Discovered at an 7.1 AU from the Sun, prediscovery images were found showing the comet at 13AU from the Sun. This was quickly noted to be remarkable. The comet was very active at these distances which is unusual.
Many comets have large outbursts at great distance only to quieten down. Hale-Bopp did not. It reached naked eye visibility at 2AU from the Sun and continued to brighten.
If it had passed as close to Earth as Comet Hyakutake (C/1996 B2) did in 1996, then the comet's tail would have spanned the entire sky and it would have been brighter than a full moon. However, it made a closest approach to Earth that would have rendered many lesser comets totally invisible and still spanned half the sky with its two tails, although the longest reaches of the tail were too faint to be visible. Its nucleus was 50km in diameter, nearly three times the size of Comet Halley.
The first naked-eye observation was on May 20, 1996 and a December 1997 naked-eye observation means the comet remained visible without aid for 569 days, or about 18 and a half months. The previous record had been set by the Great Comet of 1811, which was about 9 months.
The comet probably made its last perihelion 4200 years ago, following the 1997 perihelion passage, it will next return to the inner solar system 2380 years later as, no doubt, another spectacular comet.
The cult group, Heaven's Gate choose the appearance of the comet as a signal for their mass suicide.