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By John Zarrella, CNN

If newly unveiled plans pan out, a man and a woman may represent humanity on one journey that has never been attempted before: a mission to Mars.

“It’s incredibly feasible. It’s not crazy talk," Taber MacCallum, CEO of Paragon Space Development Corp., told CNN.

MacCallum and millionaire Dennis Tito announced their plans Wednesday to send a couple of earthlings on a 501-day trip in a spacecraft that would fly by the red planet. The proposal was unveiled at the National Press Club in Washington.

The mission would lift off in 2018, they said. It would not involve landing on Mars, making the proposed journey infinitely easier than putting people on the planet's surface, which NASA wants to do later this century. But the spacecraft would pass within 100 miles of the planet.

Tito has founded the Inspiration Mars Foundation, a nonprofit organization spearheading this effort. No stranger to space, the one-time NASA engineer became in 2001 the first space tourist flying on a Russian rocket to the International Space Station.

The public-private initiative could, according to MacCallum, use an existing rocket and capsule.

“If you take existing chemistry and technology and add some improved technologies," MacCallum told CNN, "you can get a mission together.” A life support system also would have to be developed.

The group is not asking NASA for money, he said.

“This is a philanthropic effort to be done for America,” MacCallum said. It could be accomplished for under $1 billion, he said, a figure that’s cheap compared with the tens of billions of dollars a NASA landing on Mars would cost.

At Wednesday's press conference, the panel mentioned selling media rights and finding sponsorship as well as other forms of fund-raising. It was noted that a 6-year-old boy already made a contribution, sending in $10 and calling this mission "my Apollo."

Despite MacCallum’s optimism, pulling off such a feat within five years is no small task.

Besides life support for the crew, one of the biggest challenges would be the return into the Earth's atmosphere. Heat shielding for a high speed re-entry hasn’t been tested. NASA isn’t even testing its new system on the Orion spacecraft until next year at the earliest. Orion is in development to take astronauts back to the moon and on to Mars.

And there’s also concern about radiation exposure. The man and woman whom MacCallum and Tito want to send would likely be a married couple. Because of the radiation risk, MacCallum said, they’d be older and “out of the childbearing years.”

Dr. Jonathan Clark, a former NASA flight surgeon and chief medical officer of Inspiration Mars

Foundation, said the crew should be selected six months to a year before the mission to allow time for a full health screening. And the mission planners will have to prepare for the possibility of a crew member perishing.

"If we wanted a guarantee, we wouldn't be doing this," he said.

Water and oxygen will be recycled in flight, so the crew will be drinking and breathing the same resources over and over throughout the journey, Inspiration Mars representatives said.

"No two people will have ever been more alone than the crew of this mission," Miles O'Brien, press conference moderator and former CNN correspondent, said at the event.

The year for the mission was chosen because Mars then will be 36 million miles away, about as close as it ever gets to Earth.

But consider: The humans who have traveled the farthest from Earth were the Apollo astronauts - nearly a quarter-million miles to the moon. Next to the Mars journey, that’s like a walk around the block.

Still, Tito said Wednesday, "This is a challenging but attainable goal for advancing human ... knowledge. Now is the time."

More: What we've done on Mars, and what's next

CNN's Sophia Dengo contributed to this report.