A common definition of space is known as the Kármán Line, an imaginary boundary 100 kilometers (62 miles) above mean sea level. In theory, once this 100 km line is crossed, the atmosphere becomes too thin to provide enough lift for conventional aircraft to maintain flight. At this altitude, a conventional plane would need to reach orbital velocity or risk falling back to Earth.
The U.S. military and NASA define space differently. According to them, space starts 12 miles below the Kármán Line, at 50 miles above Earth's surface. Pilots, mission specialists and civilians who cross this boundary are officially deemed astronauts.