 The narrator speculates that for millions of years (evidenced by dust buildup around its forcefield) the artifact has been transmitting signals into deep space, but it ceases to transmit when, sometime later, it is destroyed "with the savage might of atomic power".The narrator hypothesizes that this "sentinel" was left on the Moon as a "warning beacon" for possible intelligent and spacefaring species that might develop on Earth.In "", the operation of the sentinel is activated when sunlight touches it for the first time after it is dug up.