There are forces in a space laboratory which act on experiments in
gravity-like ways. "G-jitter" is the term commonly employed to lump
all of these gravity-like forces together ("residual acceleration" is
also used for this phenomenon). Some of the forces causing g-jitter
change slowly with time. For example, consider the Shuttle which
typically flies in an orbit about 260-300 nautical miles above the
earth. Even at 300 nautical miles, there is an atmosphere (although a
thin one). As the Shuttle flies through the air, there is a resistance
to that motion called atmospheric drag. The location of an experiment
relative to the Shuttle's center of gravity also causes small local
accelerations from forces known as gravity-gradient or tidal forces.
Other residual accelerations are cyclic in nature, or occur only over
small distinct intervals. These accelerations are caused by such
things as rocket firings, crew exercise and motion, and machinery
operation. Because there are so many different sources for g-jitter,
the local "gravity" which acts on an experiment changes quite a bit,
both in magnitude and direction. On earth, we can generally figure out
which direction is "down", but in space, the direction that the body
force is pulling from changes dramatically. The instantaneous
magnitude of the residual acceleration is about 1-1000 ug on the
Shuttle (where 1 ug = 0.000001 times the gravitational force at sea
level), depending on activity levels on the Shuttle.
Emily S. Nelson
NASA Lewis Research Center
21000 Brookpark Road
Cleveland, Ohio 44135
Office: (216) 433-3268
Further Reading
E.S. Nelson, "An examination of anticipated g-jitter on Space Station
and its effects on materials processes." NASA Technical Memorandum
103775, May 1991.
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A. John Mallinckrodt http://www.intranet.csupomona.edu/~ajm
Professor of Physics mailto:ajmallinckro@csupomona.edu
Physics Department voice:909-869-4054
Cal Poly Pomona fax:909-869-5090
Pomona, CA 91768-4031 office:Building 8, Room 223