Great Comets

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.

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 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 always exceptions. Comet Hale-Bopp was a large and bright enough to be seen from urban locations, but it actually didn’t get very close to the Sun! On the other hand,  Comet Hyakutake in 1996 was brighter than Hale-Bopp, but it was smaller in size, and it only appeared spectacular because it flew by very close to Earth.

There were several main reasons why Hale-Bopp overshadowed the brighter Hyakutake. First, Hyakutake’s show was only spectacular for a few days, while Hale-Bopp was visible to the naked eye for a record breaking 18 months. Two, the longer period of visibility meant much more media attention (and infamy due to the mass suicides of the Heaven’s Gate cult). Third, internet use was taking off in 1997, and thousands of Hale-Bopp images were posted.

These two were the last great comets visible in the northern hemisphere. When there is one visible over the northern hemisphere, you can imagine the attention it will get! I can already picture the amount of grainy smartphone pictures being posted over social media!

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, calculations are made based on its rate of brightening at their far away position, and how close they will get to the Sun. Often times, if the comet is initially predicted to get really bright, the rate of brightening slows the closer it gets to the Sun, 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 1973/74 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 baby boomers especially remember the media hype. Even though it was visible to the naked eye, it hardly looked impressive and became synonymous with disappointment. Because of the public backlash, astronomers grew wary of raising public awareness over potential great comets, and three years later, Comet West (also known as the Great Comet of 1976) went largely unreported. If you were of sound and mind in 1976 and don’t remember hearing about Comet West, that’s why!

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 predicted to get very bright due to an extremely close fly by of the Sun, with some even going as far as saying, “it could get brighter than the Moon!” But then its rate of brightening slowed down considerably, and when it passed 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, but not “great” show.

Even reliable comets have been duds in the past. While Halley’s Comet is famously known to visit the inner solar system every 75-76 years, it had a very disappointing showing in 1986. Compared to the spectacular appearance in 1910 thanks to its close proximity to Earth, the 1986 return saw the comet much further away than usual, in fact the comet and Earth were on opposite sides of the Sun! Therefore it was simply too dim to see from light polluted areas, and those that did see the comet described it as “just a smudge.”

So when the next great comet appears, it will be one that everyone is talking about! The wait may be long, but it the memories will last a lifetime! 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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