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AEROSPACE MEDICINE 101:
ADAPTATIONS TO SPACEFLIGHT
Tamarack R. Czarnik, M.D.
OVERVIEW
During the Mercury program, duration of spaceflights ranged from minutes
to hours. Apollo flights to the Moon extended spaceflights from hours
to days, and the United States' Skylab program observed exposures to space
conditions lasting for weeks to months. The Mir space station program
has provided data on the effects of spaceflight for over a year of continuous
exposure.
As duration of spaceflight has lengthened, we've come to recognize a number
of changes that the human body goes through as a result of exposure to
the space environment. For the most part, these changes appear to be physiological
rather than pathological; that is, changes tend to keep the body functioning
correctly, rather than strictly injurious effects of the new operating
conditions.
But as we throw ourselves (at speeds exceeding 28,000 kilometers per hour)
from one environment to another, our bodies adapt only gradually, and
perhaps incompletely. When we try to function while incompletely adapted,
our bodies fail us.
The extent of this paper, then, will be to examine our new operating conditions
(the space environment), the adaptations and effects of these new conditions
on our bodies, and which adaptations/effects may potentially be disabling
('show-stoppers") to future astronauts. This article is only a brief and
incomplete overview, and readers are encouraged to e-mail me with questions
requiring more in-depth knowledge.
THE SPACE ENVIRONMENT
Many of the environmental conditions beyond Low Earth Orbit are immediately
fatal: the body cannot adapt to the lack of air and pressure, and must
be provided artificial atmosphere and sufficient pressure through spacecraft
design. Space (being essentially a vacuum) has no ambient temperature,
but extreme radiant heating and cooling results in lunar temperatures
from -180 to +100 C (-292 to + 212 F) , and in deep space this drops to
a few degrees above Kelvin (-270 C or -454 F). Every spacecraft, from
Vostok I in 1961 to today's shuttle, has to be designed to shield us from
immediately fatal exposures.
While space is a near vacuum, it is by no means empty. Micrometeoroids,
most of them less than 1/1000th of a gram, bullet through space at high
speeds, packing tremendous kinetic energy capable of punching through
spacecraft walls. Radiation, whether as the continuous flood of Galactic
Cosmic Radiation or the occasional surge of Solar Particle Events, can
cause radiation sickness, cancer and death. Each presents its own potentially
life-threatening concerns.
Gravity and weight are functions of proximity to Earth (or any other large
mass). In Low Earth Orbit, gravitational (centripetal) force is exactly
balanced (due to the craft's velocity) by inertial ('centrifugal') force,
resulting in a balance of near-zero gravity. Further from Earth, the force
of gravity falls rapidly to near-zero with increasing distance, resulting
in microgravity.
Finally, less well studied are the psychological concerns of prolonged
spaceflight, including the effects of prolonged confinement, separation
from social supports, tedium, reduced sensory stimuli and loss of privacy.
While frequently passed over in favor of better-understood threats, psychological
factors are believed to have played a role in three medical evacuations
from Soviet missions, and even rigorous screening programs have not precluded
incidents of severe psychopathology during submarine missions. Study of
behavior during long tours on submarine or polar science stations (like
the Mars Society's proposed Devon Island facility) is now underway to
identify effective screening, education and countermeasure options.
ADAPTATIONS TO SPACEFLIGHT
While the human body cannot adapt to such conditions as anoxia (lack of
oxygen), hypobaria (lack of pressure) and radiation, it can and does adapt
to weightlessness. These alterations in body equilibrium are here divided
into short-term (hour to days) and long-term (weeks to months) changes.
In the first few minutes of weightlessness, the body's fluids shift dramatically.
Since gravity is no longer keeping fluid (in the forms of blood and extracellular
fluid) pooled in the legs, fluid redistributes up into the chest and head.
This causes the characteristic 'puffy face' and 'birds legs' appearance
of astronauts, as well as nasal congestion (probably contributing to loss
of sense of smell as well).
The body's sense of balance is partially due to the inner ear (semicircular
canals) and feeling of weight in certain joints (proprioception). With
the loss of gravity, both sensations are disabled: fluid in the semicircular
canals no longer convey feelings of rotation or 'up and down', and there
is conflict between what we see and what we 'sense'; for instance, your
eyes can tell you you're upside-down, but your 'balance' says you are
tumbling. This conflict between what you see and what you feel causes
Space Adaptation Sickness, or 'space-sickness': early onset of vomiting,
headache, and a sick feeling that subside suddenly after 2 or 3 days in
space.
As the first day of spaceflight wears on, the kidneys try
to compensate for what they perceive as excess fluid by excreting more
of it, resulting in abnormally low fluid volume. Mineral concentrations
increase, resulting in increased risk of kidney stones. The body then
decides it has too many red and white blood cells for its volume, and
proceeds to eliminate some of these, resulting in relative anemia.
In the next few days, the body adapts further to its new conditions. Without
gravity to compensate for, muscles do not need to work so hard to move
things, and muscular atrophy occurs. The heart is also a muscle; since
blood no longer has weight, the heart 'eases up', becoming weaker. Levels
of Thyroid hormone increase, which speeds up metabolism and contributes
to body mass loss. Immune system function (both cell- and antibody-mediated)
decreases, and capacity for exercise decreases.
As days turn into weeks, both muscle and bone readjust. Stress in our
bones keeps them strong; without gravity, stress is lost and bone is reabsorbed
at the rate of 1-2% per month (depending on the individual and site of
measurement). Muscles atrophy and lose protein, further contributing to
loss of body mass. Many countermeasures have been tried to stop or reverse
this readjustment, but even the most successful only partially slow its
advance (more on countermeasures in a later article).
As the initial excitement of launch and the new environment wear off,
psychological adaptations can cause problems. Soviet investigators described
three phases of psychological adaptation to spaceflight, based on long-term
flights of Salyuts and Mir. The 'acute' (first month) phase involves adjustment
to the new and busy environment. An intermediate phase is marked by increasing
fatigue and loss of motivation, irritability and emotional lability ('asthenia').
If countermeasures are not instituted, a final 'long-duration' stage of
hypoactivity, feelings of isolation and worsened asthenia proceeds. Sleep
is prolonged and disturbed, depression and hostility worse, and productivity
plummets. Both cosmonauts and astronauts have become hostile, refused
to work and become more withdrawn during long-duration missions. Not all
researchers share Dr. Zubrin's optimism concerning psychological problems
in long-duration spaceflight.
On return to a gravity field, the body must readapt to the unaccustomed
conditions. To make matters worse, during reentry Shuttle crews encounter
about 1.5 G's, half again Earth's normal gravity. On returning to Earth,
10% of astronauts (and most cosmonauts, who've been up longer) cannot
stand upright. Most feel unsteady and weak; many vomit. Almost all have
difficulty accurately pointing to and manipulating switches and buttons
in front of them: as gravity increases, the arms feel unaccustomedly heavy
and awkward, and pilots 'overshoot' the controls.
Clearly, the human body undergoes dramatic changes in function during
prolonged exposure to space and spaceflight. But so what? Who cares if
astronauts weigh less, have a puffy face and are a quart or two low? Will
any of it affect the mission, how they function away from Earth, or be
life-threatening on Earth? That is the subject of our final section, 'Show-Stoppers'.
SHOW-STOPPERS
'Show-Stopper' is a term often used casually to refer to
any condition that would preclude the safe continuation of a mission,
resulting in a mission that is aborted or never attempted, or that would
prevent the astronaut from ever safely returning to Earth. To give an
example: when Neil Armstrong made the first landing on the Moon, computer
failure caused him to overshoot his landing area. Heart pounding and less
than a minute's fuel remaining, he overrode the computer and manually
guided the 'Eagle' over the cratered and deadly surface below him, skimming
the lunar surface, searching for a safe place to put down. By virtue of
years of training and a cool head, he made it.
Now let's say Neil is landing on Mars, a trip 1,000 times as far and taking
50 times as long. But now he's dehydrated from months in space, disoriented
and vomiting from the sudden onset of gravity, unable to accurately manipulate
controls (due to the unaccustomed weight in his arms). Does he still make
it?
Several adaptations to long-duration spaceflight appear to have the potential
to become show-stoppers. We will examine several of these more closely.
Bone Loss While most of the above-mentioned adaptations disappear after
the first 3 days in weightlessness, Microgravity-Induced Bone Loss is
the exception. As our time in space increases, we've found that bone loss
due to weightlessness is extremely variable from person to person, but
relentless and cumulative. We typically lose from 1 to 2% of our calcium
(depending on the person, countermeasures, etc.) per month in space, and
so far we've never hit bottom. This means that an astronaut making the
30 month trip to and from Mars could potentially lose 30 to 60% of his
calcium, greatly increasing the risk of bone fractures. Risk of kidney
stones, another potential show-stopper, is also increased.
Fortunately, it's not likely to be as bad as that. Though bone loss does
not stop, there are indications that the rate of loss slows down. Some
partial protection is likely from the 38% gravity on Mars, and though
no countermeasure is entirely effective, exercise and other countermeasures
(discussed in a subsequent paper) can help. Vladimir Titov and Musa Manarov
were weightless for 366 days (Soyuz TM-4, 1987-88), yet a few months after
returning were travelling the world and appearing none the worse for the
experience.
Dizziness (Vestibular Dysfunction) Though muscle atrophy and cardiac
deconditioning sound more serious, dizziness and disorientation due to
a return to gravity is more likely to scrub a mission. Unlike motion sickness,
we cannot predict on Earth who will develop disabling disorientation in
space. The astronaut's body has adapted to weightlessness by becoming
dehydrated, and has learned to interpret conflicting visual and vestibular
(i.e. 'balance') cues. But on landing on Mars, astronauts will go from
0 G's to almost 0.8 G's to 0.4 G's. As stated above, returning astronauts
are dizzy, disoriented, and often nauseous. And as Neil Armstrong proved,
landing can put heavy demands on a pilot.
Some progress has been made in preventing this potentially disabling dizziness.
Astronauts on the Shuttle now drink a salt solution before reentry, which
improves their hydration. Injectable Promethazine is on-board and relatively
effective, and countermeasures are proving helpful. Exercise, 'load-suits',
LNBP and other ways to avoid return-to-gravity dizziness will be discussed
later.
Psychiatric Concerns Although passed over as negligible in "The
Case for Mars", psychiatric dysfunction is a real concern of many researchers.
Prolonged missions aboard Mir have been accompanied by lethargic, grouchy
and disinterested cosmonauts, and one French astronaut aboard the Shuttle
attempted an EVA - without a spacesuit! Tales of crew conflicts and interpersonal
battles rarely make the public headlines, but are not news to NASA Flight
Surgeons. Aresnauts (astronauts to Mars) would be isolated for almost
3 years, in a tiny living space, virtually without privacy: testing on
this scale would be considered inhumane on Earth!
Again, ongoing research on submarine and Mars Analog Stations (like the
Mars Society's proposed facility on Devon Island) is making headway on
screening, identifying and treating potential psychological conflicts,
and countermeasures (such as daily communication with family, scheduled
'down time' and on-board recreation) are being developed. Psychiatric
'show-stoppers' is one area the Russian Space Agency has taken seriously
longer than NASA, and another area needing consideration
CONCLUSIONS
Space is a dangerous place. Some of the dangers, like lack of air, lack
of pressure and radiation, we cannot adapt to, and we must protect ourselves
from. Our bodies do adapt to many new conditions, but these adaptations
cause problems when we try to move to another new environment (e.g. space
to Mars, or return to Earth). In general, our bones and muscles (including
the heart) weaken, and our fluids shift to the upper body (resulting in
dehydration, relative to Earth-normal). Without our usual balance cues
we can become dizzy and disoriented, though this lasts only a day or two.
Psychiatric problems can crop up, due to the isolation, stress and loss
of privacy. Several adaptations which can potentially abort a mission
are identified, including bone loss, dizziness and psychiatric problems.
So what can be done to avoid these stumbling blocks? That is the subject
of our next paper, Aerospace Medicine 102: Countermeasures. See you in
two months!
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