Posey's Moonshot

Why Astronauts in Training Spend Time in the Altitude Chamber

Decompression and the resulting hypoxia are the worst-case scenarios of spaceflight. Here's why.

Some of the more amusing yet embarrassing experiences I have had during my commercial spaceflight training have been in the altitude chamber. For those who might not be familiar with the altitude chamber, it is essentially a vacuum chamber that's large enough to hold several people. By reducing the air pressure inside the chamber, it is possible to recreate the conditions at any altitude from sea level to space.

Oddly enough, my first experience with the altitude chamber didn't even involve me being inside it. At the time, I was brand-new to the commercial space program and the instructor wanted to use the altitude chamber as a tool for showing me why a properly functioning spacesuit is so important.

For this particular exercise, the instructor placed a clear plastic mannequin into the altitude chamber and filled it about halfway up with water. We then exited the altitude chamber and watched through a window as all of the air was removed from the chamber. As the air pressure decreased and approached that of space, the water inside of the mannequin begin to boil -- not just a little bit, but furiously. This didn't happen because the water was hot, but because the air pressure inside of the chamber was so low.

The instructor explained in no uncertain terms that the fluids inside my body would boil just as hard if I suffered a catastrophic suit failure in space. Needless to say, this was a very sobering introduction to just how dangerous spaceflight truly is. You can see the video here.

Later that day, I had the chance to actually spend time in the altitude chamber. Because space is a lethal environment, the instructors rarely -- if ever -- recreate spaceflight conditions while humans are in the chamber. Instead, the chamber altitude was set to match the cruising altitude of a commercial airliner. At that altitude, there is plenty of air to breathe but the air is at a low enough pressure that your body has trouble getting enough oxygen into the bloodstream. The reduced blood oxygen level quickly causes you to become hypoxic.

Hypoxia (low blood oxygen saturation) affects everyone differently. Some people experience tingling or numbness in their fingertips. Other people might feel nauseous. Still others feel drunk or giddy. The purpose behind my session in the altitude chamber was to learn what slow-onset hypoxia feels like so I could recognize it if it ever happens in the real world.

The secondary purpose of this particular exercise was to see for myself just how dangerous slow-onset hypoxia truly is. Hypoxia tends to come on slowly and becomes debilitating and dangerous as it progresses. It's extremely important to recognize the symptoms of hypoxia early on so you can take the appropriate countermeasures while you are still coherent enough to do so.

Why is hypoxia so dangerous? The obvious answer is that you can't live without oxygen, but there's actually more to it than that. As your blood oxygen levels decrease, your mind has an increasingly difficult time handling cognitive workloads. The instructors demonstrated this by putting me into a hypoxic state, then asking me to solve a few simple math problems and work my way through a maze.

Even though the math problems were probably third-grade level, I didn't answer a single one correctly. I also failed to complete the maze. I did really well for about the first half of the maze, but after that, I backtracked, hit a dead end and kept on going, making my own exit from the maze. The funny thing is, at the time, I remember thinking that I was doing really well.

The instructors also asked me to complete a shape ball. Designed for toddlers, a shape ball is a plastic ball with various shapes cut out. The idea is to place blocks of various shapes into the matching holes in the ball. Under normal circumstances, completing a shape ball would be effortless; like I said, it was meant for young children. Even so, I struggled with the exercise, managing to correctly place only a few of the blocks.

Another interesting side effect of hypoxia is that it has an adverse impact on your ability to see in the dark. During another session in the altitude chamber, the instructors induced a milder case of hypoxia. This time around, I wasn't nearly as mentally impaired as before. Rather than asking me to solve math problems or play with children's toys, the instructors said they were going to test my night vision. They dimmed the lights and gave me a few minutes for my eyes to get used to the dark. The instructor then showed me a color wheel and asked me how many colors I could see.

I was about to blow the instructor's mind. I have always had extremely good night vision, and I see almost as well in the dark as I do during the day. I confidently told the instructor that I could see six colors. At that point, he turned on the lights and showed me that there were actually about 30 colors on the wheel.

On another occasion, I spent some time in a different altitude chamber in another part of the country. Rather than repeating the exercises I had done before, the instructors wanted to give me a taste of how hypoxia would impact my ability to perform critical tasks related to aviation. My job was to pilot a simulated aircraft while in the altitude chamber. An air traffic controller was issuing rapid-fire instructions to me and several other pilots in the chamber with me. I had to listen to each instruction, recognize whether it was intended for me and (if it was) read it back to the controller and perform the assigned task. This usually meant changing my altitude and heading.

For whatever reason, I didn't become hypoxic quite as quickly as I had in the past, which meant that I was able to keep up with the air traffic controller's instructions for a while. I won't go so far as to say that I was doing high-precision flying, but I was doing a good enough job to get by. Eventually, though, hypoxia began to take its toll.

Remember when I said that one of the side-effects of hypoxia is a feeling of drunkenness or giddiness? Well, that's what ended up happening to me. I actually ended up radioing air traffic control and asking for permission to do a barrel roll.

In case you're wondering, the controller was amused by my request and granted permission. Somewhat surprisingly, I was able to do a barrel roll, even in my hypoxic state. The controller thought it was funny and ended up asking one of the other pilots if he wanted to try it, too. The whole thing devolved into some sort of hypoxic aerobatic competition.

My sessions in the altitude chamber were really eye-opening. Going in, I always assumed that explosive decompression was one of the worst things that could happen (and in space, it is). In an aircraft, however, slow decompression is far more dangerous. I have experienced both types of decompression in various altitude chamber sessions.

Explosive decompression happens quickly. There is a loud bang, a rush of air, a bit of fog, then everything seems normal. You can see what explosive decompression looks like in this video. Hypoxia does eventually become a problem following an explosive decompression, but your time of useful consciousness is usually long enough to allow the aircraft to be brought to a safe altitude where hypoxia is not an issue. Slow decompression, however, can sneak up on you. You might not even realize that you are becoming hypoxic until you reach the point of serious impairment.

That's what the sessions in the altitude chamber were all about -- learning to recognize the signs of slow-onset hypoxia while you are still coherent enough to do something about it.

About the Author

Brien Posey is a 20-time Microsoft MVP with decades of IT experience. As a freelance writer, Posey has written thousands of articles and contributed to several dozen books on a wide variety of IT topics. Prior to going freelance, Posey was a CIO for a national chain of hospitals and health care facilities. He has also served as a network administrator for some of the country's largest insurance companies and for the Department of Defense at Fort Knox. In addition to his continued work in IT, Posey has spent the last several years actively training as a commercial scientist-astronaut candidate in preparation to fly on a mission to study polar mesospheric clouds from space. You can follow his spaceflight training on his Web site.

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