President Barack Obama had big shoes to fill Thursday when he unveiled NASA's new focus on sending astronauts beyond the moon to the asteroids and Mars. Almost every presidential plan for space exploration pales in comparison to the vision of the man who kicked U.S. space ambitions into high gear.

The U.S. space program got its legs when President John F. Kennedy urged the nation to send a man to the moon, during a speech on May 25, 1961. It's a bit unfair for Kennedy's successors such as Obama, but that has since become the model to follow, said Roger Launius, space history curator at the Smithsonian's National Air and Space Museum.

"He's the fellow who stood in front of Congress and said we'll land a man on the moon and return him safely to Earth within the decade, and he did it," Launius noted. "We tend to judge any succeeding president by that standard, but how often does that happen?"

Kennedy benefited from unique circumstances in which a nation gripped in the Cold War became convinced of the need to win the space race, Launius explained. The competition with the Soviet Union gained a "wartime footing" which has not come up again since.

Without the outside urgency, a president's words and determination might only go so far in realizing a new vision for U.S. space exploration.

"In my mind, presidential leadership is necessary but not sufficient to carry out these activities," Launius told SPACE.com.

The human spaceflight question

Just what NASA should do with human spaceflight may represent the toughest issue for any presidential vision of space exploration.

"The fundamental problem [for the human program] is the rationale," Launius said. He observed that the coolness factor usually loses out to tight budgets or other national concerns.

Waning public and Congressional interest helped kill the Apollo program, which first sent astronauts to the moon. President Richard Nixon also showed limited interest in NASA until the early 1970s, when the space shuttle option came along.

"If Nixon didn't approve the shuttle, he was basically saying astronauts were obsolete and it was time to send them home," Launius said, recalling the arguments provided by Nixon advisor John Ehrlichman.

Nixon's backing of the shuttle program coincided with his reelection bid in 1972. The fact that election battleground states such as Florida, Texas and California are home to so many NASA contractors has likely played a recurring role in presidential plans, Launius said -- even if it has probably not been the deciding factor.

Either way, that presidential decision represented a game changer for NASA. It also committed the U.S. space agency to a reusable vehicle which "kept us in low Earth orbit for 40 years," according to Launius.

Headed for Mars someday

NASA's commitment to the shuttle did not prevent presidents from laying out grand plans for space exploration. In 1989, President George H.W. Bush urged a "long-range, continuing commitment" during the 21st century to build a permanent moon base, and then to go on to Mars. NASA's cost estimate for the plan came to a staggering $500 billion spread over several decades. The plan failed to win over either Congress or international partners.

More limited goals for NASA led to the "faster, better, cheaper" strategy which carried over into the next presidential administration.  President Bill Clinton's plan then "officially removed human exploration from the national agenda," according to Steve Dick, NASA's former chief historian.

One of the most dramatic shifts for NASA's focus may not have emerged from any presidential plan at all. Scientists announced the discovery of possible fossils of microbial Martian life in the Allan Hills 84001 meteorite in 1996, which President Clinton then announced with fanfare.

Despite controversy over the meteorite findings, the event served as the "kickoff for a succession of Mars missions," Launius pointed out. Some of those robotic explorers include the enduring Spirit and Opportunity rovers, as well as the Phoenix Mars Lander which confirmed water ice in the Martian arctic.

President George W. Bush followed up on his father's space plans by announcing yet another broad plan to return humans to the moon, and then head for Mars. Some scientists criticized the plan as an underfunded effort, while Mars advocates saw the return to the moon as a dead-end detour.

Time for a new hope

NASA's Constellation Program to return humans to the moon grew out of the second President Bush's plan, until President Obama replaced it this year with his own vision. Launius sees the new plan as a possible game changer -- but only if the president can pull it off.

"This is what they did with Apollo, too: the leapfrog approach," Launius said. Just as Kennedy's vision pushed NASA to move quickly beyond low Earth orbit to the moon, so Obama's vision might skip the lunar return trip and head directly for the asteroids and Mars.

Obama's plan also represents the first serious move to put low Earth orbit activities in the hands of commercial spaceflight providers, and ideally frees NASA to look beyond and resume space exploration.

Launius compared the new NASA-commercial partnership to the vision of the film "2001: A Space Odyssey," where private companies such as the now-defunct Pan Am airlines and IBM had taken over low Earth orbit operations. But history also still weighs heavily on the outcome.

"The skeptic in me says the landscape is littered with failed plans," Launius admitted. Not to mention a certain shadow looming above all presidential successors.