microgravity vs. zero gravity - was STS-73 'beam' observation

Philip Chien (kc4yer@amsat.org)
Wed, 1 Nov 1995 00:43:39 -0400

Jeff Hunt asked:
>While we are talking about the shuttle, why are they called microgravity
>experients; why not zero gravity experiments?

zero gravity implies something which can't be measured (e.g. something
which is zero).

With today's ultra-sensitive accelerometers it's quite easy to measure
small gravitational forces and accelerations.  It turns out that anything
in space does have some small accelerations, due to the fact that the Earth
isn't a point mass, pertubations from the Sun, Moon, and even Jupiter,
etc., solar wind, and other environmental effects.  At lower altitudes the
atmospheric drag is also an external force.  On the shuttle there are
additional forces, primarily movements by the crew, pumps, and other
systems.  So all of these external forces make zero gravity not so zero.
Hence the microgravity term, which indicates that there is a force which -
although small - can be measured.  On a microgravity oriented shuttle
mission like the STS-73 USML-2 mission the typical linear RMS gravitational
forces are on the order of 10 E-6.

Philip Chien, Earth News - space writer and consultant  PCHIEN@IDS.NET
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