CAPE CANAVERAL, Fla. - President Obama boldly predicted Thursday that his new plans for space exploration would lead American astronauts on historic, almost fantastic journeys - in his lifetime - to an asteroid and then to Mars, relying on rockets and propulsion still to be imagined and built.

"I expect to be around to see it," he said of pioneering U.S. trips starting with a landing on an asteroid - a colossal feat in itself - before the long-dreamed-of Mars expedition.

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 say his planned changes will instead deal a staggering blow to the nation'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 shuttle flights.

By 2025, he said, the nation 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 those.

Asteroids zip by Earth fairly often and have occasionally smacked it with disastrous results. For example, asteroids have been blamed for 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.

George Washington University space scholar John Logsdon, who has served on several NASA advisory boards, said he thought Obama "said all the right things" in declaring a commitment to space exploration.

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

Obama said he was "100 percent 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 shuttle program ends.

He acknowledged criticism for his drastic changes to NASA's direction. But "the bottom line is: Nobody is more committed to manned space flight, the human exploration of space, than I am," he said. "But we've got to do it in a smart way."

Obama said the space program is not a luxury but rather a necessity for the U.S.