Outer space

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The boundaries between the Earth's surface and outer space.

Outer space (often called space) consists of 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. There is no clear boundary between Earth's atmosphere and space, as the density of the atmosphere gradually decreases as the altitude increases.

For practical purposes, the Fédération Aéronautique Internationale has established the Kármán line, at an altitude of 100 kilometers (62 mi), as a working definition for the boundary between aeronautics and astronautics. This line was chosen because, as Theodore von Kármán calculated, a vehicle traveling above that altitude would have to move faster than orbital velocity to derive sufficient aerodynamic lift from the atmosphere to support itself. The United States designates people who travel above an altitude of 50 miles (80 km) as astronauts. During re-entry, roughly 120 kilometers (75 mi) marks the boundary where atmospheric drag becomes noticeable, depending on the ballistic coefficient of the vehicle.

Contrary to popular understanding, outer space is not completely empty, that is, it is not a perfect vacuum. Rather, it contains a low density of particles, predominantly hydrogen plasma, as well as electromagnetic radiation. Hypothetically, it also contains dark matter and dark energy.


Origins of terminology

The term outer space was first recorded by H. G. Wells in his novel First Men in the Moon in 1901.[1] The shorter term space is actually older, first used to mean the region beyond Earth's sky in John Milton's Paradise Lost in 1667.[2]


Outer space is the closest natural approximation of a perfect vacuum. It has effectively no friction, allowing stars, planets and moons to move freely along ideal gravitational trajectories. But no vacuum is truly perfect, not even in intergalactic space where there are still a few hydrogen atoms per cubic centimeter. (For comparison, the air we breathe contains about 1019 molecules per cubic centimeter.) The deep vacuum of space could make it an attractive environment for certain industrial processes, for instance those that require ultraclean surfaces; however, it is currently much less costly to create an equivalent vacuum on Earth than to leave the Earth's gravity well.

Stars, planets, asteroids, and moons keep their atmospheres by gravitational attraction, and as such, atmospheres have no clearly delineated boundary: the density of atmospheric gas simply decreases with distance from the object. The Earth's atmospheric pressure drops to about 1 Pa at 100 kilometers (62 mi) of altitude, the Kármán line which is a common definition of the boundary with outer space. Beyond this line, isotropic gas pressure rapidly becomes insignificant when compared to radiation pressure from the sun and the dynamic pressure of the solar wind, so the definition of pressure becomes difficult to interpret. The thermosphere in this range has large gradients of pressure, temperature and composition, and varies greatly due to space weather. Astrophysicists prefer to use number density to describe these environments, in units of particles per cubic centimeter.

All of the observable universe is filled with large numbers of photons, the so-called cosmic background radiation, and quite likely a correspondingly large number of neutrinos. The current temperature of this radiation is about 3 K (−270.15 °C; −454.27 °F).

Contrary to popular belief,[3] a person suddenly exposed to the vacuum would not explode, freeze to death or die from boiling blood, but would take a short while to die by asphyxiation (suffocation). Air would immediately leave the lungs due to the enormous pressure gradient. Any oxygen dissolved in the blood would empty into the lungs to try to equalize the partial pressure gradient. Once the deoxygenated blood arrives at the brain, death would quickly follow.

Humans and animals exposed to vacuum will lose consciousness after a few seconds and die of hypoxia within minutes. Blood and other body fluids do boil when their pressure drops below 6.3 kPa, the vapor pressure of water at body temperature.[4] This condition is called ebullism. The steam may bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture. Ebullism is slowed by the pressure containment of blood vessels, so some blood remains liquid.[5][6] Swelling and ebullism can be reduced by containment in a flight suit. Shuttle astronauts wear a fitted elastic garment called the Crew Altitude Protection Suit (CAPS) which prevents ebullism at pressures as low as 2 kPa.[7] Water vapor would also rapidly evaporate off from exposed areas such as the lungs, cornea of the eye and mouth, cooling the body. Rapid evaporative cooling of the skin will create frost, particularly in the mouth, but this is not a significant hazard. Space may be cold, but it's mostly vacuum and can hardly transfer heat, so the main temperature worry for space suits is how to get rid of naturally generated body heat.

Cold or oxygen-rich atmospheres can sustain life at pressures much lower than atmospheric, as long as the density of oxygen is similar to that of standard sea-level atmosphere. The colder air temperatures found at altitudes of up to 3 kilometers (1.9 mi) generally compensate for the lower pressures there.[4] Above this altitude, oxygen enrichment is necessary to prevent altitude sickness, and spacesuits are necessary to prevent ebullism above 19 kilometers (12 mi).[4] Most spacesuits use only 20 kPa of pure oxygen, just enough to sustain full consciousness. This pressure is high enough to prevent ebullism, but simple evaporation of blood can still cause decompression sickness and gas embolisms if not managed.

Rapid decompression can be much more dangerous than vacuum exposure itself. Even if the victim does not hold his breath, venting through the windpipe may be too slow to prevent the fatal rupture of the delicate alveoli of the lungs.[4] Eardrums and sinuses may be ruptured by rapid decompression, soft tissues may bruise and seep blood, and the stress of shock will accelerate oxygen consumption leading to hypoxia.[8] Injuries caused by rapid decompression are called barotrauma. A pressure drop as small as 13 kPa, which produces no symptoms if it is gradual, may be fatal if occurs suddenly.[4]

Space versus orbit

To perform an orbital spaceflight, a spacecraft must travel faster than it must for a sub-orbital spaceflight. A spacecraft has not entered orbit until it is traveling with a sufficiently great horizontal velocity such that the acceleration due to gravity on the spacecraft is less than or equal to the centripetal acceleration being caused by its horizontal velocity (see circular motion). So to enter orbit, a spacecraft must not only reach space, but must also achieve a sufficient orbital speed (angular velocity). For a low-Earth orbit, this is about 7,900 m/s (28,440.00 km/h/17,671.80 mph); by contrast, the fastest airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was 2,200 m/s (7,920.00 km/h/4,921.26 mph) in 1967 by the North American X-15[9]. Konstantin Tsiolkovsky was the first person 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 11,000 m/s (39,600.00 km/h/24,606.30 mph) 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. The minimum altitude for a stable orbit around Earth (that is, one without significant atmospheric drag) begins at around 350 kilometers (220 mi)) 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, but at present, this speed is several times greater than anything within reasonable technology.

A common misconception is that people in orbit are outside Earth's gravity because they are "floating." They are floating because they are in "free fall": they are accelerating toward Earth, along with their spacecraft, but are simultaneously moving sideways fast enough that the "fall" away from a straight-line path merely keeps them in orbit at a constant distance above Earth's surface. Earth's gravity reaches out far past the Van Allen belt and keeps the Moon in orbit at an average distance of 384,403 kilometers (238,857 mi).


Space being not a perfect vacuum, its different regions are defined by the various atmospheres and "winds" that dominate within them, and extend to the point at which those winds give way to those beyond. Geospace extends from Earth's atmosphere to the outer reaches of Earth's magnetic field, whereupon it gives way to the solar wind of interplanetary space. Interplanetary space extends to the heliopause, whereupon the solar wind gives way to the winds of the interstellar medium. Interstellar space then continues to the edges of the galaxy, where it fades into the intergalactic void.


Aurora australis observed by Discovery, May 1991.

Geospace is the region of outer space near the Earth. Geospace includes the upper region of the atmosphere, as well as the ionosphere and magnetosphere. The Van Allen radiation belts also lie within the geospace. The region between Earth's atmosphere and the Moon is sometimes referred to as cis-lunar space.

Although it meets the definition of outer space, the atmospheric density within the first few hundred kilometers above the Kármán line is still sufficient to produce significant drag on satellites. Most artificial satellites operate in this region called low earth orbit and must fire their engines every few days to maintain orbit. The drag here is low enough that it could theoretically be overcome by radiation pressure on solar sails, a proposed propulsion system for interplanetary travel. Planets are too massive for their trajectories to be affected by these forces, although their atmospheres are eroded by the solar winds.

Geospace is populated at very low densities by electrically charged particles, whose motions are controlled by the Earth's magnetic field. These plasmas form a medium from which storm-like disturbances powered by the solar wind can drive electrical currents into the Earth’s upper atmosphere.

During geomagnetic storms two regions of geospace, the radiation belts and the ionosphere, can become strongly disturbed. These disturbances interfere with the functioning of satellite communications and navigation (GPS) technologies. These storms increase fluxes of energetic electrons that can permanently damage satellite electronics, and can also be a hazard to astronauts, even in low-Earth orbit.

Geospace contains material left over from previous manned and unmanned launches that are a potential hazard to spacecraft. Some of this debris re-enters Earth's atmosphere periodically.

The absence of air makes geospace (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 light from about 13.7 billion years ago—almost to the time of the Big Bang—to be observed.

The outer boundary of geospace is the interface between the magnetosphere and the solar wind. The inner boundary is the ionosphere.[10] Alternately, geospace is the region of space between the Earth’s upper atmosphere and the outermost reaches of the Earth’s magnetic field.[11]


Outer space within the 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 cosmic rays, which include ionized atomic nuclei and various subatomic particles. There is also gas, plasma and dust, small meteors, and several dozen types of organic molecules discovered to date by microwave spectroscopy. Interplanetary space is defined by the solar wind, a continuous stream of charged particles emanating from the Sun that creates a very tenuous atmosphere (the heliosphere) for billions of miles into space. The discovery since 1995 of extrasolar planets means that other stars must possess their own interplanetary media.


Main article: Interstellar medium

Interstellar space is the physical space within a galaxy not occupied by stars or their planetary systems. The interstellar medium resides – by definition – in interstellar space.


Intergalactic space is the physical space between galaxies. Generally free of dust and debris, intergalactic space is very close to a total vacuum. Some theories put the average density of the Universe as the equivalent of one hydrogen atom per cubic meter[12][13]. The density of the Universe, however, is clearly not uniform; it ranges from relatively high density in galaxies (including very high density in structures within galaxies, such as planets, stars, and black holes) to conditions in vast voids that have much lower density than the Universe's average. The temperature is only 2.73 K (−270.42 °C; −454.76 °F) Kelvin[14]. NASA's COBE mission (Cosmic Background Explorer) measured the temperature as 2.725 K (−270.43 °C; −454.77 °F) +/- 0.002 K.

See also


  1. Outer. Etymonline. Retrieved January 13, 2009.
  2. Space. Etymonline. Retrieved January 13, 2009.
  3. Human Body in a Vacuum. NASA. Retrieved January 13, 2009.
  4. 4.0 4.1 4.2 4.3 4.4 Richard M. Harding. 1989. Survival in Space: Medical Problems of Manned Spaceflight. (London, UK: Routledge. ISBN 0415002532).
  5. Charles E. Billings, (1973). "Barometric Pressure". in James F. Parker and Vita R. West, (eds.) Bioastronautics Data Book, Second Ed. (NASA. NASA SP-3006.)
  6. Geoffrey Landis, Human Exposure to Vacuum. sff.net. Retrieved January 13, 2009.
  7. P. Webb, 1968. The Space Activity Suit: An Elastic Leotard for Extravehicular Activity. Aerospace Medicine. 39:376–383.
  8. Tamarack R. Czarnik, Ebullism at 1 million feet: Surviving Rapid/Explosive Decompression. sff.net. Retrieved January 13, 2009.
  9. Linda Shiner, X-15 Walkaround: A short guide to the fastest airplane ever. Air & Space Magazine. Retrieved January 13, 2009.
  10. Report of the Living With a Star Geospace Mission Definition Team. NASA. Retrieved January 13, 2009.
  11. LWS Geospace Missions. NASA. Retrieved January 13, 2009.
  12. Keay Davidson and George Smoot. 1993. Wrinkles in Time. (New York, NY: Avon. ISBN 9780688123307), 158-163.
  13. Joseph Silk. 2001. Big Bang. (New York, NY: Freeman. ISBN 9780716742463), 299.
  14. COBE website. NASA. Retrieved January 13, 2009.


  • Billings, Charles E. 1973. "Barometric Pressure," in James F. Parker and Vita R. West eds. Bioastronautics Data Book, Second Edition. Washington, DC: NASA. NASA SP-3006.
  • Davidson, Keay, and George Smoot. 1993. Wrinkles in Time. New York, NY: Avon. ISBN 9780688123307.
  • Harding, Richard M. 1989. Survival in Space: Medical Problems of Manned Spaceflight. London, UK: Routledge. ISBN 0415002532.
  • Silk, Joseph. 2001. Big Bang. New York, NY: Freeman. ISBN 9780716742463.
  • Webb, P. 1968. The Space Activity Suit: An Elastic Leotard for Extravehicular Activity. Aerospace Medicine 39:376–383.

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