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With NASA's space shuttle program coming to an end and the future of its manned space flight in doubt, President Obama is facing intense pressure over his budget priorities for the space agency.

Last month, Obama killed NASA's Constellation program, which would have created a shuttle successor to ferry astronauts to the space station and, ultimately, to the moon. Instead, Obama has directed NASA to turn to commercial companies for getting astronauts into orbit and, instead, focus its own efforts on deep-space exploration.

A senior NASA manager said Tuesday it wouldn't be hard to add more shuttle flights, provided the government is willing to keep paying $200 million a month.

NASA's three shuttles are scheduled to retire in September, after four more trips to the International Space Station. Some in Congress, however, are pushing for additional missions to fill the gap between the end of the shuttle program and the country's next manned spaceship, whatever and whenever that might be.

But the president is encouraging private companies to develop spacecraft to deliver astronauts to the International Space Station. Two companies are moving forward with testing, but in the interim, NASA foresees a gap in space activity.

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"It's amazing that we're headed down a path where we're not going to have any vehicles at all to launch from the Kennedy Space Center for an extended period of time," John Shannon, NASA's space shuttle manager said at a news conference. "And to give up all the lessons learned, the blood, sweat and tears that we have expended to get the space shuttle to the point where it is right now where it is performing so magnificently," he said.

But even critics of the president have applauded his decision to change NASA's mission.

"I have been one of the most critical people of anything Obama has done. But on this one, he's moving in the right direction," said Edward Hudgins, the director of regulatory studies at the Cato Institute. "Sorry, but the Constellation program was going to repeat all the mistakes NASA has made in the past."

Hudgins said that NASA should have focused on deep-space exploration after the moon landing instead of building space shuttles and a space station that have seen costs skyrocket without much to show for it.

Hudgins said privatizing space activities will eventually lead to a brave new world in which space flights are nearly as common and affordable as air flights.

"This is where the action is, and it's only when private entrepreneurs put their money at risk they figure out how to bring down the cost of goods and services," he said.

But Obama's plan also is drawing fierce criticism, especially in Florida where some 20,000 jobs alone would be lost if the space shuttle program shuts down at the end of this year.

Obama plans to visit Florida on April 15 to talk up his space vision.

Taxpayers have already spent $9 billion over five years developing the program. Critics of the presidents' plan claim he has no vision for space travel, no firm goals.

"If we don't have goals, we're just going to be adrift," said Sen. George Lemieux, R-Fla. "And what I'm afraid of is we're going to lose all of these great scientists that work in Florida and other states around the country and we're going to give up our preeminence in space to the Chinese and the Russians. Shame on us if we do that."

But Florida's other senator, while concerned about the president's plans, insists there is a future for NASA.

"I can tell you that the perception is that the president has killed the manned space program," Sen. Bill Nelson, D-Fla., said. "Well in fact, Mr. President, that's the farther thing from President Obama's mind. He is an enthusiastic fan of the space program."

Fox News' Steve Centanni and The Associated Press contributed to this report.