A married couple could be sent to Mars in as little as five years' time as part of a private mission backed by the world's first space tourist.
The voyage would take the husband and wife astronauts as close as 100 miles from the surface of Mars, but would not actually land them on the Red Planet.
They will need to get along - the expedition would see them cooped up for 16 months in a cramped space capsule.
Inspiration Mars, a non-profit venture, aims to make the most of the close approach of the orbits of Mars and the Earth, which happens once in a generation.
US space agency Nasa will not be involved.
Instead, the project's backers - including US multimillionaire Dennis Tito - intend to use a private rocket and space capsule and some kind of habitat that might be inflatable.
The aim is to use a basic design that could take people to Mars for a fraction of what it would cost Nasa to do.
The crew members will have no lander to take them down to the planet and no space suits for any space walks.
They will have minimal food, water and clothing, and their urine will be recycled into drinking water.
"This is not going to be an easy mission," explained chief technical officer and potential crew member Taber MacCallum.
The trip will get initial funding from Mr Tito, who in 2001 paid around \$20m (£13m) to spend seven days in orbit as a crew member of ISS EP-1, a visiting mission to the International Space Station.
Mr MacCallum would not say how much the overall Mars flight would cost, but outsiders put the price tag at more than \$1bn (£660m).
He said the decision to send a married couple into space was both symbolic and practical, as they would need each other's emotional support on the long, cramped voyage.
Mr MacCallum was part of a team that lived for two years in Biosphere 2, a giant terrarium - or sealed environment - on Earth that was supposed to replicate a mission on another planet.
The mission timeline, which is set out in a technical paper, calls for a launch on January 5, 2018, a Mars fly-by on August 20, 2018, and a return to Earth on May 21, 2019.
Professor Scott Hubbard, Nasa's former Mars mission chief, said the paper was "long on inspiration, short on technical details", but what was in there was correct.