Comet Hale–Bopp (formally designated C/1995 O1) is a long-period comet that was one of the most widely observed of the 20th century and one of the brightest seen for many decades.
Alan Hale and Thomas Bopp discovered Comet Hale–Bopp separately on July 23, 1995, before it became visible to the naked eye. It is difficult to predict the maximum brightness of new comets with any degree of certainty, but Hale–Bopp exceeded most predictions when it passed perihelion on April 1, 1997, reaching about magnitude −1.8. Its massive nucleus size made it visible to the naked eye for a record 18 months. This is twice as long as the Great Comet of 1811, the previous record holder. Accordingly, Hale–Bopp was dubbed the Great Comet of 1997.
The comet was discovered independently on July 23, 1995, by two observers, Alan Hale and Thomas Bopp, both in the United States.
Hale had spent many hundreds of hours searching for comets without success, and was tracking known comets from his driveway in New Mexico when he chanced upon Hale–Bopp just after midnight. The comet had an apparent magnitude of 10.5 and lay near the globular cluster M70 in the constellation of Sagittarius. Hale first established that there was no other deep-sky object near M70, and then consulted a directory of known comets, finding that none were known to be in this area of the sky. Once he had established that the object was moving relative to the background stars, he emailed the Central Bureau for Astronomical Telegrams, the clearing house for astronomical discoveries, at around 6:50 UT, shortly after his observations.
Bopp did not own a telescope. He was out with friends at Vekol Ranch, about 140 kilometres (90 miles) south of Phoenix (near Stanfield), observing star clusters and galaxies when he chanced across the comet while at the eyepiece of a homemade 17.5-inch Dobsonian telescope owned by his friend Jim Stevens. He realized he might have spotted something new when, like Hale, he checked his star maps to determine if any other deep-sky objects were known to be near M70, and found none. He alerted the Central Bureau for Astronomical Telegrams about two hours after Hale through a Western Union telegram. Brian G. Marsden, who had run the bureau since 1968, laughed, "Nobody sends telegrams anymore. I mean, by the time that telegram got here, Alan Hale had already e-mailed us three times with updated coordinates."
The following morning, it was confirmed that this was a new comet, and it was given the designation C/1995 O1. The discovery was announced in International Astronomical Union circular 6187.
Hale–Bopp's orbital position was calculated as 7.2 astronomical units (AU) from the Sun, placing it between Jupiter and Saturn and by far the greatest distance from Earth at which a comet had been discovered by amateurs. Most comets at this distance are extremely faint, and show no discernible activity, but Hale–Bopp already had an observable coma. A precovery image taken at the UK Schmidt Telescope in 1993 was found to show the then-unnoticed comet some 13 AU (1.9×10^9 km or 1.2 billion miles) from the Sun, a distance at which most comets are essentially unobservable (e.g., Halley's Comet was more than 100 times fainter at the same distance from the Sun). Analysis indicated later that its comet nucleus was 60 ± 20 km (37 ± 12 miles) in diameter, approximately six times the size of Halley's Comet.
Its great distance and surprising activity indicated that comet Hale–Bopp might become very bright when it reached perihelion in 1997. However, comet scientists were wary—comets can be extremely unpredictable, and many have large outbursts at great distances only to diminish in brightness later. Comet Kohoutek in 1973 had been touted as a "comet of the century" and turned out to be unspectacular.
Hale–Bopp became visible to the naked eye in May 1996, and although its rate of brightening slowed considerably during the latter half of that year, scientists were still cautiously optimistic that it would become very bright. It was too closely aligned with the Sun to be observable during December 1996, but when it reappeared in January 1997, it was already bright enough to be seen by anyone who looked for it, even from large cities with light-polluted skies.
The internet was a growing phenomenon at the time, and numerous websites that tracked the comet's progress and provided daily images from around the world became extremely popular. The Internet played a large role in encouraging the unprecedented public interest in comet Hale–Bopp.
As the comet approached the Sun, it continued to brighten, shining at 2nd magnitude in February, and showing a growing pair of tails, the blue gas tail pointing straight away from the Sun and the yellowish dust tail curving away along its orbit. On March 9, a solar eclipse in China, Mongolia and eastern Siberia allowed observers there to see the comet in the daytime. Hale–Bopp had its closest approach to Earth on March 22, 1997, at a distance of 1.315 AU (196.72 million km; 122.24 million mi).
As it passed perihelion on April 1, 1997, the comet developed into a spectacular sight. It shone brighter than any star in the night sky except Sirius, and its dust tail stretched 40–45 degrees across the sky. The comet was visible well before the sky got fully dark each night, and while many great comets are very close to the Sun as they pass perihelion, Comet Hale–Bopp was visible all night to Northern Hemisphere observers. The next time the Northern Hemisphere observed a comet similarly as bright was Comet Tsuchinshan–ATLAS in October 2024.
After its perihelion passage, the comet moved into the southern celestial hemisphere. While Comet Hale–Bopp reached perihelion on April 1, 1997, it achieved its maximum visual brightness between April 2 and April 4, 1997. During this window, the comet reached a peak apparent magnitude of roughly −1.8, making it the brightest object in the night sky other than Sirius. This period of maximum brilliance occurred slightly after perihelion due to a "solar heating lag," where the intensified heat from its closest solar approach continued to maximize the outgassing of its dust and ion tails. At this peak, the comet's dust tail stretched significantly across the sky and remained visible to the naked eye even in light-polluted urban areas.
Inhabitants of the Southern Hemisphere could see the comet gradually fade from view during the second half of 1997. The last naked-eye observations were reported in December 1997, which meant that the comet had remained visible without aid for 569 days, or about 18+1⁄2 months. The previous record had been set by the Great Comet of 1811, which was visible to the naked eye for about nine months.
The comet continued to fade as it receded, but was still tracked by astronomers. In October 2007, ten years after the perihelion and at a distance of 25.7 AU from the Sun, the comet was still active as indicated by the detection of the CO-driven coma. Herschel Space Observatory images taken in 2010 suggest comet Hale–Bopp is covered in a fresh frost layer. Hale–Bopp was again detected in December 2010 when it was 30.7 AU away from the Sun, and in 2012, at 33.2 AU from the Sun. The James Webb Space Telescope observed Hale–Bopp in 2022, when it was 46.2 AU from the Sun.
The comet likely made its previous perihelion approximately 4,200 years ago, roughly the year 2215 BC. The estimated closest approach to Earth was 1.4 AU, and it may have been observed in ancient Egypt during the 6th-dynasty reign of the Pharaoh Pepi II (Reign: 2247 – c. 2216 BC). Pepi's pyramid at Saqqara contains a text referring to an "nhh-star" as a companion of the pharaoh in the heavens, where "nhh" is the hieroglyph for long hair.
Hale–Bopp may have had a near collision with Jupiter in 2215 BC, which probably caused a dramatic change in its orbit, and 2215 BC may have been its first passage through the inner Solar System from the Oort cloud. The comet's current orbit is almost perpendicular to the plane of the ecliptic, so further close approaches to planets will be rare. However, in April 1996 the comet passed within 0.77 AU (115 million km) of Jupiter, close enough for its orbit to be measurably affected by the planet's gravity. The comet's orbit was shortened considerably to a period of roughly 2,399 years, and its next return to the inner Solar System will be around the year 4385. Its greatest distance from the Sun (aphelion) will be about 354 AU, reduced from about 525 AU.