In December, NASA put out a call for adventurers interested in interplanetary exploration. “NASA is on an ambitious journey to Mars and we’re looking for talented men and women from diverse backgrounds and every walk of life to help get us there,” Charles Bolden, NASA administrator and a former astronaut, said in the announcement. More than 18,000 people answered the call, and between now and mid-2017, that vast pool of applications will be cut to 120 finalists, who will vie to become part of NASA’s next class of eight to 12 astronauts.

The physical requirements for space travel are fairly straightforward — general stamina and good health — but the psychological requirements are every bit as important and have become a bigger focus as the space program aims to send people on longer missions that venture much further from Earth. The process of selecting NASA’s space travelers has evolved since the space program began in the 1950s. America’s first astronauts were mostly fighter pilots and “were selected by balls and charisma,” author Mary Roach wrote in “Packing for Mars.” As spaceflight became more routine, she wrote, astronauts faced a new challenge: boredom. “Funny thing happened on the way to the moon: not much,” Apollo 17 astronaut Gene Cernan once wrote. With this in mind, NASA shifted its approach for selecting astronauts to look for people who could not only perform under pressure, but who could also tolerate tedium.

How do you test someone’s suitability for interplanetary travel and habitation? The Japanese Aerospace Exploration Agency, Roach reported, requires astronaut candidates to perform an origami test — they must make 1,000 tiny paper cranes to pass. “Deterioration of accuracy shows impatience under stress,” JAXA psychologist Natsuhiko Inoue told Roach.

At NASA, the precise details of how this latest group of astronauts will be selected remain under wraps, but the types of traits the organization is looking for are no secret. Team orientation, emotional stability, and the ability to live and work in small group environments are among the qualities sought in astronaut candidates, University of Houston industrial psychologist Kelley Slack said during a presentation at the American Psychological Association annual meeting in Denver in August. The finalists for NASA’s next astronaut class will be brought to Johnson Space Center for psychological testing, experimental interactions, psychiatric interviews and other assessments, she said.

First and foremost, NASA selects the people it will send into space based on their competency at specific jobs. “It is a mission, and there’s work that needs to be done,” said Pete Roma, a psychologist at the Institutes for Behavior Resources and Johns Hopkins University School of Medicine who has worked on a NASA project. “Technical competence is critical,” he said. “If you trust the people you’re working with to do their jobs and you respect their skills, that helps the crew sustain their performance for the long term.”

Other important traits, Roma said, include conscientiousness, attention to detail, a strong work ethic, agreeableness, an openness to other people and an ability to handle disagreements. A mission to Mars would last at least two-and-a-half years, and a classic “Type A” fighter pilot type might not have the personality to handle isolation well or cooperate with others in a very small group, Roma said. In fact, long-haul astronauts may handle missions better if they’re introverted, he said. Some people feel energized by spending time alone as well as working with others, and those are ideal candidates, he said.

If you want to go to Mars, you’d better be prepared to handle some serious isolation. That kind of spaceflight will require a complete disconnection from your home planet. “One of the favorite activities of astronauts [at the International Space Station] is to sit in a cupola and photograph and look at the Earth,” Slack said during her American Psychological Association talk. But unlike travelers to the moon or the space station, astronauts headed to Mars won’t be able to see Earth up close, if at all, for long periods. Providing a virtual window that would show images of Earth, with its rotations and even seasons, might help astronauts stay psychologically connected to the planet, Slack said, but it’s hard to know in advance how people will respond to this separation.

“A mission to Mars is much more analogous to an exploration mission at sea or in Antarctica than to an ISS flight,” Roma said. To explore the factors needed to create a cohesive, successful team on such a mission, NASA has funded a program called Hawaii Space Exploration Analog and Simulation (HI-SEAS), which operates simulated long-duration planetary surface missions.

In August, the HI-SEAS IV completed its yearlong simulated mission, in which six people lived inside a simulated Mars habitat: a 36-foot-wide dome on Hawaii’s Mauna Loa volcano. During the project, the participants engaged in daily routines and work similar to what astronauts would do on a real mission.

One thing that trips to the ISS have shown is that people cooped up in a space vehicle need to engage in purposeful tasks. “Astronauts don’t like to do things that don’t feel meaningful,” Slack said during her talk. “They don’t want busy work — they want real work.” At HI-SEAS IV, the work included preparing food, testing equipment, tracking the use of resources and performing geological field work.

Although participants had some access to the internet and the outside world (they could venture outside the dome wearing simulated spacesuits), they had to endure a 20-minute delay (in each direction) for all communications. “It’s not just the physical isolation from the world — there’s also zero ability to connect in real time,” said Roma, who works with HI-SEAS. Whereas astronauts at the International Space Station can host video chats with people on Earth, the speed of communications for Mars-bound astronauts will make ’90s-era dialup look good.

A Mars mission will test the limits of its crew members to an extreme degree. “These people aren’t only your co-workers, they’re also your roommates,” Roma said. There’s no escape. Hell is other people, as Jean-Paul Sartre observed, and psychologists like Roma take that into consideration. You can’t bring in six people to not only work together but also live together and expect no tension, Roma said. “There’s always going to be some kinds of conflicts.”

Researchers involved in HI-SEAS are working on ways to defuse conflicts, and the first step is spotting them before anyone blows up. A team led by psychologist Steve Kozlowski at Michigan State University has developed wireless badges that include proximity sensors. The idea is that everybody on the crew would wear one, and the badges then provide a metric for quantifying how much time people spend near one another. “It’s a way to measure team cohesion,” Roma said. The data could serve as an early-warning system to flag when someone is isolating herself or avoiding a particular person.

Similarly, Roma and his colleagues have developed a kind of computer game to check in with a crew periodically. The group gets together to play the game, which is based on behavioral economic principles and measures how the team is working together at that moment. “It’s like checking the team’s oil every once in a while,” Roma said. “If it points to a problem, then you can start deploying countermeasures.”

Some of the tools under development for handling stress include virtual-reality systems that can help people feel more socially connected and combat sensory monotony. These programs might also allow users to experience a relaxing sense of immersing themselves in nature, even if they’re millions of miles from Earth.

How much time scientists have in which to test strategies like this depends on who you ask. On Tuesday, Elon Musk made news by announcing that his company, SpaceX, will build a rocket that could take humans to Mars as soon as 2022. NASA’s timetable aims for the first human Mars mission in the 2030s.

Programs such as HI-SEAS can help researchers understand the challenges posed by a Mars mission and test potential coping strategies, but there’s no getting around the fact that astronauts headed to Mars will far outstrip the limits anything humans have previously accomplished. “To boldly go where no one has gone before” — that’s still an aspiration that inspires imagination and awe.