Instead of vague assurances of eventual future exploration below Earth orbit, Mr. Obama announced dates and destinations for American astronauts to reach in the future. But the goals would be achieved long after Mr. Obama leaves office: a visit to an asteroid around 2025, reaching Mars by the mid-2030s.

“The bottom line is, nobody is more committed to manned spaceflight, to human exploration of space than I am,” he said. “But we’ve got to do it in a smart way and we can’t just keep on doing the same old things we’ve been doing and thinking that’s going to get us where we want to go.”

Mr. Obama’s budget request to Congress in February proposed a major shift for the space agency: canceling the program started five years ago to send astronauts back to the Moon and turning to private companies for carrying astronauts to the International Space Station.

Mr. Obama responded to criticism that having NASA rely more on commercial companies was too risky, and also stated that the agency would start developing a heavy-lift rocket by 2015.

“Step by step, we will push the boundaries not only of where we can go but what we can do,” Mr. Obama said. “In short, 50 years after the creation of NASA, our goal is no longer just a destination to reach. Our goal is the capacity for people to work and learn, operate and live safely beyond the Earth for extended periods of time, ultimately in ways that are more sustainable and even indefinite.”

His speech contained few surprises as White House officials disclosed crucial aspects of it two days ago. It contained small concessions to critics of the new plan; Mr. Obama is now proposing to revive the Orion crew capsule, which the administration had planned to cancel, as a lifeboat for the space station.

“This Orion effort will be part of the technological foundation for advanced spacecraft to be used in future deep space missions,” Mr. Obama said.

Bohdan Bejmuk, a retired Boeing executive who served on the blue ribbon committee that reviewed the human spaceflight program last year, said the concessions were welcome, adding that the president’s plan was “better than it was a few days ago.”

Mr. Obama promised $40 million to help retrain workers in and around the Kennedy Space Center who will lose their jobs when the space shuttles are retired.