If you were to categorize the ISS, you’d probably put it in the same semantic bucket as a house. It’s a place where people live for extended periods of time, and though it’s moving at 4.76 miles every second, it’s still quite habitable. And yet, it would be surprising if the ISS were home to exactly the same bacteria as a terrestrial house. A home gets its microbes in part from the other organisms that live there, but also from the outside world. Incoming air through a window brings some bacteria, as does the dirt tracked in from the yard and the groceries brought home each week. Exposure to our atmosphere, with all its contaminants, affects the kinds of bacteria that colonize a home.