Of all the comets that have passed the Earth in the last two centuries, this one came closest to our planet —5 million kilometers. That may seem like a great distance, but in cosmic terms it’s a near miss.
    Comets are important to astronomers because they consist of the material that composed the early solar system, before the Sun was fully formed. An enormous configuration of comets, called the Oort Cloud, orbits the Sun well beyond Pluto—so far away that the cloud’s members remain frozen and invisible. Once in a great while, however, the orbit of one is disturbed and it enters the inner solar system. As a comet approaches the Sun, gas and dust from its nucleus evaporate; solar wind and radiation push the