The intruder came from beyond. A nebulous celestial body as old as the universe itself, it had been born in a vast cloud of ice, rocks, dust, and gas when the outer planets of the solar system were formed 4.6 billion years ago. Soon after its scattered particles had frozen into a solid mass one mile in diameter, it began streaking silently through the emptiness of space on an orbital voyage that carried it around a distant sun and halfway to the nearest stars again, a journey lasting many thousands of years from start to finish.
The comet’s core, or nucleus, was a conglomeration of frozen water, carbon monoxide, methane gas, and jagged blocks of metallic rocks. It might accurately be described as a dirty snowball hurled through space by the hand of God. But as it whirled past the sun and swung around on its return path beyond the outer reaches of the solar system, the solar radiation reacted with its nucleus and a metamorphosis took place. The ugly duckling soon became a thing of beauty.
As it began to absorb the sun’s heat and ultraviolet light, a long comma formed that slowly grew into an enormous luminous blue tail that curved and stretched out behind the nucleus for a distance of 90 million miles. A shorter, white dust tail more than one million miles wide also materialized and curled out on the sides of the larger tail like the fins of a fish.
