The voyage would be a cosmic no-frills flight that would take the husband-and-wife astronauts as close as 100 miles to Mars, but it would also mean being cooped up for 16 months in a cramped space capsule about half the size of an RV.
The private, nonprofit project, called Inspiration Mars, will get initial money from multimillionaire Dennis Tito, the first space tourist. The team would not say how much the overall flight would cost, but outsiders put it at more than $1 billion.
NASA will not be involved. Instead, the project's backers intend to use a private rocket and space capsule and some kind of habitat that might be inflatable, employing an austere design that could take people to Mars for a fraction of what it would cost NASA to do with robots, officials said.
The crew members will have no lander to go down to the planet, and no spacesuits to go out for any spacewalk. They will have minimal food and clothing, and their urine will be recycled into drinking water.
"This is not going to be an easy mission," chief technical officer and potential crew member Taber MacCallum said in an interview. "We called it the Lewis and Clark trip to Mars."
It also involves a huge risk, more than a government agency like NASA would normally permit, officials concede.
"It's a risk well worth taking," MacCallum said. He said it harkens back to the days when people took risks when it was meaningful, and he said it could be an inspiration, especially to students.
As for why a couple will make the flight, "this is very symbolic and we really need it to represent humanity with a man and a woman," MacCallum said.
He said if it is a man and a woman on such a long, cramped voyage, it makes sense for them to be married so that they can give each other the emotional support that 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."
The project aims to capitalize on the once-in-a-generation close approach of the two planets' orbits. The mission timeline is set out in a technical paper to be presented next month at a scientific meeting. It calls for a launch on Jan. 5, 2018, a Mars flyby on Aug. 20, 2018, and a return to Earth on May 21, 2019.
In a statement, NASA spokesman David Steitz said the venture validates President Barack Obama's decision to rely more on private sector ingenuity to explore space, and is "a testament to the audacity of America's commercial aerospace industry and the adventurous spirit of America's citizen-explorers."
He said "NASA will continue discussions with Inspiration Mars to see how the agency might collaborate on mutually beneficial activities."
Stanford University professor Scott Hubbard, NASA's former Mars mission chief, said that the team's technical paper is "long on inspiration, short on technical details. What is there is correct."
"It's sort of an audacious thing to say, 'I'm going to fly by Mars in five years,'" said MacCallum, who was part of a team that lived for two years in Biospshere 2, a sort of giant terrarium on Earth that was supposed to replicate a mission on another planet.