The world's first space tourist, US multimillionaire Dennis Tito, unveiled plans Wednesday to send a manned mission to Mars and back, targeting a launch date less than five years away.

The two space travelers wouldn't land on the Red Planet -- or even enter its orbit -- just fly through the vicinity and back, a trajectory Tito said would take 501 days, thanks to a rare planetary alignment.

The US space agency has aimed for the 2030s in its vague projections for a manned mission to Mars, and is focusing in the shorter term on sending robots, like the Curiosity rover that landed with much fanfare last summer.

Tito's non-profit Inspiration Mars, by contrast, is starting essentially from scratch, with neither a vehicle nor a clear source of funding.

Still, the mission is "achievable," insisted Taber MacCallum, the foundation's chief technology officer and the head of Paragon Space Development Corporation.

"Experts have reviewed the risks, rewards and aggressive schedule, finding that existing technologies and systems only need to be properly integrated, tested and prepared for flight."

By foregoing a landing, the mission lessens the risks and simplifies the maneuvering required.

Such a mission would likely cost between $1 billion and $2 billion, according to Robert Zubrin, president of the Mars Society, a group that campaigns for the exploration and settlement of the planet.

Inspiration Mars said it is not looking to NASA for the money, but will instead raise funds through "private, charitable donations."

In a statement, the foundation said it would act as the mission's "primary contractor," gathering technology, expertise and skills from a variety of companies and individuals, including Paragon and Applied Defense Solutions.

"Human exploration of space is a critical catalyst for our future growth and prosperity," said Tito, 72.

"This is 'A Mission for America' that will generate knowledge, experience and momentum for the next great era of space exploration."

NASA welcomed the Tito's announcement, saying the program is "a testament to the audacity of America's commercial aerospace industry and the adventurous spirit of America's citizen-explorers," and that it would continue talks on possible a collaboration with Inspiration Mars.

But space expert John Logsdon was less enthusiastic, saying the proposed mission was "not impossible, but implausible."

Logsdon, a former director of the Space Policy Institute at George Washington University, highlighted major challenges for financing and the technical execution, as well as the high risk for the crew.

"Once they start on a trip, they cannot come back... or turn around," he said.

In 2001, Tito was the first non-astronaut to fly into space, when he bought a seat on a Russian Soyuz mission for a week-long stay at the International Space Station.