Quite frankly, as big of a space addict as I am, I had no idea NASA did any underwater training beyond their famous Neutral Buoyancy Lab in Houston. Written by neuroscientist and diving physiology expert Csilla D'Agostino, she details both her surprise at being selected for NEEMO as well as the work they did underwater for a full two weeks. In selecting the  23rd NEEMO expedition team, commanded by Italian astronaut Samantha Cristoforetti, NASA was looking for a non-astronaut with both relevant field work experience and a strong diving background, and D'Agostino happened to fit the bill. This isn't that surprising - NASA looks favorably on scuba certification when selecting astronauts. In fact, back in the 1950s when NASA was still debating selection criteria for their first batch of astronauts, some thought expert scuba divers would hold the right stuff for going to space, as a lay person might experience a sort of "rapture of the deep" floating high above the Earth and risk severe mental impairment. NASA ultimately decided military test pilots were their best bet, but scuba diving still brings a lot of relevant skills

For example, scuba divers must ascend to the surface very gradually, as the water pressure compresses nitrogen gas into the diver's body. Ascend too quickly, and the nitrogen forms dangerous bubbles that can rupture key organ structures and cause decompression sickness. Similarly, since astronauts breathe pure oxygen in their spacesuits (normal air is 78% nitrogen 21% oxygen, but there's no point carrying nitrogen in the suit), before any launch or EVA the astronauts need to spend hours pre-breathing pure oxygen to purge any nitrogen from their bodies. Coincidentally, I just read Apollo 11 astronaut Michael Collins' fantastic autobiography Carrying the Fire, and in it he describes in excruciating detail (pun intended!) how during his rookie spaceflight on Gemini 10, his left knee started throbbing mid-flight due to nitrogen decompression:

"... my left knee hurts, a throbbing ache that began a couple of hours ago, gradually worsened, and is now holding steady at a moderate but very uncomfortable level of pain. I think it is nitrogen coming out of solution in the tissues and creating little bubbles which press on my nerves... true, I denitrogenated for a couple of hours before liftoff, but the time varies considerably with the individual, and I must be more sensitive than most - or perhaps some air leaked into my suit as I transferred my hoses from the portable oxygen supply to the spacecraft oxygen supply.

That's all irrelevant anyway; the point is what to do now..."

   

Apart from pressurization, D'Agostino and the NEEMO 23 team researched various other scientific questions that must be well understood before we establish long-term presence in deep space, such as cognitive and sensory performance, sleep quality, cardiovascular rhythm, and gastrointestinal microbiomes. One point I found interesting, and which D'Agostino highlights as the most challenging part of the mission for her, was the weight of the bulky gear underwater - even with the buoyancy, a 32lb helmet plus 60lbs of gear is best followed by a deep-tissue massage for sore shoulders! To add insult to injury, NASA added an extra 20lbs just to simulate lunar conditions

It kind of makes me wonder: although we've extensively studied the detrimental effects of zero-gravity on the International Space Station (see the Scott Kelly twins study!), I don't think we have a good understanding of reduced gravity on the human body. For example, is 16% Earth gravity on the moon enough to support normal bodily function for year? If not, how about 37% on Mars? With a manned base on the lunar South Pole seemingly the most logical next step in human space exploration, hopefully we'll have answers to these questions soon!