I was breathing just fine through my life support systems, but I kept feeling an urge to double check that everything was ok with my "spacesuit" before heading out on this "spacewalk." I have worn many different types of spacesuits on Earth during my career, but for the first time in my life, I knew that I couldn’t just stop to breathe the air outside if there were an emergency. That's because I was underwater. On this excursion, I was leading a simulated space mission underwater called SEA STAR. My "spacesuit" was actually scuba diving gear, and the simulated spacewalks, or EVAs (Extra-Vehicular Activities), included outings where we explored underwater environments. Being underwater, if we encountered a major problem with our gear, we would have had to treat it as seriously as if we were astronauts on the International Space Station. There was no immediate "out." This was serious.Before this underwater mission, I had worked as the commander of nearly 40 analog missions that simulated crewed missions to the moon and Mars. The goal of these types of missions is to test what living and working in lunar or Martian environments might be like. We study everything from the psychology of living in extreme conditions, or in isolation, to the technology that astronauts might use to complete those missions successfully.Testing spacesuit technology is also an important part of these analog space missions. These suits are usually a much simpler version of the ones astronauts use in space, which can weigh around 310 pounds (141 kilograms) when fully equipped with life support systems. Analog spacesuits usually weigh a maximum of 30 pounds (14 kg). While there are many differences between suits designed for use in space and these suits designed for testing on Earth, the biggest one is that, on an analog mission, crew members can open their helmets and breathe the outside air if necessary.Scuba "spacesuit" equipment for simulated "spacewalks" during the SEA STAR mission. (Image credit: Michaela Musilova)Astronauts in space don’t have that advantage. But neither do aquanauts, those performing a simulated space mission underwater. Aquanauts spend at least 24 hours underwater in a so-called state of saturation. That’s a physiological condition achieved by a person who breathes underwater long enough for the body’s tissues to absorb the maximum possible amount of inert gas (like nitrogen), for a specific depth and pressure. Water leaking into our suits or into our underwater habitat can pose serious risks for the crew, as well as health problems related to the higher pressures underwater, amongst other challenges unique to this environment.This is what pushed me to be extra thorough, and perhaps even a bit paranoid, when preparing for my scuba diving "spacewalks." While scuba diving, the dangers of your equipment failing or decompression sickness can be as lethal as some of the difficulties astronauts face during their spacewalks. That’s why we follow strict protocols when preparing to go on underwater EVAs and during the outings themselves. A SEA STAR crewmember performing a simulated "spacewalk" underwater. (Image credit: Michaela Musilova)As an experienced scuba diver, I wasn’t concerned with the scuba diving aspect of the mission itself. In many ways, it was much easier than some dives in pitch black water I performed in the open ocean or cave dives. But for these EVAs, I was more on edge because I was the Commander of the SEA STAR crew living 22 feet (7 meters) underwater in the station, called the Jules’ Undersea Lab, in Florida. Being responsible for other people’s lives made me extra cautious and also more worried than usual when diving.Despite the many challenges of these underwater EVAs, these experiences and lessons learned can teach us a lot about what we still need to improve for future long-duration space missions.Our aquanaut "spacesuits" were essentially the same as scuba gear, with the exception of two important accessories that aren’t always necessary for normal dives. Those were gloves and flashlights. We needed the gloves as protection from a number of factors in the environment around us, like barnacles and other sharp natural formations. They also made using cameras and vacuums for environmental cleaning underwater more challenging, which was a good approximation for how gloves limit astronauts’ dexterity during spacewalks. The underwater environment around us had very low visibility, which is why the flashlights were a mandatory component of our EVAs. They helped us to navigate through the darkness and find each other. Even still, with just the small flash of light we could only see a couple of feet ahead of us. That is why it was also vital that we worked together and moved side-by-side. Good communication skills were another crucial component of our underwater simulated space mission. Before each EVA, we prepared a plan, talked about backup plans and went over emergency procedures. During the EVA, we used standard scuba diving sign language to communicate, and we created some new mission-specific signals as well. This visual communication is very useful during simulated space missions on land, too, especially in situations when our communications technologies have not worked properly.No matter how much we prepared, though, the fear of the unknown and of getting lost can still get to a crew. A common safety protocol when diving is to safely return to the surface if you get lost or separated from your team and cannot find them. However, during the SEA STAR mission, we planned on staying underwater for the full five-day duration of our mission. That meant that such an option was out of the question, except of course in emergency scenarios.Personally, I enjoyed the challenge of the low visibility in new terrain. It allowed me to hone my orientation and navigation skills, as well as my abilities as a leader in challenging environments. Though I was more vigilant than I perhaps needed to be, once I dove out of our submerged station, I went straight into my favorite mode: exploration. Despite not being able to see very much ahead of me, and all while keeping a watchful eye on my crewmembers, I finally truly felt like an astronaut on a spacewalk. Darkness was all around me, but my determination and curiosity propelled me toward exploring the unknown.You can follow Dr. Michaela Musilova at her website and on social media: @astro_michaela (Instagram, TikTok and Threads), @Dr. Michaela Musilova (Facebook) and @astro-michaela.bsky.social (Bluesky).