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."