Bush called late Wednesday for a new space vessel capable of traveling to the moon as early as 2015. But in terms of financing, the space agency will have only an additional billion dollars at its disposal over five years, in addition to its annual budget of 15.4 billion dollars.
Traveling to Mars "is expensive and risky. The United States may not spend the money this will take, or people may lose interest" in the program, warned James Lewis, director of the Technology and Public Policy program at the Washington-based Center for Strategic and International Studies (CSIS).
Peter Wilson, a space exploration policy expert at the Rand Corporation, conceded that it "will require a huge effort by the administration to mobilize the public opinion over this. It's a 20-to-30-year program, not a crash program like Apollo, that took only 10 years."
As if attempting to demonstrate the feasibility of Bush's dream, the US robot Spirit made its first steps on Mars overnight Wednesday. But the two-robot rover mission, expected to continue for three months on the Red Planet, cost just 820 million dollars.
But the notion of manned missions to the moon and Mars has also raised a host of other questions -- including what will happen in the interim between when the space shuttle is phased out, as Bush has called for, and when a new orbital space plane takes its place.
"The program is a little confusing, because the new vehicle is not ready before 2014 but the shuttle stops in 2010," said Wilson.
Asked about this four-year gap, NASA Administrator Sean O'Keefe said that, although the shuttle has been grounded following last year's space shuttle Columbia disaster, the International Space Station crews are being transported to the station by Russian Soyuz rockets.
"Our Russian partners have excelled at assuring the flight of Soyuz spacecraft," O'Keefe said.
He gave no details on the new space vehicle, answering questions on the topic with a simple "We'll see." The orbital space plane now being developed is not designed to go to the moon.
Several experts stress that keeping the shuttle in service until 2010 will be costly, with maintenance eating up a portion of the budget that could be devoted to the new spacecraft.
The shuttle programs "outlived their usefulness and were just a black hole for money," Lewis said.
Aside from the funding concerns, the United States could also come up against some tough competition from other countries in reaching for the moon and Mars, experts say.
"Being passed by one of these countries would damage US prestige. This might be enough incentive to actually get us back to the moon and then to Mars," Lewis said.
Several times in recent months, China has mentioned plans to send people to the moon, and on Thursday Russia reacted to the US announcement a day earlier by saying it had the know-how to relaunch its space exploration programs.
But convincing Congress to approve the budget will be the real challenge. Bush's father, former president George Bush, had his space hopes dashed in 1989, when Congress rejected his 20-year, 400-billion-dollar plan for manned missions to the moon and Mars.
"New financial resources will be hard to secure from Congress," said Christian Beckner, project manager of CSIS's Human Space Exploration Initiative.
He added that "any reorganization of the NASA centers will be difficult and will require a strong political commitment by the White House."
On Thursday NASA announced the organization's first changes aimed at meeting the new goals.
Retired admiral Craig Steidle was nominated to the job of associate administrator, and will head a new office, the Office of Exploration Systems.
The office is to "set priorities and direct the identification, development, and validation of exploration systems and related technologies," NASA said in a statement.
The Office of Aerospace Technology will now be called the Office of Aeronautics, reorganized "to reflect NASA's commitment to aviation research and aeronautics technologies for the nation's civil and defense interests," NASA said.