Where are the most important places to explore in the solar system?
The leading answers to that question have shifted over the years, driven by scientific discoveries, public curiosity, technological realities, and political agendas. For human exploration, the list of plausible destinations has always been short: Earth-orbit and the Moon. (Mars is achievable, no doubt, but we are not at all technologically ready to go there right now.) For robotic probes, the list started in the same place but kept going: inward to Mercury and the Sun itself, outward past Neptune and Pluto.
For the most part, though, we've ignored the other 99.9999 percent of the objects in the inner solar system: the asteroids. There are, by current estimates, nearly two million asteroids more than a kilometer in diameter. Collectively, they represent a landscape greater than the surface of the Moon, but we'd never seen one up close until 1991. ...
