The outermost layer of the earth's atmosphere.

The exosphere goes from about 400 miles (640 km) high to about 800 miles (1,280 km). The lower boundary of the exosphere is called the "critical level of escape", where atmospheric pressure is very low (the gas atoms are very widely spaced) and the temperature is very low.

Considered to be the final layer where the atmosphere fringe begins at 435 miles (700 kilometers) above the earth's sea level.