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One way is NASA has a special jet (affectionately known
as the "barf wagon") that simulates zero G by free falling in
a parabolic arc. That can produce zero G for about 30 seconds
to a minute.
John Hawley
The other way is in a "neutral-bouyancy" tank of water,
where the weight of the astronaut plus gear is just right
to float in the middle of the water, so it almost feels
like being in zero gravity. Except that water's pretty
different from a vacuum.
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