HOUSTON (AP) — 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.