President Obama told a supportive crowd at the Kennedy Space Center on Thursday that NASA should aim to send astronauts to explore asteroids beyond the moon by 2025 and visit Mars in the following decade.

Responding to critics who said his proposed changes to NASA would “kill” the human space program, the president said, “I am 100 percent committed to the mission of NASA and its future.”

He said that the Constellation space program proposed and initiated by the Bush administration was substantially over budget and behind schedule, and was not going to provide a sustainable path to deep space.

Speaking at NASA’s operations and checkout building, Obama said the agency could not proceed in the “same old way,” and that it needed to bring along commercial space entrepreneurs to handle transport missions to the international space station so the agency would be freed up to think and reach much further.

He said the ultimate goal was to land astronauts on Mars and, he said, “I expect to be around to see it.”

The president focused on plans to design and develop different kinds of “heavy lift” rockets that can take astronauts out of low Earth orbit and into deep space. He wants Congress to approve plans to spend more than $3.1 billion in the next five years on the project, and said NASA would select and begin to build the selected model in 2015.

Obama’s NASA plans, which represent one of the most significant changes in the agency’s history, have not been well received in Congress nor among some former astronauts, including iconic Apollo-era figures including Neil Armstrong and James Lovell. They released a letter this week calling the scrapping of Constellation “devastating” to American plans in space. But the administration is hoping the Kennedy Space Center visit will begin to change the dynamic, and there were some early signs Thursday that that is possible.

Sen. Bill Nelson, D-Fla., for instance, toned down his previous criticisms and said Obama is moving in the right direction. Nelson is chairman of the Science and Space Subcommittee, which oversees NASA.

Former astronaut Sally Ride, the first American woman in space and a member of the blue-ribbon Augustine Commission that cast doubt on the prospects for success under the Constellation program, was one of several recent astronauts to weigh in on the president’s side.

She called the plan “a bold strategic shift that will enable NASA to return to its roots: developing innovative technologies aimed at enabling human exploration and tackling the truly challenging aspects of human spaceflight — venturing beyond Earth orbit, beyond the Earth-Moon system, and into the solar system.”

During his speech, Obama said that while his administration had put a freeze on almost all discretionary spending, it has budgeted $6 billion extra for NASA over five years.

Reflecting the mood of the day, White House deputy press secretary Bill Burton said, “The president believes good policy is good politics.”

Obama said that while the administration was jettisoning much of Constellation, which has cost $9 billion already, he wanted to keep work going on a slimmed-down version of the Orion spacecraft. The capsule will be launched without a crew to the international space station on one of the commercial rockets used by the military, and then kept tethered there as an escape vehicle.

“There are also those who have criticized our decision to end parts of Constellation as one that will hinder space exploration beyond low Earth orbit,” the president said. “But by investing in groundbreaking research and innovative companies, we have the potential to rapidly transform our capabilities — even as we build on the important work already completed, through projects like Orion, for future missions. And unlike the previous program, we are setting a course with specific and achievable milestones.”

Obama’s speech was interrupted 15 times by applause, including after passages expressing “100 percent” commitment to NASA, the Mars landing mission and a budget increase for the space agency.

The speech represented his first detailed public comments about the future of NASA since the agency’s 2011 budget was released to controversy and opposition. NASA administrator Charles Bolden and science adviser John Holden have explained, defended and elaborated on the plans, but the president had remained silent while critics accused him of trying to “kill” the American human space exploration program.

A White House update of plans for NASA released Tuesday made some concessions to critics, but they were not always well received.

Sen. Richard Shelby, R-Ala., said in a statement that “the President’s new plan unmistakably shows that NASA’s management team does not understand the issues at stake. While the Administration may have finally realized that its initial budget request was a complete disaster, the new plan, from the same team, still ends human space flight. This new plan does not represent an advancement in policy or an improvement upon the Constellation program, but a continued abdication of America’s leadership in space.”

Shelby is from one of several Gulf Coast states that will lose NASA and other jobs tied to the space program under the Obama plan. Overall, the NASA budget will increased by $1 billion a year over the next five years under the 2011 budget proposal, but more of the funds will be distributed to science, aeronautics and commercial entrepreneurs than under former President George W. Bush.

Florida officials have been lobbying hard for additional space shuttle flights to the international space station. The Bush administration decided to end the shuttle program and scheduled its last flight for later this year. But in a conference call with reporters Thursday, Sen. George LeMieux, R-Fla., said that top officials at the Kennedy Space Center told him enough resources are in place for one or two additional shuttle flights.

One of the potential major beneficiaries of the administration’s turn to the commercial space industry to resupply and perhaps ferry crew to and from the space station is Elon Musk, president and founder of SpaceX. The company’s Falcon 9 is being prepared for a springtime test launch from Cape Canaveral. “I believe this address could be as important as President Kennedy’s 1962 speech,” Musk said. “For the first time since Apollo, our country will have a plan for space exploration that inspires and excites all who look to the stars. Even more important, it will work.”