“Wow,” Harwood said.

“Yeah,” Shannon said. “Wow.”

They sat in silence with the information, struggling to fathom the shape and weight of it, for a very long time.

Later that morning, Nathan Jones, an emergency-room physician in Springfield, Ill., received the call that he had so fervently awaited and so deeply dreaded. His thoughts turned immediately to his wife, Kacie, and their three sons, who were 8, 10 and 12. You get only 18 years with your kids, he told himself. If you accept this opportunity, you’ll have to give up one of them.

And yet … he couldn’t possibly turn down NASA. Mars, he had convinced himself, was his destiny. As a child, he dreamed of walking across an alien planet in a state of wonder; he hoped to attend space camp, but his family couldn’t afford it. Once his sons were old enough, he took them to Cape Canaveral for a rocket launch.

When he told Kacie the news, she nearly burst into tears.

This Mars mission, CHAPEA, would not actually go to Mars. But the success of CHAPEA (“Crew Health and Performance Exploration Analog”) will hang on the precision with which it simulates the first human expedition to Mars — an eventuality that NASA expects to occur by 2040.

That people will travel to Mars, and soon, is a widely accepted conviction within NASA. The target date for the initial human mission has drifted slightly — in a 2018 report commissioned by Congress, NASA estimated that the first human beings would land on Mars “no later than the late 2020s” — but the certainty has not wavered, even if technical hurdles remain. Rachel McCauley, until recently the acting deputy director of NASA’s Mars campaign, had, as of July, a punch list of 800 problems that must be solved before the first human mission launches. Many of these concern the mechanical difficulties of transporting people to a planet that is never closer than 33.9 million miles away; keeping them alive on poisonous soil in unbreathable air, bombarded by solar radiation and galactic cosmic rays, without access to immediate communication; and returning them safely to Earth, more than a year and half later. Many other problems involve technical details so arcane that McCauley wouldn’t even know how to begin explaining them to a well-intentioned journalist lacking an advanced engineering degree. But McCauley does not doubt that NASA will overcome these challenges. What NASA does not yet know — what nobody can know — is whether humanity can overcome the psychological torment of Martian life.

Enter CHAPEA. Instead of asking questions about aeroshell sensor design and terrain-relative navigation, it promised to ask questions about people. For 378 days, four ordinary people would enact, as closely as possible, the lives of Martian colonists, receiving directives, feedback and near-total surveillance from mission control. They would eat astronaut food, conduct basic experiments, perform maintenance duties, respond to endless surveys and enjoy highly structured down time. This level of extreme verisimilitude is necessary to ensure that the experiment accurately determines whether human beings can thrive while living millions of miles from everybody they’ve ever known.

Experimenters wanted to learn whether crew members could eat low-salt, prepackaged astronaut meals for hundreds of days without losing their appetite, weight and positive attitude. Whether they could live in harmony with strangers in a confined space. Whether they could preserve a cohesive professional environment when they are out of contact with Earth for as long as three weeks at a time. Such questions are of paramount importance, because no mission to Mars can succeed if its inhabitants cannot maintain their health, their happiness and, most critical of all, their sanity.

And so before NASA can safely judge whether astronauts will thrive on Mars, NASA must first determine whether astronaut-imitators can thrive on a stage set designed, with maximum fidelity, to look like Mars.

“Mars is calling!” began the announcement that NASA published on its website in August 2021. Unlike most NASA missions, CHAPEA was open to the general public, or at least a reasonably broad swath of it: citizens or permanent residents between the ages of 30 and 55 with a master’s degree in a STEM field. Applicants were told to expect the experience to be “mentally demanding.”

Among the not-insignificant percentage of the country that idolizes NASA, this news was tantamount to learning that Willy Wonka would open his mysterious factory to five lucky contest winners. NASA offered four golden tickets to Mars — or rather Mars Dune Alpha, a 1,700-square-foot habitat built inside a warehouse at the Johnson Space Center in Houston.

The habitat was constructed as future Mars dwellings will be constructed: by 3-D printer. For “ink,” Martian colonies will use Martian regolith. Because NASA does not possess sufficient quantities of Martian rock, CHAPEA used a proprietary, airtight cement-based material called Lavacrete, which extrudes from a 3-D printer layer by layer, like orange toothpaste. (Though Lavacrete can be printed in any color, NASA engineers chose to dye the habitat that peculiar hue of orange misleadingly called “Martian red.”)

At one end of the rectangular habitat, four identical 6-by-11-foot cells serve as bedrooms. In the middle lies the “lounge,” a small room with a television and four reclining chairs. The other end is occupied by several desks with computer monitors, a medical station and a crop garden. The vegetables are not intended for subsistence but for mental health: Growing plants, one CHAPEA researcher said, may “provide psychological benefits for astronauts living in isolated, confined environments away from Earth.” Rooms have different ceiling heights, in order, according to its builder, to “avoid spatial monotony and crew member fatigue.” A hatch opens to a Martian backyard: a tented sandbox of reddish sand and two treadmills, to be used for “spacewalks” by virtual-reality-goggled crew members. The walls of the backyard are painted with a mural of Martian cliffs. There are no windows.

The duration of the experiment is the most glaring violation of verisimilitude. Orbital geometries dictate that the shortest possible round-trip mission to Mars will last about 570 days, a scenario possible once every 15 years, next in 2033; a typical Martian tour of duty will last at least 800 days.

To preserve the integrity of the experiment, NASA has refused to disclose any additional details about what the crew will experience during their 378-day confinement, which will end on July 6, 2024. NASA has emphasized only that participants will experience “resource limitations, equipment failure, communication delays and other environmental stressors.” But if Alyssa Shannon and Nathan Jones were to take NASA at its word about its dedication to realism, they could assume that certain conditions would have to be present. Crew members on a mission to Mars will, for instance, have to form durable emotional bonds with total strangers, relying on them for the comforts and consolations of the relationships they abandoned on Earth. Crew members will have to respond to every emergency themselves, without the possibility of intervention, or even guidance, from a mission command too distant to reply promptly to an S.O.S. They would have to come to terms with their inability to care for a sick child, comfort an upset spouse or visit a dying parent.

Future Mars voyagers will not only have to tolerate these conditions. In order to win the privilege of long-distance space travel, they will have to pursue the opportunity with devout, single-minded purpose. They will have to want to travel to Mars more than almost anyone else in the world. They will have to embrace the knowledge that, for at least 570 days, they will be the most isolated human beings in the history of the universe.

Alyssa Shannon had fantasized about colonizing Mars since childhood. She spent weeks on the floor of her bedroom playing with a Lego spaceship that converted into a Martian base station. Later she read Ray Bradbury’s “Martian Chronicles,” James S.A. Corey’s Expanse series and Kim Stanley Robinson’s Mars trilogy — any Martian sci-fi she could find. She knew she could tolerate hardship and extended periods in isolation: She was an avid backpacker, having hiked the John Muir Trail in 23 days and trekked across Spain in 40. She would miss cooking — her specialty was whole-wheat sourdough pizza — but she was willing to sacrifice her culinary passion in service of humanity’s future. Her partner, Jake, understood. Her decision to apply, he said, “reaffirmed what I knew about her: When it comes time to do something important, requiring a major commitment, she’s the kind of person who will follow through.” While she waited to hear back from NASA, Alyssa didn’t discuss it much: The prospect was almost too exciting to bear.

Nathan Jones, the father of three, told his identical twin, Matthew, that he felt the mission had been designed for him — and that he had been made for the mission. Matthew agreed. Nathan could talk to anyone and seemed to solve any problem he faced. He had spent years as a night-shift paramedic, saving lives in the backs of speeding ambulances. He had volunteered on medical missions in the jungles of Honduras, treating health emergencies for members of remote Indigenous tribes without being able to speak their language (or, for that matter, much Spanish). Jones was the emergency specialist in his household too, responsible for repairing every leak, dysfunctional appliance and clogged toilet. He figured he could handle Mars — or, at least, “Mars.”

Kacie, his wife, wasn’t certain she could handle it. When Nathan announced that he had applied, she was dumbfounded. Why, she asked, would you choose to leave our family for a year?

Another version of this question was posed by various professional observers of the American space program: the historians, ethicists and NASA consultants who spend much of their professional lives imagining the future of space exploration and planetary colonization. What, they wondered, did NASA hope to learn from CHAPEA that it did not know already?

The psychic perils of separation from one’s social world are well understood. “Don’t we already know what isolation does to people?” asks J.S. Johnson-Schwartz, a professor of philosophy at Wichita State University who studies the ethics of space exploration. “What uncertainty exists about what’s going to happen when you lock people inside a room for a year? Just because the room is painted to look like Mars doesn’t mean it’s going to change the results.”

The findings to which Johnson-Schwartz referred were from the last 80 years of isolation research, a field of study initiated during World War II, when the British Royal Air Force grew concerned about pilots’ performance during solo reconnaissance flights. Officers noticed that the longer a pilot stayed in the air, the fewer German submarines he detected. The psychologist Norman Mackworth determined that the monotony of the mission was responsible. But inattention wasn’t the worst of it: Monotony weakened the pilots’ competence in even the most basic tasks.

Mackworth’s conclusions inspired a series of studies by the psychologist Donald O. Hebb at McGill University in Montreal, in which male students earned $20 a day to lie on a bed in a lighted, soundproofed gray cubicle. Hebb confirmed Mackworth’s findings and added a disturbing new wrinkle. Monotony didn’t only cause intellectual impairment. It led to “change of attitude.”

At first Hebb’s students slept a lot and ruminated on their studies and their personal problems. Later they fell into reminiscences, recreating movies they had watched or trips they had taken. Some counted to incredibly large numbers. Eventually, however, they lost the ability to focus. Several students reported “blank periods” during which they did not have a single thought.

Next came the hallucinations: a procession of marching squirrels hauling sacks over their shoulders. Nude women frolicking in a woodland pool. Giant eyeglasses marching down a street. An old man wearing a battle helmet in a bathtub rolling across a field on rubber wheels. Dogs, endless dogs. One student complained of a phantom “sucking my mind out through my eyes.” The delusions made the students vulnerable to manipulation. When played recordings about ghosts, poltergeists and ESP, they were far more likely to believe such phenomena were real, even long after the experiment ended.

Hebb’s findings inspired a boom of isolation studies. Subjects were confined within iron lungs, water tanks and subterranean caves; the results were consistent. “These experiments were extremely useful to many different people,” says Jeffrey Mathias, a historian of science at Cornell University, who is writing a book about the history of isolation research. Besides attracting neuroscientists and psychologists, the research also drew the interest, and funding, of the U.S. intelligence community. The C.I.A. incorporated findings into their practice of “coercive counterintelligence interrogation,” or what today might be called “brainwashing” or “torture.”

The isolation studies were also closely monitored by the Air Force, which directed the nascent U.S. space program before the creation of NASA in 1958. Worried that spaceflight might drive astronauts insane, the Air Force conducted the first iteration of a CHAPEA-like experiment at the Air Force’s School of Aeronautic Medicine in San Antonio in 1955. Prospective astronauts were enclosed for a week within a spaceship cockpit slightly larger than a coffin perpetually illuminated by bright fluorescent lights. The airmen were assigned an overwhelming number of technical tasks and, in some cases, given huge doses of amphetamines.

Their experience followed a familiar trajectory: Initial high spirits gave way to what one researcher called a “gradual increase in irritability,” which abruptly flipped into “frank hostility.” Many participants, including a few who hadn’t taken speed, hallucinated. One pilot saw “little people” perched on the instrument panel. “I can’t say if I thought they were alive or not,” he said. “I really don’t know.” Another pilot abandoned the experiment after three hours and demanded psychiatric care.

Similar studies followed — in blackened anechoic chambers and pill-shaped capsules dangling from high-altitude balloons — before the entire line of inquiry was put to rest by Project Mercury. During the successful solo missions that marked the formal start of the American space program in the early 1960s, astronauts did not suffer from any obvious psychological distress, placating Hebb’s researchers. All future long-duration expeditions remained in Earth’s orbit, allowing crew to communicate easily with family and friends; the International Space Station flies about as far from Earth as Manhattan is from Washington. Although government agencies, particularly those concerned about crew performance aboard nuclear submarines, continue to examine the effects of isolation, NASA did not.

NASA had not solved the problem of isolation in outer space. It realized it did not need to solve it. At least not until half a century later, when a new challenge presented itself: a human mission to a planet so distant that a cry for help would have to travel through the solar system for 22 minutes before it was heard.

It was the lag in communication that particularly worried the partners and families of the CHAPEA crew. All contact with the habitat would be delayed by the amount of time that it would take to beam information hundreds of millions of miles from Earth to Mars. Even the tersest exchange (“How’s it going?” “OK.”) would take 44 minutes.

But 44 minutes was the best-case scenario, because any communication will have to flow through a single node. Every unit of information — not just messages but surveillance footage, audio recordings, experimental and biostatic records — will have to wait its turn in a digital queue, with precedence given to the most urgent signals and the smallest packets of data. The upshot was that anything approaching a normal human conversation with an Earthling was unthinkable. The most modest digital postcard — a short, grainy video of a child blowing out a birthday candle — might take weeks to arrive. And during one three-week period in the middle of the experiment, representing the farthest distance (more than 250 million miles) between the two planets, there would be no contact at all.

Alyssa Shannon’s partner, Jake, the cybersecurity expert, dedicated himself to gaming the digital traffic snarl. “I have to figure out how to make sure my stuff goes faster than everyone else,” he said. “I know enough about tech to get the lowest bit rate possible. The lowest-grade image quality will travel faster. Black and white instead of color. I need to calculate the smallest transmittable unit that’s still me, smiling.”

Nathan Jones emphasized to NASA’s experimenters that he wanted to be kept as busy as possible. He didn’t want too much idle time to worry about his wife and their sons — how, when they were having tough days, he wouldn’t be able to give them “Dad hugs.” He didn’t want to dwell on the lost band performances, piano recitals, cross-country meets and soccer games, or about how his oldest son might be six inches taller by the end of his Martian sojourn. Nor did he care to consider what his friends in central Illinois, who responded to the news of his mission with bafflement and concern, might think. “That’s been the hardest part,” he said. “Their jaws hit the floor. They ask Kacie: ‘Why would you let your husband do this? How are you going to be OK?’ This looks crazy to a lot of people. Maybe it is. It’s not the kind of thing folks around here do.”

Kacie alternated among feelings of anger, fear, grief, defeatism, pride and resolve. There were times when she told Nathan that he shouldn’t go or that she wouldn’t let him go. “As a mother,” she said, “I don’t know that I could even consider leaving my children for a year.” But ultimately she was won over by his enthusiasm.

In the months before the crew was sealed within the habitat — the moment of “ingress,” NASA called it — Nathan threw himself into an extensive “Honey do” list. He worked in the backyard garden, planting tomatoes, cucumbers, blackberries, melons and strawberries for his family to harvest in his absence. He taught them how to garden and weed and clip the hedges. After he left for his final month of training in Houston, Kacie noticed that her sons would stand in the yard and survey the plot with their hands on their hips, in subconscious mimicry of their father.

Nathan also renovated two bathrooms, reconstructed the family car’s carburetor, replaced fixtures and trimmed the lower branches of the pine trees. He gave Kacie the passwords to their accounts and detailed directions on how to file their taxes. He taught her how to use the chain saw. He paid a professional photographer to take a family portrait and over spring break splurged for a Disney cruise. He drafted birthday and holiday cards, gifts and letters for every month (“We’re halfway there!”; “One month to go!”). He hid additional Post-it notes under couch cushions and under mattresses, or in places that Kacie might encounter in moments of stress, like the circuit breaker. “You can do it,” he wrote on the note he hid inside his toolbox. “You got this.”

A final envelope he addressed to Kacie, to open on their 15th wedding anniversary.

Jones and Shannon respected NASA’s discretion about the mission. But if they had wanted better to imagine the next year of their lives, they could have read up on a previous series of Mars simulations that shared some of CHAPEA’s objectives. The Hawaii Space Exploration Analog and Simulation (HI-SEAS) experiment was conducted with NASA funding between 2013 and 2017 in a domed habitat on the reddish slope of the Mauna Loa volcano, 3,000 feet below the observatory there that keeps a continuous measurement of the concentration of carbon dioxide in our atmosphere. Civilians were selected to live inside the habitat for as long as 12 months at a time. HI-SEAS studied the nutritional and “psychosocial” benefits of various meal plans, as well as the volunteers’ behavior and mental acuity and the coping strategies they developed to withstand confined isolation.

“Once Upon a Time I Lived on Mars,” a memoir-in-essays by Kate Greene, one of HI-SEAS’ original crew members, includes chapters titled “On Boredom,” “On Isolation” and “Dreams of Mars, Dreams of Earth.” Greene describes how the crushing monotony of the mission changed her. “Somewhere along the way,” she writes, “mental fatigue had become my baseline state.” The crew had difficulty sleeping, were disturbed by the constant monitoring and recording and found that the scheduled leisure time “felt a little forced.” Minor irritations began to madden Greene: the sound of sandals on the stairs, the way a crew member grazed her shin when crossing her leg under the table. She found herself desperately missing quotidian aspects of life on Earth, where she left behind her wife, aging parents and an ailing brother. The smell of fresh pineapple, in a routine sensory test, was enough to make her cry.

HI-SEAS followed Mars500, the longest Mars simulation yet attempted. Administered by Russia’s ingenuously nomenclatured Institute of Biomedical Problems, Mars500 locked six male crew members together for 520 days, between June 2010 and November 2011, in a faux spacecraft and a faux landing module, and on a faux Mars. The Russian experimenters had hypothesized that, over time, the astronauts would lose motivation, work less effectively and suffer intensifying feelings of isolation from family and friends. After the experiment concluded, the scientists announced that their hypotheses had been “largely confirmed.” Crew members lost trust in the commanders and mission control when communications grew less frequent, developed nutritional problems and grew homesick and depressed. “The 520 days are really not easy to get through,” Wang Yue, a Chinese participant who lost 22 pounds and much of his hair, told China Daily. “It’s impossible to stay happy all the time. After all, I’m human, not a robot.”

Despite the consistency of results, the appetite for Mars simulations appears insatiable. CHAPEA is one of more than a dozen current analogue experiments NASA is participating in, including HERA, a 650-square-foot habitat that regularly houses four participants for as long as 45 days in confined isolation. Since NASA ended its participation in HI-SEAS, a conglomerate of public and private organizations has staged 12 additional missions on Mauna Loa. For nearly a quarter-century, the nonprofit Mars Society has directed research stations in the Utah desert and on a remote island in northern Canada. Mars analogues have been conducted on Dome C of the Antarctic Plateau, in a semiarid tract of northeastern Brazil, in the northern Sahara, within Austria’s Dachstein ice caves and in the Dhofar region in the Sultanate of Oman.

“We’ve seen similar things happen many times,” acknowledges Kelly C. Smith, a philosopher at Clemson University who specializes in the ethics of space exploration and advises NASA, which has no ethicists on staff. “But that doesn’t necessarily mean they’re a waste of time. The stakes are higher than in the past, after all. We’re doing this because we’re planning missions to other worlds.”

It is likely that the first travelers to Mars will have a similar psychological profile to that of Shannon, Jones and the two other participants selected by NASA for the crew: Ross Brockwell, a public-works operations manager in Chesapeake, Va., and Kelly Haston, a stem-cell biologist in the San Francisco Bay Area. All four were not only NASA enthusiasts and in perfect physical health but habitually sought out extended periods of isolation. Brockwell routinely retreated to a camp he had built on undeveloped land in Virginia, living off the grid. Haston is an ultramarathoner, having run some 70 trail races in the last decade, including several hundred-milers. Loneliness was something she had read about in books but never, as far as she could recall, experienced. A passion for isolation might have been as important to NASA’s screening process as educational attainment and blood glucose levels.

The CHAPEA participants should further benefit from their devotion to the cause. Louise Hawkley, an expert on social isolation at the University of Chicago, emphasizes that psychological responses are heavily influenced by whether people choose isolation or have it thrust upon them. A prisoner sentenced to life would be expected to suffer more than a monk who takes a vow of silence. But Hawkley points out that the participants’ loved ones, however supportive they might be, lacked the same autonomy: “Even if the crew member is fine, what happens to the family left behind?” Hawkley wondered if NASA will study the psychological effects of the mission on the families.

It will not. Nor did CHAPEA’s architects seem to have a strong grasp of the history of isolation research. In interviews, they discounted the predictive value of previous experiments, including HI-SEAS. “I don’t believe they were doing the performance metrics that we’re doing,” says Grace Douglas, CHAPEA’s principal investigator, who admitted she wasn’t “fully familiar” with the previous four-year experiment. “Our metrics are going to be at a higher level of detail and more extensive. The resource plan is more accurate.”

Rachel McCauley was the NASA official responsible for funding CHAPEA. When asked what she hoped to learn about human psychology, she dismissed the premise of the question. “The big reason why I funded it,” she said, “is because I need an even more refined answer to the question, How much food does it really take for a Mars mission?”

What about the mission’s psychological aspect? The monotony? The loneliness?

“I’m a hardware person first,” McCauley said. She is, to be precise, a solid-propulsion systems engineer. She has the distinction of being the member of our species who has been most responsible for determining the best method to catapult humanity to Mars. In order to do so, she had to know how much weight a spaceship will carry. McCauley could estimate, down to the milligram, the mass of every nut and bolt, every antivortex baffle and cargo-bay door. But how many corn tortillas and yogurt packets will four astronauts, under psychological duress, consume in 378 days? That question, or some version of it, was what McCauley needed answered. She also needed to know how much clothing they’ll need. Clothes are heavy.

Mathias, the isolation historian, was not surprised to learn that the psychological questions were a secondary consideration for NASA. But his skepticism about CHAPEA went further. Mathias questioned whether any experimental rationale could justify yet another isolation study. “I wonder if the scientific value of these simulation experiments is beside the point,” he said. The experiments, instead, seemed to him “a way of willing the colonization of Mars into being. A form of wish fulfillment — or cosplaying, to put it less poetically. This is about satisfying an urge. There seems to be a compulsion to keep repeating these fake Mars missions until we actually do it. There’s something very beautiful about this idea, but also very macabre at the same time.”

The analogue experiments reflect the utopian promise of our Martian future. For a human mission to Mars is not the highest ambition of the space program. It is just the beginning, a small step for mankind before the giant leap of planetary colonization.

Five months before CHAPEA’s call for applications, Dennis Bushnell, then chief scientist at NASA Langley Research Center and a nearly 60-year veteran of NASA, published “Futures of Deep Space Exploration, Commercialization and Colonization: The Frontiers of the Responsibly Imaginable.” Martian colonization has always been imaginable, particularly to this nation of colonizers. But in his paper Bushnell noted that the prospect has in recent years “moved from extremely difficult to increasingly feasible.” Colonization has also become increasingly desirable, because of “possibly existential societal issues, including climate change, the crashing ecosystem, machines taking the jobs, etc.” — the et cetera perhaps reflective of the obviousness of planetary decline.

A more surprising aspect of the paper is Bushnell’s prediction for how the physical hostility of Mars will be overcome: Colonists will “morph into an altered species.” He cites projections that suggest that “travelers that colonize Mars will, over time, due to the reduced g and radiation exposure, evolve into Martians.” The ultimate promise of NASA’s Mars mission is the chance to begin again — if not, exactly, as human beings, then as Martians.

There is a beautiful and macabre poetry to this rationalization. “Utopia,” after all, derives from the Greek: ou (“not”) and topos (“place”). If we manage to inhabit the not-place of Mars, enjoying a carefree life of not-problems, not-regret and not-environmental-ruin, it makes sense that we should be not-people. We should be Martians. Let people, with all their baggage and fragility and foolishness, stay home.

Mathias likened the incessant Mars analogue experiments to a traumatic repetition: a compulsion to restage a trauma in an irrational, futile attempt to undo a profound damage. “The urge to try to recreate a perfect world is always going to be about rehearsing what we got wrong here,” he said. “We’re not chasing Mars. We’re mourning Earth.”

In late May, a month before sealing themselves within the habitat, the four crew members and two alternates reported to Houston for a final month of training and evaluation. Three weeks before the ingress, NASA hosted a “family weekend” for the crew’s loved ones. The visitors were given a tour of the Johnson Space Center. They met a real astronaut, saw replicas of spaceships, walked around in the red sandbox that crew members would use for their “spacewalks” and asked questions directly of CHAPEA’s lead researcher, Grace Douglas. The three Jones boys were proud to learn how their father was helping to shape the future of humanity.

But the most valuable part of the weekend, the families agreed, was the chance to meet one another. During a barbecue by the hotel pool, they shared their anxieties about the coming year. They exchanged techniques for managing stress and pledged to keep in close contact through a private Facebook page.

On Jake Harwood’s final evening in Houston, Alyssa Shannon prepared a shrimp salad in the hotel kitchenette. It was bittersweet: the last meal she would fix in more than a year. Before leaving Oakland, she had frozen about a dozen feasts for Jake and their friends to enjoy during her absence. She would miss cooking. There would be no pizza on Mars.

The couple gazed out the window at a full moon. There would be 13 more, Jake told her, before she returned from Mars. He would be counting down the full moons until they saw each other again.

They awoke at dawn and watched the sun rise. Alyssa drove him to the airport. “It was hard to say goodbye,” Jake said, if not as hard, he anticipated, as their final phone call before the ingress, which he referred to as the “big one.” But Alyssa’s final phone call from Houston came five days earlier than he expected.

Alyssa announced that NASA had removed her from the mission. The investigators pulled her into a room and told her that she had been “excluded from continuing.” She would be replaced by one of the alternates, Anca Selariu, a microbiologist in the U.S. Navy. Alyssa did not know why she had been removed. The investigators refused to tell her, she said. They said only that their decision had not been based on her performance. They added that sometimes, in the final tests before a mission, they found something that was not “medically serious” but might present a hazard. Like an increased risk of kidney stones.

“Do I have an increased risk of kidney stones?” Alyssa asked.

Kidney stones was just an example, the investigators insisted. But they refused to say more, lest they compromise the integrity of the experiment.

Alyssa doubted that she had been torpedoed by a medical condition. She wondered instead if she wasn’t “exactly the right mix of introvert and extrovert they were seeking.” Or perhaps they had grown concerned about the crew’s social dynamic? If so, Alyssa couldn’t say why. The investigators, she said, told her that she could make up any excuse she wanted, and they wouldn’t deny it. “But lying is so unsatisfying,” she said. “And you have to remember the lie. It’s too challenging. I want to go to the truth. There was a reason, and they couldn’t tell me what it was.”

The uncertainty plagued her, but not as much as the loss she felt from the death of a dream she had nurtured since the Lego Martian colonies of her childhood. She couldn’t help feeling wounded. “This has been hard on my ego,” she said. “It’s a big upheaval. It’s been uncomfortable.” She sighed. “But I have to trust that my departure is for the best of the mission. By stepping back I’m just serving in a different way.”

Her sudden banishment led to some logistical awkwardness at home. “When an astronaut comes back,” Kate Greene wrote, “Earth isn’t where it was.” When Alyssa came back, she found herself suddenly without a job, income or home. Her hospital had promised her a position in 13 months, but in the meantime someone had been hired to replace her. Nor would NASA pay her the full stipend she had been promised, which she says was about $60,000. She didn’t qualify for unemployment benefits. And she had rented her apartment for a year. Though she knew she would be able to move in with Jake, they hadn’t previously decided to live together.

Jake could not disguise his excitement. He met her at the airport and brought her to his house, where they shared a pizza.

Alyssa, an indefatigable optimist, began brainstorming over dinner. Perhaps she would use the sudden windfall of free time to set out on a major backpacking adventure or a cross-country road trip. Maybe she would begin a new career. Or maybe she wouldn’t go back to work — ever. Jake listened, humoring her. Then, with great tenderness, he proposed that she take a couple of weeks to herself before deciding what to do with the rest of her life.

On the afternoon of Sunday, June 25, the couple opened NASA’s YouTube channel. The four crew members stood on a platform in front of the habitat. They wore black jumpsuits embossed with the reddish CHAPEA mission patch: Mars Dune Alpha, rendered not inside a Houston warehouse but at the foot of a Martian sierra, the same mountain range painted on the wall of the sandbox.

“The knowledge we gain here will help enable us to send humans to Mars and bring them home safely,” Grace Douglas said. The crew members expressed their gratitude to NASA. When Anca Selariu said, “I just can’t believe that I’m here,” Alyssa teared up.

As soon as Nathan Jones began speaking about his family, he broke down. Kelly Haston patted his shoulder. “To my wife and kids,” he finally said, through a sob, “I love you to Mars and back.”

Douglas opened the door to the habitat. It was not a special hatch with airlocks or anything: It was just a plain white office door. The crew, waving, entered. Douglas shut the door firmly behind them.

From inside the sealed habitat, the crew could be heard whooping with joy.

In Springfield, Kacie Jones was watching with her sons. She had felt it was important that she be alone with the boys, without any extended family, not knowing how they would respond to the sight of their father leaving for a year. In the end, the boys were fine. Kacie was not. But about 22 minutes after the habitat door closed, she received a text message. It came from Mars.

“I love you,” Nathan wrote.

Kacie took a deep breath. “We’re finally in it,” she told herself. “Which means now we can move forward.” She took the boys for tacos, put them to sleep and set the alarm clock so that she had enough time, in the morning, to get them ready for camp.

At Jake’s house in Oakland, after Alyssa closed the laptop, there was a moment in which they did not know what to do with themselves. They figured Alyssa’s family would worry about her, so she put on a costume spacesuit and dressed Bun Bun, a stuffed rabbit that she had planned to bring to Fake Mars, in a tiny NASA spacesuit. Jake snapped portraits and sent them to her family to let them know she was all right. Or at least that, once the sting of missing out on a year on Mars had subsided, she would be all right. That staying on Earth, with her recipe collections and Bun Bun and her devoted partner, might not be such a terrible outcome after all.

Then she baked a whole-wheat sourdough pizza, and she and Jake ate it, together.