Comets - A Brief Primer

in #steemstem3 years ago (edited)


Comet C/2014 Q2 (Lovejoy) discovered by the author on August 17, 2014, had developed a complex ion tail shown here. This photo was a total of 510 seconds exposure using a Celestron C14, with a QHY9 camera and Hyperstar reducer on January 8, 2015

One of the most beautiful sights in the night sky is the appearance of a bright comet that is visible to the naked eye. It is a rare event that occurs maybe once in a decade. Centuries ago, well before man-made light pollution, the appearance of a large comet could instill fear in many cultures and some believed them to be omens of war. William Shakespeare even wrote in his play Julius Caesar, "When beggars die there are no comets seen; The heavens themselves blaze forth the death of princes."

Today comets are thought to hold valuable clues to the origin of the Solar System, and some believe even the very water in our oceans and the carbon-based building blocks of life originated from comets. It is for this reason that comets are the focus of considerable research, culminating in the recent highly successful Rosetta mission to Comet 67P/Churyumov-Gerasimenko. Within the last few decades large strides have been made in our understanding of comets, thanks to space missions to comet’s such as ESA’s Rosetta.

A Brief Overview

There are 3 components that make up an active comet. Firstly, the heart of the comet, the nucleus, which is solid core that contains the volatiles that fuel the comet. Then there is the coma, which is a cloud of rarefied gas and dust that forms around the nucleus. It is sometimes referred to as the head of the comet. Finally, there is a tail that trails in an anti-sunward direction from the coma.

The following infographic gives an overview of those components. But we will examine the components in more interesting detail below.


At the comet’s heart is the nucleus, a solid body typically 1-10km across made of a mixture of frozen gases, rock, dust and various carbon compounds. Common ices found on comet nuclei include water and Carbon Monoxide, however other ices have been found such as ammonia, ethane, methane, Carbon Dioxide and even ethanol. These ices help fuel the formation of a comet as nucleus approaches the sun. Thanks to various space missions as well as ground-based radar observations we have a fairly good idea about the size, shape and composition of comet nuclei.

Here are some of the observations that have been made of comet nuclei.


On March 14, 1986, European Space Agency’s Giotto mission passed just 596km from the nucleus of the most famous comet of them all, Comet Halley. Because the comet was highly active, the spacecraft was showered with small debris as it approached, one impact spun the spacecraft off it’s axis another impact destroyed the main imaging camera! Luckily, before this happened Giotto was able to send back several good images. These images revealed the nucleus to very dark (darker than coal even) and peanut shaped with dimensions of 15km x 10km.


Giotto’s image (HMC68) of the nucleus of Halley’s Comet. Credit: ESA (European Space Agency)

Comet 9P/Tempel

NASA’s Deep Impact mission performed a rather unique experiment where they slammed a 300kg Copper projectile into the 8 x 5km nucleus of Comet Tempel as they monitored the effects of the impact. Several years later in 2011, NASA’s Stardust-NExT mission also performed a flyby of Comet Tempel that enabled it to image the area where the copper projectile had impacted in 2005. Based on the fact that the visible crater was flatter and more subsided that expected, scientists concluded that the comet nucleus was fragile and porous in nature.


Please click image to view a movie of the actual impact

The Deep Impact mission slammed a 300kg copper projectile into the nucleus of Comet Tempel on July 4, 2005. Image Credit:NASA


Nucleus of Comet 67P/Churyumov–Gerasimenko. Copyright: ESA/Rosetta/NavCam

ESA sent their Rosetta spacecraft to Comet 67P/Churyumov–Gerasimenko where it monitored the comet continuously for over 2 years from 2014 to 2016. Our knowledge of comets grew enormously as a result. There were many interesting findings including that the ratio of deuterium to hydrogen in the water from the comet being significantly different to the earth. this has significant implications on theories of the origin of the solar system since the origin of terrestrial water would appear to be from a different source to the comet’s.



Coma of Comet C/2015 ER61 (PANSTARRS) fluoresces strongly green in the image from Apr 7, 2017. Image 210x8seconds with C14 f2 scope with QHY183c camera. Credit:Author

When distant from the sun a comet looks like an asteroid, however, as it approaches the sun its surface begins to warm which in turns starts to sublimate its constituent ices which include Water, Carbon Dioxide and others. In the vacuum of space most ices sublimate, that is pass from the solid to gas phase without becoming a liquid. Because of outgassing dust and ice grains are also released, and these all combine to form a large cloud around the nucleus which typically extends 100,000 km or more, which is about the size of Jupiter. During the Rosetta mission it was found that Water and Carbon Dioxide liberated from the nucleus of Comet 67P were dissociated from free electrons, creating other products (like Diatomic Carbon and Hydrogen, among others).

Often coma shine a vivid green, something some people can see even by direct observation by eye. Spectroscopic observations of coma reveal the main cause of this to be fluorescing diatomic carbon (C2) and Cyanogen (CN) which are presumably the byproduct of the dissociation of ices containing Carbon (CO2) and Nitrogen (Ammonia) from the nucleus surface. This can be seen in the following spectra which was obtained by renown French amateur astronomer Christian Buil. For more information on Spectra and Spectroscopy please click on this link. I may well do a separate article on Astronomical Spectroscopy because it is quite fascinating and a lot of fun to do.


Comet Spectra. Observe how the C2 (Diatomic) has 2 bright emission lines in the green. Credit: Christian Buil

Surround the nucleus and coma, but not visible in the visual spectrum is a large cloud of Hydrogen atoms, which extends well beyond the normal coma and most likely is derived from the dissociation of water from the nucleus. It can only be detected from outside the earth's atmosphere because of the wavelengths it emits. In the case of Comet Hale-Bopp in 1997 the Hydrogen envelope extended 1 Astronomical Unit, the distance between the sun and the earth!

Comet Tails


Comet Hale-Bopp’s distinctive white dust tail contrasts with the dimmer blue gas tail. The Andromeda Galaxy M31 is also visible bottom right of this image, as are 2 plane trails. This image was taken late March, 1997, from north of Los Angeles, Ca. Image Credit:Author

The tail is the most well recognised feature of a comet, although often they are either absent or small. There are two main types of tails, the ion or gas tail and the dust tail. In very active comet’s like Comet Hale-Bopp in the image above both tails are strongly formed.

Ion Tail


The blue ion tail in this image shines from Carbon Monoxide fluorescing. Solar wind rapidly drags the tail in the direction opposite the sun. Credit: Author

As a comet gets closer to the sun it’s is subjected to greater heating which causes outgassing to increase. Additionally, some of the released gases begin to ionise and as these are charged, they are carried by the solar wind, a stream of charged particles from the sun, at velocities of the order of 500 km per second. This forms what is known as the ion tail, and it always points away from the sun because of the direction of the solar wind. Ionised CO (CO+) in the tail glows a distinctive blue color when the sun’s radiation causes it to fluoresce.

Dust Tail


A classic curved dust tail (Comet McNaught in 2007). The tail is redden due to low elevation. Credit: Author

Outgassing from the comet's nucleus also causes Dust and ice grains to be lifted off the nucleus are also pushed away from the sun, but the velocities are much lower than experienced with the ion tail so that the motion of the comet in it’s orbit can cause the tail to lag opposite to the comet’s motion. This causes the dust tail to form, which is essentially a broad sheet of dust that is more uniform and diffuse than an ion tail, and has an almost white color.


Comet Arend-Roland's dust tail in 1957 was viewed under unusual geometry which meant it could be seen in front of the comet as well. Credit:APPLAUSE

Other types of tail

Both Comet Hale-Bopp (1997) and Comet McNaught (2007) were both observed to have unusual tails made of Neutral Sodium atoms. The mechanism of formation for these is unknown. I was lucky to photograph the Sodium tail of Comet McNaught because I had a very clear sky (it is only one of 2 amateur photos I'm aware of that caught it)


A highly contrast boosted photo Comet McNaught’s faint Neutral Sodium tail imaged by the author with a 200mm f2.8 lens and a 130 second exposure and a 350D on Jan 20, 2007. Only the red channel is shown. Credit: Author

Orbits and Origins

Unlike the planets which travel in near circular orbits in a similar plane to the earths, comets tend to have orbits that are much more elliptical and often quite inclined to the earths orbital plane. Halley’s Comet, for instance has an orbit that takes it close to within the orbit of Venus all the way out past Neptune’s orbit over a period of 76 years. A generic orbit is shown in the following diagram is typical of a comet like Halley.


The orbit of a typical comet. Perihelion is the point at which the comet is closest the sun, and inclination is how tilted the orbit is relative to the plane of the earths

Work done in 1950’s by astronomer Jan Oort demonstrated that a large population of comets have aphelia (most distant point from the sun) around the 50-200,000 AU distance. An AU is a measurement of distance equal to the sun to earth distance, or about 150,000,000 km. When these aphelion points are plotted they tend to describe a shell which we now call the Oort cloud as shown in the following diagram. It is postulated that an encounter with a nearby star can perturb comet’s inward towards the inner solar system.

Image Credit Nasa

One implication of the highly elliptical orbits of comets is the fact that a comet approaching the sun will encounter increasingly fierce solar radiation that changes at in inverse square law to the distance of the sun. Therefore, halving the distance to the sun means that solar radiation increases 4 times (2 squared). The result of this is that 4 times as much sunlight is being reflected/re-emitted from 4 times as much gas/dust, so the brightness of a comet will tend to increase as much as 16x for each halving of the solar distance (or at a inverse 4th power)!


This was a brief overview on the topic of comets. In the future I plan to do a number of additional articles including some information on how I came to discover a number of comets, as well as how professionals are interested in finding PHA's (potentially harmful asteroids).


  1. Magnani, Loris & F. Ahearn, M. (1986). CO/+ Fluorescence in Comets. Publications of the Astronomical Society of the Pacific. 97. . 10.1086/131694.

  2. Oort, J. H. (1950).The structure of the cloud of comets surrounding the Solar System and a hypothesis concerning its origin. Bulletin of the Astronomical Institutes of the Netherlands, vol. 11, p. 91-110 (1950)

  3. NASA Releases Images of Man-Made Crater on Comet.


Wow! So happy you are posting on Steem, this was fascinating to read and you have a frickin' comet named after you hurtling around the solar system! I really like your clear explanations and this was so well illustrated. I just learned a lot :)

Much love - Carl

Thank you Carl. I'm glad you enjoyed it! Hope you also had a chance to view the impact video of Comet 9P/Tempel I linked, thats pretty cool :)

Thanks for the comment, I had completely missed that there was a video if I clicked on the image. That was indeed really cool!

You got nominated in my latest article TOP 5 trending Science Accounts you should follow today!
Feel free to check it out:

Woow, thank you so much for interesting content. Unfortunately, because of man-made light pollution we can't see the beauty of night sky. Pictures are awesome. Waiting for the article about PHA!

Thank you @aleksandraz ! I posted the PHA article here .

wow very nice post. I really like.

That's a lot to read at one time, but I like everything there is to do with space. You'll notice on my blog I am a time traveler volunteer. Traveling through space and time would be cool.

I had to cut a lot out as it was to not turn it into a book (!). Where all travelling through time whether we like or not :)

This post is one of the best I've read here on Steemit, excellent work. How a writeup on comets from someone who has actually discovered comets isn't at the top of Steemit I'll never know. In particular I had no idea that the sodium tails existed (or even that sodium was present on comets at all). I often wonder what kind of objects are out there in the Oort cloud that we don't see. Keep up the great work!

I'm curious - how did you end up discovering a new comet? Were you specifically looking for a comet, or did you find it while observing something else?

Thank you so much @proteus-h I try my best. It takes me a few days to put these together, do some of the graphics, etc. Time consuming but enjoyable and a good distraction from everyday life! You don't do too badly yourself, I was actually thinking about your Aluminium battery idea just before while driving from the shops!

To answer the new comet discovery, I actually started writing about that with this article but it started to get too big because I had to introduce what a comet was, as well as why an amateur can still find them given big budget professional surveys are also sweeping the sky. The original article is coming, I just want to do it right and make it interesting without overwhelming the reader. For the time being let me answer your question simply: I deliberately search for comets and have found 6 with my own telescopes, and 11 with data from the SOHO spacecraft. Kindest, Terry

Oh my goodness! You are THE Lovejoy who discovered Comet Lovejoy! Wow I am gobsmacked! I wish I can take those amazing comet photos like you and discover comets like you!

Yes! I am in the process of putting together an article on how I discovered those comets. Stay tuned!

extraordinary postings deserve to be trending

extraordinary post is very appropriate to be trending, hopefully I also one day later my post can be trending like you.

I liked your post, I love everything that has to do with space. Go through my channel check my last post in which I talk about the asteroid AJ129.

Saw your post and upvoted! Good work @bysamuel11 :)

Wonderful post Sir. Your post is completely different from other posts because the information other posts contain , one learns or get to know in their daily life but this is the post which something we hardly get to know through regular sources. Thanks for providing such information . :)

extraordinary post is very appropriate to be trending, hopefully I also one day later my post can be trending like you.

Wow, you put a lot of work into this, too bad it is based entirely on antiquated theories that do not stand up to the observations. Nonetheless, I will still upvote it for the excellent effort. I stopped reading at the mention of ice because comets have no ice. None was discovered when they landed a probe on a comet. Which if one pays close attention to that landing there were some troubles, all of which were predicted by Wal Thornhill the day before the event. This slushy snowball theory does not work especially since comets have been observed with tails at a distance considered too far from the sun therefore too cold to melt any alleged ice.
When they landed the probe on the comet it was a rough landing as they expected a slushy snowball but found a rock. Prior to landing, they shot a charge at the comet to investigate its interior but before the charge made contact there was a flash and then the explosion which blinded the cameras because it was brighter than expected.

Rather than I explaining it pedantically, listen to Wal Thornhill's presentation with an open mind and if so inclined leave a comment. And lest I forget Karl Popper has something to say too.


Bollocks. There is a huge amount of ice on comets. Carbon Monoxide drives comet activity on distant comets, water closer in, and even silicate rock when close enough to the sun. We have known this for a very long time because we can observe their emission signature in a spectrograph (i.e a century before Rosetta went to 67P).

With all due respect; can a spectrograph tell the difference between a water molecule and disassociated hydrogen and oxygen atoms especially at those distances involved?

It is folly to compare the electric universe theory to flat earth as by doing so you are demeaning many prominent scientists of the past century from Sir Fred Hoyle to Irving Langmuir, Hannes Alfven, Kristian Birkeland and many more. I note that you are neither knighted nor are you a Nobel laureate and seem to have no regard for the wisdom of Karl Popper. It also suggests an emotional attachment to your beliefs, which is anathema to scientific progress. I have no doubt that you are a very learned man with a well organized intellect and I, in no way, question your accomplishments, but, even geniuses can be wrong.

Plasma constitutes 99.6% of the observable universe. Double Layers, Alfven lectured at the Marshall Space Flight Center on Double Layers in Astrophysics, "should be treated as a new type of celestial object". ~

There are only two ways to produce a magnetic field, one is with the use of a dynamo, the other is through electric current, just about every instance of a so-called "magnetic rope" or a magnetic field in space is an indicater of electric currents. Double layers allow for a separation of charge and plasma is highly conductive with very little resistance. Alfven is also known for his modelling of the heliospheric current sheet as a "ballerina skirt"

The undulatory nature of this configuration has a conductor(plasma) moving through magnetic fields which anyone with a basic electrical background knows produces electric current.

Kristen Birkeland mocked and demeaned by the likes of the great Lord Kelvin(because he believed space was empty) in his day for his postulations regarding electric currents in space was proven correct decades later with the observation of Birkeland currents in the ionosphere. His experimental work with his terrelaa showed significant evidence of the electric nature of the sun and the solar system, Saturn's rings etc.
A paper by Dr.Don Scott modelling Birkeland currents as having double layers rotating in opposite directions.

I often see those of the standard model try to dismiss the electric potential in space by saying things such as the charge is "nearly balanced" therefore neutral. This is like saying a woman is nearly pregnant. Either it is or it isn't. If the charge is nearly balanced that means it is slightly out of balance, and considering double layers and the highly conductive nature of plasma, any slightly out of balance charge has an electrical potential. To further the argument, a battery has charge balanced but connect the poles with a load and you have current flow.

So, are you in the game or out of the game as Karl Popper puts it? Do you believe the science is settled? Are you a modern day Lord Kelvin clinging to antiquated beliefs? In the end it doesn't really matter to me what you choose to believe and if any of my words find you offended, well, I apologize. As I do hope that we can, at the very least, agree to disagree respectfully.


Can you post references from Sir Fred Hoyle, Irving Langmuir, Hannes Alfven and Kristian Birkeland regarding Electric Comet Theory? I mean where they directly commented on it? I'm betting you can't....

So, what are you willing to bet?

It took me a while but I found a link to Birkeland's theories on comets.

This is all in his book 'The Norwegian Aurora Polaris Expedition, 1902-1903'

By the way, I did see your fakebook post referring to me. It left me feeling sorry for you.


Flag down spam

Lol. You would win that bet because electric comet theory was not their forte per se. But all those mentioned were supporters of electric theories pertaining to astrophysics, whether it is electric-plasma models of the solar system, the sun and the universe in general as I somewhat already outlined. And being as they supported the electric universe hypothesis it is not a stretch to think that they would not contradict the electric comet hypothesis.

But, I understand why you ask such a question as it is merely an attempt at avoidance as no-one likes to face the possibility that after years of investment they may be wrong.

One of the reasons I like talking to those with a different opinion than mine is that it challenges me in my opinion when they take the intellectual scientific route. What you have done is absolutely not that. You could talk science with me and critically discuss the ideas I have presented but instead, you have done the opposite, you asked a question which you already knew the answer to. Oh well. I guess I am not worthy. lol

I guess you are, as Karl Popper said, no longer in the game as a scientist.

Looking forward to your next post.

Flagged as spam.

Lol. Thank you for proving my point.


What is your end motive here? This seems like a very accusative post for essentially winding down to saying that comets have rocks instead of ice. Ice is, of course, essentially a rock.

If comets were made out of ordinary rock as in rocks found on Earth, how do you explain the extremely visible outgassing tail? Such outgassing makes perfect sense when you consider that the comet came from the far outer solar system where ices don't melt. The ice then sublimates when it approaches the sun as sunlight increases. If rocks outgassed like this, the moon would have an atmosphere. The Philae lander itself showed that the landing site contained hard water ice. If you're going to make claims that the ice tails have been observed at distances too far to sublimate, you'll need sources to back that up. If it isn't ice, what is it? Isn't ice the simplest answer?

It's "Electric Comet Theory". Comet sciences answer to the flat earth theory. From the people that DIDN'T bring you all the wonderful things science gave us, like medicine, internet, computers, cars, etc :)

The outgassing tail is due to a difference of charge picked up when having left the heliosphere(becomes -Q) and when it re-enters the heliosphere, the Q+ of the solar wind causes ionic action stripping hydrogen and oxygen atoms from the comet. Since the moon is firmly ensconced in the mostly positive charge of the solar wind it does not experience what a comet experiences.

There's a few issues I see right away.

  • Where is the negative charge coming from when the comet leaves the solar system? A positively charged object doesn't lose charge unless it is so charged that it can eject electrons to the vacuum.

  • If the moon (and the rest of the inner solar system) were all positively charged, and no charge transfer took place to neutralize them, all of the inner planets and moons would repel each other via the electrostatic force. The moon would slowly be pushed out of Earth's orbit. If the charge was able to neutralize via electrons ejecting across the vacuum, then the force would be gone but so also would the charge.

  • Finally, how is this is a simpler explanation than icy rocks from far from the sun where ice doesn't melt coming closer and sublimating gas out into the tail? What does it explain that the comets being made of ice doesn't? The simplest answer that explains everything about a process is usually correct.

Thanks for commenting.
The negative charge is what exists outside of the heliosphere which is itself predominantly a positive charge due to the solar wind. When comets leave the heliosphere they pick up the negative charge and when they re-enter the heliosphere, the negative charge just picked up by the comet interacts with the positive charged solar wind producing the coma.

We do not know what the charges are of all the planets. Earth is predominantly a negative charge. We do know that the solar wind is a predominantly positive charge.

It is simpler because it fits the observations such as comas on comets too far from the sun for there to be any heat to cause an alleged sublimation and the fact there was no ice or snow when they landed on a comet and even lead scientists at ESA have said recently that we need to rethink the slushy snowball hypothesis.

The simplest answer is usually correct, but that answer has to be proven and the model needs to predict observations. This isn't the case with the slushy-snowball hypothesis.

"Here on the left is an artist’s impression of a comet surface before the first flyby of the nucleus of a comet on March 14, 1986. It shows icy vapours wafting into space. In the centre is an artist’s impression of the Philae lander on the surface of comet 67 P where you can see the surface still appears icy, the only concession to images of other comet nuclei being the crater and ridges. On the right is the real surface of comet 67 P, which is actually blacker than photocopier toner. But despite the stark reality, the story of comets remains unchanged."...Wal Thornhill

" “Pinnacles range from tens of meters to over 100 m in height, and they have varied shapes including spires with pointed tops near the resolution of the images. The pinnacles were not anticipated land forms on primitive bodies, and their origin on Wild 2 is a mystery."...ESA scientists recounting their observations of comet Wild 2 not fitting their model's predicition.

"Returning to comet Halley in 1986, water molecules are supposed to sublimate (change directly from solid to gas) off the comet nucleus in the heat of the Sun. Later, ultraviolet light from the Sun is thought to split the water molecule into OH and H. So we should expect more H2O near the nucleus than OH. However, the Vega 2 spacecraft found the reverse, which “may indicate the existence of parents of OH other than H2O.” "..W.Thornhill

The small size of the dust particles from comet Halley was a surprise. “The dust particle mass spectra do not exhibit the expected low-mass cutoff at 10-14 gm; instead they continue to rise to 10-16 gm.” “The most striking feature is the large number of low-mass particles.” “Indeed, the first particles encountered at the ‘fringes’ of the coma had the lowest masses measured, instead of the higher masses predicted by the ‘fountain’ model first introduced by Eddington and later widely developed to predict the mass distribution of cometary dust.” ** Low mass particles fit with electrical sputtering of surface atoms and molecules but not with the standard model of gas jet dispersal of interstellar dust grains trapped in dusty ice.**

most of these quotes come from a single source, if truly interested you should check it out.


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