I'd always wanted to visit Mars. Instead I got Hawaii. There, about 8,200 feet above sea level on Mauna Loa, sits a geodesically domed habitat for testing crew psychology and technologies for boldly going. I did a four-month tour at the NASA-funded HI-SEAS—that's Hawaii Space Exploration Analog and Simulation—in 2013, and a new 8-month mission is scheduled to start in October. It's a long time to be cooped up, “so the psychological impacts are extremely important,” habitat designer Vincent Paul Ponthieux says. The key to keeping everybody sane? A sense of airiness. Yep—even on Mars, you're going to need more space.

__Power System | __ Solar panels supply power and charge the batteries for the habitat. If juice levels fall below 5 percent, a hydrogen fuel cell kicks in.

__Workshop & Airlock | __ Crew can use the 3-D printer to make hair clips, replacement parts, and anything else they forgot back on Earth. This area is also the door to the surface; they simulate depressurization and pressurization before and after sorties.

__Composting Toilets | __ Repurposed poop (sans pathogens) from one mission might be plant food for the next one.

__Bedrooms | __ Six pie-slice-shaped staterooms each contain a mattress, a desk, and a stool. Clothing goes under the bed, which sits at the wide side of the slice. Cozy like a closet.

__Workout Area | __ Everyone exercises in shifts, often to videos like P90X and Insanity. Other workouts: juggling and balloon volleyball.

__Communications | __ Mars is up to 24 minutes away as the photon flies, so crews have NASA-issued email addresses with an artificial delay and access to a web made of cached, nondynamic pages.

__High Ceilings | __ The 36-foot-diameter dome has a living area of about 1,000 square feet, and the second level is a loftlike partial floor. To long-term inhabitants, these spaces appear to shrink over time, so high ceilings are crucial.