Aerostat meant anything that hung in the air. This was an easy trick to 
pull off nowadays. Nanotech materials were stronger. Computers were 
infinitesimal. Power supplies were much more potent. It was almost difficult not to build things that were lighter than air. Really simple things 
like packaging materials—the constituents of litter, basically—
tended to float around as if they weighed nothing, and aircraft pilots, 
cruising along ten kilometers above sea level, had become accustomed to the 
sight of empty, discarded grocery bags zooming past their windshields (and 
getting sucked into their engines). As seen from low earth orbit, the upper 
atmosphere now looked dandruffy. Protocol insisted that everything be made 
heavier than need be, so that it would fall, and capable of being degraded by 
ultraviolet light. But some people violated Protocol.