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Comets: Everything you need to know about 'dirty snowballs' of space

Comets are some of the most spectacular sights in the night sky. Here's everything you need to know about them.

Comets: Everything you need to know about 'dirty snowballs' of space
Illustration of a shooting star, meteor. ikonacolor/iStock

Comets have been in the news a lot lately, largely because of the recent Netflix hit Don't Look Up, which centers on the threat of a comet impact as an allegory for human inaction on climate change.

But comets are some of the most spectacular sights in the night sky, and so have been a source of both dread and fascination for millennia.

It's only been in the last few hundred years though that we've started making sense of them and understanding what they are, where they come from, and how they interact with the rest of the solar system.

So, what are comets then? Let's dig in and find out all we know about these "dirty snowballs" of the solar system.

What is a Comet?

The basic definition of a comet, according to NASA, is a "cosmic snowball" of frozen gases, rock, and dust that orbits the Sun.

They are remnants left over from the formation of the solar system some 4.6 billion years ago, ranging from a few miles to tens of miles wide when in their "frozen" state.

As a comet orbits the Sun, however, these gases heat up and produce a huge luminous head of sublimated gas that can be larger than whole planets. These gases flow behind the comet as it orbits closer to the Sun and leaves brightly illuminated trails that can be millions of miles long.

When seen from the surface of the Earth, a comet's "tail" can stretch for some distance across the night sky or just above the horizon during the twilight hours before sunrise and after sunset.

What is Special about Comets?

Comets: Everything you need to know about 'dirty snowballs' of space
Comet C/2014 Q2 Lovejoy is a long-period comet discovered in 2014. | Source: John Vermette/Wikimedia Commons

Comets mostly fascinate us because of their brightly colored tails, leading to all kinds of ancient beliefs about their true nature.

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Among societies where the night sky had special cultural resonance (which is pretty much every human society we know of) and where intricate charts were constructed to map the heavens, having a comet appear suddenly in the sky was a very special event for both astronomers and regular people alike.

We've learned much more about the true nature of comets than our ancestors could ever have hoped to understand, but they are no less fascinating for us because of that.

In fact, because of our scientific advances, we are able to see comets in ways our ancestors never could and we have even landed a probe on one to better understand its composition (though the probe couldn't manage to properly anchor itself to the comet). 

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Comets also rarely pass through the outer solar system, much less the inner solar system near to us, so we take special notice of them when they do come our way.

There are far more asteroids near us than there are comets, though it is believed that there are ultimately many billions more comets out there than there are asteroids.

Why are Comets Called Dirty Snowballs?

Comets: Everything you need to know about 'dirty snowballs' of space
Source: Davewhite7/Wikimedia Commons

Comets are often referred to as "dirty snowballs" as a quick way to describe them to non-astronomers who might not know all the facts about them. This is mostly owing to the composition of a comet, thanks to the way in which it formed and where.

While comets are made mostly of ice formed after gases in the early solar system were pushed out from the Sun by solar winds into the outer part of the solar system, where it is considerably colder.

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At these distances, gases like hydrogen, nitrogen, oxygen, are able to freeze, and so began to crystalize and form chunks of ice. While still orbiting the Sun in an accretion disk full of rocks, dust, and gases, these chunks of ice accumulated other non-gaseous material as they went, essentially mixing in dust and rock into their growing body of ice.

Solid, rocky material from the early solar system that didn't become planets, moons, or asteroids eventually got rolled up into comets, making them "dirty".

How is a Comet Different from an Asteroid?

The main difference between a comet and an asteroid is its composition.

Comets are predominantly ice with some rocky material and dust weaved throughout as well as sprinkled along its surface. Asteroids, meanwhile, are almost entirely made of solid rock and metals, with very little ice added into the mix.

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Asteroids simply orbit too close to the Sun, generally speaking, for a considerable amount of ice to have accumulated in an asteroid, though ice can exist deep beneath the surface. In especially large asteroids like Vesta or Ceres, the amount of subsurface ice might be considerable, though likely not nearly enough to make up a majority of its mass.

The other major difference between a comet and an asteroid is where they originate.

Nearly all of the asteroids in the solar system are found in the asteroid belt between Mars and Jupiter, or as trojan asteroids of Jupiter (which means they co-orbit the Sun with Jupiter at one of Jupiter's five Lagrange points).

Comets, meanwhile, mainly originate in the Kuiper belt, which is a belt of ice and rocky material beyond the orbit of Neptune, or in the Oort Cloud, which is a hypothetical sphere of icy debris that surrounds our solar system but which is still under the waning influence of the Sun's gravity.

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The Oort Cloud, if it exists, might stretch between 10,000 astronomical units (AU; about 93 million miles, or the distance from the Earth to the Sun) or as far as 100,000 AU, which is nearly two light-years away.

Another major difference between comets and asteroids is their relative sizes. While very large asteroids like Vesta can be hundreds of miles across, most asteroids are not nearly as large and can be as small as a couple of dozens of meters across.

Comets: Everything you need to know about 'dirty snowballs' of space
The 'megacomet' Bernardinelli-Bernstein is believed to be about 100km to 200km in diameter | Source: NOIRLab/NSF/AURA/J. da Silva

Comets, meanwhile, are typically about 10km wide but can be as large as 100km wide, as in the case of recently discovered comet Bernardinelli-Bernstein. There might be even larger comets in the Oort Cloud, but until they get knocked loose and into the inner parts of the solar system, we have no way of knowing that they are there.

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Once a comet gets knocked into the solar system proper, either by a passing star or some other cause, the Sun's gravity pulls it more or less directly towards it, producing highly elliptical orbits.

Asteroids have spent 4.6 billion years orbiting pretty close to the Sun and so have fairly circular orbits much like planets have. A comet's orbit, meanwhile, looks like a nearly flattened circle, and in the case of some very long-period comets, their "orbit" isn't an orbit at all.

If their approach is "steep" enough and they approach close enough to the Sun, the slingshot effect of the Sun's gravity can send a comet flying out of the solar system entirely. That has a lot to do with the composition of the comets themselves as well, since very close approaches to the Sun will sublimate a lot of a comet's ice, thereby reducing its mass and making it harder for it to escape the solar system.

This last part actually leads us to the final major difference between comets and asteroids: "active" comets have an expiration date. While comets are leftover from the formation of the solar system, all but a few comets orbit far enough away from the Sun that they can stay frozen and remain largely unchanged.

Active comets though, those that get knocked into closer orbit with the Sun, lose mass every time they pass through the solar system. Bernardinelli-Bernstein, with its 100km diameter, is very likely a "newly arrived" comet that has only made a pass or two through the solar system before its current visit.

Halley's comet, meanwhile, is about 15km in diameter. Given its orbital period of about 76 years, it has passed through the inner solar system nearly 30 times at least, with records of its appearance possibly going back as far as 466 BCE.

If Halley's comet began its active life in the Oort Cloud, then it would have been substantially larger during its initial passes before having most of its mass boiled away by the Sun. It will likely spend about 7,300 years as an active comet over its entire lifetime, of which about 2,250 years have already passed. Eventually, it will simply evaporate away in the heat of the Sun, which is the fate of most comets, but not all. 

As comets pass close to the Sun and their ice sublimates, the dust on their surface may be left behind over time, leading to a buildup on the surface that actually reflects or absorbs sunlight away from the icy parts of the comet. In time, this buildup might entirely protect the ice inside it at which point the comet is no longer considered to be active.

In either case, once a comet "activates", the clock is ticking on its remaining life. Meanwhile, unless an asteroid gets eaten up by the Sun or collides with a planet or moon, it will remain largely unchanged.

What Are the Four Types of Comets?

There are generally four types of comets, with a further division for periodic comets between short and long periods.

Periodic comets (designated as P-comets) are subdivided into two categories: short- and long-period.

Short-period comets have tighter orbits that put their observable period at under 200 years and generally orbit as far as Neptune, but not as far as the Kuiper belt. Long-period comets have an orbital period greater than 200 years and generally orbit beyond Neptune and into the Kuiper belt. Some especially long-period comets originating in the Oort Cloud can have periods of millions of years.  

Periodic comets are generally defined as those that have been observed more than once during their perihelion passage (that is, their closest approach to the Sun). These are also called Halley-type comets, since the first comet ever identified, by Edmond Halley, is a comet of this type and is also fairly typical of short-period comets.

Non-periodic comets (designated as C-comets) are long-period comets that generally have an orbital period of 1,000 years or greater but which don't have velocities high enough to exit the solar system. Due to their high eccentricities and the angle of their semi-major axis, they may have gravitational interactions with planetary bodies and minor and dwarf planets in the major asteroid and Kuiper belts that affect their orbital trajectories. This produces orbital periods that vary with each pass, making them all but impossible to predict or identify in historical comet records.

Comets without a meaningful orbit are those that have parabolic or hyperbolic orbits that typically see the comets exit the solar system. By definition, these comets will only pass through the solar system once before being slingshot by the Sun's gravity out into interstellar space.

These comets are believed to originate in the Oort Cloud, and since the Oort cloud is believed to entirely encircle the solar system, they can approach the Sun from any direction and typically do so with retrograde trajectories.

Lost comets are comets who had a significant and identifiable record of perihelion approaches in the past, but which have since failed to reappear as expected, likely due to their disintegration at the end of their life.

Why Were Comets Considered Bad Omens in History?

Comets: Everything you need to know about 'dirty snowballs' of space
The Bayeux Tapestry in France, which records the appearance of Halley's Comet in 1066 ahead of William the Conquerer's invasion of England. | Source: Wikimedia Commons

Historically, the night sky has been an object of endless human fascination. In the ancient world, astronomy was closely connected to astrology and was considered one of the most serious sciences of the ancient world.

For many in the ancient world, the night sky provided insight into the future or the intentions of the gods, and so disturbances of the regular order of things in the night sky tended to catch the attention of astrologers.

Nothing was more disruptive then than a comet, which appeared suddenly and seemingly disconnected from anything else in the sky. Naturally, people here on Earth tried to make sense of what was streaking across the night sky and often looked to Earthly matters for an explanation.

This often involved people taking comets to be a type of omen, either good or bad. Famously, in 1066, Halley's Comet made its perihelion approach and left a visible trail in the night sky just as William of Normandy was set to invade England from France.

To the Anglo-Saxons who were on the receiving end of William's invasion, Halley's Comet presaged calamity. Meanwhile, for the Norman invaders, the comet's tail appeared to light the way to England, giving a heavenly mandate for their invasion. In fact, it inspired a rallying cry among William's troops: Nova stella, novus rex, or, "a new star, a new king." 

In many ways, where you stood in relation to events would largely determine how you felt about the sighting of a comet, historically speaking.

"Comets have a long history, usually as omens and bearers of bad news," Woody Sullivan, a professor of astronomy at the University of Washington, said in 1997 after the appearance of comet Hale-Bopp.

But on the other hand, the death of Julius Caesar was marked by a comet and this was taken by the Romans as a sign of his divinity. And Napoleon made a fuss about the appearances of comets and some of his early military victories.

"'Awe-full' might be a better way to describe the impact of comets. It is often taken to mean dread, but it also can indicate greatness."

So why are comets so associated with disaster and calamity in history? Part of this might be who is writing the histories. If you are a royal historian writing up current events and you see a comet in the sky, the odds are that you're going to look for what has changed, or what did change after the fact.

Typically, like many in the modern world, historians often ignored good news (at least good news that was not related to their ruler) and focused on the bad news of the day. There's a saying in journalism that no one covers the planes that land safely, and the same could be said of historians.

Now, if you're a royal historian, possibly one of the few literate people in the kingdom or empire, and you're reading other historians associating this comet or that comet with a plague or famine or royal death, seeing a comet in the night sky might send you off looking for a negative event that happened at around the same time. And if you want to find something wrong, you will always find something. So, historians would likely then blame whatever it was on the comet, which then gets passed down to posterity. Never mind that royals die all the time when comets aren't present and that famines and plagues strike more often in the absence of comets than under them.

It wasn't until the 16th century, really, that comets started to lose the dread that many had associated with them. By the time of the Scientific Revolution, the learned class at least had come to understand them as celestial phenomena that could be studied rather than feared.

And by the time Halley identified his comet in the historical record and predicted its return, the mundanity of comets became well-established, even if they were no less fascinating to look at. 

Which Comet is the Rarest?

Comets: Everything you need to know about 'dirty snowballs' of space
Comet C/2011 L4 PanSTARRS in 2013 | Source: Astronomy Education Services/Gingin Observatory via NASA

It really does depend on what you mean by rarest. Comets with no discernable orbits only pass through the solar system once before they are shot out of the solar system by the slingshot effect of the Sun's gravity. Technically, those would be one-of-a-kind comets that you can only see once and never again, so any of these could count as the rarest comet ever. 

However, if we're talking about long-period comets, then comet C/2012 S4 PanSTARRS is likely the longest period comet we know of given that it has the largest listed aphelion of any object on the JPL Small-Body Database. However, its listed aphelion distance of about 500,000 AU, or about 8 light-years from the Sun, was reached as a "generic near-perihelion unperturbed two-body solution that assumes the Sun and comet are the only two objects in the Universe."

This is almost certainly not a comet that we will ever see,  since any comet beyond 2 light-years is generally considered to have broken free of our solar system.

That won't necessarily be the case with C/2012 S4 PanSTARRS, since the determination for whether it will be ejected from the solar system largely depends on the epoch, or reference time, used to calculate its orbit.

Some calculations show that C/2012 S4 PanSTARRS will not be ejected from the solar system after all, in which case its orbital period might be more than 100 million years.

What Comets Come Nearest to Earth?

Comets: Everything you need to know about 'dirty snowballs' of space
Source: E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria/Wikimedia Commons

Comets, like asteroids, can make very close approaches to Earth, and have even hit the Earth in the past.

In the modern era of comet observation, which is classified in this case as after 1950, five comets have passed within 14 lunar distances (LD) of the Earth, with the closest being P/SOHO 5 (1999 J6), which passed about 4.7 LD from Earth on June 12, 1999.

The comet P/PANSTARRS 51 (2016 BA14) passed as close as 9.22 LD on March 22, 2016, while 289P/Blanpain passed within 9.68 LD on December 11, 2003.

On May 11, 1983, comet C/IRAS-Araki-Alcock (1983 H1) passed within 12.14 LD of Earth, and on March 21, 2016 (that must have been a rough week), 252P/LINEAR passed within 13.93 LD of Earth.

Will these comets come as close to Earth again on their next pass through the solar system? We don't know, but it is so unlikely as to be dismissed.

Comets simply don't sync up with Earth's orbit in such a way that we can say which comets come "closest" to us on a regular basis. All we can say is which have come closest in the past and, hopefully, identify those that will come close to us in the future.

Where is Halley's Comet Right Now?

Comets: Everything you need to know about 'dirty snowballs' of space
Source: NASA, ESA, and Z. Levay (STScI)

Right now, Halley's Comet is approaching its aphelion (furthest distance from the Sun), some ways out beyond the orbit of Neptune.

In about 40 years' time, roughly July 2061, Halley's Comet will reach its perihelion, and will actually be situated between the Earth and the Sun at our closest approach to it.

This might produce a spectacular sight of it along the horizon at twilight though since we will be closer to it at this point than at any other time during its pass through the inner solar system.

Will Halley's Comet Hit Earth?

Comets: Everything you need to know about 'dirty snowballs' of space
Halley's comet passing Earth in 1986 | Source: NASA/W. Liller

Not any time soon, if ever. In addition to its orbit being tilted by 18 degrees off the plane of the Sun's ecliptic, Halley's comet passes over Earth's orbit at only two points. Even then, it would be millions of miles above us when it passed. That said, lots of things can influence the orbit of comets, and it's anticipated that Halley's comet has many dozens of passes through the solar system left in it.

Could planetary interactions down the road add up to enough of a change in its orbit that it threatens Earth? That is possible, but it is highly unlikely.

Ok, Fine, Sure. But What Would Happen If Halley's Comet Hit Earth?

Comets: Everything you need to know about 'dirty snowballs' of space
Source: Max Pixel

See, we know what you're here for.

Assuming that something nudged Halley's Comet into a collision course with Earth, there are some factors to consider. Halley's Comet, like most comets, is traveling incredibly fast. Faster than any known asteroid, in fact. When it last passed through in 1986, at perihelion Halley's Comet was traveling at 122,000 miles per hour.

That's about 54 kilometers a second, and with an average density of about 0.6 grams per cubic centimeter at roughly 11 kilometers wide, the impact would be in the mass extinction territory, though not quite in the planet killer class you'd get if the asteroid Vesta hit the Earth, for example.

Still, it wouldn't be good, not by any stretch of the imagination. Given the size of Halley's Comet, it wouldn't matter whether it hit land or the ocean, the result would be more or less the same.

With a comet that large traveling at that speed, the atmosphere would put up almost no resistance whatsoever. The ocean would be as effective a cushion as a sidewalk puddle is against a large stone.

Let's assume the comet hits land, though. You'd end up with a final impact crater about 85 kilometers wide and just over a kilometer deep, with a more narrow, but deeper, intermediate crater of molten material immediately after impact.

The impact would vaporize or melt about 1,870 cubic kilometers of material, half of which would remain in the crater, the other half ejected out into the atmosphere.

At a distance of 1,000 kilometers, the fireball would appear about 21 times larger than the Sun. At that distance, clothing, paper, grass, and plywood construction ignites from the thermal radiation, and anyone exposed (which would be everyone, whether they were inside or not), would suffer third-degree burns over much of their body.

The impact 1,000 kilometers away would generate an earthquake of 10.1 on the Richter Scale, which is stronger than any earthquake ever recorded in human history.

About 50 minutes after the impact, an air blast would arrive with a maximum wind velocity of 591 miles per hour—enough to knock down multi-story construction, blowdown bridges, level single-family homes, and flatten about 90 percent of the trees while stripping the remainder of their branches and leaves.

The ejecta thrown into the air would block sunlight for an extended period of time, enough to kill off much of the plant life on Earth. Anything that depended on these plants would soon die off after that, creating a mass extinction event very similar to that which ended the reign of the dinosaurs 66 million years ago.

It's not a foregone conclusion that all larger life forms would die though. After all, mammals survived a similar calamity when the non-avian dinosaurs were wiped out and went on to thrive. Maybe insects would come up after us and form entire insectoid civilizations in our absence. It's entirely possible. 

When Was the Last Time a Comet Hit the Earth?

Comets: Everything you need to know about 'dirty snowballs' of space
Source: Pixabay

It is generally hard to tell once an asteroid or a comet hits the Earth what exactly it was before it hit.

While comets are largely made of ice, they do have a lot of other material in them that we would similarly see in asteroids, so analyzing the geological record for telltale signs of an impact wouldn't necessarily distinguish between a comet and an asteroid.

What's more, smaller comets or comet fragments are more likely to "airburst" in the atmosphere, given their composition, so an impact crater might not be left behind as evidence, though such airbursts can be just as devastating as a surface impact.

In short, it's hard to say specifically which impacts in the past were comets or asteroids, so it's hard to tell when the last comet impact occurred.

It's suspected that a comet or several comet fragments—known as the Clovis Comet—impacted around 13,000 years ago, triggering the Younger Dryas period that wiped out many of the remaining megafaunas on Earth and forcing humans to adopt agriculture in response.

While that theory has gained some evidence in its favor in recent years, it isn't entirely accepted at this time.

What if a Comet Hit the Moon?

Comets: Everything you need to know about 'dirty snowballs' of space
The far side of the Moon | Source: NASA/Goddard/Arizona State University

A comet hitting the Moon wouldn't be any less energetic than if it hit Earth, though it would obviously have different effects.

Let's use Halley's Comet as an example. Given that, we know the orbit of Halley's Comet very well, we'll likely have predicted the impact long in advance. At a minimum, it would be an incredible light show, especially if it occurred during a full moon when the Moon is on the Earth's night side and the impact wasn't on the completely opposite side of the Moon.

As for the effect it would have on the Moon itself, here's what would likely happen.   

For one, there is no life on the Moon, so there is no threat of an extinction-level event or anything like that. Also, there is effectively no atmosphere, so you wouldn't get the kind of shock wave wrapping around the Moon as you would here on Earth.

It would still be devastating for the Moon, selenologically, but not so much that it would fundamentally change anything about the Moon's orbit, rotation, or composition. 

The Moon has a similar chemical composition to the Earth, so the impact dynamics would be more or less the same.

The amount of energy released by the impact wouldn't be any less than if it impacted Earth, so it would still eject about 1,900 cubic kilometers of material, though much more of it would exit the crater, given the Moon's significantly lower escape velocity.

The final impact crater on the moon would likely be about 55 kilometers across and just over a kilometer deep. 

For context, this wouldn't be anywhere close to the largest impact crater on the Moon, the South Pole-Aitken basin. 

Formed about 3.5 billion years ago, the South Pole-Aitken basin is about 2,500 kilometers wide and 13 kilometers deep. That means that depending on how it struck, Halley's Comet could fit entirely inside this one crater basin upon impact.

Still, the impact would be phenomenal, and much of that material would eventually make its way to Earth since Earth's gravitational pull is much greater than the Moon's.

How much material would depend on which side of the Moon the comet hit, and if the near side of the Moon was struck, then a considerable amount of material would rain down on Earth. Most of this material would burn up in the upper atmosphere, producing a spectacular meteor shower, but the larger pieces could pose a much more serious threat.

The meteor that exploded over the Russian city of Chelyabinsk in 2013 was only about 20 meters across but generated an explosion equal to about 500 kilotons of TNT, or about 35 Hiroshima bombs.

It is not known if the Tunguska meteor was a larger meteor that passed very, very close to the surface and then skimmed off or a smaller meteor that actually hit, but it generated a 12-megaton explosion that is about 20% less than the yield of the Castle Bravo nuclear device detonated by the US Military in 1954, the largest nuclear device ever detonated by the United States.

Now, could fragments of the Moon that large hit the Earth after such an event? Yes, but they would be rare. Also, speed is an important factor in determining the final impact energy of an object, so the speed of those larger chunks would play a major role in determining the ultimate energy their explosions in our atmosphere release.

Now, if the impact is on the far side of the Moon or along the "sides" of the visible disc that we can see during a full moon, more of that material is going to fall back onto the Moon's surface, especially the heavier pieces, so what reaches us might largely or entirely burn up in the atmosphere.

The resulting crater would be molten for some time, providing a stunning contrast with the rest of the Moon's surface, and might even be visible with the naked eye.

Can a Comet Hit the Sun?

Comets: Everything you need to know about 'dirty snowballs' of space
The Parker Solar Probe spotted the comet NEOWISE on July 5, 2020, shortly after its closest approach to the Sun | Source: NASA/Johns Hopkins APL/Naval Research Lab/Parker Solar Probe/Brendan Gallagher

Absolutely, a comet can crash into the Sun. Depending on its orbital characteristics, speed, and its mass, a comet that comes too close to the Sun could end up smacking right into the super-hot atmosphere of the Sun and burn up.

“There’s no reason for it not to happen,” NASA's Karl Battams told New Scientist in 2015. “The Sun is a pretty big target, and there’s enough stuff around flying in the solar system.”

There is a lot cutting against a comet flying into the Sun though, at least to the point of "hitting" it.

Remember, comets are mostly made of different ices, so flying directly at the Sun is going to boil away a lot of that ice. Depending on the comet's composition, this could lead to the comet breaking apart entirely long before it even gets close to the Sun.

Even if something did survive the outer reaches of the Sun's corona, the temperatures there are enough to vaporize rock, so only the largest comets would really have a chance of even coming close to hitting the Sun.

Are There Comets in 2022?

So, if this has whetted your appetite for comets, you're in luck. There are going to be some exciting comets to see in 2022, though almost none of them will be visible with the naked eye. However, the comet is designated C/2017 K2 (PanSTARRS) will make its way into the inner solar system this year, approaching closest to Earth on July 14, 2022. Another PanSTARRS Comet, C/2021 O3, might be in the binocular range in late April or early May. At the very least, for most comets, you'll need binoculars and a dark sky with a clear view of the horizon at twilight, since most are really only visible as they approach the Sun.

Whether any given comet will be visible is always a difficult question to answer, since you need to know the comet's composition and how much ice is exposed to know whether there will be a visible tail. In any event, there will be plenty of opportunities to do some comet watching in the months and years ahead and enjoy these truly spectacular celestial marvels. 

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