The Horizon was a gleaming silver dart set adrift in the vast blue-black ocean of outer space, its hull pocked with scars of a thousand micro-meteoroids, its name stenciled beside a stylized image of a sunrise.

Within, Captain Maeve Carter, engineer Lucas Kim, and pilot Alyssa Torres made a family of three—the only human warmth for millions of kilometers in any direction. They were not explorers, not in the classical, starry-eyed sense. NASA had rebranded them "planetary guardians," their mission simple and dire: patrol the solar system and neutralize any incoming space debris with the potential to wipe out a continent or, on a bad day, the entire human race.