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.
Given that it was so easy to make things that would float in air, it was not much of a stretch to add an air turbine. This was nothing more than a small propeller, or series of them, mounted in a tubular foramen wrought through the body of the aerostat, drawing in air at one end and forcing it out the other to generate thrust. A device built with several thrusters pointed along different axes could remain in one position, or indeed navigate through space.
Each aerostat in the dog pod grid was a mirror-surfaced, aerodynamic teardrop just wide enough, at its widest part, to have contained a pingpong ball. These pods were programmed to hang in space in a hexagonal grid pattern, about ten centimeters apart near the ground (close enough to stop a dog but not a cat, hence "dog pods") and spaced wider as they got higher. In this fashion a hemispherical dome was limned around the sacrosanct airspace of the New Atlantis Clave. When wind gusted, the pods all swung into it like weathervanes, and the grid deformed for a bit as the pods were shoved around; but all of them eventually worked their way back into place, swimming upstream like minnows, propelling the air turbines. The 'bines made a thin hissing noise, like a razor blade cutting air, that, when multiplied by the number of pods within earshot, engendered a not altogether cheerful ambience. Enough wrestling with the wind, and a pod's battery would run down. Then it would swim over and nuzzle its neighbor. The two would mate in midair, like dragonflies, and the weaker would take power from the stronger. The system included larger aerostats called nurse drones that would cruise around dumping large amounts of power into randomly selected pods all over the grid, which would then distribute it to their neighbors. If a pod thought it was having mechanical trouble, it would send out a message, and a fresh pod would fly out from the Royal Security installation beneath Source Victoria and relieve it so that it could fly home to be decompiled.
As numerous eight-year-old boys had discovered, you could not climb the dog pod grid because the pods didn't have enough thrust to support your weight; your foot would just mash the first pod into the ground. It would try to work its way loose, but if it were stuck in mud or its turbines fouled, another pod would have to come out and replace it. For the same reason you could pluck any pod from its place and carry it away. When Hackworth had performed this stunt as a youth, he had discovered that the farther it got from its appointed place the hotter it became, all the while politely informing him, in clipped military diction, that he had best release it or fall victim to vaguely adumbrated consequences. But nowadays you could just steal one or two whenever you felt like it, and a new one would come out and replace it; once they figured out they were no longer part of the grid, the pods would self-scramble and become instant souvenirs.
This user-friendly approach did not imply that grid-tampering went ignored, or that such activities were approved of. You could walk through the grid whenever you chose by shoving a few pods out of the way— unless Royal Security had told the pods to electrocute you or blast you into chum. If so, they would politely warn you before doing it. Even when they were in a more passive mode, though, the aerostats were watching and listening, so that nothing got through the dog pod grid without becoming an instant media celebrity with hundreds of uniformed fans down in Royal Joint Forces Command.
Unless it was microscopic. Microscopic invaders were more of the threat nowadays. Just to name one example, there was Red Death, a.k.a. the Seven Minute Special, a tiny aerodynamic capsule that burst open after impact and released a thousand or so corpuscle-size bodies, known colloquially as cookie-cutters, into the victim's bloodstream. It took about seven minutes for all of the blood in a typical person's body to recirculate, so after this interval the cookie-cutters would be randomly distributed throughout the victim's organs and limbs.
A cookie-cutter was shaped like an aspirin tablet except that the top and bottom were domed more to withstand ambient pressure; for like most other nanotechnological devices a cookie-cutter was filled with vacuum. Inside were two centrifuges, rotating on the same axis but in opposite directions, preventing the unit from acting like a gyroscope. The device could be triggered in various ways; the most primitive were simple seven-minute time bombs.
Detonation dissolved the bonds holding the centrifuges together so that each of a thousand or so balhisticules suddenly flew outward. The enclosing shell shattered easily, and each ballisticule kicked up a shock wave, doing surprisingly little damage at first, tracing narrow linear disturbances and occasionally taking a chip out of a bone. But soon they slowed to near the speed of sound, where shock wave piled on top of shock wave to produce a sonic boom. Then all the damage happened at once. Depending on the initial speed of the centrifuge, this could happen at varying distances from the detonation point; most everything inside the radius was undamaged but everything near it was pulped; hence, "cookie-cutter."
The victim then made a loud noise like the crack of a whip, as a few fragments exited his or her flesh and dropped through the sound barrier in air. Startled witnesses would turn just in time to see the victim flushing bright pink. Bloodred crescents would suddenly appear all over the body; these marked the geometric intersection of detonation surfaces with skin and were a boon to forensic types, who could thereby identify the type of cookie-cutter by comparing the marks against a handy pocket reference card. The victim was just a big leaky sack of undifferentiated gore at this point and, of course, never survived.
Such inventions had spawned concern that people from Phyle A might surreptitiously introduce a few million lethal devices into the bodies of members of Phyle B, providing the technically sweetest possible twist on the trite, ancient dream of being able instantly to turn a whole society into gravy. A few inroads of that kind had been made, a few mass closed-casket funerals had been held, but not many. It was hard to control these devices. If a person ate or drank one, it might end up in their body, but it might just go into the food chain and get recycled into the body of someone you liked. But the big problem was the host's immune system, which caused enough of a histological fuss to tip off the intended victims.
What worked in the body could work elsewhere, which is why phyles had their own immune systems now. The impregnable-shield paradigm didn't work at the nano level; one needed to hack the mean free path. A well-defended clave was surrounded by an aerial buffer zone infested with immunocules— microscopic aerostats designed to seek and destroy invaders. In the case of Atlantis/Shanghai this zone was never shallower than twenty kilometers. The innermost ring was a greenbelt lying on both sides of the dog pod grid, and the outer ring was called the Leased Territories.