There were an even dozen of the air-borne 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. Then a thirteenth object winked into being, began the encircling, closing spiral of descent. A sphere resembling the warden-globes, it was a hundred times their size, and its orbit was purposefully controlled by instruments under the eye and hand of a human pilot. Four men were strapped down on cushioned sling-seats in the control cabin of the Western Alliance ship, two hanging where their fingers might reach buttons and levers, the others merely passengers, their own labor waiting for the time when they would set down on the alien soil of Topaz. The planet hung there in their visa-screen, richly beautiful in its amber gold, growing larger, nearer, so that they could pick out features of seas, continents, mountain ranges, which had been studied on tape until they were familiar, yet now were strangely unfamiliar too. One of the warden-globes alerted, oscillated in its set path, whirled faster as its delicate interior mechanisms responded to the awakening spark which would send it on its mission of destruction. A relay clicked, but for the smallest fraction of a millimeter failed to set the proper course. On the instrument, far below, which checked the globe's new course the mistake was not noted. The screen of the ship spiraling toward Topaz registered a path which would bring it into violent contact with the globe. They were still some hundreds of miles apart when the alarm rang. The pilot's hand clawed out at the bank of controls; under the almost intolerable pressure of their descent, there was so little he could do. His crooked fingers fell back powerlessly from the buttons and levers; his mouth was a twisted grimace of bleak acceptance as the beat of the signal increased. One of the passengers forced his head around on the padded rest, fought to form words, to speak to his companion. The other was staring ahead at the screen, his thick lips wide and flat against his teeth in a snarl of rage. "They ... are ... here...."