gazing up this pastel column , so incredibly slim and graceful , it seemed unthinkable that anything so delicate could withstand the pressures which it had been designed to meet on friday-the howling stream of the most powerful controlled explosion that had ever been attempted ; the impact of the sound barrier ; the unknown pressures of the atmosphere at 15,000 miles an hour ; the terrible shock as it plunged back from a thousand miles up and hit the atmospheric envelope of the earth .