Outer 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. Although outer space is certainly spacious, it is now known to be far from empty, and filled with a tenuous plasma.

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

• 4.6 km (15,000 ft) - FAA requires supplemental oxygen for aircraft pilots and passengers
• 5.3 km (17,400 ft) - Half of the earth's atmosphere is below this altitude
• 16 km (52,500 ft) - Pressurized cabin or pressure suit required
• 18 km (59,000 ft) - Upper limit of the Troposphere
• 20 km (65,600 ft) - 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 (78,700 ft) - Regular aircraft pressurization systems no longer function
• 32 km (105,000 ft) - Turbojets no longer function
• 45 km (148,000 ft) - Ramjets no longer function
• 50 km (164,000 ft) - Stratosphere ends
• 80 km (262,000 ft) - Mesosphere ends
• 100 km (328,000 ft)- Aerodynamic surfaces no longer function

Reentry from orbit begins at 122 km (400,000 ft).

Types of space

• Interplanetary space
• Interstellar space

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 around the Earth 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 7.9 km/s (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).

See also

Wikiquote has a collection of quotations by or about:

space

• Outer Space Treaty
• Karman line
• Astronaut wings
• Difference between sub-orbital and orbital spaceflights
• space and survival
• space colonization
• space exploration
• space science
• space technology
• astronomy