Outer space (often referred to as space) may be defined as the relatively empty regions of the universe outside the atmospheres of celestial bodies. There is no universally accepted legal definition of outer space (see space law). A working definition accepted by many legal experts assumes that outer space generally begins at the lowest altitude above sea level at which objects can orbit the Earth, approximately 100 kilometres (62 miles) at a region known as the Kármán line. This article is directed less at law, and more at technical issues regarding outer space and getting to and from it. A space launch vehicle, for example, is a general term for a device that can move objects into space, either for suborbital scientific work, to put things in satellite orbits, or to send objects into escape velocity from Earth. "Space launch vehicle" excludes weapons, such as ballistic missiles, whose trajectory goes through outer space, but does not achieve orbit.
The case is made, not infrequently, that it may not be advisable to have a rigorous legal definition, given that conceptual space law, technology, and case law relevant to space continue to emerge. Today's working definitions tend either to define space by excluding that which is not considered space. Space law treaties do not contain a formal definition, but, with reasonable good faith by nations, appear to work.
Definitions based on Earth's atmospheric layers
One method to define  space is to define the layers of Earth's atmosphere, and assume space is above the highest level. Unfortunately, the top of the highest atmospheric layer is not rigorously defined.
From the lowest to the highest, these are the five primary layers (and their inter-layer boundaries) of the Earth's atmosphere:
- The troposphere is the lowest layer of the atmosphere and extends from the Earth's surface to about 7 kilometres (4 miles) high at the north and south poles and 17 kilometres (11 miles) high at the equator. This is where we live and all of our weather occurs in the troposphere. The troposphere contains about 80% of the total mass of the atmosphere. In fact, 50% percent of the total mass of the atmosphere is located in the lowest 5 to 6 kilometres (3.1 to 3.7 miles) of the troposphere. The cruising altitude of commercial airliners is usually about 9 to 10 kilometres (5.6 to 6.2 miles) which is close to the top of the troposphere. The tropopause is the boundary between the troposphere and the stratosphere. Together, the troposphere and the tropopause are known as the lower atmosphere.
- The stratosphere extends from the tropopause's height of 7 to 17 kilometres (4 to 11 miles) to a height of about 50 kilometres (31 miles) and contains about 19% of the atmospheric gases. The ozone layer resides in the lower portion of the stratosphere, though the thickness of the layer varies seasonally and geographically. The stratopause is the boundary between the stratosphere and the mesosphere. The atmospheric pressure here is 1/1000th of the atmospheric pressure at sea level.
- The mesosphere extends from stratopause's height of about 50 kilometres (31 miles) to a height of about 80 to 85 kilometres (50 to 53 miles). The gases continue to become less and less dense with height. However, the gases in the mesosphere are still dense enough to slow down meteors hurtling into the atmosphere, where they burn up and leave fiery trails in the night sky. The mesopause is the boundary between the mesosphere and the thermosphere. It is the coldest place on Earth, with a temperature as low as -120 °C at times.
- The thermosphere extends from the mesopause's height of 80 to 85 kilometres (50 to 53 miles) to a height of between 500 and 1000 kilometres (310 and 620 miles) and is referred to as the upper atmosphere. The gases of the thermosphere are increasingly less dense than in the mesosphere. The International Space Station orbits in this layer, between 320 and 380 kilometers (200 and 235 miles) from the Earth's surface. The Kármán line, named after the physicist Theodore von Kármán, is in the thermosphere at an altitude of 100 kilometres (62 miles) above the Earth and is often referred to as the boundary between Earth's atmosphere and outer space. The thermopause is the boundary between the thermosphere and the exosphere.
- Extends from the thermopause's height of 500 to 1000 kilometres (310 to 620 miles) up to a height of 10,000 kilometres (6,200 miles) above the Earth's surface. The exosphere is the outermost layer of the atmosphere and it is in this layer that our man-made satellites orbit the Earth. From this layer, atoms and molecules escape into the void of space. There is no definite boundary between the exosphere and outer space. It slowly becomes less dense (i.e., more empty) and fades into outer space.
Definitions based on aircraft and satellite performance
According to the International Civil Aviation Organization, an aircraft is “any machine that can derive support in the atmosphere from the reactions of the air other than the reactions of the air against the earth’s surface.” The maximum altitude at which a machine can derive support from the reactions of the air is presently estimated at about twenty one miles by the ICAO Secretariat. By exclusion, that suggests that outer space begins at approximately 21 miles, but this is a moving target as new techniques evolve for reaching high altitudes.
Another approach is to define the lowest level of outer space as the lowest altitude at which a satellite can stay in orbit, even an orbit that will degrade fairly quickly due to atmospheric resistance. That altitude is approximately 95 miles, but a satellite equipped with altitude-keeping thrusters may be able to orbit at 75 miles. A variant treats the boundary as the point at which flight depends more on orbital mechanics and less on aerodynamic effects.
- Barrett, Raymond J. (May-June 1973), "Outer Space and Air Space: The Difficulties in Definition", Air University Review
- The Layers of the Atmosphere From the website of the National Oceanic and Atmospheric Administration (NOAA) and the National Weather Service (NWS) The primary source for this section.
- This definition is accepted by the Fédération Aéronautique Internationale (FAI), an international standard setting and record-keeping body for aeronautics and astronautics.
- U.S. Statement, Definition and Delimitation of Outer Space And The Character And Utilization Of The Geostationary Orbit, 40th Session in Vienna, April 2001