The process of pressurizing the space between the Dragon and the station (the “vestibule”) takes several hours and is important to do correctly. The danger Dragon poses to the station is not over: a mistake in this procedure could cause depressurization. So Samantha and I work through the steps one by one, making sure to do it right. First we check the integrity of the seal between the station and the Dragon by introducing air into the opening between them, a bit at a time. As when we arrived in the Soyuz, if the air pressure inside the vestibule were to decrease, even a tiny bit, that would indicate that the seal has been compromised and that opening the hatch would mean venting our breathable air out into the cosmos.
After a number of iterations of this process—introduce air, wait, measure pressure; repeat—we declare the seal safe, but we will wait until tomorrow to open the hatch. That step requires its own sequence of exacting steps. I’ve seen crews push themselves to get through the entire process because they were so eager to get into their care packages and fresh food. The process takes hours, though, and especially after the morning we spent with capture, it doesn’t seem like a good idea to push things—there’s too much risk of making a mistake. It will take us the next five weeks to unload all the cargo.
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WHEN I FLOAT into my CQ for a moment to check my email, it’s the first time I’ve had the chance to pause and think today. The carbon dioxide level is high today, nearly four millimeters of mercury. I can check it on the laptops and see exactly what the concentration of CO2 is in our air, but I don’t need to—I can feel it. I can sense the levels with a high degree of accuracy based only on the symptoms I’ve come to know so well: headaches, congestion, burning eyes, irritability. Perhaps the most dangerous symptom is impairment to cognitive function—we have to be able to perform tasks that require a high degree of concentration and attention to detail at a moment’s notice, and in an emergency, which can happen anytime, we need to be able to do those tasks right the first time. Losing just a fraction of our ability to focus, make calculations, or solve problems could cost us our lives. And we are still learning about the long-term effects of breathing so much CO2. It may cause cardiovascular problems and other issues in the future that we don’t yet understand.
My tumultuous relationship with carbon dioxide has been going on as long as I’ve been flying in space. On my first shuttle mission, which was seven days, I was responsible for changing the lithium hydroxide canisters on board that scrubbed the CO2 from the air. I remember each time I changed the canisters, once in the morning and once at night, I would become aware soon after how fresh the air was—only then did I realize that we had been breathing bad air. Part of our training in advance of flying on the shuttle was meant to let us experience and recognize the symptoms of high CO2; we each went into a booth in the flight medicine clinic to put on a breathing mask that gave us slowly increasing levels of carbon dioxide.
On my second flight into space, again on the space shuttle, I became more aware of how CO2 was affecting me and talked with my crewmates about their symptoms. I could pinpoint those moments when the CO2 was the highest just from the way I felt. I decided to make an effort to start a more serious conversation about its effects. A new space station program manager had just been appointed, and soon after I was back on Earth I helped arrange to bring him on a visit to a Navy submarine under way in the Florida Straits. I thought the submarine environment would be a useful analogy for the space station in a number of ways, and I especially wanted my colleagues to get an up-close look at how the Navy deals with CO2. What we learned on that trip was illuminating: the Navy has their submarines turn on their air scrubbers when the CO2 concentration rises above two millimeters of mercury, even though the scrubbers are noisy and risk giving away the submarine’s location. By comparison, the international agreement on ISS says the CO2 is acceptable up to six millimeters of mercury! The submarine’s chief engineering officer explained to us that the symptoms of high CO2 posed a threat to their work, so keeping that level low was a priority. I felt that NASA should be thinking of it the same way.
When I prepared for my first flight on the ISS, I got acquainted with a new carbon dioxide removal system. The lithium hydroxide cartridges were foolproof and reliable, but that system depended on cartridges that were to be thrown away after use—not very practical, since hundreds of cartridges would be required to get through a single six-month mission. So instead we now have a device called the carbon dioxide removal assembly, or CDRA, pronounced “seedra,” and it has become the bane of my existence. There are two of them—one in the U.S. lab and one in Node 3. Each weighs about five hundred pounds and looks something like a car engine. Covered in greenish brown insulation, the Seedra is a collection of electronic boxes, sensors, heaters, valves, fans, and absorbent beds. The absorbent beds use a zeolite crystal to separate the CO2 from the air, after which the lab Seedra dumps the CO2 out into space through a vacuum valve, while the Node 3 Seedra combines oxygen drawn from the CO2 with leftover hydrogen from our oxygen-generating system in a device called Sabatier. The result is water—which we drink—and methane, which is also vented overboard.
Terry Virts and me working on the Seedra in the Japanese module of the ISS Credit 6
The Seedra is a finicky beast that requires a lot of care and feeding to keep it operating. It wasn’t until I was a month or so into my first mission aboard the space station that I started to correlate the symptoms I was feeling to specific levels of CO2. At two millimeters of mercury I feel okay, but at around three I get headaches and start to feel congested. At four, my eyes burn and I can feel the cognitive effects. If I’m trying to do something complex, I actually start to feel stupid, which is a troubling way to feel on a space station. Above four millimeters of mercury, the symptoms become unacceptable. The levels can creep up for different reasons. Sometimes the Seedra has to be shut off because the space station’s orientation isn’t allowing us to collect enough solar energy to power it. For example, when a Progress resupply spacecraft docks, the solar arrays need to be turned edge on so their surface area doesn’t get blasted by its thrusters. At other times there’s no clear reason for Seedra going on strike. Sometimes, it’s just broken.