A spacewalk occurs when an astronaut leaves their spacecraft while in space.
But in fact, they only ever (deliberately) do this while still in another spacecraft -
a tightly fitting one we call a spacesuit. So, conceptually it's hard to rigorously
define the difference between a spacesuit and a spaceship (I am not convinced
that definitions involving flexibility are meaningful.) Nevertheless, to date
there have not been any cases that are really ambiguous.
In particular, for my purposes I define a `spacewalk', loosely, as an activity carried
out in vacuum conditions protected at most by a (not rigourously defined) spacesuit.
And by `vacuum conditions', I specifically mean an ambient pressure of less than
50 mbar (5 kPa). I chose this arbitrary value to be `basically vacuum'; I note
that the point at which US spacewalkers are told they are near enough to vacuum
to open the airlock hatch is 34 mbar, not too different. On the
other hand, during Apollo NASA used 241 mbar to mark start and end of spacewalks,
and currently uses 345 mbar as a point to pause depressurization for leak checks,
so I don't claim I have a strong argument for this particular value.
When airlocks are repressurized, the pressure usually rises quite quickly above
the 50 mbar level, but random pressure fluctuations in a depressurized airlock
are much less than this. This means that my definition doesn't create large numbers
of spurious mini-spacewalks at either end of a real spacewalk (if you picked 1 mbar,
for example, this might be a problem).
My definition means that I include what NASA calls `intravehicular activity',
where astronauts work inside a depressurized section of a spacecraft. My
view is that a spacewalk can be either when you go outside the spaceship to
enter space, or equally well when you let space into the spaceship.
My 50 mbar rule means that activity on the Martian surface
(with a pressure of 6 mbar) would count as spacewalking, but
activity on the surface of Venus or Titan, with their much higher
atmospheric pressures, would not. This is not to say that humans operating
in an unbreathable atmosphere is not dangerous and/or not interesting, just
that I choose to treat it as a different thing from a spacewalk. (It is in some
ways more comparable to undersea diving, which also involves an unbreathable
ambient environment).
Clearly, by my definition, activity on the lunar surface (Lunar EVA or LEVA), where the typical
pressure is orders of magnitude below my threshold, is included.
For `spacewalk' I sometimes use the alternative but ugly term `depressurization event'. I don't
like NASA's term `extravehicular activity' (EVA) since my focus is on the vacuum, not the fact that
the astronaut is outside. The Russian term is
ВКД
(VKD) which stands for
внекорабельная деятельность
(vnekorabel'naya deyatel'nost') which is a direct translation of EVA. Less formally they say
Выход в открытый космос
(exit to open space) which has some of the same problem but is at least a bit less unpoetic.
I hereby (Oct 2023) propose a new interpretation of the abbreviation EVA: Exoatmospheric Vacuum Activity,
in other words an activity that occurs in a vacuum (outside a pressurized spaceship, for now
not counting a suit) and not in a ground vacuum chamber (hence, exoatmospheric).
Since 1981, NASA has measured spacewalk duration from when the suits went to internal
power until the start of airlock repressurization. Prior to this, they usually
used the time from depressurization to 3.5 psi until repressurization to 3.5 psi.
Roskosmos and its precedecessors use the time from hatch opening to hatch closure,
and China (which got its initial spacesuit tech from Russia) appears to use the same rule.
As a historian, however, I balk at using different measurs for different agencies.
When asking questions like `what is the longest spacewalk' you really need to use
the same rules for everyone. The
accuracy
of my '50 mbar depress to 50 mbar repress' rule is lower than that
of the values given according to the agency rules (because I have to guess my values
sometimes) but my values have the advantage of being
unbiased.
In any case, I provide the values needed for ALL the different measurement rules so that the reader
can pick their own.