U.S. President Barack Obama boldly predicts his new plans for space exploration will lead American astronauts on historic, almost fantastic, journeys to an asteroid and then to Mars — in his lifetime — relying on propulsion systems still to be imagined.

President Barack Obama speaks about the future of NASA at the Kennedy Space Center in Florida on Thursday. (John Raoux/Associated Press) "I expect to be around to see it," he said Thursday about pioneering U.S. trips starting with a landing on an asteroid — a colossal feat in itself — before the long-dreamed-of expedition to Mars.

He spoke near the historic Kennedy Space Center launch pads that sent the first men to the moon, a blunt rejoinder to critics, including several former astronauts, who contend his planned changes will instead deal a staggering blow to the country's manned space program.

"We want to leap into the future," not continue on the same path as before, Obama said as he sought to reassure NASA workers that America's space adventures would soar on despite the impending termination of space shuttle flights.

Obama did not predict a Mars landing soon. But he said that by 2025, the country would have a new spacecraft "designed for long journeys to allow us to begin the first-ever crewed missions beyond the moon, into deep space."

"We'll start by sending astronauts to an asteroid for the first time in history," he said. "By the mid-2030s, I believe we can send humans to orbit Mars and return them safely to Earth. And a landing on Mars will follow. And I expect to be around to see it."

The biggest criticisms of Obama's plans have been that they have lacked details and goals. Thursday's speech was an attempt to answer, especially since an asteroid is the next step away from Earth's reach.

Asteroids zip by Earth fairly often and have occasionally smacked the planet with disastrous results. For example, asteroids have been blamed for the extinction of the dinosaurs.

Landing on an asteroid would give scientists a better idea of how to handle a future killer asteroid that could wipe out much of life on Earth. Also, it would be a feat sure to win great attention — and there is far less gravity than the moon, meaning it would be easier and cheaper to leave.

"I think he said all the right things" in declaring a commitment to space exploration, said George Washington University space scholar John Logsdon, who has served on several NASA advisory boards. "I don't know what more you could have asked for."

Criticism continues

But several Republicans, including Senator David Vitter of Louisiana and Representative Rob Bishop of Utah, assailed Obama's plan and speech, calling his plans "job-killing."

"The president's new plans for NASA are flat-out irresponsible," Vitter said. "He has evidently decided … that it's time for us to simply walk away from manned space exploration for the foreseeable future, with no clear timeline for returning or for achieving any of our goals for deep space exploration."

Obama said he was "100 per cent committed to the mission of NASA and its future." He outlined plans for federal spending to bring more private companies into space exploration after the soon-to-end space shuttle program.

He acknowledged criticism for his drastic changes to the space agency's direction. But, he said, "The bottom line is: Nobody is more committed to manned space flight, the human exploration of space, than I am. But we've got to do it in a smart way; we can't keep doing the same old things as before."

Obama said the space program is not a luxury, but a necessity, for the United States.

He noted that the Kennedy Space Center has inspired the country and the world for half a century. He said NASA represents what it means to be American — "reaching for new heights and reaching for what's possible" — and is not close to its final days.

Obama sought to explain why he aborted 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 country'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."

The Obama space plan relies on private companies to fly to the space station, giving them almost $6 billion US to build their own rockets and ships. It also extends the space station's life by five years and puts billions into research to eventually develop new government rocket ships for future missions to a nearby asteroid, to the moon, Martian moons or other points in space.

Those stops would be stepping stones on an eventual mission to Mars itself.

Praise for Aldrin

Among his most vocal critics has been Neil Armstrong, the first person to walk on the moon. Obama did not mention Armstrong, who didn't attend the speech, but he did praise Buzz Aldrin, one of Armstrong's Apollo 11 crewmates and the second man to walk on the moon.

Aldrin flew in with Obama on Air Force One.

Obama also said his administration will rescue a small part of the moon program: its Orion crew capsule. But instead of taking four astronauts to the moon, the not-yet-built Orion will be slimmed down and used as an emergency escape pod for the space station.

Obama spoke in the vast launch complex's Operations and Checkout building — the place where Orion would be prepared for launch.

The president said: "This Orion effort will be part of the technological foundation for advanced spacecraft to be used in future deep space missions. In fact, Orion will be readied for flight right here in this room."

White House science adviser John Holdren summed up Obama's program as "a faster pace in space, more missions to more destinations sooner, at lower cost."