A
vacuum is a
space devoid of
matter.
The word is derived from the Latin adjective vacuus for "vacant" or "void". An approximation to such vacuum is a region with a gaseous
pressure much less than
atmospheric pressure.
[1] Physicists often discuss ideal test results that would occur in a
perfect vacuum, which they sometimes simply call "vacuum" or
free space, and use the term
partial vacuum to refer to an actual imperfect vacuum as one might have in a
laboratory or in
space. In engineering and applied physics on the other hand, vacuum refers to any space in which the pressure is considerably lower than atmospheric pressure.
[2] The Latin term
in vacuo is used to describe an object that is surrounded by a vacuum.
...
Outer space has very low density and pressure, and is the closest physical approximation of a perfect vacuum. But no vacuum is truly perfect, not even in interstellar space, where there are still a few hydrogen atoms per cubic meter.
[5]
Stars, planets, and moons keep their
atmospheres by gravitational attraction, and as such, atmospheres have no clearly delineated boundary: the density of atmospheric gas simply decreases with distance from the object. The Earth's atmospheric pressure drops to about 32 millipascals (4.6×10−6 psi) at 100 kilometres (62 mi) of altitude,
[36] the
Kármán line, which is a common definition of the boundary with outer space. Beyond this line, isotropic gas pressure rapidly becomes insignificant when compared to
radiation pressure from the
Sun and the
dynamic pressure of the
solar winds, so the definition of pressure becomes difficult to interpret. The
thermosphere in this range has large gradients of pressure, temperature and composition, and varies greatly due to
space weather. Astrophysicists prefer to use
number density to describe these environments, in units of particles per cubic centimetre.
