Forty years ago Monday, Neil Armstrong made his "giant leap for mankind." Since that triumphant moment, astronauts in the U.S. space program have gone no farther.
The first footsteps on the moon — made by Armstrong on July 20, 1969, on the mission known as Apollo 11— came 3½ years before the last ones. Since then, astronauts have been stuck close to the Earth, mostly circling a few hundred miles overhead in a spacecraft that's little more than a glorified cargo truck.
So now what?
That question preoccupies NASA and worries the Obama administration. The president said in March that NASA is beset by "a sense of drift." Even some of the men who once walked on the moon are divided on how to proceed. Options could include going back to the moon, landing on an asteroid, shooting for Mars or even ending human exploration of space altogether.
Former president George W. Bush tried to give NASA a sense of purpose, ordering the agency in 2004 to retire the space shuttle and return humans to the moon. The public yawned. Bush never publicly mentioned the plan again and didn't add much to NASA's budget for it.
NASA is still trying to carry out Bush's goals, but the effort is in doubt. At the White House's request, a panel of independent space experts is giving NASA's human spaceflight program a top-to-bottom review. The panel, which will make recommendations at the end of the summer, could tell Obama that NASA is on track. Or, it could send the agency back to the drawing board.
No matter what the panel decides, the federal deficit and competition from programs such as health care mean that NASA is unlikely to get enough money to do anything truly ambitious. Already Obama's proposed budget for 2010 shows that the administration plans to slash funding later this decade for the rocket and spacecraft needed to take astronauts back to the moon.
If that stands, it's a "an absolutely going-out-of-business budget," says former NASA official Scott Pace, now at George Washington University.
Many space historians and even NASA veterans agree that the glory days of Apollo — which spawned countless songs, movies and books — can't be recaptured. Gone is the vast budget for building spaceships. Gone is the Cold War with the Soviet Union, which unified the nation and lent urgency to the effort to put an American on the moon.
"The Apollo program was such a success because it did have complete support," Aaron Cohen, a top Apollo official, said last month at an MIT symposium on the 40th anniversary of man's first step on the moon. "This may be very difficult to achieve in the near future."
America "is a different place" now than during Apollo, says Rep. Bart Gordon, D-Tenn., head of the House Science and Technology Committee. "We were in a space race with the Soviet Union. (Apollo) was about geopolitics, not space exploration."
All the same, polls regularly show that Americans have a warm feeling for the human spaceflight program and don't want it to end. That means figuring out what astronauts should do next. Should they forge outward into the solar system, despite the huge cost and a soaring deficit? And if so, where?
The decision is not just technical, says David Mindell, who directs MIT's Department of Science, Technology and Society.
"It's emotional and it's political, because human spaceflight is primarily a symbolic activity," he says. "If you really are looking strictly (at the) technical, you wouldn't be sending people."
Some possible destinations for human space explorers include:
Yes, America has been there. That doesn't mean it's not worth going back, say scientists and an astronaut who's been to the lunar surface. Humans went to the moon six times from 1969 to 1972, spending fewer than 13 days there. Lunar advocates say that's hardly time enough to plumb the moon's mysteries.
Sending humans back to the moon could help unlock the secrets of the early solar system, says Jack Burns, a University of Colorado astronomer. The forces that shaped the Earth have not scarred the lunar surface, making the moon a pristine record of how planets formed, he says.
Burns scoffs at the idea that because Americans have landed on the moon, there's no reason to go back. "It's like Thomas Jefferson sending Lewis and Clark to the West, and … people saying, 'We're done, we don't need to go there anymore,' " he says.
NASA's plans for the moon include not just short, Apollo-style stopovers but eventually a moon base. The agency hopes to send the astronauts back to the moon around 2020.
Operating a moon base would allow astronauts to practice living on another planet, NASA's Jeff Hanley says. Crews would need that experience before pressing on to Mars, the long-term goal of most space enthusiasts.
"The fastest way to get to Mars is through the moon," says Harrison Schmitt, who in 1972 was one of the last two men on the moon. "We need to learn how to work in deep space again. That's what the moon does for us."
It may sound crazy, but preliminary NASA studies indicate it's possible to send humans to visit asteroids, huge chunks of rock and gravel that orbit the sun.
Telescopes have spotted at least nine asteroids that astronauts could reach using the spaceship and giant rocket NASA is designing to return humans to the moon, says the space agency's Rob Landis, who headed a study of such missions. Total travel time would be 90 to 180 days, he says. That's much longer than the six-day round trip to the moon but much shorter than the one-year round trip to Mars.
Asteroids, unlike the moon, have negligible gravity, so a spaceship could fly to an asteroid and just pull up next to it. Then an astronaut could clamber out and explore. Going to the moon requires not just a spaceship but an expensive lander, one equipped with rockets so it could blast off from the lunar surface.
There's a big incentive to learn more about asteroids: They could wipe out humanity. A wallop from even a medium-size asteroid could unleash as much energy as a large nuclear bomb, NASA says. Many scientists blame a collision with a huge asteroid for the extinction of the dinosaurs 65 million years ago.
Asteroids also are of interest because they're loaded with minerals that could be useful for space crews headed into the solar system, says Russell "Rusty" Schweickart, who flew on the 1969 Apollo 9 mission that tested the lunar module.
"Asteroids are a combination of long-term resource, potential threat and great scientific interest," he says. "In my mind, (that) sells a heck of a lot better to the general public than going back to the moon."
Scientists have debated the existence of life on Mars for more than a century. Mars boosters say it's time to settle the arguments by sending humans to the Red Planet. Humans would learn "whether we're … in a living universe, where life is common, or a dead universe," says Robert Zubrin, president of the Mars Society, a group dedicated to Mars exploration.
In all the vast universe, the Earth is the only place known to support life. Mars may be the next best place to nurture living things. It's not only the most Earth-like planet but also has stores of water, as confirmed by a NASA robot last year.
Two of Apollo 11's three crewmembers are Mars partisans. "As celestial bodies go, the moon is not a particularly interesting place, but Mars is," Apollo 11 astronaut Michael Collins said in a statement from NASA.
A return to the moon is "not very attractive," says Buzz Aldrin, who was the second man on the moon. "After 50 years, do we want to be known for returning to the moon?"
He favors human colonization of Mars. He envisions crews testing their skills and spaceships on Phobos, a Mars moon, before pushing on to the Red Planet.
A Mars trip may seem far-fetched, but such a mission was proposed by the first President Bush in 1989. The idea went nowhere then, but the younger President Bush's 2004 plan for NASA also included the goal of sending humans to Mars.
NASA took that directive so seriously that its engineers started work on a giant rocket so powerful it could launch spacecraft not just to the moon but also to Mars. Work on the rocket is on hold while the Obama administration sorts out its plans for space.
Even some strong supporters of space exploration say the best place to send America's astronauts would be nowhere at all.
Opponents of human spaceflight say robots can do the job just as well as astronauts, pose no safety worries and work cheaply. Sending humans into space isn't worth it, they say.
"The cost and risks are just too high," says physicist Robert Park of the University of Maryland, who wants NASA's manned program to be phased out.
Human space exploration also has run into trouble in Congress. In its spending bill for 2008, lawmakers ordered NASA not to spend any money to study sending humans to Mars.
"Manned space travel adds far more cost than is justified in terms of scientific return," says Rep. Barney Frank, D-Mass. Frank says he doesn't want to end the astronaut program but doesn't want to send humans to Mars or the moon. He'd restrict astronauts to tasks robots can't handle, such as the recent upgrade of the Hubble Space Telescope by a seven-astronaut team.
Opposition to NASA's astronaut program stretches across the political spectrum. Republican Newt Gingrich, former speaker of the House, wrote in Aviation Week & Space Technology last year that NASA should get out of the business of sending humans to space to make way for private space entrepreneurs.
For NASA, the most opposition may be from the people who pay the bills: the public. In a 2005 USA TODAY poll, 58% opposed spending money on a human mission to Mars.
Americans may support human spaceflight, but they don't make it a high priority, says historian Roger Launius of the Smithsonian Institution's National Air and Space Museum. Nor do political leaders, he says. "That leaves us in low-Earth orbit for the foreseeable future," Launius says "I hope it doesn't come to that, but I'm afraid it might."