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Surpassing the longest operating duration of previous ion engines by 5,000 hours, NEXT has operated for more than 35,500 hours, more than four years of cumulative time. In addition, over 600 kilograms of xenon propellant have been expended, twice as much as originally required. These two measures are essential to accomplishing long-duration scientific space exploration missions.
In an atmosphere jets/rockets or any other propellant based flying machine gains thrust from
the exhaust pushing against the air.
One more way to realize that it isn't "pushing against the air" is to do this: Sitting in your chair, try to push yourself out of it with your hands....just "pushing" against the air.....won't work, will it?