Exosphere

The exosphere is the outermost, thinnest layer of Earth's atmosphere, beginning around 600 kilometers above the surface and gradually fading into the vacuum of interplanetary space, with no sharply defined upper edge. In this region, the atmosphere is so sparse that individual gas particles, mostly light hydrogen and helium atoms, travel on long, ballistic paths and rarely collide with one another — behaving more like objects in orbit than like a fluid gas. Because collisions are so rare, some particles moving fast enough on an upward trajectory can escape Earth's gravity entirely and drift off into space, a slow but continuous process called atmospheric escape. The exosphere has no true boundary with space; instead, scientists typically define its outer edge as the point where it becomes indistinguishable from the surrounding solar wind and interplanetary medium, sometimes placed as far as 10,000 kilometers from Earth's surface. Many satellites, including geostationary communications satellites, orbit within or above the exosphere. Because it sits at the very edge of Earth's gravitational and magnetic influence, the exosphere is also where Earth's own extended hydrogen envelope, called the geocorona, can be detected glowing faintly in ultraviolet light by spacecraft looking back at Earth from deep space.