Space
Long-Period Comets: Messengers From the Edge of the Solar System
Comet Hale-Bopp, which dominated the 1997 night sky, had not been seen by any human being before. Its previous perihelion passage was in approximately 2215 BCE, before writing systems existed in most of the world. It had spent the intervening 4,200 years falling inward from the outer solar system at speeds so slow that a bicycle could have overtaken it. When it finally arrived, its pristine composition — unchanged since the solar system formed 4.5 billion years ago — gave scientists a direct sample of the raw material from which planets were built. Long-period comets are the oldest, least-altered objects available for study. They are time capsules from the beginning.