to stand on the edge of precipices or tall bridges staring into the abyss and then jumping, more than one astronaut has expressed the same fascination by the free-falling view of space afforded by space walks.
Space walkers have expressed a strange sensation when floating in space with Earth below and the entire universe above them. Right from the start, some space walkers expressed a reluctance to return to the safety of their space station. America's first space walker, Ed White, had to be ordered back into his space station by the director of Mission Control. According to Dr. Tamarack R. Czarnik, who wrote an online article titled "Medical Emergencies in Space," White reportedly sighed and said to Mission Control, "It's the saddest moment of my life."
In 1977 this compulsion to stare with fascination into the void almost turned deadly for rookie cosmonaut Yuri Romanenko. During his stay aboard Salyut 6 with Georgi Grechko, a space walk was scheduled; Grechko would space walk while his partner, Romanenko would remain inside the airlock, monitoring medical readings. But Romanenko's curiosity got the better of him; he reportedly stuck his head out of the hatch and then began drifting farther and farther out. When he started drifting by, Grechko realized his friend's safety line was not attached, and Romanenko was drifting off into space. By leaning over as Romanenko drifted by, Grechko was able to grab hold of his loose safety line and pull him back in.
NASA is aware of this strange and interesting phenomenon. One of the reasons for the tether cord is to prevent space walkers from drifting off into space, where they would die within a few hours and then remain in orbit for years before falling back to Earth. Nonetheless, NASA officials remain vigilant about the possibility of an astronaut, mesmerized by the abyss of space, disconnecting his or her tether and drifting away.
who had been on board Mir for four months, began experiencing fits of anger, insomnia, and withdrawal from other crew members. According to fellow American astronaut Jerry M. Linenger, "He was hurting, he was, in essence, depressed." 24
Stress resulting when a fire broke out aboard Mir led Linenger himself to become increasingly withdrawn and isolated; eventually he even refused to participate in voice communications with ground control. Space historian Bryan Burrough observes in his book Dragonfly: An Epic Adventure of Survival in Outer Space , "Linenger's voice is high-pitched and shrill; he sounds as if he is on the verge of some kind of breakdown." 25
Many psychological problems on the ISS and Mir stemmed from cultural and political differences between the Russian and American crews. Part of the problem was the inability of the two crews to communicate effectively because no one was completely fluent in both Russian and English. This is of particular import for the ISS, where crew members of different nationalities must live together, perform experiments of various types together, and operate the spacecraft together in a confined place for three to six months. Political conflicts between Russian and American politicians over matters on Earth still occasionally spill over on the ISS, causing shouting matches among members of the crew.
Space station experiments since the 1970s have yielded solid results for understanding the psychological and physical stresses placed on astronauts. Some of what has been learned has also been applied to medicine on Earth for the benefit of the public. Although NASA managers and researchers are excited about their record to date, they are equally excited about a whole set of experiments in other disciplines as well.