Comet over Wind Point Lighthouse 3/15/97
description...Notice the two tails. The faint blue one off to the left is called the "Ion tail". The ion tail is made of gasses which are primarily influenced by the solar wind. The brighter tail is made of dust which is influenced more by gravity and acceleration. You might imagine sand and smoke being streaming out of a car window as it careens around a corner. The comet itself (nucleus) is only about 20 miles across but the cloud of dust and gases which comprise its "tail" and "coma" is much larger. Hale-Bopp was over 120 million miles away at it's closest. This is further than the sun (93 million miles.) so when you see it's size in the sky, you can almost imagine how big the tail is.
A comet could cause problems if it's nucleus hit the earth. Short of that it can only influence us through our own fears.
It was rumored that William the Conqueror saw "Halley's Comet" as a good sign, while Harold King of England saw it as a sign of sure defeat in the battle of Hastings in 1066.