Astronaut Perspectives: An Overview
I joined NASA’s astronaut corps in 1978 and made my first space flight on the shuttle Discovery in April 1985, which lasted 7 days. This was the 16th flight of the Space Shuttle, and although a lot more is known today about the effects of space flight on the human body than was known then, people already understood many of the most significant short-term impacts of space flight. Our crew was well briefed on most of the physiological changes reported by earlier shuttle crews, and we knew many of the phenomena that we could expect to experience ourselves. This article describes my personal experience with some of the most common phenomena, such as fluid shift and neurovestibular disturbances. Hopefully it will put a personal touch on the more technical articles found elsewhere.
When I was nearing the launch date of my first flight, an experienced astronaut suggested to me that I might ease the accommodation to the fluid shift from the legs to the head and upper body that occurs in weightlessness by raising the foot of my bed and sleeping in a six degree head-down position during the week-long preflight medical quarantine period. I knew that this head down position is used to simulate weightlessness in bedrest studies, and it seemed to make logical sense, so I decided to try it. Unlike the subjects in bedrest studies, however, I could not stay in bed all day, since I had to continue training and attend briefings. Whether or not a week of continual bedrest would help accommodation, I cannot say. However, it certainly did not help me.
After the overwhelming experience of my first rocket ride into orbit, my first sensation after main engine cutoff was that I was upside down, probably because of the fluid rushing into my head. This was a much stronger sensation than I had had going to sleep in a six-degree head down posture. The urge to urinate over the first 2 days was also much stronger than anything I had experienced during my head down sleep. I did not suffer much of a headache, which some astronauts have reported, but I cannot ascribe this to my “preflight accommodation.” I concluded that trying to prepare for the fluid shift of weightlessness to ease the process once in orbit is not worth the inconvenience of rearranging ones sleeping position. The actual experience of the fluid shift was mildly unpleasant, but nothing I could not tolerate. Even for people who have more discomfort than I did from the initial fluid shift, I think that “practicing” for fluid shift seems a bit like practicing bleeding or practicing banging your head against the wall. If these unpleasant experiences occur, you have to deal with them. But subjecting yourself to them in advance does not in any way make the actual experience less uncomfortable.
Another aspect of fluid shift is the expansion of the fluid-filled disks in the spinal column. Combined with the absence of gravity loading on the skeleton, the spine grows longer. I was not aware of this during my first flight, but over the first few years of shuttle flights, numerous astronauts reported lower back pain during the first few days in orbit. Sleep stations were modified so that you could pull your legs up towards your chest and hold them there with a bungee cord, which seemed to relieve lower back pain for some people. On my second and subsequent flights, I measured my height in orbit and found that I grew from my normal 6′ 2″ (188 cm) to 6′ 4″ (193 cm). I experienced mild lower back discomfort during the first day or so, on only two of my five space flights. Interestingly, I experienced mild back discomfort after landing on one of the three flights where I had experienced no back problem on-orbit. On two flights, I experienced no back discomfort either in orbit or after landing.
During my fourth space flight, STS 61, in which I did three spacewalks as part of the rescue/repair of the Hubble Space Telescope, I measured myself both before and after each spacewalk. I like a tight spacesuit, where the shoulder straps keep my feet pressed tightly down into my boots, giving more control over the suit. Before each shuttle flight, EVA crewmembers gave their spacesuits a test run in a vacuum chamber. This was a confidence building measure, especially for astronauts who had never done an EVA. The only thing done to the suits after this final test was to increase the length of the torso slightly, to compensate for the anticipated height growth in weightlessness. My suit length was increased by 1½” (3.8 cm), so that with my actual height growth of 2″ (5 cm), I was guaranteed the tight fit that I desired. The spacesuit was sufficiently tight that after each spacewalk I had red welts from the shoulder harness. What I found from the measurements was that after each spacewalk, my height had decreased by ~3/4″ (~1.8 cm), presumably due to my spine being compressed in the suit. I regained my “normal” space height within a few hours. The flight surgeons expressed an interest in this, although I do not think it had any real operational significance in terms of modifying any procedures.
The other aspect of fluid shift in weightlessness is that the pressure sensors in the aorta and carotid arteries sense an excess amount of fluid in the body. These sensors do not know that they are in space, so they assume that the fluid excess extends over the entire body, and the body responds accordingly, decreasing thirst and increasing urination. Astronauts lose from one to two liters of body fluid before reaching “in-space equilibrium.” I experienced all these effects, just as predicted. We were also briefed on the problem of postlanding hypotension, where under the influence of gravity blood flows back to the legs, leaving too little blood getting to the brain. This apparently caused many astronauts in the early days to be unable to stand up for more than a few minutes before getting presyncope symptoms or in some cases being unable to stand up at all without passing out.
The solution to this problem came to be known as “pre-entry fluid loading.” Astronauts would drink one to two liters of water, depending on body size, to make up for the fluid lost during the first few days of space flight. This would allow the body to maintain adequate blood pressure to supply the brain with enough blood to prevent syncope. However, we were told that pure water would not be absorbed properly into the bloodstream. Instead, the fluid had to have an isotonic salt content, matching the natural salinity of body fluids. We could either drink salt water or take salt tablets with the pure water we were drinking.
It is instructive to relate the anecdotal account of the first trial of this countermeasure. The medical community had proposed a traditional experiment, whereby some of the crew would perform the fluid-loading countermeasure and others would not. In order to obtain the required statistical significance, this experiment would have to be conducted over numerous shuttle flights. The crew who first tried this countermeasure reported in their postflight debrief to the astronaut office that the astronauts who fluid loaded were in far better condition after landing than those who did not. Apparently, the message was so strong that on the next flight, no crewmembers volunteered to be controls, since everyone wanted to fluid load. So the medical community had to give up on their desire to demonstrate statistical significance, and from that time on, fluid loading became standard procedure.
I followed the fluid loading protocol during all of my flights, and I was always in good shape after landing, never having any problems with light-headedness while standing up, walking around, or even climbing up and down the ladder between the middeck and the flight deck. However, the large amount of salt we had to take along with the water produced unpleasant stomach sensations. Before my fifth space flight, I asked the flight surgeons if there might be some more palatable liquid onboard that had enough salt to produce the desired fluid loading effects. We had many powdered fruit drinks, but these were mostly loaded with sugar, not salt, and were not appropriate for fluid loading. However, on consultation with the food providers, the flight surgeons determined that the powdered chicken soup we had onboard had a sufficiently high salt content to be effective. Therefore, before my final reentry to Earth’s gravity, I consumed 2 liters of chicken soup, which seemed to work as well as the water and salt tablets had, with less stomach discomfort. I only wished that my grandmother, who had recently died at age 106, had still been alive. She had always told me that chicken soup was a cure for almost anything, and now we had living proof of this from NASA!
One of the things that the flight surgeons were interested in determining was whether or not returning crews would be in good enough shape to evacuate the shuttle on their own in case of a landing emergency such as a fire or a leak in one of the tanks containing toxic substances. I always tested myself by getting up and walking around right after landing and, on a few flights, by climbing up and down the ladder connecting the middeck and the flight deck. I could tell that my heart rate was accelerated, but I always felt that I would be capable of getting away from the shuttle quickly after landing should that be necessary. However, on most of my flights I observed numerous crewmates who would not have been able to do this. Some seemed to lack the muscular strength to move their now much heavier bodies. Others tried to stand up, felt light-headed, and had to sit down again, despite everyone having followed the fluid loading protocol. A few had serious vertigo, so much so that they had to be given anti-vertigo drugs in order to be able to stand up.
For me, one of the strangest aspects of space flight occurred after landing and seems to have been a manifestation of what is technically referred to as the “tilt-translation phenomenon” (Young et al. 1986). Despite all the briefings we received, nobody had mentioned this effect before my first flight, so my experiences were totally unanticipated. After unstrapping my seat harness, I started to remove my helmet, which involved tilting my head forward and back. Instead of the normal feeling of a slight to-and-fro tilt, what I experienced was a violent linear motion of my head backwards, then a sensation of bouncing off the aft cabin wall and hurtling forward. Thinking back to preflight briefings, I remembered that first-time pilots were recommended not to move their heads during reentry, and I realized immediately that this was some sort of neurovestibular phenomenon where gravity was being misinterpreted by my vestibular system. At that time, I did not know the details of the phenomenon, but being of a scientific bent, after removing my helmet, I started moving my head in various directions to try and analyze the sensations. I did not sense any rotational confusion, so I am not sure that this phenomenon is related to the vertigo that plagues some returning astronauts. I only felt a linear back-and-forth motion, which made my head seem like a tennis ball in active play.
On standing up, I realized that I could not rely on my internal sense of balance to stay upright and move around, since even a slight tilting of my head caused a strong sensation of linear motion. However, I was able to tell myself to ignore what I was feeling and just concentrate on what my eyes were telling me, which as far as I could tell had not been affected by my return to gravity. This allowed me to walk around the cabin, although my motion was far from smooth. By the time the ground crew had entered the cabin and we were ready to exit the shuttle and descend the stairs, I noticed that this strange sensation had already decreased considerably. There seemed to be an almost exponential falloff in its intensity. I think that this phenomenon is responsible for recently landed astronauts walking around with legs somewhat further apart than normal, reeling a bit when going around corners like sailors getting on dry land after a long sea voyage. Flight surgeons were obviously aware of this problem, because when I entered the shower room in crew quarters, there was a large sign saying “Do Not Shower Alone”. This was clearly due to the reasonable fear that we might lose our balance and fall over, but, as can be imagined, it was the source of many jokes that day. After a few hours, the phenomenon seemed to have faded away, although doctors told me that they could still track effects on eye motion for a day or two. Astronauts returning from 6 months on the space station have told me that this phenomenon lasted much longer for them. If there is an exponential falloff, as I perceived, then the time constant seems to be related to the length of time spent in space. Whether or not the effect of flight duration ever reaches a plateau, I do not know, but for shuttle flights, at least, it demonstrates the importance of getting rapid access to the crew to measure readaptation to gravity, since by the time I went in for medical tests, I felt that many symptoms had already faded.
On subsequent flights, I knew what physiological symptoms to expect both at the beginning of weightlessness and on returning to Earth. I think there is definitely a learning curve and a bodily memory of the effects of space flight. Because of the Challenger disaster, I had to wait 5 years before my second space flight. But aside from the fullness in my head caused by fluid loading, which will always occur, I felt as if the first day of my second space flight was like the eighth day of my first flight. The intervals separating my remaining three flights were much smaller, so I cannot tell if there is a limit beyond which the body memory of space flight fades, but as would most retired astronauts, I would be happy to volunteer for another space flight to test this hypothesis!
- Young LR, Oman CM, Watt DGD, Money KE, Lichtenberg BK, Kenyon RV, Arrott AP (1986) M.I.T./Canadian vestibular experiments o the Spacelab-1 mission: 1. Sensory adaptation to weightlessness and readaptation to one-g: an overview. Exp Brain Res 64:291–298Google Scholar