We tend to think of Earth’s atmosphere as a vast protective shield, but astronauts looking back from the ISS see something far more fragile: a thin blue-green line, like the skin of an apple wrapped around a basketball, separating every living thing we know from the complete inhospitality of space.

We tend to think of Earth’s atmosphere as a vast protective shield, but astronauts looking back from the ISS see something far more fragile: a thin blue-green line, like the skin of an apple wrapped around a basketball, separating every living thing we know from the complete inhospitality of space.

From the ground, the atmosphere can feel almost limitless. It is the sky above every roof, the weather moving across oceans, the air column that scatters sunlight into blue. Seen from the International Space Station, though, that same system becomes visually small. It appears as a narrow band of colour on the curved edge of Earth, with black space beyond it. That is the tension inside one of the most repeated impressions from orbital flight: Ear…