weems.umass-coins@UDel-Relay@sri-unix (11/30/82)
From: Charles Weems <weems.umass-coins@UDel-Relay>
This latest sequence of notes seems to have split into the questions of
"What would really happen to an unprotected human in a sudden
depressurization?" and "Why do spacesuits need to be so bulky?".
I thought I'd throw in my two cents worth on each of these subjects.
First, what would happen in a depressurization? Someone asked if any
experiments had been done. The answer is, unfortunately yes. According one
of my biologist friends, some German scientists conducted a series of low
pressure experiments on prisoners from the concentration camps during WW2.
These were slow depressurizations. The results weren't pleasant.
Let's assume that you're in a spacecraft, breathing pure oxygen at about
6psi. (The effects are only worse if the pressure is higher or the gas mix
isn't pure O2.) Lets also assume that the loss of pressure isn't instan-
taneous but is still fairly rapid -- say it takes 15 seconds to reach zero.
Then start timing. One thing that will happen is that the gas trapped
in the lungs will try to expand. If you hold your breath (which isn't hard,
we can easily lock up quite a bit more lung pressure than is good for us),
the pressure will force your diaphragm down, displacing your viscera.
(remember, your diaphragm is used to expanding your lungs). At some point
the lungs would rupture, causing severe internal bleeding. This could be
compensated for by some sort of corset or other pressure garment around the
torso. (But we're assuming you're just wearing you're overalls.) The other
alternative is to release the gas pressure from the lungs as quickly as
possible, but in a controlled manner. If you didn't panic and had practiced
doing this, it wouldn't be too hard (your lungs won't burst instantly, and
besides, we're assuming that the pressure doesn't reach zero all that
fast). You would need to get the lung pressure down to about 2psi to
be manageable by your diaphragm. At this point the gas mix becomes
important since a normal atmosphere mix at 4psi is about as low as you can
go without reversing the direction of oxygen transfer across the membrane
in your lungs. With pure oxygen, 2psi should keep it going the right way.
(Although other things will start to happen, but we'll get to that in a
bit.) It will also be important for you to widely open your mouth to
clear the pressure from your eustachian tubes and inner ear. If you have
a slightly stuffed up head, too bad, your eardrums will burst. Almost
immediately, your nose will start to bleed -- the vessels in your nasal
lining are very sensitive to pressure differentials. Other thin membranes
will also start to bleed very quickly. These include your lips and mouth
lining (since you had to open your mouth), excretory tract linings and
(in women) vaginal lining. Perhaps most disabling will be that the insides
of your eyelids, tear ducts and even your eyeballs will start to bleed (all
those little vessels that make your eyes look bloodshot will burst very
quickly). You will also have problems with any gas in your digestive tract.
This will expand or displace the viscera and cause severe cramps.
(Although that will probably be among the least of your worries.) At 2psi
the linings in your lungs will start to bleed but not massively (the higher
the original pressure the greater the bleeding will be).
Assuming that you don't pass out from the pain, or lose control, how long
could you last? One limiting factor is how long you could remain conscious.
Once you lose control and the pressure in your lungs escapes, they
will hemorrhage massively and there will be no chance of reviving you.
Considering various reflex reactions and the loss of blood to the brain due
to embolisms, 15 seconds is a reasonable estimate. You would probably be
severely impeded in any effort to restore the pressure after the first 10
seconds. Assuming you spend the first 5 seconds reacting and venting your
lungs and eustachian tubes, that doesn't leave much time to act.
Just for argument's sake, lets say that you were wearing a protective
face mask and a corset, and that your exposed mucous membranes were
also protected. How well would your skin act as a space suit? Not very
well. The skin is less than one percent as permeable as the lining in
your lungs, but that's still fairly permeable. Someone pointed out that
when they go swimming, they come out thirsty because they don't absorb
water. All this implies is that the skin is relatively impermeable to
water (good thing too, or you'd drip all over an awful lot). Actually
it's not even all that impermeable to water. For one thing it's covered
with pores through which you sweat. It's also that permeability that makes
your skin shrivel up when you keep it in water for a long time. It is very
permeable to quite a number of things including a variety of carcinogenic
solvents and most gases (your skin actually breathes to a small extent --
this helps make up for the fact that blood vessels don't run right at the
surface). The real problem, however is that your skin is very elastic. Due
to the large internal fluid pressures it would ballon up almost instantly.
This is how the decrease in pressure is transferred through the skin at first.
The pressure decrease results in the fluid pressure within the capillaries
bursting them. This is further aided by bubbles of gas forming (disolved
gasses in the fluids come out of solution) which block flow in the
capillaries, thus causing local pressure increases which burst more
capillaries. Thus soon after depressurization the small vessels near the
skin's surface would begin to rupture due to the formation and expansion of
bubbles of gases dissolved in the blood. (It should be noted here that this
mechanism is the same one at work in the membrane bleeding discussed above.
The vessels are simply much closer to the surface and more numerous in these
areas. This is really what people mean when they refer to the boiling of
body fluids in a vacuum. Although the fluids themselves will evaporate too,
the process will not be quite as rapid due to surface effects. Note that it
doesn't matter that the vessel walls can hold the fluid pressures under
normal external pressure. For one thing that external pressure is a real
help to them. They will expand when that pressure is removed. For another
the vessel walls are also permeable to the disolved gases.) The vessel
rupturing would proceed at a much slower rate than in the lungs, however it
would still be very quick. In addition, there will be some genuine 'boiling'
of body fluids, although it might be better to call this rapid evaporation.
The combination of this with the sudden reduction in pressure in tissue near
the surface will lead to rapid and intense cooling. At some point fairly
early on, some of that tissue will actually freeze. This will block flow in
the deeper vessels and cause some rupturing there. Even the frozen fluids
would continue to evaporate through sublimation. To an observer, you would
first appear to ballon up like a Macy's Parade baloon character, then a cloud
of 'steam' would appear around you. The length of time that you could survive
can be looked at two ways. There is the time, after which, the damage is
too great to be repaired and death will inevitably occur though perhaps not
for some time if pressure is restored. Then there is the time at which you
would expire if no pressure restoration occured. The maximum time for
recovery is somewhere in the range of 15 to 30 seconds. (Although with
anything over 15 seconds, the quality of the recovery might be questionable.)
The time at which death will occur is somewhere around 45 seconds. In both
of these cases, the limiting factor has become the point at which embolisms
stop the flow of blood to the brain. In an explosive decompression this can
happen in as little as 5 seconds. The above figures assume that the pressure
doesn't go to zero instantaneously, but at some nonetheless rapid rate
(reaching zero in, say, about 15 seconds). Unconsciousness would occur within
a second or two after blood flow to the brain stopped (the neurons sense and
react to this very quickly) and death would follow very soon after. Hence,
although Dave in the 2001 scenario might have survived (the suit he had on
would have been a great help) he would not have come out of the air lock
fit and trim and ready to tear HAL apart. Because of his missing helmet,
he would have probably lost his vision due to vessel breakage in his retina
his eardrums would have ruptured (and bled) and he would have had
a really bad nosebleed, bleeding lips and mouth (and his hair would have been
pretty messed up too). Not a pretty sight.
OK, lets assume you're wearing some skin tight elastic suit that covers
your body and seals cleanly with a face mask, but is permeable to gasses.
This prevents your internal pressure from ballooning your body. Now how long?
Still not very long. Again, the gas permeability will allow the gasses near
the surface to come out of solution. This will transfer the pressure drop
inside fairly quickly (albeit not quite as quickly) and cause bubbles to
form in the blood which will eventually cut off the flow to the brain. Since
the whole body is involved, these build up very rapidly. Perhaps this would
push the times to about 30 seconds to recover and 60 seconds until death.
The cooling will also occur in this case, so a case of nearly total frostbite
might also result. I doubt that anyone could survive such a condition even
if the pressure was restored quickly. Note, however, that exposing a small
part of the body, sufficiently far from the brain (as the blood flows),
wouldn't necessarily be fatal. The embolisms would tend to disperse before
they reached the brain. The exposed area would probably be severely injured
but (if the exposure wasn't too long) might actually recover with proper
treatment.
Now, there are several reasons why current space suits are gas pressured
and so bulky. First of all, gas pressured suits are more comfortable. The
circulating gasses help remove perspiration and the various waste products
in it. (some of the components of perspiration are actually toxic) If we
had a gas impermeable skinsuit, it would have to be permeable to perspiration
and all of its toxic components. If it were, then evaporative cooling would
become a problem and require additional heating energy to compensate. (Except
when you were in direct sunlight, when it would still not be sufficient to
cool you.) Another problem is that an evaporative system like this would
pollute the vacuum around the spacecraft. This wouldn't be a permanent
condition, but it would prevent such things as infrared telescope work while
anyone was doing an EVA. Without the evaporation, all that's needed to keep
the astronaut warm is a good layer of insulation. In fact the current suits
don't have any heating systems -- they (like the shuttle) only need to
provide for cooling. The astronaut provides more than enough heat when well
insulated. Another comfort factor is that the pressured suit isn't skin
tight. Imagine trying to work for any length of time with tight elastic
wrapped all around your body -- it just isn't going to stretch in all of
the directions you need to move in. (If it did, it wouldn't keep you from
expanding.) Hence you'll get fatigued fairly quickly. The gas pressured
suit on the other hand may be a little stiff, but you can move it just about
as easily in any direction -- and it's not going to have all of that skin
friction and elastic resistance to muscle flexure and skin movement.
Lets assume too that you could make this thing with built in insulation,
cooling and a non-stick internal surface (except in the gloves and other
places where some internal skin friction is needed). (This kind of thing
gives materials science people nightmares, by the way.) We still have
to remember that we'll doing work in this thing for long periods on EVA.
Thus it will be necessary to have extra wear protection in areas that rub
together (such as armpits, crotch, insides of joints, etc.). Unfortunately
these also happen to be areas of maximum perspiration and will need more
permeability. It would be nice too to have several layers of back-up
pressure holding material or perhaps just a fast puncture sealing property
in the skinsuit. It would also be nice to have something that stopped
some of the radiation -- a foil of any dense material would do to stop the
low energy alpha particles. Finally the skinsuit material would have to
be amenable to shaping or to cutting and joining in a gas tight manner
(and with no weakness at the seams).
The current space suits accomplish all of this by using several layers
of different materials, combined with a gas pressurization and circulation
system. Even if a miracle skinsuit fabric was developed, it's unlikely
that it would be used because it still doesn't provide air for the skin
to 'breathe'. As mentioned above, the skin actually breathes to compensate
for the fact that blood vessels don't run right on the surface. Although
it wouldn't kill you to have the skin sealed off for a fairly long period,
it would get rather uncomfortable on any long EVA's.
The conclusions we can draw from all of this are: The skin makes a
very poor spacesuit and even in the best of conditions couldn't keep you
alive for more than a minute. The materials do not yet exist to make a
non-gas-pressurized suit that would be practical for long periods. It would
be possible to make such a suit for short EVA's or emergency use, but it
would probably be even more difficult to put on, and too uncomfortable to
wear continuously as a safety backup.
chip weems
weems.umass-coins@udel-relay