Explore "The Defiant Agents" by Andre Norton on Textopian. Read with annotations and highlights.
There were an even dozen of the airborne guardians, each following the swing of its own orbital path just within the atmospheric envelope of the planet which glowed as a great bronze-golden gem in the four-world system of a yellow star. The globes had been launched to form a web of protection around Topaz six months earlier, and the highest skill had gone into their production. Just as contact mines sown in a harbor could close that landfall to ships not knowing the secret channel, so was this world supposedly closed to any spaceship not equipped with the signal to ward off the sphere missiles.
That was the theory of the new off-world settlers whose protection they were to be, already tested as well as possible, but as yet not put to the ultimate proof. The small bright globes spun undisturbed across a two-mooned sky at night and made reassuring blips on an installation screen by day.