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According to a recent news article a group of USC students are attempting to launch a rocket "in to space" with a planned height of 62 miles. Making them "the first group of students to successfully launch a rocket into space,"

The ISS being at about the 250 mile height, and weather balloons reaching heights of 25 miles or more, space would seem to begin someplace between those two heights. Where does "space" begin?

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  • $\begingroup$ One might say that Earth is in space. The rock group Foreigner says "I stole a ride on a passing star" (orbiting the Sun). $\endgroup$ – Oscar Lanzi Jun 5 at 12:32
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We are "in space", in fact everything that exists and has a physical presence is. But what we usually mean by it is to describe "outer space" conditions of near (or hard) vacuum, where atmospheric pressure is already low enough to affect matter differently than under true atmospheric conditions, for example at atmospheric pressure below triple point of water. But where "outer space" begins, as you might expect by now, is a matter of opinion and there isn't any clear line to be drawn. For example, Wikipedia on Outer space has this to say:

There is no firm boundary where space begins. However the Kármán line, at an altitude of 100 km (62 mi) above sea level, is conventionally used as the start of outer space in space treaties and for aerospace records keeping.

Kármán line, named after the Hungarian-American engineer and physicist Theodore von Kármán, is the theoretical altitude at which the Earth's atmosphere becomes too thin for aeronautical purposes. But this is merely one arbitrarily set boundary, and where "outer space" begins could as well be defined differently, for example by the mentioned atmospheric pressure where water can sublimate directly from ice to gas phase without the transition to a liquid first. Again quoting Wikipedia, this time on the Triple points of water:

The gas–liquid–solid triple point of water corresponds to the minimum pressure at which liquid water can exist. At pressures below the triple point (as in outer space), solid ice when heated at constant pressure is converted directly into water vapour in a process known as sublimation.

So there it is again, mentioned as where the "outer space" begins. And it could, of course, be defined by other means as well. For example by what arbitrary atmospheric boundaries we set, at the edge of any altitude that we name atmospheric layers with a different term, designating different atmospheric conditions, maybe at the edge of Stratosphere and Mesosphere at 50 km? Yet again quoting Wikipedia on Uncertainties of Mesosphere:

The mesosphere lies above the maximum altitude for aircraft and below the minimum altitude for orbital spacecraft. It has only been accessed through the use of sounding rockets. As a result, it is the most poorly understood part of the atmosphere. The presence of red sprites and blue jets (electrical discharges or lightning within the lower mesosphere), noctilucent clouds and density shears within the poorly understood layer are of current scientific interest.

So it again mentions the Kármán line, but gives us an additional clue that "outer space" might as well be defined by where atmospheric weather phenomena stops. This could go on forever really, so I'll stop with these three examples.

We can draw a few conclusions though:

  • First, that it seems the internationally accepted standard for defining where "outer space" actually begins is the Kármán line, at an altitude of 100 km (62 mi) above sea level, and that we have agreed on certain treaties, namely the Outer Space Treaty, based on this definition.
  • Second, that any boundary where the "outer space" begins is exclusively arbitrary and might vary depending on your own definitions and use case.
  • And lastly, that we have not yet agreed on any of these arbitrary notions of where "outer space" begins, that would be equally applicable to atmospheric conditions of other celestials. I.e. at what altitude is considered to be "outer space" above the Earth will not equally apply to what might be this boundary for, say, the Atmosphere of Mars that has surface atmospheric pressure on average only slightly larger than the water triple point, or only about 0.6% of the Earth's mean sea level pressure.
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  • $\begingroup$ I would propose one definition as outside of any Earth atmospheric drag, which would mean that the ISS is not in that "space" (maybe the thermosphere/exosphere transition at 700 km altitude?) But it doesn't seem to be established enough to have even a name. Another border for an "inner space" as in non-interplanetary space, would be cislunar or geocentric space and reach out to the Earth-Moon-Lagrange point two. This has some practical significance for space travel and is somewhat established. $\endgroup$ – LocalFluff Mar 8 '14 at 10:26
  • $\begingroup$ That boundary between the thermosphere and exosphere is called thermopause (or exobase). But yes, you're right, where outer space begins could as well be defined there, too. $\endgroup$ – TildalWave May 28 '15 at 23:29
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    $\begingroup$ So what altitude is the equivalent of the triple-point of water? $\endgroup$ – wallyk May 29 '15 at 5:27
  • $\begingroup$ "pressures below the triple point (as in outer space)" does not imply outer space starts at the point the pressure is below the triple point. $\endgroup$ – JCRM Nov 21 '17 at 9:24
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Concurrent with TildalWave's reply I too say 'Karman line'(100 kilometres (62 mi) above the Earth's sea level). Apart from the fact the K-line is legally so recognized an alternative definition of 'in space' is covered in the same Wikipedia article

... any vehicle at this altitude would have to travel faster than orbital velocity in order to derive sufficient aerodynamic lift from the atmosphere to support itself, neglecting centrifugal force). ...

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The beginning of outer space and spaceflight is something hard to classify because the boundary between an atmosphere and the vacuum of space is very fluid. A space border is to be defined according to what one considers air and space / what one considers important in these matters. The space border may be an air pressure border rather than an altitude border, and should apply to all celestial bodies.

One of the following altitudes and pressures might be set as a border between an atmosphere and outer space / between air- and spaceflight (some of them are based on my personal spaceflight experience in Orbiter2016):

60,000 ft (18.3 km) The Armstrong limit above which the outside air pressure is so low that you need a pressurized suit (like a spacesuit). Water would boild at the temperature of your body. So your body considers the space above the Armstrong line to be vacuum and you can't survive without a pressurized suit or cabin anymore. 90% of the Earth's atmosphere's mass is below you. FAA airspace legislation ends at the Armstrong limit. The sky gets very dark already above 60,000 ft and you'd see the brightest stars and planets at noon. The Armstrong limit marks the begin of nearspace, a transition area between airspace and outer space. If you consider it the space border already, the following celestial bodies would count as bodies with atmospheres: Venus, Earth, Titan, and the four gas giants.

115,000 ft (35 km) The triple point of water. Above that altitude water can't exist in liquid state outside anymore. Water ice would sublime (evaporate), not melt. The triple point of water is at a pressure of about 611.7 pa (0.088 psi). At this altitude there's also the upper boundary of the ozone layer above which there's little block of UV radiation. Above that altitude the sky is completely black and won't get any more blacker. You'd see all bright enough stars and planets at noon (such as the Orion in summer). Jet aircraft can't fly leveled anymore and the altitude record for a jet plane is a MiG-25M about 8,000 ft (2.5 km) above that altitude. If you consider the pressure of the triple point of water to be the space boundary, you are to add Mars to the list of bodies with considerable atmospheres.

32 mi (51.5 km) The stratopause (stratosphere-mesosphere border). Temperature stops increasing and starts decreasing with altitude. Above 32 mi, air pressure drops below 0.01 psi. If you consider this or a lower altitude the space border, note that Yuri Gagarin wasn't the first man in space for you. The first man in space would be American pilot Joseph Walker who reached slightly more than 32 mi in the X-15 on 30 March 1961, a few days before Gagarin's spaceflight.

200,000 ft (61 km) As I conclude from flying in Orbiter2016, above about that altitude, pressure drops below 0.003 psi. There it is so low that you can't hear anymore, there is no sound and one is essentially deaf above that altitude. Outside only, since sound would still travel through your spacecraft of course. Also above circa 200,000 ft the ionosphere begins. Balloon flight is no longer possible. The highest unmanned balloon reached an altitude of 173,900 ft (53 km) and the highest manned one (flown by Alan Eustace) reached about 136,000 ft (41.5 km). Above 200,000 ft you may become weightless in your spaceplane without having to push the yoke. See this answer for clarification.

71 km (230,000 ft) This is circa the lowest perigee I reached in Orbiter2016 and continued orbiting the Earth. The orbit didn't change much, it remained pretty stable.

50 mi (80.47 km) This is the space border as defined by NASA, the USAF and the FAA. It is the mesopause (mesosphere-thermosphere border): temperature stops decreasing and starts increasing again. Pressure falls below 1 Pa / 0.00015 psi above that altitude. It is defined as where you have to put more effort into rocket-powered flight rather than air buoyancy. Astrodynamics take over from aerodynamics around that altitude. If you consider the space border here, you must add Pluto, Eris and Triton to celestial bodies that have a considerable atmosphere.

83.6 km (51.9 mi) Theodore von Kármán calculated that at that altitude the atmosphere becomes too thin to support aeronautical flight.

53 mi (85.3 km) This is circa where in Orbiter2016 my spacecraft starts to glow when re-entering from orbit. I suppose the Space Shuttle started to glow around that altitude too. I regain rudder control around that altitude.

57 mi (91.5 km) The original Kármán line: a vehicle's speed to generate lift must be orbital speed. Aerodynamic lift is 2% while 98% of the vehicle's weight is carried by centrifugal force. While circular orbits are impossible at that altitude, an spacecraft in an elliptical orbit can attain a perigee at 230,000 ft and remain it quite stable.

100 km (62.14 mi) What is currently referred to as the Kármán line and set by the FAI as space border. It is just the nextmost double-0-value in metric units in order to make the "Kárman line" "more memorable", lacking any basis in physical properties.

65 mi (105 km) In Orbiter2016, my spacecraft's gravimeter begins to read a more considerable g-force around that altitude when re-entering from orbit (or when having an elliptical orbit with a perigee reaching that low).

118 km (73 mi) Quote from Wikipedia 1: "In 2009, scientists reported detailed measurements with a Supra-Thermal Ion Imager (an instrument that measures the direction and speed of ions), which allowed them to establish a boundary at 118 km (73 mi) above Earth. The boundary represents the midpoint of a gradual transition over tens of kilometers from the relatively gentle winds of the Earth's atmosphere to the more violent flows of charged particles in space, which can reach speeds well over 268 m/s (600 mph)."

120 km (75 mi) This is where in Orbiter2016 my spacecraft starts to experience significant atmospheric drag when re-entering from orbit. If you set the space border at this altitude or higher, you must include Io into the list of bodies with considerable atmospheres.

400,000 ft (122 km) NASA's re-entry altitude for the Space Shuttle, defined as the beginning of more significant atmospheric drag.

93 mi (150 km) Above that altitude, a stable circular orbit is possible.

450 mi (700 km) The thermopause / exobase (end of collisional atmosphere). Above circa that altitude the atmosphere becomes an exosphere which no longer behaves like gas. The molecules don't collide with each other and are dispersed away from Earth by solar wind, reaching escape velocity. If you consider this the space border, you must include Callisto into the group of bodies with considerable atmospheres. You would also have to classify only the following flights as spaceflights: Gemini 10, Gemini 11, Apollo 8 and Apollo 10-17. All other spaceflights wouldn't count as any.

10,000 km (6,214 mi) End of the exosphere. Above that altitude there's quite an absolute vacuum. If you consider this the space border, only Apollo 8 and Apollo 10-17 would you have to count as spaceflights.

35,786 kilometres (22,236 miles) Geostationary orbit. While this has nothing to do with air pressure/density and vacuum, some equatorial countries claimed legal right on the territory up to the Geostationary orbit altitude.

As for me, I consider 200,000 ft (61 km) the space border. The least plausible to me are the one at 100 km and the one at geostationary orbit, due to the reasons written above.

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    $\begingroup$ While the info is interesting, I don't think this answers the question other than saying "it's arbitrary", whereas @Tildal's answer gives the internationally agreed altitude. Also, references please $\endgroup$ – Rory Alsop Jun 5 at 13:07
  • $\begingroup$ @RoryAlsop I wouldn't say the FAI boundary is internationally agreed. As written, the USA use the 50 mi border and NASA also used a re-entry altitude at 400,000 ft. There is no official international boundary, and the FAI boundary is not good for the reasons written. I can link the Armstrong limit but some of the altitudes I found out myself while others (like the thermopause) aren't necessary to be linked. The link to Wikipedia concerning the 118 km border mentions other possible space borders. $\endgroup$ – LoveForChrist Jun 5 at 13:21
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    $\begingroup$ Useful summary of the various limits, +1 $\endgroup$ – Organic Marble Jun 5 at 15:13
  • $\begingroup$ @OrganicMarble Thank you. If anyone finds out another altitude/pressure limit that might be considered as a space border, feel free to add it into my answer. $\endgroup$ – LoveForChrist Jun 5 at 15:15
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    $\begingroup$ You can't survive just below the Armstrong limit at 60,000 ft (18.3 km) without a suit breathing pure oxygen. The maximum height for breathing pure oxygen without a suit is lower, about 11,900 m (39,000 ft) to 15,000 m (49,000 ft). Just below the Armstrong limit the lungs are filled with 99 % water vapor and only 1 % oxygen. $\endgroup$ – Uwe Jun 23 at 18:14
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To contradict everyone else, the US Air Force used an altitude of 50 miles when awarding the Astronaut Badge to pilots of the X-15.

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    $\begingroup$ Heh, you're not really contradicting it, more confirming the arbitrarily defined boundary point. Even the same Astronaut Badge is now awarded to military pilots that went past the Kármán line, and the 50 mile requirement was from the 60's, defined by National Advisory Committee for Aeronautics (NACA). Also explained in the answer to What was the first man-made object to enter space? $\endgroup$ – TildalWave May 31 '14 at 11:55
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    $\begingroup$ The US Air Force did that in order to contract everyone else? My goodness! ;-) $\endgroup$ – uhoh Apr 21 '19 at 10:08
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Outer space begins at the Kármán line (correctly stated by @TildalWave). This has been accepted by Fédération Aéronautique Internationale (FIA). Quoting Wikipedia:

The Kármán line, or Karman line, lies at an altitude of 100 kilometres (62 mi) above the Earth's sea level, and commonly represents the boundary between the Earth's atmosphere and outer space. This definition is accepted by the Fédération Aéronautique Internationale (FAI), which is an international standard setting and record-keeping body for aeronautics and astronautics.

But it is not where atmospheric weather phenomena ceases. Weather phenomena typically ceases in the thermosphere which reaches 8-15 km above the Earth (18 km above equator), but stratospheric weather phenomena does exist, e.g. nacreous or polar stratospheric clouds which can form as high as 25 km above the mean sea-level. Still, that's far below the Kármán line at 100 km above the Earth's surface.

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  • $\begingroup$ This answer could be improved by expanding the section on weather, and providing reliable references for the data. $\endgroup$ – James Jenkins May 31 '14 at 8:00
  • $\begingroup$ I think you mean the troposphere, not the thermosphere. The Karman Line itself lies within the thermosphere, as do many low Earth orbits (including, for example, the ISS.) The troposphere is from the ground to the tropopause and is where most weather phenomena take place. $\endgroup$ – reirab Feb 26 '19 at 10:33
  • $\begingroup$ Nocturnal luminescent clouds are located in the mesosphere around 50 miles high where the U.S. space border is set. en.wikipedia.org/wiki/Noctilucent_cloud $\endgroup$ – user35272 Apr 23 at 17:22

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