A vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than standard atmospheric pressure.
Much of outer space has the density and pressure of an almost perfect vacuum. It has effectively no friction, which allows stars, planets and moons to move freely along ideal gravitational trajectories. But no vacuum is perfect, not even in interstellar space, where there are only a few hydrogen atoms per cubic centimeter at 10 fPa (10−16 Torr). The deep vacuum of space could make it an attractive environment for certain processes, for instance those that require ultraclean surfaces; for small-scale applications, however, it is much more cost-effective to create an equivalent vacuum on Earth than to leave the Earth's gravity well.
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. In low
earth orbit (about 300 km or 185 miles
altitude) the atmospheric density is about
100 nPa (10-9 Torr), still sufficient to
produce significant drag on satellites.
Most artificial satellites operate in this
region, and must fire their engines every
few days to maintain orbit.
All of the observable universe is filled
with large numbers of photons, the
so-called cosmic background radiation,
and quite likely a correspondingly large
number of neutrinos. The current
temperature of this radiation is about Click to enlarge
3 K, or -270 degrees Celsius or -454 degrees Fahrenheit.
Before NASA's new spacecraft, Orion, carries the next generation of explorers into
space, it first will make a shorter journey to the world's largest vacuum chamber.
In this massive, cathedral-like structure, it must endure a variety of rigorous challenges.
Called the Space Power Facility, the vacuum chamber resides at NASA Glenn
Research Center's Plum Brook Station in Sandusky, Ohio. It measures 100 feet in
diameter and towers at 122 feet tall. Its immense size and ability to simulate the vacuum
of space make it ideal for testing the Orion crew exploration vehicle.
Orion is part of NASA's Constellation Program to send human explorers back to
the moon and then onward to Mars and other destinations in the solar system.
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