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?


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.
  • $\begingroup$ 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
  • $\begingroup$ 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

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). ...


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.

  • 1
    $\begingroup$ 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
  • $\begingroup$ The US Air Force did that in order to contract everyone else? My goodness! ;-) $\endgroup$ – uhoh Apr 21 '19 at 10:08

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.

  • $\begingroup$ 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

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