Comet Hale Bopp
Comet Hale-Bopp (formally designated C/1995 O1) was arguably the most widely observed comet of the twentieth century, and one of the brightest seen for many decades. It was visible to the naked eye for a record 18 months, twice as long as the previous record holder, the Great Comet of 1811.
Hale-Bopp was discovered on July 23, 1995, at a great distance from the Sun, raising expectations that the comet would brighten considerably by the time it passed close to Earth. Although predicting the brightness of comets with any degree of accuracy is very difficult, Hale-Bopp met or exceeded most predictions when it passed perihelion on April 1, 1997. The comet was dubbed the Great Comet of 1997.
The passage of Hale-Bopp was notable also for inciting a comet-related panic. Rumours that an alien spacecraft was following the comet gained remarkable currency, and inspired a mass suicide among followers of the Heaven's Gate cult.