WASHINGTON — For fans of space exploration, the 40th anniversary of Apollo 11's mission to the moon is a celebration mingled with melancholy.

For all the promised "giant leap for mankind" the mission foretold, the prophesied future of moon bases and journeys to Mars, Jupiter and beyond is still science fiction.

The last of six moon landings, bringing two men each time to the lunar surface, was in 1972. Since then, no one has left low Earth orbit, much less ventured to Mars. For many advocates, there is a consensus the National Aeronautics and Space Administration is suffering from what President Obama in March called "a sense of drift."

The astronauts who made the first moon landing are still alive, and so are many of the 600 million people around the world who watched the ghostly images of Neil Armstrong and Edwin "Buzz" Aldrin Jr.'s first bounding steps on the dusty lunar surface on July 20, 1969. But the generations born since then have other interests: A YouTube clip of the first moon walk has 2 million views; Michael Jackson moonwalking to "Billie Jean" has 20 million.

John Olson, who oversees NASA's future plans as its director of the Exploration Systems integration office, is quick to point out the agency's recent successes:

• The Hubble telescope has provided breathtaking images of the universe.

• Two remotely controlled rovers on Mars have discovered strong evidence of water there.

• At this moment, 13 astronauts — the biggest-ever gathering in space — are in orbit installing new components of the international space station.

Yet the Apollo program easily looms over it all; it is hard to love a robotic rover. Even NASA's post-Apollo triumphs have been overshadowed by two space shuttle disasters: Challenger's destruction on launch in 1986 and Columbia's disintegration on re-entry into Earth's atmosphere in 2003.

"The U.S. civilian space effort has moved forward for 30 years without a guiding vision," an independent board investigating the Columbia accident wrote in 2004.

FUTURE PLANS

The United States now has a plan for space exploration that includes a return to the moon by 2020, and last week the Senate confirmed NASA's new administrator, Charles F. Bolden Jr., a former astronaut who has vowed to restore the agency's sense of mission.

But some prominent supporters remain dissatisfied.

Aldrin himself critiqued the nation's space exploration strategy in an op-ed piece in The Post, saying that, like Apollo, "this plan will prove to be a dead end littered with broken spacecraft, broken dreams and broken policies."

He said that to rekindle pride in its program, the United States needed to embrace a vision that, for the moment, sounds impossible: a homestead on Mars.

"With all due deference — my gosh, he is the second man to walk on the surface of the moon — that doesn't mean that going to the moon isn't a worthy goal," Olson responded in an interview.

Getting back to the moon will be difficult. Human space exploration has always moved in tandem with political realities.

In 2004, President George W. Bush proposed a voyage to Mars, but critics saw it as a ploy to distract the public from the war in Iraq, and it was hardly referred to thereafter.

Even President John F. Kennedy, who issued the challenge to land on the moon by the end of the 1960s, framed his goal in the context of a Cold War rivalry with the Soviet Union that intensified when the Communist regime hurled the Sputnik satellite into orbit in 1957.

"I am not that interested in space," Kennedy told James Webb, NASA's administrator, in late 1962. "I think it's good. I think we ought to know about it. But we're talking about fantastic expenditures."

WORTH THE EXPENSE?

Most Americans would seem to agree. Even during NASA's heyday in the 1960s, when the agency's expenditures stood at 4.5 percent of the federal budget and it employed nearly 400,000 civil servants and contractors, polls showed most Americans didn't think Apollo was worth the expense, according to Roger Launius, the chairman of the space history division at the National Air and Space Museum.

(The only exception was after the landing in 1969, when a poll showed a bare majority of 53 percent of Americans supporting the spending.)

"The bottom line was that the political will was not based on a desire to explore space per se," Launius said. "Nobody objected to that, but that was not the reason."

Absent the fury of the space race, now NASA employs about half as many people as it did at its peak and its budget is less than 1 percent of the federal budget.

To return to the moon would take renewed political support and a clear vision of how to get there. The latter question is one subject taken up by the Human Space Flight Plans Committee, which is expected to offer recommendations for a space strategy in August. But when asked what it would take to rally support behind an aggressive push into space today, historians and experts were mostly stumped.

"As Americans we tend to respond best when we think there's a crisis," Launius said. "But, you know, it has to be a clear threat."