Why Astronauts & Divers are at Risk for Decompression Sickness a.k.a. the Bends
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".
How Divers get the Bends
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.)
Why Astronauts are at Risk for Decompression Sickness
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.
What is Done to Prevent the Sickness
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: https://www.livescience.com/animals/whale_bends_041224.html.
Picture Credit: https://aerospacescholars.jsc.nasa.gov/