Thermosphere: The thermosphere of a terrestrial planet is the fourth and final member of the atmosphere. On Earth, the thermosphere starts atop the mesosphere at approximately 85 kilometers above mean sea level. The upper limit of the thermosphere is reasonably presumed to terminate approximately 95 kilometers above mean sea level. In fact, most of Earth’s satellites orbit the planet within the thermosphere. This part of the atmosphere comprises high ultraviolet rays and X-rays from the sun. This radiation produces high heat conditions, up to 2,000 degrees celsius. Although the thinness of the air brings the temperatures back down to items or beings within the thermosphere. The thermosphere is currently considered the final member of the atmosphere. However, geologists and physicists have modernly defined other layers of the Earth’s gaseous envelope, beyond the thermosphere. These are the exosphere and ionosphere.