Vor — James Blish

The glowing ovoid plummeted downward, the whole sky screaming like a metallic banshee as the air boiled away from its sides. A supersonic bang broke over the forest, but the thing was already gliding down to Earth. As the ovoid settled in among the trees, the entire forest burst into flames. A thin black line appeared and etched a perfect circle on the side of the metal egg, then thickened into a door. The air was rent by a searing hiss. Slowly, the strange being - soon to be known to all mankind as VOR - began to emerge . . .