In the Mars500 experiment, a collaboration between the Russian, European and Chinese space agencies, a crew of six men was locked in a chamber near Moscow for 520 days to simulate a Mars trip. The crew received no fresh food or fresh air and could not see the sun.

A recent study in the journal Proceedings of the National Academy of Sciences found that most crew members experienced sleep disturbances of some kind but found a trend toward increased sleep and rest as the mission went on. One crew member appeared to live on a 25-hour day instead of 24. Researchers also noted "behavioral aspects of torpor" in the crew, akin to hibernation of some birds and animals.

In his Antarctica studies, Palinkas and colleagues found that in a psychologically healthy group, about 5 percent of people experience clinically significant psychological symptoms. These include depression, anxiety, substance abuse problems, sleep disorders and adjustment disorders relating to not getting along with crewmates.

However, most people do well, he said, and see a lot of benefit from participating in the experience -- "definitely an increase in feelings of self-confidence, self-efficacy -- the belief that if they can handle this, they can pretty much handle anything."

In Biosphere 2, where Poynter's days involved pruning shears, plants and a two-way radio, the crew of eight didn't exactly live in psychological peace.

Poynter remembers standing in the sweet potato field one day, and "it was like a time portal opened." In her mind, she was a child again, having an argument with one of her brothers. Such vivid flashbacks have also occurred to people living in Antarctica, she said.

The Biosphere 2 group eventually sought and received psychological counseling via telephone.

Bored, bored and bored 40 minutes later?

"We've got to select for people that can maintain upbeat and happy attitude in the face of adversity and that are resilient," Poynter said.

Aboard the International Space Station, astronauts often enjoy just gazing out the window at Earth from afar. Canadian astronaut Chris Hadfield has a popular Twitter account that he uses to post extraordinary photos of the planet.

But a couple en route to Mars would see Earth shrink until it's a dot among many others in the lonely sky.

"There's going to be, I'm quite sure, a feeling of separation that no one has experienced before," Suedfeld said.

Don't forget the communication delay. A message may take as long as 40 minutes to travel to Earth and back, and there may be times when communication with the ground is not possible at all.

If a crew member is having a breakdown and someone on Earth is trying to counsel him or her, Kring says, it will take 20 minutes for a message such as "try to think calming thoughts" to reach the spacefarer and another 20 minutes for an acknowledgment to get back to Earth.

One solution to the boredom question could be time-release entertainment packages, Suedfeld said. In other words, some movies or music will not become "unlocked" until a given amount of time has passed so that the couple doesn't blow through all of their entertainment at once.

Poynter said the Biosphere 2 crew created music using the sounds from the animals and machinery in the biosphere. Music-making or reading would be options on the Mars journey, too, she said.

Eventually, presumably, Mars would come into view.

"To look down on Mars and be the first people to see, potentially, our future home planet --" Poynter interrupted herself to let out a high-pitched "Oooo!" -- "gives me chills just thinking about it."

... And other health issues

Besides mental factors, gravity differences in space present a slew of physiological challenges that aren't easily simulated on Earth over long periods of time. There's evidence that the visual system can be impaired, and the weakening of bones and muscle may occur, Kring said.

How the two people aboard the space capsule would deal with these sorts of issues, on top of mental stress, remains to be seen -- as does whether the mission will actually launch as envisioned in January 2018.

"I'm all for supporting this, and I certainly am a big proponent of human spaceflight, but I wonder if all of this can be figured out in five years," Kring said.