If all goes as NASA — and Elon Musk — have planned, at some point in the not-too-distant future, a group of astronauts will begin a years-long round trip to Mars. In NASA’s plan, during each six-month (or more) leg of the journey, the members of a small crew will strap themselves into a cramped spacecraft that offers limited opportunities for recreation, distraction or privacy. As they get farther from Earth, they’ll be increasingly isolated from everything they’ve ever known. Real-time communication with mission control or family members will become impossible.

All of that is a recipe for psychological stress, even above and beyond what astronauts have already experienced. So scientists are trying to identify the unique mental pressures that would accompany a trip to Mars so they can select crews who will cope the best, prepare them to handle the difficulties they will face, and learn how best to help them when they’re millions of miles away.

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Obviously, the first trip to Mars will be unprecedented. So for clues to what astronauts will face, researchers at NASA and other organizations are relying on various analogs. They’ve studied long-term missions to the International Space Station and to other isolated, extreme environments where humans live in confined spaces — Antarctica, for example — and they have conducted controlled studies that shut participants in environments designed to mimic the conditions in space in order to identify potential sources of conflict. According to Nick Kanas, a psychiatry professor emeritus at the University of California, San Francisco and the author of “Humans in Space: The Psychological Hurdles,” some of the stressors that come with long-duration space missions include confinement, isolation, the inherently dangerous environment, and monotony. And there are interpersonal stressors, too — conflicts that could arise within a team or with the ground crew.

The NASA selection process includes psychiatric and psychological screening designed to weed out people with behavioral health issues. A NASA report released last year notes that “while mood and anxiety disturbances have occurred, no behavioral emergencies have been reported to date in space flight.” A longer mission, though, “greatly increases” the chance that an adverse cognitive or behavioral condition or psychiatric disorder will emerge. That means that in addition to screening candidates, it’s crucial to anticipate problems and have a plan to offer support or therapy from afar. The selection panel also looks to identify the people who are most suited to the new challenges of long-haul missions — those who have attributes like working well in teams and understanding how to live peacefully in a group as well as emotional stability and composure under stress.

It’s not clear exactly when potential behavioral and interpersonal problems are most likely to pop up. In one simulation called Mars 500, which was conducted in Russia, six male participants — similar in age, career and education to ISS astronauts and selected by the Russian Federation, European Space Agency and China National Space Administration — lived together in a closed chamber for 520 days. They answered weekly questionnaires to gauge their moods, depression levels, conflicts with others, reaction time, stress and fatigue. Researchers found that the response to stressors varied among participants; one man, for example, showed at least some symptoms of depression in 93 percent of the mission weeks. Two crew members showed no signs of behavioral disturbances or psychological distress during the simulation.

But crew members’ coping abilities — or lack thereof — showed themselves pretty early on in the mission, said David Dinges, professor and director of the unit for experimental psychiatry at the University of Pennsylvania’s Perelman School of Medicine and an author of the study. That suggests it might be easier to pick up on these vulnerabilities in much shorter confinement studies, raising the chances of identifying candidates who are either suboptimal or who might need extra support during the mission.

Some, but by no means all, research has found that on longer space missions and in other extreme environments, psychological and interpersonal difficulties seem to ramp up after the halfway point. “The idea is that some crew members arrive at this milestone with relief that things are going well, only to realize that there is still another half to go before they will be home,” Kanas wrote in a paper on potential psychosocial issues during a Mars mission. Kelley Slack, an industrial-organizational psychologist and a member of NASA’s behavioral health and performance astronaut-selection team, said that while this so-called third-quarter effect doesn’t seem to occur consistently, mission organizers might be wise to provide care packages, special events (perhaps done through virtual reality) or other treats to boost spirits at a time when astronauts might be particularly vulnerable.

Some of the potential pitfalls of a Mars trip are the same as those of previous missions or analogs, only compounded. For example, astronauts will experience microgravity — which can be disorienting and uncomfortable — for much longer. Confinement and isolation are always a problem, but the habitable space in the International Space Station is big — about 15,000 cubic feet. That’s the volume of a roughly 1,700-square-foot home with 9-foot ceilings, but because astronauts are weightless, that third dimension serves as habitable, floatable space. The Space Station also offers opportunities for multiple exercise and leisure activities, Slack said. We don’t know exactly how big a Mars vehicle will be, but NASA is planning for something that will offer a fraction of the living space on the ISS — current recommendations are for at least 883 cubic feet of habitable space per person, or about 5,300 cubic feet for a six-person crew. (This document shows a proposed layout.) To put that in perspective, I live with two other people (one of them much smaller than the average astronaut), and in our smallish Brooklyn apartment we each have about 2,850 cubic feet of living space. Of course, we can’t float to take advantage of all of it.

Storage space will be at a premium, which will make it hard to take, for example, a variety of foods to maintain sensory stimulation. That may sound like a small thing, but imagine the mood you’d be in if you were stuck eating barbecue-flavored energy bars for breakfast for months on end.

Astronauts’ sleep cycles, too, can be thrown out of whack, which has wide-ranging implications. When people sleep badly — whether they’re not getting enough sleep or their sleep-wake schedule has drifted off the 24-hour cycle, which can happen in the absence of regular stimuli like sunrise and sunset — that affects our brains and behavior, Dinges said. “Emotions deteriorate, it can be harder to concentrate, you may feel like others are picking on you.” During the Mars 500 simulation, the same two crew members who experienced no behavioral or psychological problems also showed no sleep or circadian disturbances.

Using blue-spectrum light to promote wakefulness and turning that light off to promote sleep can help keep people on a regular, consistent sleep-wake cycle, but Dinges said there’s some evidence that even when lighting cues are used, a subset of space travelers may still experience poor sleep quality that affects their stress and exhaustion levels. Scientists are trying to figure out why, Dinges said. (Sleep is likely to be an issue once astronauts reach Mars as well.)

Then there are the things that the analogs can’t fully capture. For one: the full significance of the two-way 25- to 44-minute communications gap between Earth and Mars (this varies depending on the planets’ positions). Practically speaking, that makes it tough to maintain a normal back-and-forth with support crew or friends and family at home; at minimum, conversational styles will need to be adjusted. Slack said her group at NASA is looking into whether a modified form of behavioral therapy — perhaps delivered the way some apps now offer therapy via text messages, allowing patients and therapists to respond on their own time frame — might be effective for astronauts who need help with stress or other mental health issues. And there’s also ongoing research into using virtual reality to interact with family members as avatars in non-space settings. For example, an astronaut could create a VR treasure hunt, hiding Easter eggs for a child to discover later. But even with these adaptations, it’s hard to know how astronauts will handle the communication delay. Earthbound experiments can simulate the time lag — and several have, including Mars 500 and HI-SEAS, a NASA-funded program simulating Mars missions in Hawaii — but it’s impossible to simulate how someone will feel when they know that the delay exists because they are hundreds of millions of miles away from their fellow Earthlings.

That sense of isolation will likely only be heightened by something called the Earth-out-of-view phenomenon, which happens when our home planet diminishes in size until it becomes indistinguishable from other astral bodies. “No one has ever lost complete contact with Earth visually,” Kanas said. At the very least, not being able to see our planet could take a big source of enjoyment out of the trip, since a survey of 39 astronauts and cosmonauts found that the top positive effect of going to space was gaining a stronger appreciation of Earth’s beauty and fragility. On the space station, the top leisure activity is looking out at Earth, either photographing it or just watching it roll by, Slack said.

We can only imagine how astronauts might feel as Earth dwindles in the window. Will that shrinking dot inspire them as interplanetary explorers? Or will it fill them with existential dread? There’s only one way to find out.