Over the last half century, spacecraft have visited every planet and their major moons, as well as two dwarf planets and more than a dozen asteroids and comets. Thanks to high-res images, we know these worlds intimately and can appreciate what makes each of them unique. These days, fewer than 3 in 10 Americans are old enough to recall a time when our neighboring worlds were indistinct dots in even the most powerful telescopes.

And yet, even before there were spacecraft to show us, in the