What 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 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% 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. Even now, we've visited just a dozen of them. The asteroids didn't get much love.
That's about to change: There currently eight dedicated asteroid missions underway or in development (nine if you include MMX, a Japanese mission to Mars's inner moon, Phobos, which might be a captured asteroid). Hayabusa2 is about to swing past Earth next week, dropping of samples of asteroid Ryugu over Australia. OSIRIS-REx will follow behind with a larger cache of rocks that it recently collected from another small asteroid, Bennu.
The next round of asteroid missions will try out a bunch of unusual styles of exploration. Lucy will visit the Trojan asteroids that move in the same orbit as Jupiter. The Psyche mission will travel to the asteroid Psyche—a mysterious object that appears to be composed almost entirely of metal. DESTINY+ will head to Phaethon, a "rock-comet" asteroid that appears to be crumbling because it passes so close to the Sun. NEA Scout will use a solar sail to navigate to a near-Earth asteroid.
Most dramatic of all, the
DART spacecraft will ram full-speed into a small asteroid Dimorphos in 2022. The goal is to test out a technique for deflecting a dangerous asteroid if we discover one coming our way; four years later, the
Hera probe will follow up to assess the damage.