"We're going to have to do it with NASA, and probably a certain amount of government funding," said Dennis Tito, the investment guru and one-time space passenger who kicked off the Mars flyby project earlier this year. "But probably within the scope of the current budget."

Tito and other leaders of Inspiration Mars provided an update on their plans on Friday at the Mars Society's annual convention at the University of Colorado in Boulder. In cooperation with the Mars Society and NASA's Ames Research Center, they also announced an engineering design contest that gives student-led teams a chance to lay out proposed mission architectures for the 501-day flyby.

The top-rated team gets $10,000, plus an expense-paid trip to next year's Mars Society meeting. There'll also be cash prizes for four runner-up teams. Check out the Mars Society website for deadlines and details.

Mission in flux
The contest serves as one indication that Inspiration Mars is still a project in flux. Chief technology officer Taber MacCallum told NBC News that the mission architecture is still under study, and the crew selection process isn't due to begin until next year.

But at least one thing is not in flux: the 2018 launch date. That's because the mission is being designed around a rarely occurring "free return" trajectory that would send a spacecraft within 100 miles (160 kilometers) of the Martian surface and bring it back to Earth without any extra rocket blasts along the way. Such an opportunity won't come again until 2033.

The need to launch in 2018 — or hold off for another 15 years — means Tito, MacCallum and their colleagues need all the help they can get. They're already getting lots of free advice from NASA, and taking advantage of the know-how gained from 13 years of operations on the International Space Station.

"This mission would not be possible without the ISS," MacCallum said. "We're really taking ISS technology and adapting it to a two-person crew for that duration. We would love to test our systems on the International Space Station. It'd be a crime not to."

MacCallum said Inspiration Mars might even hand over responsibility for crew selection and training to NASA — with the understanding that the astronauts would have to be a man and a woman, most likely a couple beyond childbearing age, so that both genders of the species would be represented on the mission.