On the way to Mars, Neil Scheibelhut stopped by Wal-Mart for mouthwash and dental floss. “We’re picking up some last-minute things,” he said.

Scheibelhut is not actually an astronaut leaving Earth. But three hours later, he and five other people stepped into a dome-shaped building on a Hawaiian volcano where they will live for the next eight months, mimicking a stay on Mars.

This is part of a NASA-financed study, the Hawaii Space Exploration Analog and Simulation, or Hi-Seas. The goal is to examine how well an isolated group of people can get along and work together.

When astronauts finally head toward Mars years from now — NASA has penciled the 2030s — it will be a long and lonely journey: about six months to Mars, 500 days on the planet, and then six months home.

“Right now, the psychological risks are still not completely understood and not completely corrected for,” said Kimberly Binsted, a professor of information and computer science at the University of Hawaii at Manoa and the principal investigator for the project. (She is not in the dome.) “NASA is not going to go until we solve this.”

Several mock Mars missions have been conducted. A simulation in Russia in 2010 and 2011 stretched 520 days. Four of the six volunteers developed sleep disorders and became less productive as the experiment progressed. The Mars Society, a nonprofit group that promotes human spaceflight, has run short simulations in the Utah desert since 2001 and is looking to do a one-year simulation in the Canadian Arctic beginning in 2015.

Hi-Seas has already conducted two four-month missions, and next year, six more people will reside for one year inside the dome, a two-story building 36 feet in diameter with about 1,500 square feet of space. It sits in an abandoned quarry on Mauna Loa.

To simulate the operational challenges, the crew is largely cut off. Their communications outside the dome are limited to e-mail, and each message is delayed by 20 minutes, simulating the Mars to Earth lag.

The crew can check a few websites, such as bank accounts, to ensure that their Earth lives do not fall apart. A cellphone can make emergency calls.

About 150 people applied to participate. Binsted said the three men and three women, ages 26 to 36, were chosen to have a similar mix of experience and backgrounds as real NASA astronauts, and many aspire to go to space.

Commander Martha Lenio is an entrepreneur looking to start a renewable-energy consulting company. Other members are Jocelyn Dunn, a Purdue University grad student; Sophie Milam, a grad student at the University of Idaho; Allen Mirkadyrov, a NASA aerospace engineer; and Zak Wilson, a mechanical engineer. Scheibelhut had worked on the first Hi-Seas mission ground support crew.

“I dream about being an astronaut, and this might be the closest I ever get,” Dunn said.