Outer space

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Layers of Atmosphere (NOAA)
Layers of Atmosphere (NOAA)

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 the 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 Federation Aeronautique Internationale has established the Kármán line at an altitude of 100 km (62 miles) as the working definition for 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.


Milestones on the way to space

  • Sea level - 1 bar of atmospheric pressure
  • 4.6 km (15,000 ft) - FAA requires supplemental oxygen for aircraft pilots and passengers
  • 5.0 km (16,000 ft) - 0.5 bar of atmospheric pressure
  • 5.3 km (17,400 ft) - Half of the Earth's atmosphere is below this altitude
  • 8.8 km (29,035 ft) - Summit of Mount Everest, the highest mountain on Earth
  • 16 km (52,500 ft) - Pressurized cabin or pressure suit required
  • 18 km (59,000 ft) - Boundary between troposphere and stratosphere
  • 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
  • 24.7 km - Altitude record for manned balloon flight
  • 32 km (105,000 ft) - Turbojets no longer function
  • 45 km (148,000 ft) - Ramjets no longer function
  • 50 km (164,000 ft) - Boundary between stratosphere and mesosphere
  • 80 km (262,000 ft) - Boundary between mesosphere and thermosphere
  • 100 km (328,084 ft) - Kármán line, defining the limit of outer space according to the Fédération Aéronautique Internationale. Aerodynamic surfaces no longer function due to lack of atmospheric pressure
  • 120 km (400,000 ft) - First noticeable atmospheric effects during reentry from orbit
  • 200 km - Lowest possible orbit with short-term stability (stable for a few days)
  • 350 km - Lowest possible orbit with long-term stability (stable for many years)
  • 690 km - Boundary between thermosphere and exosphere

Types of space

Space does not equal orbit

A common misunderstanding about the boundary to space is that orbit occurs by reaching this altitude. There is a major difference between sub-orbital and orbital spaceflights, however. Achieving orbit requires orbital speed, and this 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).

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