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Finally, once in a great while ("a small number out of more than 1,700 missiles" according to Vandenberg Air Force Base), an extra added bonus of a "twilight phenomenon" occurs. This spectacular event features the colors of the rainbow (specifically, green, blue, white and rose-colors) emanating from the top of the contrail. It is created when unburned missile propellant and water freezes in the upper atmosphere, and those frozen crystals are illuminated at the right angle by the sun, which must be below the horizon by 30-60 minutes of time.