The air that surrounds the Earth is called the atmosphere. It is held in place by the planetÕs gravity and consists of mostly nitrogen (78%) and oxygen (21%). There are several different layers of atmosphere. The lowest level is the troposphere [0 to 16 km (0 to 10 mi.) above ground level]. This level is where most of the worldÕs weather takes place. Above the troposphere is the stratosphere [16 km to 50 km (10 mi. to 31 mi.) above ground level]. The air is much thinner here, and temperatures hover around 0¡ C (32¡ F). The next level is the mesosphere [50 to 80 km (31 mi. to 50 mi.) above ground level]. The air starts to get very cold here, rarely rising above -100¡ C (-148¡ F). The uppermost level of the atmosphere is the thermosphere, which extends from 80 km (50 mi.) above the ground all the way to outer space. The air in the thermosphere is actually very hot, rising as high as 1000¡ C (1832¡F)! However, the air is so thin here and is spread out over such a wide area that we would not be able to feel this heat.
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When ET separation occurs, the Shuttle is in the thermosphere, about 110 km (68 mi.) above the surface of the planet.
