KENNEDY SPACE CENTER, Florida — President Barack Obama Thursday set a bold new course for the future of US space travel, planning to send American astronauts into Mars orbit within the next three decades.

And he sought to quell a storm of outrage which met earlier plans unveiled by his administration, vowing before NASA staff that he was "100 percent committed" to their mission and the US space agency's future.

"As president, I believe that space exploration is not a luxury, it's not an afterthought in America's quest for a brighter future. It is an essential part of that quest," he said at the Kennedy Space Center in Florida.

The US president was making a whirlwind trip to the heart of the US space industry after stinging criticism of his decision to drop the costly Constellation project which had aimed to put Americans back on the moon.

Obama, who was accompanied by astronaut Buzz Aldrin, the second man to set foot on the moon, said his administration would pump six billion more dollars into the NASA budget over the next five years.

But he had specific ideas how it should be spent.

"We should attempt a return to the surface of the moon first, as previously planned. But I just have to say, pretty bluntly here, we've been there before. Buzz has been there," Obama said.

"There's a lot more of space to explore and a lot more to learn when we do," he said, to loud applause.

"By 2025 we expect new spacecraft designed for long journeys to allow us to begin the first ever crew missions beyond the moon into deep space.

"So, we'll start by sending astronauts to an asteroid for the first time in history. 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."

In a nod to critics who say the new approach will costs jobs and undermine American leadership in space exploration, Obama said he was retaining part of the Constellation project, the Orion capsule.

Obama said he had instructed NASA administrator Charles Bolden to immediately begin the design of a rescue vehicle using technology already developed for the Orion capsule.

And the United States would also invest some three billion dollars in research on a heavy-lift rocket to send crew capsules and supplies into deep space, with the design to be finalized by 2015.

His plan includes ramping up "robotic exploration of the solar system, including a probe of the sun's atmosphere, new scouting missions to Mars and other destinations, and an advanced telescope to follow Hubble," he said.

Obama also pledged the new plan would create some 2,500 jobs along the so-called space coast in the next two years -- aiming to bring new hope to a region blighted by high unemployment.

Critics, including the first man on the moon Neil Armstrong, were angered by Obama's decision earlier this year to scrap the bloated and behind-schedule Constellation program.

The aging US space shuttle fleet, which carries astronauts to the International Space Station, is due to be grounded at the end of the year, leaving the United States to hitch rides on Russian spacecraft to the International Space Station until a replacement is developed.

"Without the skill and experience that actual spacecraft operation provides, the USA is far too likely to be on a long downhill slide to mediocrity," Armstrong wrote in a letter, co-signed by two other astronauts.

Another powerful critic was Senator Richard Shelby, the top Republican on the subcommittee that has oversight of NASA spending.

"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 said in a statement Wednesday.

But Obama hit back Thursday saying: "We will actually reach space faster and more often under this new plan in ways that will help us improve our technological capacity and lower our costs.

"The bottom line is, nobody is more committed to manned space flight, to human exploration of space, than I am. 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 somehow it's going to get us to where we want to go."