with activity and sounds. 

Some come from the pervasive plumbing, wiring, and mechanical hardware 
that fills every corner and cranny. If you were aboard, you'd hear 
air-circulation fans whirring, electric motors humming, pumps 
switching on and off--constant reminders that you're living in the 
bowels of a giant machine, switched "on."

Others come from the people themselves: an astronaut chugs-away on a
spring-mounted exercise bike to prevent muscle atrophy; crew members
move around, talk, bump into things, work with clanking metal tools;
tinny-sounding music plays from little speakers to keep the crew's
spirits up amid their gray, metallic surroundings. 

All these little sounds lend a kind of machine "personality" to the
spacecraft--like the distinctive feel of your own car. To astronauts
the hubbub is familiar, comforting, better by far than dead silence.
But to scientists with experiments onboard, the sounds are a sign of
something possibly unwelcome. Every clank, hum and buzz corresponds to 
a tiny jolting acceleration. 

"They are the "g's in the machine."
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Experiments are done in space (more often than not) to escape
acceleration. On Earth objects are accelerated downward at a rate of
9.8 m/s2 or 1-g ("one gee"). That constant pull is responsible for
effects such as convection and sedimentation that can complicate
chemistry and physics experiments--effects that scientists would like
to leave behind on Earth. Yet after all the trouble of going to space
to get a
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