Honeymoon on Mars, anyone?

A non-profit foundation wants to recruit a man and a woman - possibly a married couple - for a bare-bones, 501-day journey to Mars and back that would start in less than five years, project organisers said.

The mission, expected to cost upwards of $1-billion, would be privately financed by donations and sponsorships.

Project founder Dennis Tito - a multimillionaire who, in 2001, paid $20-million for a trip to the International Space Station - said he would pay start-up costs for two years to begin development of life-support systems and other critical technologies.

"This is not going to be an easy mission," chief technical officer and potential crew member Taber MacCallum said.

"We called it the Lewis and Clark trip to Mars."

The spacecraft will be bare-bones, with about 17m³ of living space available for a two-person crew. The trying circumstances include: no showers, limits on toilet paper and clothing, drinking water made from the crew members' recycled urine and sweat and almost no privacy.

But the flight also comes with never-before-seen views of Mars. And there's ample time for zero-gravity sex in space, something Nasa doesn't like to talk about.

As for why a man and a woman will be selected, "this is very symbolic and we really need it to represent humanity", MacCallum said.

He said if it is a man and a woman on such a long, close-quarters voyage, it makes sense for them to be married so that they can give each other the emotional support they will probably need when they look out the window and see Earth get smaller and more distant: "If that's not scary, I don't know what is."

On January 5 2018, a rare planetary alignment will allow a craft to loop around Mars, coming as close as about 241km to the planet's surface, before returning to Earth.

The launch window for the mission opens on January 5 2018. The next opportunity is not until 2031.

If launch occurs on January 5 2018, the capsule would reach Mars 228 days later, loop around its far side and slingshot back towards Earth. The return trip would take 273 days and end with an unprecedented 51119km/h slam into Earth's atmosphere.

Once the spaceship is on its way, there is no turning back.

"If something goes wrong, they're not coming back," MacCallum said.