NASA is making progress on its journey to send humans to Mars, but it will take tenacity, flexibility and the cooperation of scientists and engineers from around the world.

"We're on pace... to send American astronauts to Mars in the 2030s," said NASA administrator Charles Bolden, speaking at the Humans to Mars Summit in Washington, D.C. "We could say we don't know today how to get to Mars. But we know enough... This plan is clear. This plan is affordable. And this plan is sustainable."

Bolden was just one of the NASA leaders who spoke about how feasible it will be to meet the space agency's goal of getting astronauts to the surface of Mars by the 2030s.

NASA "Stay the course and make course corrections as you get there. If we go back and try to start all over now, we are toast... Have faith. We're going to get there," said NASA administrator Charles Bolden.

A former astronaut and various administrators talked about the cost involved, the technology that will be needed and the risks that will accompany any human trip to the Red Planet.

Bolden emphasized that now is not the time to falter.

"You start out and modify your route as you go rather than go back to the beginning... where you face no hope of getting to Mars," Bolden said. "Stay the course and make course corrections as you get there. If we go back and try to start all over now, we are toast... Have faith. We're going to get there."

William Gerstenmaier, NASA's associate administrator for human exploration and operations, noted this morning that while the space agency is making "huge progress" with Orion -- NASA's new exploration craft -- and its heavy-lift launch technology, the agency needs to stay open to receiving help from others.

"That big budget increase is not coming," said Gerstenmaier, so NASA can't afford to create everything itself. The agency must embrace new ideas, whether they're from a university, another space agency or from Elon Musk of SpaceX. "This won't happen from any one of us but from all of us working together. We can't lock onto one approach... If we do that, we'll be on Mars."

NASA's leaders explained that the work being done today with robotics, space probes and work on the International Space Station is about testing new technology and teaching scientists about what it will take for the long journey to Mars and back, as well as ultimately for a long stay on Mars.

Bolden also noted that NASA has been criticized for not having enough of a plan for Mars and for not having a big enough budget to make a human journey there realistic.

He said neither critique is accurate.

"Past estimates don't apply," Bolden said. "They're irrelevant. ... We can show that it's affordable."