U.S. President Barack Obama said Thursday he was "100 per cent committed to the mission of NASA and its future" as he outlined plans to get more private companies involved in space exploration after the space shuttle program is shut down.

This image provided by NASA shows astronaut Clayton Anderson at the International Space Station on Tuesday. Obama said the space program is not a luxury but a necessity for the U.S. (Associated Press)

"We want to leap into the future," Obama said at a press conference at the Kennedy Space Center in Merritt Island, Fla., outside of Orlando.

The president sought to reassure NASA workers that America's space adventures would soar on despite the termination of shuttle flights.

Obama acknowledged criticism, even from some prominent astronauts, for his drastic changes to the space program but insisted he is not abandoning space exploration.

He sought to explain why he aborted former president George W. Bush's return-to-the moon plan in favour of a complicated system of public-and-private flights that would go elsewhere in space, with details still to be worked out.

"We've been there before," Obama said of the nation's moon landings decades ago. "There's a lot more of space to explore."

He said his administration would support continued manned exploration of space "not just with dollars but with clear aims and a larger purpose."

Obama's plan is to increase NASA's budget by $6 billion over five years and shift the responsibility for space transportation to private companies, which would be encouraged to "compete to design and build and launch new means of carrying people and materials out of our atmosphere." They would be responsible not just for designing and building the rockets and spaceships, which they largely do already, but also for conducting flights to the International Space Station.

"We will extend the life of the International Space Station likely by more than five years while actually using it for its intended purpose: conducting advanced research that can help improve the daily lives of people here on Earth, as well as testing and improving upon our capabilities in space," Obama said.

"This includes technologies like more efficient life support systems that will help reduce the cost of future missions. And in order to reach the space station, we will work with a growing array of private companies competing to make getting to space easier and more affordable."

The plan would see the government directing billions of dollars into research to eventually develop new government rocket ships for future missions: to an asteroid near the station, to the moon, to Martian moons and other points in space.

Those missions would be stepping stones toward an eventual mission to Mars.

"The bottom line is: nobody is more committed to manned space flight, the human exploration of space, than I am," Obama said. "But we've got to do it in a smart way; we can't keep doing the same old things as before."

He said that by 2025, he expects U.S. space exploration to reach beyond the moon and farther into the solar system's reaches.

During his visit to the space center, Obama visited the launch pads where U.S. space voyages begin. He said the space program is not a luxury but a necessity for the nation.

The Kennedy Space Center launched Americans into space and has inspired a nation for half a century, Obama said.

NASA represents what it means to be American, Obama said: "reaching for new heights and reaching for what's possible."