- For other meanings of the term space, see space.
Outer space (also called just space), as a name for a region, refers to the relatively empty parts of the Universe, outside the atmospheres of celestial bodies. The term outer space is used to distinguish it from airspace and terrestrial locations.
As Earth's atmosphere has no abrupt cut-off, but rather thins gradually with increasing altitude, there is no definite boundary between the atmosphere and space. The altitude of 100 kilometers or 62 miles established by the Federation Aeronautique Internationale is the most widely used definition as the boundary between atmosphere and space. In the United States, persons who travel above an altitude of 50 miles (80 kilometers) are designated as astronauts. 400,000 feet (75 miles or 120 kilometers) marks the boundary where atmospheric effects become noticeable during re-entry. (See also Karman line.)
Milestones on the way to space
- 10,000ft (3 km) - FAA requires supplemental oxygen for aircraft pilots and passengers
- 5.3 km (17,400ft) - Half of the earth's atmosphere is below this altitude
- 16 km (52,500ft) - Pressurized cabin or pressure suit required
- 18 km (59,000ft) - Upper limit of the Troposphere
- 20 km (65,600ft) - Water at room temperature boils without a pressurized container (the popular notion that bodily fluids would start to boil at this point is false because the confines of the body generate enough pressure to prevent actual boiling)
- 24 km (78,700ft) - Regular aircraft pressurization systems no longer function
- 32 km (105,000ft) - Turbojets no longer function
- 45 km (148,000ft) - Ramjets no longer function
- 50 km (164,000ft) - Stratosphere ends
- 80 km (262,000ft) - Mesosphere ends
- 100 km (328,000ft)- Aerodynamic surfaces no longer function
Reentry from orbit begins at 122 km (400,000ft).
Space does not equal orbit
A common misunderstanding about the boundary to space is that orbit occurs by reaching this altitude. Orbit, however, requires orbital speed and can theoretically occur at any altitude. Atmospheric drag precludes an orbit that is too low.
Minimal altitudes for a stable orbit begin at around 350 km (220 miles) above mean sea level, so to actually perform an orbital spaceflight, a spacecraft would need to go higher and (more importantly) faster than what would be required for a sub-orbital spaceflight.
Reaching orbit requires tremendous speed. A craft has not reached orbit until it is circling Earth so quickly that the upward centrifugal "force" cancels the downward gravitational force on the craft. Having climbed up out of the atmosphere, a craft entering orbit must then turn sideways and continue firing its rockets to reach the necessary speed; for low Earth orbit, the speed is about 7.9km per second (18,000 mph). Thus, achieving the necessary altitude is only the first step in reaching orbit.
The energy required to reach velocity for low earth orbit (32 MJ/kg) is about twenty times the energy to reach the corresponding altitude (10 kJ/km/kg).