At first you might not think
astronauts riding in the space shuttle or on the space station would have much
in common with people who dive underwater for fun or for a living, but there are
some similarities. In both cases they
must bring along their supply of life sustaining air along with a suit to
protect them from the harsh environment they are immersed in. Underwater, you have to contend with
hypothermia due to the water temperature being lower than your body
temperature, and the pressure, which increases as you dive deeper. In space, you have an extreme version of the
temperature problem—down to about –250F/-160C in the shadows and up to about
+250F/+120C on the sunny side. In addition,
you have a vacuum, micrometeorites and radiation that your suit has to protect
you from. Working in a vacuum means
that the astronauts must wear a pressurized suit and it is here that they share
a common hazard with divers—the “bends” or “decompression sickness”.
You’ve probably seen more than one
movie or TV show depicting a diver racked in pain after surfacing too quickly
from a deep dive. What is happening is
that while they were happily chasing the fishes in the watery depths, the
increased pressure forced the nitrogen gas in the air they were breathing to
dissolve into the tissues in their bodies.
When they quickly surfaced, the dissolved nitrogen turned back into a
gas forming bubbles, which can damage the organs in which they formed. (A good
example of this is when you open a bottle of soda—especially a warm one— and
all of a sudden a gusher of bubbles and foam erupts. This is the carbon dioxide, which was dissolved in the liquid at
a much higher pressure, being released because of the sudden drop in pressure
as the cap is removed.)
Divers have to contend with the
large pressure changes as they dive into deeper waters, but the astronauts
don’t face a high-pressure environment, so how do they risk the bends? The space shuttle is a sealed environment
and when in orbit, the astronauts breathe an oxygen/nitrogen (20%/80%) mixture
at a pressure of 14.7 psi (101 kPa) (also known as 1-atmosphere of pressure),
just like we do on the surface at or about sea level. Problems arise once they have to do an EVA (Extra Vehicular
Activity). The astronauts don their EMU
suits (Extravehicular Mobility Units) and if they pressurized the suits at the
normal 1 atmosphere they would look something like a Macy’s Thanksgiving Day
balloon with their arms and legs sticking straight out of the suit—they would
not be able to bend their arms and legs!
Certainly not a sight becoming any astronaut! The solution is to drop the pressure in the suit to a point that
they can comfortably move about and have enough oxygen available to function at
the level of physical activity the task requires. This working pressure for the spacesuit is about 4.3 psi (30
kPa). Now you can see where the
pressure differential comes from. It’s
not as dramatic a change in pressure as diving a 100 meters below the ocean,
but it is enough to allow the dissolved nitrogen to be released.
To help the astronauts acclimate to the lower
pressure in their suits they can “prebreathe” pure oxygen for 4 hours prior to
the EVA. Basically this means that they
go about their affairs wearing a respirator, which provides 100% oxygen for the
time period required. If they exercise
while they are on oxygen they can reduce the prebreathe time to about 2 hours. Alternatively, the shuttle’s environmental
pressure can be dropped to about 9.5 psi for 12 hours prior to the EVA, which
allows the space-walking astronauts to prebreathe oxygen for only 30 minutes
before they enter their space suits.
The purpose of this is to eliminate any dissolved nitrogen in the body that
may come out when they put on their EMU’s.
In the event that an astronaut does
have a problem with the bends, the EMU can be pressurized up to 8 psi over the
cabin pressure, and the suit effectively becomes a mini-hyperbaric chamber to
slow the release of the nitrogen back into the body.
So, whether you are diving for
treasure in Davy Jones’ locker or you are doing maintenance on the outside of
the International Space Station you now know that you are susceptible to the
bends in either environment. Oh, and by the way, did you know that scientists
now have evidence that indicates that sperm whales also suffer from the
bends? Even these mighty leviathans of
the deep appear not to be immune from the bends.
Here is a link that will provide
more information on whales and the bends: http://www.livescience.com/animals/whale_bends_041224.html.
Picture Credit: http://aerospacescholars.jsc.nasa.gov/HAS/cirr/Images/suitlg.jpg