HOUSTON- With the space shuttle now history, NASA's next great mission is so audacious, the agency's best minds are wrestling with how to pull it off: Send astronauts to an asteroid in less than 15 years.

The challenges are innumerable. Some old-timers are grousing about it, saying going back to the moon makes more sense. But many NASA brains are thrilled to have such an improbable assignment.

And NASA leaders say civilization may depend on it.

An asteroid is a giant space rock that orbits the sun, like Earth. And someday one might threaten the planet.

But sending people to one won't be easy. You can't land on an asteroid because you'd bounce off - it has virtually no gravity. Reaching it might require a NASA spacecraft to harpoon it. Heck, astronauts couldn't even walk on it because they'd float away.

NASA is thinking about jetpacks, tethers, bungees, nets and spiderwebs to allow explorers to float just above the surface of it while attached to a smaller mini-spaceship.

Such a ship - something like a "Star Trek" shuttlecraft melded with a deep sea explorer with pincer-like arms- is needed just to get within working distance of the rock. That craft would have to be big enough for astronauts to live in for a week or two. They'd still need a larger habitat for the long term.

It would take half a year to reach an asteroid, based on current possible targets. The deep space propulsion system to fly such a distance isn't perfected yet. Football-field-sized solar panels would help, meaning the entire mothership complex would be fairly large. It would have to protect the space travelers from killer solar and cosmic ray bursts. And, they would need a crew capsule, maybe two, for traveling between the asteroid complex and Earth.

And all those parts - mini-spaceship, habitat/living area, crew capsule, solar arrays and propulsion system - would have to be linked together in the middle of space, assembled in a way like the International Space Station but on a smaller scale.

Beyond all those obstacles, NASA doesn't even know which asteroid would be the best place to visit.

All this has to be ready to launch by 2025 by presidential order.

"This is the big step," said Kent Joosten, chief architect of the human exploration team at Johnson Space Center. "This is out into the universe, away from Earth's gravity completely... This is really where you are doing the 'Star Trek' kind of thing."

It has the dreamers of NASA both excited and anxious.

"This is a risky mission. It's a challenging mission," said NASA chief technology officer Bobby Braun. "It's the kind of mission that engineers will eat up."

This is a matter of sending "humans farther than ever before," said NASA Deputy Administrator Lori Garver. It is all a stepping stone to the dream of flying astronauts to Mars in the mid 2030s.

"I think it is THE mission NASA should embrace," said University of Tennessee aerospace professor John Muratore. "To be successful at this mission, you've got to embrace all of the technologies that you need for Mars."

Critics, including former Apollo astronauts and flight directors, have blasted President Barack Obama for canceling George W. Bush's plan to return astronauts to the moon. They dismiss talk of asteroid visits.

But that's because NASA has not done a good job of outlining the fascinating details and explaining why it is important, said astronomer and former astronaut John Grunsfeld.

"NASA doesn't have a story right now," said Grunsfeld, deputy director at the Space Telescope Science Institute. "Exploration is nothing if not the articulation of a great story."

The story begins with why NASA would want to go to an asteroid. The agency has sent small spacecraft off to study asteroids over the years and even landed on one in 2001. Just last week, a space probe began orbiting a huge asteroid called Vesta, which lies beyond Mars.

Scientifically, an asteroid is a remnant from the birth of the solar system, offering clues about how our planetary system began. Logistically, NASA wants to go to Mars, but that is distant and more difficult. So the argument is that going to an asteroid is a better testing ground than returning to the moon.

The reason NASA Administrator Charles Bolden and others give is that this mission could save civilization. Every 100 million years or so an asteroid 6 miles wide - the type that killed off the dinosaurs- smacks Earth, said NASA Near Earth Object program manager Donald Yeomans.

If NASA can get astronauts to an asteroid, they can figure out a way of changing a potential killer's orbit. They'll experiment with the safe one they land on, Braun said.

One joke going around is that dinosaurs couldn't stop catastrophe because they didn't have a space program.

"One of the statements going to an asteroid will make is that humans are smarter than dinosaurs," Grunsfeld said.

If you are going to reroute a killer asteroid, first you have to know one is coming and where it is now. And that's also a problem for NASA's mission. Astronomers figure there are about 50,000 asteroids and comets larger than 300 feet in diameter and they only know where fewer than 1 percent of them are, Yeomans said. NASA is focusing on rocks that size or larger that would come relatively close to Earth in the 2025 time frame.