Great Comets in History: Hale-Bopp, Hyakutake and NEOWISE

What makes a comet "great"

There is no official rating. NASA's Jet Propulsion Laboratory (JPL) describes a great comet as one that is visually impressive to the naked eye. It says a comet gets there in one of two ways. It may pass very close to the Sun, which releases huge amounts of gas and dust. Or it may pass close to Earth, which makes its tail easy to see. Either way, the comet has to be viewed from a dark sky, and JPL notes that the rating is subjective.

JPL keeps a table of great naked-eye comets. In recent decades it lists 1976 (West), 1996 (Hyakutake), 1996 to 1997 (Hale-Bopp) and 2007 (McNaught). The gaps show how rare the events are.

Hyakutake, 1996

NASA calls Comet Hyakutake the "Great Comet of 1996." It was discovered less than two months before its closest approach to Earth in late March 1996, when it came within about one-tenth of the Earth-Sun distance, or about 15 million km (9 million miles). NASA says it was the brightest comet to grace Earth's skies in 20 years.

Photos show a green-tinted coma and a long blue ion tail, and NASA says the ion tail seemed to have just gone through a magnetic disconnection event. The nucleus is about 5 km (3 miles) across. After its visit, planetary gravity sent the comet onto a path that NASA says will not bring it back for about 100,000 years.

A spacecraft also caught Hyakutake's tail by accident. On May 1, 1996, the Ulysses probe unexpectedly crossed it. Researchers concluded that the tail extended about 500 million km (over 300 million miles) and called it the longest comet tail ever recorded. The full story of tails, including this crossing, is in the guide to ion, dust and anti-tails.

Hale-Bopp, 1995 to 1997

Because Hale-Bopp was found so far from the Sun, observers could follow it for many months as it brightened. It reached perihelion, its closest point to the Sun, on April 1, 1997. NASA says its nucleus was about 60 km (37 miles) across. ESA gives "perhaps 40 km," so size estimates vary. Its orbit takes about 2,534 years.

ESA's SOHO spacecraft showed how large the comet's invisible parts were. A cloud of hydrogen around it was about 100 million km (62 million miles) wide, roughly 70 times the width of the Sun. Water vapor poured out of the comet at nearly 50 million metric tons per day at the peak. NASA's page also describes material thrown off in a pinwheel pattern.

The parts that made Hale-Bopp so bright, a huge coma, a hydrogen cloud and two tails, are described in the overview of what a comet is.

NEOWISE, 2020

Comet NEOWISE was discovered on March 27, 2020, by NASA's NEOWISE spacecraft, the space telescope it is named for. Perihelion came on July 3. NASA says its nucleus is about 5 km (3 miles) across and that it will not return for about 6,800 years. Its closest approach to Earth was on July 22, at about 103 million km (64 million miles). NASA called that a harmless distance.

It became the brightest comet to grace Northern Hemisphere skies in decades. A NEOWISE co-investigator said it was the brightest comet since Hale-Bopp in the mid-1990s. It was seen from North America and Europe, and from orbit: an astronaut on the International Space Station took more than 340 photos on July 5. From mid-July it appeared after sunset below the Big Dipper in the northwest sky, and NASA advised viewers to get away from city lights and use binoculars or a small telescope if needed. Never point binoculars or a telescope at the Sun without a proper solar filter made for that purpose. The guide to seeing a comet covers the planning.

NASA's satellite data showed a dust tail and possibly two ionized-gas tails, and its tails reached hundreds of thousands to millions of kilometers.

A forecast that stayed uncertain

Not every promised comet delivers. In October 2024, NASA's skywatching team wrote about Comet Tsuchinshan-ATLAS. It was closest to the Sun in late September and closest to Earth on October 13, and it was expected to be visible low in the west after sunset from October 14 to 31. NASA said it might be the brightest comet of the year and that, if its tail was well lit, it could be seen without a telescope. It also said predictions for its brightness were uncertain and that comets have a way of surprising us. Its orbit takes about 80,000 years, and it arrived from the distant Oort Cloud.

The names tell part of the story. Hale-Bopp carries the names of its two discoverers, and NEOWISE carries the name of the spacecraft that found it. Each also has a formal designation, such as C/1995 O1 for Hale-Bopp. The rules are in the guide to how asteroids and comets get their names.

Common mistakes and myths

"A great comet comes on a schedule." None of these did. Hale-Bopp's orbit lasts about 2,534 years, and NEOWISE's about 6,800.

"A close comet is a danger." Hyakutake came within about 15 million km, and NASA called NEOWISE's 103 million km pass a harmless distance. Neither threatened Earth.

"Every bright comet is a great comet." JPL says the rating is subjective and that a comet must be viewed from a dark sky to count.

"Photos show what the eye sees." Long exposures show far more tail and color than the eye does. The JPL description of a great comet is based on what a person sees without aid.

"Hale-Bopp was a quick flash." It was visible to the naked eye for about 18 months.

Frequently asked questions

How often does a great comet appear?

Rarely. JPL's table lists entries in 1976, 1996, 1996 to 1997 and 2007 in recent decades. The list is subjective, so counts vary.

Will Hale-Bopp come back?

Yes, but not for a very long time. NASA gives its orbit as about 2,534 years.

Was NEOWISE the brightest comet ever?

No. NASA says it was the brightest to grace the Northern Hemisphere in decades and the brightest since Hale-Bopp, which is a narrower claim.

How can a skywatcher prepare for the next one?

Follow NASA's monthly skywatching guide, find a dark site, and expect surprises, since forecasts can miss. Never point binoculars or a telescope at the Sun without a proper solar filter.