A mission to land humans on Mars would take a minimum of two years from lift off to homecoming, but the most difficult engineering problem isn’t how we get there; it’s what we do once we’re on our way.

In her book Packing For Mars, Mary Roach writes about the challenges of sending the glitch-prone human body into space.

Roach tells NPR’s Tony Cox that physically, space travel is incredibly taxing.

“Particularly with a long-term mission,” she says, “you have a tremendous amount of bone loss.”

Experts estimate that on a Mars mission, astronauts would lose one-third to one-half of their bone mass.

“That’s similar to if you went into a wheelchair,” Roach says. “It’s a serious consideration.”

The psychological challenges of space travel are also considerable because simple acts like eating, crossing the room or going to the bathroom can suddenly become incredibly difficult.

“These astronauts are the most high-achieving people in the world,” Roach says, “and you kind of have to send them back to preschool, because they have to re-learn everything.”

Boredom is another serious factor because of the length of missions, as we’ve learned from astronauts who spent time on the Mir Space Station or the International Space Station.

“The scenery doesn’t change, you can’t go out for a walk,” Roach says. “The repetitiveness and the lack of color just begins to seem mundane.”

Of course, tedium is less of a problem on shuttle flights when there’s much to be done. For those times when there isn’t much to do, some members of the space community are pushing for beer in space — but carbonation can be tricky.

“The gas bubbles don’t rise to the surface as they do in a glass on earth,” Roach explains. “They tend to just foam around and froth in the middle.”

And in the human stomach, if gas bubbles don’t rise to the top, then astronauts can’t burp. According to Roach, “it’s very uncomfortable.”

She would know. As part of her research, Roach flew aboard the “vomit comet,” NASA’s zero gravity flying laboratory.

“It was an extraordinary experience, because you have no weight, and all of your organs have no weight,” she says. “It’s this wonderful physical euphoria.”

Needless to say, she’d love to try it again. Copyright 2010 National Public Radio. To see more, visit http://www.npr.org/.