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
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.