This material is what makes a comet visible. Far from the sun, a comet resembles an asteroid. But as it draws closer to our solar system's central source of heat and energy, the frozen compounds in the nucleus are turned to vapor, and dust is dragged away from the nucleus by the sublimating gases. Solar wind drags ionized gas away in a slightly different direction, producing a plasma tail that may also be visible. Passing comets have been observed for millennia and recorded throughout history, Britannica notes.
Astronomers are able to calculate past and future passes of comets based on present behavior.
In this way, they have determined that Hale-Bopp has passed this way before, more than 4,000 years ago. The ancients, who believed that the heavens were fixed and unchanging, believed comets were not part of the natural order of things, and saw them as augurs of momentous events - wars, disasters, the births or deaths of kings. Since it will be several millennia before Comet Hale-Bopp is visible again, astronomers are advising earthlings to take the opportunity to get a good look this time. It's best to get completely away from city lights, MacRobert said, although the comet is bright enough to see under poor conditions. In the Texas Panhandle, look to the northwest, about 20 degrees up from the horizon.