 Vacuum
on earth
Nature uses "low vacuum techniques" in some of the functions of life
of animals, but no natural high vacuum is known on earth. Some of
these "applications" are very vital, as our own respiration, others
like the vacuum action of mosquitoes are rather bothersome.
Human beings are pumping to about 740 Torr during their respiration,
and may achieve pressures as low as 300 Torr by suction. The octopus
is able to achieve pressures of about 100 Torr (Champeix, 1965).
Vacuum in space
As the pressure of 760 Torr at sea level is a result of the "atmospheric
column", the pressure decreases with the altitude. Up to 100 km
altitude (troposphere and stratosphere) the pressure decreases quite
regularly by a factor of 10 for each increase in altitude of 15 km,
which results in a pressure of 10-3 Torr at about 90 km altitude. At
higher altitudes high vacuum exists.
The ionosphere (100-400 km) contains a large number of ionized atoms,
and its pressure decreases only by a factor of 10 every 100-200 km.
This decrease results in a pressure of about 10-10 Torr at an altitude
of 1000 km. Above 400 km, ultra-high vacuum conditions exist. Above
this altitude the pressure decreases at an even slower rate, thus at
10 000 km a pressure of about 10-13 Torr exists.
Since the average spacecraft travels at a velocity considerably in
excess of that of the average gas molecule, the pressures measured on
spacecrafts are actually determined by the spacecraft velocity and gas
particle concentration. Thus the diagram (fig. 1.2) of the high
altitude atmosphere is expressed in concentration (density) units.
The gas molecule concentration (density) is estimated to fall in the
shaded area of figure 1, since the density varies with the time of day
and the amount of solar activity. At an altitude below 200 km, the
atmosphere is essentially air. Between 200-1000 km the gas is
principally atomic nitrogen and oxygen, which may be largely ionized
at periods of solar maxima. There is some evidence of an appreciable
amount of helium at about 700-1000 km altitude. Above an altitude of
1500 km, the gas consists of neutral atomic hydrogen, protons and
electrons.
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