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When humans move to space, we are the aliens, the extraterrestrials. And so, living in space, the oddness never quite goes away. Consider something as elemental as sleep. In 2009, with the expansive International Space Station nearing completion after more than a decade of orbital construction, astronauts finally installed some staterooms on the U.S. side—four private cubicles about the size of airplane lavatories. That’s where the NASA astronauts sleep, in a space where they can close a folding door and have a few hours of privacy and quiet, a few hours away from the radio, the video cameras, the instructions from Mission Control. Each cabin is upholstered in white quilted material and equipped with a sleeping bag tethered to an inside wall. When an astronaut is ready to sleep, he climbs into the sleeping bag. “The biggest thing with falling asleep in space,” says Mike Hopkins, who returned from a six-month tour on the Space Station last March, “is kind of a mental thing. On Earth, when I’ve had a long day, when I’m mentally and physically tired—when you first lie down on your bed, there’s a sense of relief. You get a load off your feet. There’s an immediate sense of relaxation. In space, you never feel that. You never have that feeling of taking weight off your feet—or that emotional relief.” Some astronauts miss it enough that they bungee-cord themselves to the wall, to provide a sense of lying down.

Sleep position presents its own challenges. The main question is whether you want your arms inside or outside the sleeping bag. If you leave your arms out, they float free in zero gravity, often drifting out from your body, giving a sleeping astronaut the look of a wacky ballet dancer. “I’m an inside guy,” Hopkins says. “I like to be cocooned up.” Hopkins says he didn’t have unusual dreams in space, although now, back on Earth, he does occasionally dream of floating through the station. “I wish I dreamed every night of floating,” he says. “I wish I could recapture that.” Spaceflight has faded from American consciousness even as our performance in space has reached a new level of accomplishment. In the past decade, America has become a truly, permanently spacefaring nation. All day, every day, half a dozen men and women, including two Americans, are living and working in orbit, and have been since November 2000. Mission Control in Houston literally never sleeps now, and in one corner of a huge video screen there, a counter ticks the days and hours the Space Station has been continuously staffed. The number is rounding past 5,200 days. It’s a little strange when you think about it: Just about every American ninth-grader has never lived a moment without astronauts soaring overhead, living in space. But chances are, most ninth-graders don’t know the name of a single active astronaut—many don’t even know that Americans are up there. We’ve got a permanent space colony, inaugurated a year before the setting of the iconic movie 2001: A Space Odyssey. It’s a stunning achievement, and it’s completely ignored.

As a culture, we remain fascinated by the possibilities, discoveries, of space travel. The 2013 movie Gravity, starring Sandra Bullock and George Clooney, brought in $716 million at the box office and won seven Academy Awards. But we seem indifferent to what is happening in reality all the time now. Without any fanfare, we have slipped into the era of Captain Kirk and Mr. Spock. We just know the fictional characters better than the real ones. Perhaps that’s unsurprising. The Space Station is an engineering marvel, but all it seems to do is soar in circles—a fresh sunrise every 92 minutes. Scientific research on the station hasn’t yielded any noteworthy breakthroughs, and daily life there, thankfully, lacks the drama of a movie script. But all of that does the station and its astronauts a disservice: The details and challenges of life in space are weird and arresting, revealing and valuable. In them, one can begin to make out a greater purpose for the station’s 82,000 manned orbits—even if it’s not the one NASA seems to be pursuing.

The International Space Station is a vast outpost, its scale inspiring awe even in the astronauts who have constructed it. From the edge of one solar panel to the edge of the opposite one, the station stretches the length of a football field, including the end zones. The station weighs nearly 1 million pounds, and its solar arrays cover more than an acre. It’s as big inside as a six-bedroom house, more than 10 times the size of a space shuttle’s interior. Astronauts regularly volunteer how spacious it feels. It’s so big that during the early years of three-person crews, the astronauts would often go whole workdays without bumping into one another, except at mealtimes. Indeed, it’s so big, you can see it tracing across the night sky when it passes overhead (there are apps for finding it, ISS Spotter among them).

The station is a joint operation: half American, half Russian, with each nation managing its own side of the craft (the U.S. side includes modules or equipment from Canada, Japan, and Europe, and typically a visiting astronaut from one of those places). Navigation responsibilities and operation of the station’s infrastructure are shared, and the role of station commander alternates between a cosmonaut and an astronaut. The Russian and U.S.-side astronauts typically keep to their own modules during the workday. But the crews often gather for meals and hang out together after work. As a facility, a spacecraft, and a habitation, the station is most comparable to a ship. It has its own personality, its own charms and quirks. Crew members come and go, bringing their own style, but the station itself imposes a certain rhythm and tone. It has a more sophisticated water-recycling system than any on Earth. An astronaut who mixes up an orange drink for breakfast on Monday morning and urinates on Monday afternoon can use that same water, newly purified, to mix a fresh drink on Thursday. Yet the station lacks a refrigerator or freezer for food (there is a freezer for science experiments), and while the food is much better than it was 20 years ago, most of it’s still vacuum-packed or canned. The arrival of a few oranges on a cargo ship every couple of months is cause for jubilation. On the station, the ordinary becomes peculiar. The exercise bike for the American astronauts has no handlebars. It also has no seat. With no gravity, it’s just as easy to pedal furiously, feet strapped in, without either. You can watch a movie while you pedal by floating a laptop anywhere you want. But station residents have to be careful about staying in one place too long. Without gravity to help circulate air, the carbon dioxide you exhale has a tendency to form an invisible cloud around your head. You can end up with what astronauts call a carbon-dioxide headache. (The station is equipped with fans to help with this problem.)

Since the station’s first components were launched, 216 men and women have lived there, and NASA has learned a lot about how to live in space—about the difference between rocketing into zero‑G for two weeks and settling in for months at a time. Day-to-day life in space is nothing like the sleek, improvisational world that TV and movie directors have created. It is more thrilling and dangerous than we earthlings appreciate, and also more choreographed and mundane. Often those qualities coexist in the very same experience, such as spacewalking. Space is a brittle and unforgiving place—a single thoughtless maneuver can trigger disaster. NASA has reduced the risk by scripting almost everything, from the replacement of a water filter to the safety checks on a space suit. In 54 years of flying humans in space, NASA has suffered three fatal spacecraft accidents that killed a total of 17 people—the Apollo 1 capsule fire in 1967, the Challenger shuttle disaster in 1986, and the Columbia shuttle disaster in 2003. But none of those resulted from any error on the part of the astronauts. The meticulous scripting can make watching the astronauts at work boring, but NASA knows that excitement means mistakes. Even by the low estimates, it costs $350,000 an hour to keep the station flying, which makes astronauts’ time an exceptionally expensive resource—and explains their relentless scheduling: Today’s astronauts typically start work by 7:30 in the morning, Greenwich Mean Time, and stop at 7 o’clock in the evening. They are supposed to have the weekends off, but Saturday is devoted to cleaning the station—vital, but no more fun in orbit than housecleaning down here—and some work inevitably sneaks into Sunday. The exercise bike for the astronauts has no handlebars. It also has no seat. You can float a laptop anywhere you want while pedaling. From 2003 to 2010, 10 American astronauts who lived on the station kept a diary as part of a research study conducted by Jack Stuster, an anthropologist who studies people living in extreme environments. The anonymous diaries—nearly 300,000 words in all—reveal people who are thrilled by life in space, and occasionally bored, and sometimes seriously irritated. For a nation accustomed to 50 years of smiling, can-do astronauts who almost never say anything genuinely revealing about flying in space, the diaries are refreshingly frank.

“I had to laugh to myself at the procedures today,” wrote one station astronaut. To replace a light bulb, I had to have safety glasses and a vacuum cleaner handy. This was in case the bulb broke. However, the actual bulb is encased in a plastic enclosure, so even if the glass bulb did break, the shards would be completely contained. Also, I had to take a photo of the installed bulb, before turning it on. Why? I have no idea! It’s just the way NASA does things. Astronauts never tire of watching the Earth spin below—one wrote of stopping at a window and being so captivated that he watched an entire orbit without even reaching for a camera. “I have been looking at the Earth, from the point of view of a visiting extraterrestrial,” wrote another. “Where would I put down, and how would I go about making contact? The least dangerous thing would be to board the International Space Station and talk to those people first.” The diary entries make it very clear that six months is a long time to be in space—a long time to go without family and friends, without fresh food, without feeling sunshine or rain or the pleasures of gravity; a long time to be tethered to the tasks of maintaining body and station, on a ship with no bathing or laundry facilities. The entries also reveal that keeping a diary significantly improves an astronaut’s morale. During short missions, even two-week shuttle missions, the excitement of being in space never fades. On the station, NASA and the astronauts themselves have had to be more attentive to morale, simply because there’s a lot of work that’s uninteresting. The Space Station has a telephone—astronauts can call anyone they want, whenever it’s convenient—and their families get a specially programmed iPad for private videoconferences. The astronauts have private conversations with NASA psychologists once every two weeks. They also have regular conferences with NASA strength coaches—twice as frequently as they talk to the shrinks.

In space, says Mike Hopkins, “Everything is new. Simple hygiene, eating a meal, sleeping—you name it, it’s all completely different from what you’re used to.” That’s from someone who, like most astronauts, trained full-time for two years before launch, so that he would know what to expect. The Space Station is stocked with movies and has a locker filled with paperback books. But Ed Lu, one of the earliest crew members, in 2003 decided that he wasn’t going to spend his free time doing something so earthly as reading a paperback. “I don’t know if I’m ever coming back here,” Lu remembers thinking to himself. “I want to do things I can never do at home.” When Lu arrived at the Space Station—he was half of a two-person crew that kept the station active in the wake of the Columbia disaster—he’d already flown two shuttle missions, and had 21 days’ experience living in zero‑G. “I decided to learn to fly better, to learn acrobatics,” he says. “I would pick a module and say to myself, Every time I go through this module, I’m going to fly through without touching the sides. I would pick a compartment and say, Every time I go through this compartment, I’m going to do a double flip. Being on station was my third flight, and I thought I knew what I was doing. But I got an awful lot better with months of training.”

The singular experience of space is the flying—not flying the spaceship you’re in, but flying, yourself, inside it. That’s what really makes you an astronaut—not altitude, but the almost unbelievable liberation from gravity. Astronauts speak of it with bemused awe, because it is pure delight but also totally counterintuitive and sometimes inconvenient. “What’s it like to live in zero‑G?” asks Sandra Magnus, who took three spaceflights, including 130 days on the station, before her recent retirement from the astronaut corps. “It’s a lot of fun,” she says, then bursts out laughing. “I learned to carry things with my knees—tuck them between my knees and shove off. That way I had my hands free to propel myself. The thing is, in space, Newton’s laws rule your life. If you’re doing something as simple as typing on a laptop, you’re exerting force on the keyboard, and you end up getting pushed away and floating off. You have to hold yourself down with your feet.” Magnus developed calluses on her big toes because she used them continuously, in stocking feet, to navigate and position herself. Gravity is an indispensable organizing tool, she says, one you don’t appreciate until you have to live without it. “Just look around the room you’re in … There’s stuff sitting on tables, on shelves, in drawers, on the floor. In space, all of that would be all over the place.” Every single item you use needs to be secured, or it will float off. Astronauts mutter quietly about the amount of time they spend searching for misplaced equipment—“Keeping track of stuff can eat your whole day,” says the astronaut Mike Fincke—but securing everything also takes time.

Magnus liked to cook for her colleagues on the station, finding new dishes to make with the food NASA supplied, especially with the delivery of, say, a fresh onion. “It takes hours, so I could only do it on the weekend,” she says. “Why hours? Think about one thing: when you cook, how often you throw things in a trash can. How can you do that? Because gravity lets you throw things in the trash. Without gravity, you have to figure out what to do. I put the trash on a piece of duct tape—duct tape is awesome—but even dealing with the trash takes forever.” When you’re in zero‑G, all the fluids in your body are in zero‑G too, so astronauts often have a stuffy-head feeling, from fluid migrating to their sinuses; some end up literally puffy-faced. And zero‑G causes the nausea and space sickness that many astronauts have quietly suffered during the first day or two in orbit, going back at least to Apollo. Leroy Chiao, 54 and retired from the astronaut corps after four flights, describes what happens even before you float out of your seat. “Your inner ear thinks you’re tumbling: the balance system in there is going all over the place … Meanwhile your eyes are telling you you’re not tumbling; you’re upright. The two systems are sending all this contradictory information to your brain. That can be provocative—that’s why some people feel nauseous.” Within a couple of days—truly miserable days for some—astronauts’ brains learn to ignore the panicky signals from the inner ear, and space sickness disappears.

Mike Fincke has spent more time in space than any other American astronaut—381 and a half days, spread over three missions. He has done nine space walks, totaling 48 hours. When his first station posting was unexpectedly moved forward in 2004, Fincke became the first U.S. astronaut to become a father while in space. Mission Control patched him through to his wife’s cellphone while she was in labor. For Fincke, there’s nothing like the flying. “There is sheer joy in it,” he says. “Just sheer joy in flying in space. You can take the most serious 50-year-old curmudgeon and put him into orbit, in zero‑G, and he’ll smile, he’ll laugh, he’ll giggle.” Fincke has degrees from MIT and Stanford, and graduated from the U.S. Air Force Test Pilot School before becoming an astronaut. He’s disarmingly chatty compared with the stereotype, unable to suppress his exuberance about his job, even after 18 years. In 2011, he participated in a video call between the Space Station and Pope Benedict XVI in Vatican City. At the end, Fincke launched himself straight up out of the video frame, inspiring the Italian press to joke about Christ’s Ascension. “We even made the pope laugh,” Fincke says. “A little push with your big toe will take you halfway across the station. It’s like being Superman—with just the brush of a finger. It does not get old, even after 381 days.” The very quality that makes space travel so delightful also makes it invisibly dangerous. Zero‑G is harmful to the human body in insidious ways.

Significantly, astronauts lose bone mass. Bones regenerate and grow partly in response to the work they have to do each day. Without weight to support in space, the rate at which they make fresh cells slows down, and the bones thin and weaken. A postmenopausal woman on Earth might lose 1 percent of bone mass a year. An astronaut of either gender can lose 1 percent of bone mass a month. Mark Guilliams is the lead strength-and-conditioning coach for NASA astronauts. He works out of the expansive astronaut gym at Houston’s Johnson Space Center, where the 43 active American astronauts are based. “Living in zero‑G is the equivalent of a long stay in a hospital,” Guilliams says. You lose muscle mass and strength. You lose blood volume. You lose aerobic fitness, anaerobic fitness, stamina. “Spaceflight is hard on the body. Period.” It’s hard on the body because it’s so easy on the body. The antidote is vigorous, almost relentless exercise while in space. The U.S. part of the station has three exercise machines—the seatless bike, a treadmill, and a weight machine known as the ARED (advanced resistance exercise device) with a 600-pound capacity. Astronauts are scheduled for two and a half hours of exercise a day, six days a week, but most exercise seven days a week. Exercise is considered so vital that NASA puts it right on the workday schedule, although some astronauts wake up early and do it in their own time.

Mike Hopkins is a fitness buff, and he made a series of YouTube videos to show what astronaut workouts are like. The treadmill is the hardest piece of equipment to get used to, he says, because you have to be bungee-corded down to provide the sense of weight to your body that a runner on Earth would have. “You run with a backpacking harness on, and that’s attached to bungees, and you can change the load, how hard it is pulling against you,” Hopkins says. “I would try to get up to my body weight, simulating what it was like to run on Earth. But you’re carrying that load on your shoulders and hips; it’s like trying to run with a 180-pound pack on your back.” Zero‑G doesn’t make sweating any more pleasant. “You sweat buckets up there,” says Hopkins. “On the ground, when you’re riding the bike, the sweat drips off you. Up there, the sweat sticks to you—you have pools of sweat on your arms, your head, around your eyes. Once in a while, a glob of it will go flying off.” The astronauts use large wipes and dry towels to clean off. “The shower was one of those things that I missed.” Still, the sponge-bath method works just fine, and the station generally has a neutral smell. Astronauts wear fresh clothes for a week, which then become workout clothes for a week, which are then discarded with the rest of the trash. The focus on fitness is as much about science and the future as it is about keeping any individual astronaut healthy. NASA is worried about two things: recovery time once astronauts return home, and, crucially, how to maintain strength and fitness for the two and a half years or more that it would take to make a round-trip to Mars, which President Obama has said he believes NASA can do by the mid‑2030s (although there is no detailed plan). Figuring out how to get to Mars safely, in fact, underlies much of what happens on the station. “If astronauts have a 10 percent loss of strength, of cardio capacity, how much does that impair their functioning on station? Not much,” says Guilliams. “But if you’re going to Mars, that kind of loss could be critical. What would they be able to do when they land?” “I have been looking at the Earth, from the point of view of a visiting extraterrestrial … how would I go about making contact?” We don’t yet understand all the implications of long-duration spaceflight. “Five years ago,” says John Charles, of NASA’s Human Research Program, “we had an astronaut on station all of a sudden say, ‘Hey, my eyesight has changed. I’m three months into this flight, and I can’t read the checklists anymore.’ ” It turns out, Charles says, that all that fluid shifting upward in zero‑G increases intracranial pressure. “Fluid pushes on the eyeball from behind and flattens it,” says Charles. “Many astronauts slowly get farsighted in orbit.”

In fact, the station is now stocked with adjustable eyeglasses, so astronauts who don’t normally wear glasses will have them if they need them. Those who already wear glasses bring along extra pairs with stronger prescriptions. Astronauts need precise, reliable vision, so its deterioration during spaceflight is hardly a minor problem. And it’s a particularly humbling one. NASA has known about the eyesight issue for decades. “We saw this on Skylab”—the first U.S. space station, which intermittently housed astronauts for up to three months at a time from 1973 to 1974—“and on the shuttle,” Charles says. The importance of it just wasn’t clear until astronauts were regularly spending months in orbit. And at the moment, NASA doesn’t know how to fix it back on Earth. Bone mass, muscle mass, blood volume, aerobic fitness all return to normal, for the most part. But astronauts’ eyes do not completely recover. Nor do doctors know exactly what would happen to eyesight over the course of a mission four or five times longer than those of today. “We’re not watching anything else as significant as this right now,” Charles says. In March, the longest mission ever for an American astronaut will begin: a full year on the station. (Four cosmonauts have lived in space for a year or more, aboard the Russian space station Mir.) Before tackling a two-and-a-half-year trip to Mars, Charles says, “we have to see if there are any other things we aren’t anticipating—psychologically or physiologically. We have to see if there are any cliffs.” “Hey, Houston, this is Station. Good morning. We’re ready for the morning DPC.”

That’s U.S. station commander Steven Swanson, hailing Mission Control from orbit one morning last July to start another fully scheduled space workday. For Americans of a certain age, those first seven words—“Hey, Houston, this is Station. Good morning”—remain packed with a sense of romance, adventure, and unassuming competence. The astronauts are flying with the stars; Mission Control has things in hand on the ground. On shuttle missions—there were 135, stretching from 1981 to 2011—Mission Control would wake the astronauts, radioing up a burst of music to start each day. The wake-up-song tradition stretches back to Gemini, and its passing is meaningful, at least symbolically. The station is a permanent outpost, with a measure of independence. So Mission Control doesn’t shake the astronauts awake. They get up in their mini-staterooms well before any contact with Houston, and then signal the start of the workday by radioing down to Mission Control. They also end the day, by signing off for the night. When the astronauts get ready to turn in, they float through the station, switching off lights and closing window shutters, to shade their sleep from all those sunrises. Mission Control does not normally radio the station during off-hours. But it’s a thin layer of independence, as acknowledged in that next line from Swanson—“We’re ready for the morning DPC.” Every day starts and ends with a daily planning conference, during which the astronauts briefly check in with all five control centers around the world to talk about schedule glitches or pending maintenance, or look ahead to the next day. (NASA has a second facility, in Huntsville, Alabama, to handle scientific research; Moscow has a Mission Control for the Russian half of the station; and the European Space Agency and Japan’s space agency also have their own 24-hour control centers.) The astronauts may be rocketing around Earth at 17,500 miles an hour, 10 times faster than the average bullet leaves a gun, but they can’t escape regular meetings.

Although the astronauts live and work in the Space Station, they don’t fly it or otherwise control it. That’s all done from Houston and Moscow. Mission Control monitors the station’s position in space and adjusts it as necessary, using gyroscopes and thrusters; Mission Control also monitors all the onboard systems—electrical, life support, IT, communications. A vast team on the ground supports the station—more than 1,000 people for every astronaut in orbit. And while astronauts get to kick off the workday, the pace and rhythm of the day are unequivocally set by the people on the ground. Life on the station is managed via spreadsheet: every minute of each astronaut’s workday is mapped out in blocks devoted to specific tasks. When an astronaut clicks on a time block, it expands to present all the steps necessary to perform the task at hand—whether it’s conducting an hours-long experiment on the behavior of fire in zero-G or stowing supplies from a cargo ship. In its own way, the schedule can be a source of freedom for astronauts who surrender to it, but it is also a symbol of a certain tyranny. Science experiments, maintenance tasks, cargo-vehicle arrival and departure: all are set from the ground. Each astronaut’s schedule has a red line that slowly shifts across the laptop screen, left to right, showing the current time and what the astronaut should be doing at that particular moment. “The red line, no matter what I do, it just keeps moving and moving to the right,” groans the astronaut Garrett Reisman, in a tongue-in-cheek YouTube video he recorded in space. “I can’t stop it!”

Life in space is so complicated that a lot of logistics have to be off-loaded to the ground if astronauts are to actually do anything substantive. Just building the schedule for the astronauts in orbit on the U.S. side of the station requires a full-time team of 50 staffers. The schedulers get input and priorities from everyone—what is the point of a particular six-month mission? What scientific experiments will be done? What cargo ships will visit? What maintenance tasks are necessary? Getting each individual item onto the schedule requires detail and coordination. For a particular experiment, what equipment and tools are required? Where are those things stowed? How long will setup take? What steps will the astronaut need to follow to conduct the experiment? Will enough power be available at that time? Will the experiment get in the way of what another astronaut is doing? Who are the investigators on the ground? Will they need video observation? Is there sufficient bandwidth for all the video and audio being used at a given moment? Each day has dozens of individual activities requiring that level of planning, which begins 18 months out. Highly educated, highly motivated astronauts end up doing one task after another, all day long, some of them fun and intellectually challenging (doing research with ground-based scientists), some of them tedious (recording the serial numbers of the items in the trash before sending them to be burned up in the atmosphere). No one signs up to fly through space in order to empty the urine container or swap out air filters. But of course even the research is commissioned and directed by other people—the astronauts are merely highly skilled technicians.

NASA struggles to balance the independence of its astronauts with the desire to keep them on schedule. In the anonymous diary study conducted by Jack Stuster, work was the most frequent topic raised by astronauts in their journal entries, and when Stuster analyzed the work entries, scheduling was the second most frequently discussed topic, after simple descriptions of tasks. “Only 30 minutes [scheduled] to execute a 55-step procedure that required collecting 21 items,” wrote an astronaut. “It took 3 or 4 hours.” Another wrote, “It has been a pretty tedious week with tasks that were clearly allotted too little time on the schedule. Talking to [a Mission Control staffer] today, I realized he just doesn’t understand how we work up here.” Astronauts need precise vision, so its deterioration during spaceflight is a significant, and humbling, problem. That’s a pretty standard complaint, of course: Soldiers at the front line have one impression of how the war is going; military headquarters has another. Sales reps in the field see the products and the customers one way; the vice president of sales sees them differently. In part because of the care NASA takes picking astronauts and assigning them to crews, and in part because the astronauts appreciate the need to work together, they all report getting along well, and being able to quickly resolve minor disputes. But at the same time, from their perspective, it’s hard for NASA’s Space Station ground staff to understand life in space. As with anything else, the privileges and joys of working in space don’t neutralize ordinary office politics.

Stuster’s study has a whole subcategory of diary entries devoted to “praise inflation,” a phenomenon whereby the astronauts feel compelled to pass out “profuse compliments, even when undeserved,” to ground personnel, “and a general avoidance of criticizing ground personnel for deficiencies, real and perceived.” The tradition of praise goes back to the moon effort—when the astronauts were bathed in glory, and worked hard to pass that on to the army of technicians who made the mission possible. But on the station, it can sometimes grate. Wrote one astronaut: “I feel that the ground has often made my life more difficult here, thus making it hard to hand out praise on such a frequent basis.” Almost anyone who has ever had a job can imagine the eye-rolling that goes on, both in Houston and on the station: What are those guys up there/down there thinking? For the most part, such sentiments go unexpressed. Peggy Whitson, a veteran of two six-month station postings, was the chief of NASA’s astronaut office—the astronauts’ direct boss—from 2009 to 2012. Communication was something she paid close attention to, on both ends of the radio. “One thing I can tell you,” Whitson says. “Sarcasm does not work in space travel.” Scott Kelly and Tim Kopra are standing back-to-back on a steel platform in July, outfitted in NASA space suits. A yellow crane slowly lifts the platform, swings it out over the surface of a huge swimming pool, then lowers the astronauts into the water. Kelly and Kopra are going to spend most of the day—six hours—underwater, doing a practice space walk in the pool, going through every step of replacing part of the Space Station’s robotic arm. It’s a maintenance task they will do in space next November.

Kelly and Kopra spend 30 minutes getting latched into the suits, each of which weighs 230 pounds empty. A fellow astronaut, Kevin Ford, is watching. “See how each astronaut has three or four guys helping him?” says Ford, who was the commander of the Space Station for four months in 2012 and 2013. “On station, it’s just one guy suiting up two astronauts. The procedure to get into the space suits and out the hatch is a 400-step checklist. And you don’t want to skip too many of those steps.” Four hundred steps, just to get one astronaut ready to float into the station’s air lock and prepare to egress. Before a NASA astronaut starts the first minute of a space walk, he or she has spent four hours getting into the suit and checking it over. And long before that, the astronaut has rehearsed that specific six-to-eight-hour space walk five or more times on Earth, in the pool, which NASA calls the Neutral Buoyancy Laboratory. Nothing captures the strange contradictions of modern spaceflight as well as spacewalking—shoving off into space with only your wits and training, sealed into your one-person spacecraft. An EVA (extravehicular activity) is, for almost all astronauts, the ultimate professional challenge and the ultimate thrill ride. When you’re outside the station, you are literally an independent astronomical body, a tiny moon of Earth, orbiting at 17,500 miles an hour. When you look at Earth between your boots, that first step is more than 1 million feet down.

But spacewalking is also a window into how dangerous space is, how a single connector not properly mated can lead to disaster, and how NASA has grappled with that risk by wringing all the spontaneity, all the surprise, out of it. That’s why every scheduled space walk is scripted, and then rehearsed and rehearsed and rehearsed in a pool big enough to immerse two space shuttles. Working in space to construct or repair a spaceship that weighs 1 million pounds is so challenging that the station’s exterior elements have a remarkable engineering feature: although the station is made up of more than 100 components, with a surface area spanning almost three acres, most bolts the astronauts work with are a single size. That way astronauts almost never have to worry about changing sockets. Imagine constructing a whole building that way. All the scripting, the rehearsal, the design considerations—life in space isn’t just stranger than ordinary folks realize; it’s harder. Harder even than NASA has always imagined. The agency originally promised that space shuttles would fly at least 25 times a year. In actuality, the shuttle program averaged fewer than five flights a year; in the peak year, 1985, the shuttle flew nine times. It was President Ronald Reagan, in his 1984 State of the Union speech, who directed NASA to create and permanently staff a space station, which he predicted would “permit quantum leaps in our research in science, communications, in metals, and in lifesaving medicines which could be manufactured only in space.” NASA’s original vision for the station was as ambitious as it had been for Apollo or the shuttles. The station was to have seven major functions—it was to be a research lab, a manufacturing facility, an observatory, a space transportation hub, a satellite-repair facility, a spacecraft-assembly facility, and a staging base for manned missions to the solar system.