The exosphere is the outermost layer of Earth's atmosphere. It begins where the thermosphere ends, at an altitude of about 600 km (370 mi) and extends outward to the edge of space. The exosphere is where the atmospheric density is so low that the atmospheric gases are no longer considered to be part of the atmosphere. The exosphere is also where the Earth's magnetic field ends and the solar wind begins.
The exosphere does not have a distinct boundary, and its upper limit can vary depending on solar activity. During periods of high solar activity, the exosphere may expand to about 1000 km, while during periods of low solar activity, it may contract to about 500 km.
Satellites, space stations and some high altitude balloons operate in the exosphere, where they are not affected by the drag of the lower atmosphere. The International Space Station (ISS) orbits Earth at an altitude of about 400 km, which is within the exosphere. The exosphere also includes the area where space debris and defunct satellites orbits, as well as the area where intercontinental ballistic missiles (ICBMs) travel through.