There are many things that travel the cosmos: runaway stars, asteroids, planets, galaxies, space probes like the Voyagers, and comets. These interstellar travelers have long filled humans with fear, panic and dread as well as being more auspicious in their tidings.
If you ask most people to tell you about a comet they know of, chances are they're not going to come up with Comet Ikeya–Seki (C/1965 S1) or Comet West (C/1975 V1). They might, if you're dealing with someone like me, bring up Comet Hale–Bopp (C/1995 O1), but almost everyone will bring up the "most famous" comet, Comet Halley, officially designated 1P/Halley... Side note, the day I wrote this, January 12, 2022, I asked every staff member here at SMLI and Halley's Comet was the only one they could name. Told ya!
At the heart of all comets is a relatively small body composed of ice and rock that is loosely held together by gravity. Called a nucleus, this small body typically dwells deep in the solar system in something called the Oort Cloud (pronounced "ort") or the Kuiper Belt (pronounced "Kai-per"). Deep within the Kuiper Belt lurks things like Pluto and Arrokoth, the bodies visited by the New Horizons spacecraft. These "dirty snowballs" are remnants from when the solar system was created, thrown deep into the cold deep of outer space but still held onto by the sun's immense gravity. The core of these objects are known to amalgamation of rock, dust, water ice, and frozen carbon dioxide (dry ice), carbon monoxide, methane, and ammonia.
Typically, these objects have highly elliptical orbits that take them dangerously close to the sun before being flung back into the depths from whence they came, or sometimes, they crash directly into the sun! The orbital period of these interlopers often range from dozens of years to many tens of thousands of years! Whatever causes them to begin their journey towards the sun, as a comet races towards sunward, the volatiles start to warm, the ice starts to sublimate, (change from a solid to a gas) and comet starts to become visible. Once this warming starts to happen, a coma, or atmosphere, forms around the nucleus, hiding it from sight, but causing the bright head that we typically associate with a comet, as seen above in the visit from Halley's Comet in 1986 above or Comet NEAT's visit in 2002 below.
But these visitors were not always seen as a thing of beauty or something to be studied. As far back as 1910 when Halley's Comet returned, and deeper into humanity's past, these Inner-Solar System intruders were seen as a source of fear and dread. At one point in their collective history, comets were thought of as atmospheric phenomena and ill omens. They were thought to be harbingers of death of kings and other nobles, even bringing catastrophes to the lands they flew over! In fact, during the return of Halley's Comet in 1910, when Cyanogen was detected in the tail of the comet, French Astronomer Camille Flammarion claimed that the gas "would impregnate the atmosphere and possibly snuff out all life on the planet"; Cyanogen is a colorless and highly toxic gas with a pungent odor. This led to panic buying of gas masks and "anti-comet pills", among other quack ideas! Thankfully, the Cyanogen that the Earth did pass through was so incredibly diffuse that nothing ever came of it, except the Orionid meteor shower, which we see each October.
Comets dazzle the night sky with their tails, sometimes stretching over six times the distance of a full moon! But what causes them is a combination of the sun and the stuff coming off of the comet as it's warmed up. Because chemicals like carbon dioxide sublimates, it is pushed away from the nucleus by the pressure from the solar wind, creating the tail you see in the photos so far. The volatiles are vaporized and pushed away from the coma, always pointing away from the sun. However, comets, being the mysterious objects that they are, can often grow two tails as with the case with the infamous Hale-Bopp comet of 1996-1997.
Hale-Bopp showed itself at an astounding distance of 7.2 A.U. (where 1 A.U. or Astronomical Unit, is equal to the distance between the Earth and Sun); this placed it in between Saturn and Jupiter when it was first observed at approximately 669,281,812 miles from Earth! Visible with the naked eye for 18 months, Hale-Bopp showed the typical separation of the ion tail in blue and the bright, white dust tail. The streams each form their own tail, pointing in similar but unique directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the anti-tail (as you can kind of see in the Hale-Bopp image). At the same time, the ion tail, made of gases, always points in the direction of the solar wind as it is strongly affected by the magnetic field from the solar wind. To help demonstrate how the tail works, please take a gander at the image below; the "gas tail" is the ion tail mentioned.
Just like certain species of lizards, comets can lose their tails in something called a "tail disconnection event", which is exactly as you might imagine it is. For some reason, when/if a coronal mass ejection (CME) erupts from the sun or if the ion tail is properly magnetically loaded, the magnetic field lines become sufficiently squeezed and pinch the tail off before it starts to reappear again. Comet Leonard (below) demonstrated this phenomenon in a picture taken by Gerald Rhemann on Christmas Day, 2021. In this stunning image, if you look halfway down the tail, you will notice the clump and see where the original disconnect-event happened!
Or even more incredible is the video of Comet Encke (2P/Encke) from 2007 when it had its tail temporarily blown away by a coronal mass ejection. Looking closely, you can see the CME pass the head of the comet (a pale white wave-looking feature), ripping its tail off before the tail reappearing!
Whether it is a comet like Halley's, who will reach its furthest point in its orbit (aphelion) late this year before starting its return trip to reappear in July of 2061, or one like Hale-Bopp, which won't return until 4385 ± 2.0 years, these objects remind us that the solar system is a very active place. It also reminds us of the fear and delusions associated with these objects and the Heaven's Gate cult, which sadly convinced 39 people to commit suicide en masse in the hopes of being transported to a UFO they believed was behind Hale-Bopp. These mysterious objects make humanity think all sorts of interesting things about their appearance in our skies.
We know these objects aren't harbingers of doom or cataclysm but rather a chance to study that which has been preserved for billions of years, only to make a brief and hopefully stunning showcase of what can happen if a comet "decides" to become a Great Comet. Thankfully there is a collection at Wikipedia of these incredible objects in a neat list for you!
Keep watching those night skies, you never know what's going to make an appearance next!