Going to Mars? Expect to stay a while. Because of the relative motions of Earth and Mars, the pioneering astronauts who touch down on the Martian surface will have to remain there for a year and a half. For this reason, NASA has already started experimenting with a habitat fit for the long-term exploration of Mars.

Last year, students at the University of Wisconsin won the XHab competition to design and build an inflatable loft addition to a habitat shell that NASA had already constructed. The final structure now serves as a working model that is being tested in the Arizona desert.

Like any home, it's a sacred bulwark against the elements; but not just the cold, heat and pests of Arizona. A Mars habitat will have to protect astronauts from cosmic rays, solar flares and unknown soil compositions all while keeping inhabitants happy and comfortable.

"Radiation protection is our number one risk," says Kriss Kennedy, the project manager at NASA's habitat. "We have the luxury of Earth providing a magnetic field and an atmosphere." But on the moon or Mars, astronauts will be exposed to cosmic rays that damage human tissue when absorbed by the body. Solar flares, a more sudden and unpredictable burst of radiation, can kill instantaneously.

NASA is investigating ways to build an electrostatic radiation shield to protect astronauts. For now, however, the easiest solution is surrounding them with materials that absorb the onslaught of energy.

This means carefully choosing what materials to use in the habitat shell, but it will also determine how objects are arranged in the interior, says Kennedy. Food and supplies can by pushed up against the walls as extra protection. NASA is also looking at ways to repurpose discarded supplies and packaging to build up the habitat wall over time.

"We can take all the garbage and compact it into these discs that we can use on the outside for radiation protection," says Kennedy.

Which raises the more general problem of waste. In NASA's working prototype, the bathroom is not called a bathroom, but a "hygiene module." The name appropriately indicates the level of technology required in designing a bathroom that enables astronauts to tidy themselves without soiling the rest of the living quarters.

The throne resembles more of a robot docking station in the NASA habitat, just as it did on the shuttles. The toilet spirits away waste using an assortment of pneumatic tubes and gullies, which astronauts fix into proper alignment with the aid of a live video feed. This very natural process does not come naturally in space, and astronauts undergo special training to master it.

And so building a Martian or Lunar habitat involves a lot of thought about how to keep bad things out, including dirt. During the Apollo missions, lunar dust insinuated itself into the lander and command module causing respiratory problems among the crew and threatening to damage equipment.

"It's like broken or ground glass. We have to really do a lot to minimize it getting into the habitat and interacting with the crew members," says Kennedy, possibly by covering equipment and fabric with an electrostatic coating.

Reinforcing the habitat so successfully against outside elements has the negative effect of trapping in heat from metabolism and equipment. "The habitats are so well insulated that heat does not escape. So you're basically living inside a thermos bottle," says Kennedy. The NASA habitat collects heat from the air and electronic devices and sends it through a fluid loop to a set of radiators, then expel the heat out to the environment.

Even after securing the ability to live, NASA will have to address the standard of living for astronauts.

Unfortunately, recreation usually takes up a lot of space, and there is active debate in the agency about just how big to make the habitat, according to Kennedy. On a first trip to Mars it would be impossible to build from materials found on the planet.

Every component of the habitat will either have to be preplaced or arrive with the astronauts - either way it will come from Earth. Designing an inflatable loft was one way to create more space while using lighter materials with the aim of providing social, recreational, and living space for each astronaut.

These days, NASA is not quite sure where the next adventure lies-whether a good candidate for asteroid exploration will soar into our neighborhood before we summon enough political courage to send humans into deep space. Wherever we go next, we will at least be prepared to stay for a while.