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Great comets are celestial events that wow the masses! But they are rare events that are not always predictable, and one can go decades before seeing one in their life time!

I was nine years old in 1997 when comet Hale-Bopp put on a show worthy of being called “Comet of the Century.” I remember it being so visible at night that you had to be living under a rock to miss it! Even from cities, the two different colored tails were obvious, and I remember it looking like it was going “backwards” in the sky during its peak (learning a comet’s tail always points away from the Sun in the process).

It was that comet that made me want a telescope for Christmas. While I definitely got that telescope, there hasn’t been a great comet visible in the northern hemisphere since! Meanwhile, the southern hemisphere has had two! So It’s been over two decades for me!

So… Where Are They?

The Solar System is filled with comets that orbit our Sun. However, nearly all of them are not only tiny, but are also faint. Because the vast majority of comets that enter the inner Solar System are very faint, there can be a comet in the sky and we wouldn’t know it! Comets get brighter when they get closer to the Sun, and their tails get longer as well!

However, most comets that get close enough to the Sun are often lost in the Sun’s glare when they’re at their brightest. If they do reach naked eye visibility before or after, they’re still too dim to see from bright cities – thus you need to go to a dark location (that is IF the moon isn’t washing them out). There is also a distance factor as well, because many potential great comets are often too far away from Earth to put on a good show.

What makes a Great Comet?
content_Comet_McNaught_at_Paranal
Comet Mcnaught was spectacular in 2007… from the southern hemisphere.

One, a great comet is not only exceptionally bright, but is noticed by casual observers who aren’t looking for them. These are the typical characteristics:

  1. It’s big and very active! – A comet with a large and very active nucleus will outgas a lot of material when it gets closer to the Sun. The bigger and more it ejects, the better chances it will be a good show from Earth!
  2. It gets close enough to the Sun! – Most comets do not get closer to the Sun than Earth’s orbit. Comets that get closer than this distance get more heat from the Sun, and therefore eject more material! Sometimes they’re bright enough to see during the day!
  3. It gets close enough to Earth! – This should be a given, as the closer to Earth it is, the easier it is to see!

But there are exceptions. Comet Hale-Bopp was huge but actually didn’t get very close to the Sun. Still, it was very bright and it was visible in the sky to the naked eye for a record breaking 18 months!  Comet Hyakutake in 1996 was actually quite small, but because it got so close to Earth it appeared spectacular.

These two were the last great comets visible in the northern hemisphere.

So When Is the Next One?

That is the thing… nobody knows! Comets are still being studied and their brightness is hard to predict. The bigger comets typically have long orbital periods that last for thousands of years! On average, great comets occur near Earth once every decade, with the last one happening over the southern hemisphere in 2011.

Whenever a potential great comet is discovered from far away, many times calculations are made based on the rate of brightening and how close they will get to the Sun. Of course, even if the comet is initially predicted to get really bright, often times the rate of brightening slows, and by the time it is supposed to be putting on a show, it goes out with a whimper.

Two big examples are comet Kohoutek from 1974 and Comet ISON from 2013.

Fig5-CometWest_Kohoutek-e1426728195200

The one on the left is Kohoutek – the so called predicted “Comet of the century.” The one on the right is West, a comet that went largely unreported because scientists were afraid of it being “another Kohoutek”  in terms of backlash. 

Kohoutek was predicted to be the “comet of the century,” and even though it was visible to the naked eye, it hardly looked impressive and became synonymous with disappointment. Because of the media backlash, astronomers grew wary of raising public awareness over potential great comets, and Comet West, also known as the Great Comet of 1976, went largely unreported by the media.

Comet_ISON_disintegrates_node_full_image_2

This is a time lapse from SOHO – you see ISON approach the Sun from the bottom, and then when it reemerges, you can see it disintegrate as it moves further toward the top.

ISON was supposed to get very close to the Sun, and therefore get very bright. But then its brightness increased less quickly than expected, and after it made its close approach to the Sun, the comet disintegrated. Had ISON stayed intact, it would have been well placed for northern observers and still could have put on a decent show.

When the next great comet appears, it will be one that everyone is talking about! It’s only a matter of time when Earth is finally treated to one, but it is definitely something you should make sure you see on your astronomy bucket list!

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