CAPE CANAVERAL, Fla. -- Landing a man on the moon was a towering achievement. Now the president has given NASA an even harder job, one with a certain Hollywood quality: sending astronauts to an asteroid, a giant speeding rock, just 15 years from now.

Space experts say such a voyage could take several months longer than a journey to the moon and entail far greater dangers.

"It is really the hardest thing we can do," NASA Administrator Charles Bolden said.

Going to an asteroid could provide vital training for an eventual mission to Mars. It might help unlock the secrets of how our solar system formed. And it could give mankind the know-how to do something that has been accomplished only in the movies by a few square-jawed, squinty-eyed heroes: saving the Earth from a collision with a killer asteroid.

"You could be saving humankind. That's worthy, isn't it?" said Bill Nye, TV's Science Guy and vice president of the Planetary Society.

"By 2025, we expect new spacecraft designed for long journeys to allow us to begin the first-ever crewed missions beyond the moon into deep space," he said. "We'll start by sending astronauts to an asteroid for the first time in history."

NASA has tracked nearly 7,000 near-Earth objects that are bigger than several feet across. Objects bigger than two-thirds of a mile are major killers and hit Earth every several hundred thousand years. Scientists think it was a 6-mile-wide asteroid that wiped out the dinosaurs 65 million years ago.

Landing on an asteroid and giving it a well-timed nudge "would demonstrate once and for all that we're smarter than the dinosaurs and can avoid what they didn't," said White House science adviser John Holdren.