The exosphere is the outermost layer of Earth's atmosphere, where air becomes so thin that it gradually fades into the vacuum of space. It starts about 400 miles above Earth's surface and extends thousands of miles outward. At this altitude, air molecules are so spread out that they rarely bump into each other anymore. Some molecules escape Earth's gravity entirely and drift off into space, while others fall back down.

In the exosphere, there's no weather, no clouds, and no sound, since sound needs air molecules close together to travel. Some satellites orbit in the exosphere because there's so little air resistance to slow them down.

Think of it as Earth's atmospheric exit zone, where our planet's blanket of air finally peters out and space begins. When astronauts talk about “leaving Earth's atmosphere,” they are passing through or beyond the exosphere, where the atmosphere becomes almost indistinguishable from the emptiness of space.