Comet Hale-Bopp was found in 1995, beyond Jupiter's orbit

Comet Hale-Bopp was discovered on 23 July 1995, independently, by Alan Hale in Cloudcroft, New Mexico, and Thomas Bopp near Stanfield, Arizona, while it was still 7.15 AU from the Sun, more than a billion kilometres away and beyond the orbit of Jupiter.

The discovery report, International Astronomical Union Circular 6187, describes a faint, diffuse smudge of about magnitude 10.5 with some central condensation and no tail, drifting west-northwest. Hale was using a 0.41-metre reflector; Bopp was observing alongside Jim Stevens with a 0.44-metre Dobsonian telescope, and the circular notes the comet was found while observing the star cluster M70 in Sagittarius. That makes it the most distant comet amateurs had ever picked up at the time.

Comets usually stay dim beyond Jupiter, yet Hale-Bopp was roughly 1,000 times brighter than Halley’s Comet had been at the same distance, because its icy nucleus was enormous: about 60 km across, against a typical comet nucleus of no more than about 16 km, and some five times the size of the object thought to have wiped out the dinosaurs.

That size paid off. Known as the Great Comet of 1997, Hale-Bopp passed closest to the Sun on 1 April 1997 and stayed visible to the naked eye for about 18 months across 1996 and 1997.

It will not be back soon. NASA puts its orbit at about 2,534 years, so nobody alive today will see it return. Its previous pass, some 4,200 years ago, came when Egypt’s pyramids were still new and the Epic of Gilgamesh had not yet been written.