Suzanne Bell, Ph.D., a NASA-funded psychologist researching group dynamics on extended missions: A Mars trip has all the ingredients for falling in love, and researchers have seen the danger of that in simulated environments. In one incident a man made unwanted sexual advances toward a woman in the other group, and they decided to shut the hatch between the two groups. It's something NASA has to think about if they're going to send men and women to space together for three years.

Martha Lenio, Ph.D., 35, a former HI-SEAS volunteer who hopes to become an astronaut for Canada: We did have conflicts, like whose turn it was to vacuum, but nothing we couldn't work out. In general what I most worried about—like getting bored with the food since everything is freeze-dried or dehydrated—wasn't a problem at all. We'd compete on how creative we could get with taco and pizza nights. By the end of the eight months, some of my crew mates were really ready to get out, but I could have stayed longer!

Sheyna Gifford, M.D., 37, a doctor in St. Louis who is currently living in the HI-SEAS dome for a year: The researchers "back on Earth" (we consider ourselves "simulated astronauts" in "simulated space") are giving us tools to deal with the isolation. We have some virtual reality [VR] games that our loved ones at home can play too, so we can leave each other notes and photos or have treasure hunts. Another researcher is going to use VR to "send us to the beach" for a few minutes a week. That may prove very transportive!

"My son's life will change while I'm gone."

For a mission to Mars, crew members may be able to take only two shoe boxes' worth of personal belongings. But the hardest thing to leave behind? Family. Three of the astronauts are married, and two are mothers.

Koch: I'd definitely miss my husband. When I was at the South Pole, I packed boxes of things to take with me and opened one every few months, so I had something to look forward to. For Mars I'd ask my family and friends to make small surprises for me to open on designated dates. A handwritten card when you've been away 15 months can be the best thing imaginable.

McClain: [For a long-term mission today,] I'd want a way to video chat with my three-year-old son and my partner. Leaving them is the only downside to space travel; they are my everything. I'd also bring my son's stuffed monkey. He would dig seeing it floating in space.

Meir: I'd need music—the Red Hot Chili Peppers. I would definitely request mixes from a few specific friends, and I'd try to get access to The New York Times.

Mann: If I get tapped for the mission, I'll talk to my son about what I'll be doing. He's almost four now but will be a teen or in his twenties by then. His life will change while I'm gone. And that's a big sacrifice.

"From space you can't see borders."

If chosen for the expedition, these women and their fellow astronauts will make many big sacrifices. But they will also go on what could be humankind's greatest exploration to date—an experience those who came before them can only begin to imagine.

Richard Garriott de Cayeux, 54, a private astronaut who has traveled to the international space station: When you're floating in a slightly stale-smelling, noisy tin can, you have this experience astronauts call the overview effect: From space you can see how weather forms and moves, you can see Earth's tectonic plates' seams and deserts and oceans. And suddenly this place you've always thought of as vast looks small and fragile. It's impossible not to feel an urge to protect our environment and our planet.