WASHINGTON — NASA needs new technology breakthroughs to revitalize its mission to explore the universe, President Barack Obama said Wednesday as the shuttle Atlantis was preparing for its final mission.

Asked during his first-ever Twitter town hall meeting about where America stands in terms of space exploration with the shuttle program about to end after 30 years, Obama said he was "proud" of the past but eager for a new leap forward.

"We are still a leader in space exploration, but, frankly, I have been pushing NASA to revamp its vision," Obama said.

"The shuttle did some extraordinary work in low-orbit experiments, the International Space Station, moving cargo. It was an extraordinary accomplishment. And we're very proud of the work that it did," he added.

"But now what we need is that next technological breakthrough."

Obama, who axed NASA's Constellation program that would have returned astronauts to the moon, said that the United States should move beyond the space travel models it used in the 1960s for the Apollo program that first sent men to the moon.

"Rather than keep on doing the same thing, let's invest in basic research around new technologies that can get us places faster, allow human space flight to last longer," he said.

While private companies hustle to build a next generation spacecraft to tote astronauts to space again, the US space agency can spend time figuring out new ways to live longer in space and send explorers to Mars or an asteroid, he said.

"Let's start stretching the boundaries so we're not doing the same things over and over again. But rather, let's start thinking about, what's the next horizon? What's the next frontier out there?

"In order to do that, we'll need some technological breakthroughs that we don't have yet," said the US president.

Commercial companies may be able to make space travel more affordable for everyday people, while government-funded scientists focus on other work.

"Let's allow the private sector to get in so that they can, for example, send these low earth orbit vehicles into space, and we may be able to achieve a point in time where those of you who are just dying to go into space, you know, you can buy a ticket," said Obama.

"And a private carrier can potentially take you up there while the government focuses on the big breakthroughs that require much larger investments and involve much greater risk."

The space shuttle Atlantis is scheduled to make its final flight to the International Space Station later this week, though stormy weather could delay the mission by a couple of days, NASA said.

Once the shuttle program ends, the world's astronauts will have to rely on Russia to carry them to the ISS and back until private enterprise comes up with a next-generation US space capsule for cargo and crew.