From Atmosphere to Space

The atmosphere does not have a defined top, like the surface of land or ocean. Air becomes thinner with altitude until it is too thin to measure. Scientists chose an altitude within the atmosphere where they consider space begins. The most widely used definition for the beginning of space is 62 miles (100 kilometers) above sea level, in the thermosphere. In the United States, however, a person is defined as an astronaut when traveling above 50 miles (80.45 kilometers).

Space scientists and engineers defined the beginning of space at a dif­ferent altitude in the thermosphere. They chose an altitude of 400,000 feet (121,920 meters), or about 76 miles (122 kilometers), and they call it the entry interface. This is the altitude where the air is thick enough to begin heating up a spacecraft as it returns from space.

The air doesn’t stop suddenly at this altitude. There is still some air higher up, where spacecraft orbit Earth. In fact, there is enough air at those levels to slow down a spacecraft.