Billionaire Dennis Tito, tired of being told that we can’t send humans to Mars just yet, on Wednesday revealed his scheme for launching two astronauts to the red planet as early as December 2017.

Dubbed “Inspiration Mars,” the flyby mission would exploit a rare alignment of Earth and Mars that minimizes the time and the fuel it would take to get to Mars and back home again. The astronauts would come within 100 miles of the Martian surface before being slung back to Earth.

“It would be a voyage of around 800 million miles around the sun in 501 days,” Tito testified Wednesday at a hearing of the House subcommittee on space. “No longer is a Mars flyby mission just one more theoretical idea. It can be done. Not in a matter of decades, but in a few years.”

Tito is a former engineer who made a fortune in investment management and, in 2001, became the first person to pay his way into space, buying a seat on a Russian rocket. Now he’s pitching Inspiration Mars as a national priority for the United States. Grab this rare chance to go to Mars quickly or risk seeing China or Russia get there first, he told members of Congress.

Tito mentioned a backup plan that would offer Inspiration Mars four more years of development time. Another alignment of planets in 2021 offers a second chance to go to Mars fairly quickly, but the journey would last 80 days longer and require that the astronauts fly much closer to the sun, within the orbit of Venus, in one portion of the trip. That would add to the already considerable radiation hazards.

When Tito broached the idea of Inspiration Mars early this year, he thought he could use primarily private rockets and minimize the need for NASA involvement. But the feasibility study led Tito back to NASA. NASA is building a jumbo rocket, the Space Launch System, that is supposed to be ready for its inaugural, uncrewed test flight in 2017. The second launch, carrying a crew in NASA’s new Orion capsule for the first time, isn’t scheduled until 2021.

Tito’s plan would essentially borrow the SLS for the Mars mission, if NASA agreed. And NASA would have to pay for a lot of this. Tito described Inspiration Mars as a “philanthropic partnership with government.” He said private donors would probably give about $300 million for the mission, and the government would need to provide about $700 million — in addition to the money NASA is already spending, under current programs, on rocket and spacecraft development.

NASA reacted coolly to Tito’s proposal.

“Inspiration Mars’s proposed schedule is a significant challenge due to life support systems, space radiation response, habitats and the human psychology of being in a small spacecraft for over 500 days,” spokesman David Weaver said in a statement. “The agency is willing to share technical and programmatic expertise with Inspiration Mars but is unable to commit to sharing expenses with them. However, we remain open to further collaboration as their proposal and plans for a later mission develop.”