Outer space

Layers of Atmosphere - not to scale (NOAA)
Layers of Atmosphere - not to scale (NOAA)

Outer space, also called just space, refers to the relatively empty regions of the Universe outside the atmospheres of celestial bodies. Outer space is used to distinguish it from airspace (and terrestrial locations). Although outer space is certainly spacious, it is far from empty.

There is no discrete boundary between the Earth's atmosphere and space as the atmosphere gradually attenuates with increasing altitude. If the atmosphere had a constant temperature, its pressure would decrease exponentially from a sea-level value of 100 kPa ( 1 bar) toward its final value of zero. The Federation Aeronautique Internationale has established the Kármán line at an altitude of 100 km (62 miles) as a working definition for the boundary between atmosphere and space. The United States designates people who travel above an altitude of 50 miles (80 km) as astronauts. During re-entry, 400,000 feet (75 miles or 120 km) marks the boundary where atmospheric drag becomes noticeable.

Outer space within our solar system is called interplanetary space, which passes over into interstellar space at the heliopause. The vacuum of outer space is not really empty, it is sparsely filled with interesting things: several dozen organic molecules discovered to date by microwave spectroscopy, 2.7 K blackbody radiation left over from the big bang and the origin of the Universe, and cosmic rays, which include ionized atomic nuclei and various subatomic particles. There is also gas, plasma and dust, and small meteors and junk left over from previous manned and unmanned launches which are a hazard to astronauts. Some of this debris re-enters the atmosphere periodically.

The absence of air makes outer space (and the surface of the Moon) ideal locations for astronomy at all wavelengths of the electromagnetic spectrum, as evidenced by the spectacular pictures sent back by the Hubble Space Telescope, allowing us to see light from about 14 billion years ago, back almost to the time of the big bang. Pictures and other data from unmanned space vehicles have provided invaluable information about the planets from Mercury to Jupiter, Saturn and beyond, as well as asteroids and comets.

There are many artificial satellites orbiting the Earth, including geosynchronous communications satellites 35,786 km (22,241 miles) above mean sea level at the equator. Their orbits never "decay" because there is almost no matter there to exert frictional drag. There is also increasing reliance, for both military and civilian uses, of satellites which enable the Global Positioning System (GPS). A common misconception is that people in orbit are outside Earth's gravity because they are obviously "floating". If they, or indeed any satellite, were outside Earth's gravity, it would fly off and never return. They are floating because they are in free fall, the downward force of gravity being exactly counterbalanced by an outward centrifugal force due to their orbital motion. Earth's gravity reaches out far past the Van Allen belt and keeps the Moon in orbit at an average distance of 384,403 km (238,857 miles). The gravity of all celestial bodies drops off toward zero with the inverse square of the distance.

Going from sea level to outer space produces a pressure difference of only about 15 lbf/sq in, equal to surfacing from an underwater depth of about 34 ft (10 m). A person suddenly exposed to the vacuum would not explode, but would take a short while to die by asphyxiation (anoxia). Water vapor would start to boil off from exposed areas such as the cornea of the eye, and along with oxygen, from membranes inside the lungs.

There is a wealth of science fiction predicated on faster-than-light interstellar travel. But it's a good bet that the speed of light is a permanent limit that will never be exceeded, as expressed by Einstein's Special Theory of Relativity ( 1905), which has been amply confirmed by experiments and is one of the foundations of physics.

Milestones on the way to space

  • Sea level - 100 kPa (1 atm; 1 bar; 760 mm Hg; 14.5 lbf/in²) of atmospheric pressure
  • 4.6 km (15,000 ft) - FAA requires supplemental oxygen for aircraft pilots and passengers.
  • 5.0 km (16,000 ft) - 50 kPa 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 body generates enough internal pressure to prevent it.)
  • 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

Regions of outer space

  • Cislunar space
  • Interplanetary space
  • Interstellar medium
  • Intergalactic space

Space does not equal orbit

To perform an orbital spaceflight, a spacecraft must go higher and faster than for a sub-orbital spaceflight. A spacecraft has not made orbit until it is circling the Earth at a sufficiently great speed such that the weight of the spacecraft is exactly equal to the centripetal acceleration required to keep it in a circular orbit (see circular motion). It must not only rise above the atmosphere, but must also achieve a sufficient orbital speed ( angular velocity). For a low Earth orbit, this is about 7.9 km/s (18,000 mph). Robert Goddard was the first to realize that, given the energy available from any available chemical fuel, a several-stage rocket would be required. The escape velocity to pull free of Earth’s gravitational field altogether and move into interplanetary space is about 40,000 km/h (25,000 mph or 11,000 m/s). The energy required to reach velocity for low Earth orbit ( 32 MJ/kg) is about twenty times the energy required simply to climb to the corresponding altitude (10 kJ/(km·kg)).

There is a major difference between sub-orbital and orbital spaceflights. Minimal altitude for a stable orbit around the Earth, without excessive atmospheric drag, begins at around 350 km (220 miles) above mean sea level. A common misunderstanding about the boundary to space is that orbit occurs simply by reaching this altitude. Achieving orbital speed can theoretically occur at any altitude, although atmospheric drag precludes an orbit that is too low. At sufficient speed, an airplane would need a way to keep it from flying off into space.