A second sun blazed white and swelled visibly as he watched. What on Earth would have been—so many times had been—a climbing mushroom cloud was here … - Robert A. Heinlein

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A second sun blazed white and swelled visibly as he watched. What on Earth would have been—so many times had been—a climbing mushroom cloud was here in open space a perfect geometrical sphere, growing unbelievably. It swelled still larger, dropping from limelight white to to silvery violet, became blotched with purple, red and flame. And still it grew, until it blanked out the earth beyond it.
At the time it had been transformed into a radioactive cosmic cloud Circum-Terra had been passing over, or opposite, the North Atlantic; the swollen incandescent cloud was visible to most of the habitable portions of the globe, a burning symbol in the sky.