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White House Press Secretary Robert Gibbs defended President Obama on Wednesday against charges from Neil Armstrong and other famed astronauts that the president is dismantling the American space program, saying the president's "renewed strategy" will be better for the economy, astronauts and the space program as a whole.

Armstrong, the first man to walk on the moon, penned an open letter with other outer space legends blasting off at the Obama administration for plans to shelve NASA's manned moon missions.

"The ... decision to cancel the Constellation program, its Ares 1 and Ares V rockets, and the Orion spacecraft, is devastating," they wrote.

But Gibbs, citing an independent commission that found the program was over-budget and behind schedule, said Wednesday that the plan Obama is set to unveil Thursday in Florida should ease concerns that the president is somehow relegating NASA to the back of the budget.

"The president will outline a renewed strategy tomorrow in Florida that will provide more jobs for the area, greater investment in innovation, more astronaut time in space, more rockets launching sooner, and a more ambitious and sustainable space program for America's future," Gibbs said.

The White House also pitted astronaut Buzz Aldrin against Armstrong, re-releasing a statement Aldrin put out Feb. 1 defending Obama's plans.

"The simple truth is that we have already been to the Moon - some 40 years ago," Aldrin said in the statement, claiming Obama's plan would help NASA "push our boundaries to achieve new and challenging things beyond Earth."

At the Kennedy Space Center in Florida Thursday, Obama plans to announce a set of stepping-stone achievements aimed at taking the United States farther into space, along a range of destinations including lunar orbit, so-called "Lagrange points" (prime destinations for permanent, orbiting fuel depots), near-Earth asteroids, the moons of Mars, and eventually Mars itself.

While Armstrong and others derided the elimination of Constellation, the planned replacement for the aging space shuttle, Obama will announce plans to salvage a portion of it: the Orion space capsule, which was intended to house astronauts during their travel to the International Space Station and on later missions to the Moon. It also was to be capable of docking at the Space Station for six months and returning crews to Earth.

"We wanted to take the best of what was available from Constellation," a NASA official told The Associated Press as part of a White House briefing.

Orion will serve temporarily to provide standby emergency escape capabilities for astronauts on the Space Station, addressing fears from some experts that U.S. astronauts on the space station would be held "hostage" to Russian interests.

Obama hopes NASA will be able to launch the Orion vehicle within the next few years, creating an escape capability that will increase the safety of Americans on the Space Station, reduce U.S. dependence on foreign providers and simplify requirements for other commercial crew providers.

The president on Thursday will announce his commitment to choosing a single heavy-lift rocket design by 2015 and then starting its construction, officials said. This shift means NASA would launch a heavy rocket years before it was supposed to under the old Constellation plan, a NASA official said.

But the new rocket will be different from the Apollo-like Ares V rocket that the Constellation plan would have used. Instead, it will incorporate newer concepts, such as refueling in orbit or using inflatable habitats, officials said.

Overall, the Obama program will mean 2,500 more Florida jobs than the old Bush program, a senior White House official said. In addition, the commercial space industry on Tuesday released a study that said the president's plan for private ships to fly astronauts to and from the space station would result in 11,800 jobs.

But Armstrong and other astronauts continued to express concerns that scrapping the moon program would had deep consequences in terms of America's standing in the world.

Apollo 13 astronaut Jim Lovell, who signed the letter with Armstrong, told Fox News he believes Obama's plan is "short-sighted."

"We're going to be a third-rate nation. China and Russia are going to be premier," he said.

Fox News' Jeremy Kaplan contributed to this report.