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What are comets anyway?

Early memories …

I was 4 years old in late April 1957 when my mother took me out into the street one dark evening. “Can you see the comet?” she asked as she picked me up and pointed it out. This was Comet Arend-Roland and was pretty bright at about magnitude 1.5 – certainly easy enough to spot against the feeble light pollution in a small Welsh town at that time. I’m sure I didn’t know what a comet was, but being taken out into the dark like that was pretty exciting!

Comet Arend-Roland on 26 April 1957, taken with the 48-inch Schmidt camera at Mount Palomar Observatory.


I didn’t know it but the spike, or “anti-tail” pointing towards the Sun which was visible at the time, was rather unusual. It was a line-of-sight effect resulting from looking edge-on at a fan of dust.

What are comets anyway?
For millennia comets were entirely mysterious but nevertheless feared as the harbingers of disasters and plagues. It wasn’t even known if they were simply effects in the atmosphere, but in 1577 Tycho Brahe proved that they were further away than the Moon.

By the end of the 17th Century scholars realised that comets were a type of celestial object which could be studied scientifically. Newton’s used his laws of gravity and motion to compute the orbit of the Great Comet of 1680 and in 1705 his friend Edmund Halley analysed details of historical bright comets and came to an important conclusion. He noticed that bright comets seen in 1531 and 1607 had very similar orbits. Deducing that they were the same object he predicted its return in 1758. Although he died before that, in 1742, he was proved correct: it was the first time anybody had shown that comets orbit the Sun, and the 1758 comet was subsequently named after him. It has come back repeatedly since then, the most recent apparition being in 1986.

Comets are small solar system objects, typically some kilometres across, and consist of conglomerates of ices (not necessarily only water ice) and dust; they were characterised as “dirty snowballs” by cometary scientist Fred Whipple. After recent spacecraft visits this view has been refined: typically they seem to have a rocky or dusty crust several metres thick with the ices below. When their orbit brings them closer to the Sun the heat causes the ices to transfer directly from the solid to the gaseous phase. The resulting gas vents away from the comet, and carries dust with it. This has been directly seen by the Rosetta spacecraft which rendezvoused with Comet 67P/Churyumov-Gerasimenko in 2014 and stayed with it through its perihelion, the point in the orbit closest to the Sun. Rosetta found that gas does not erupt uniformly from the surface of the comet, but vents from specific active pits.

Once the gas and dust are vented into space they are exposed to sunlight: light exerts a pressure, called radiation pressure. It can be imagined to be a recoil from the impacts of photons or equivalently it can be calculated from the induced currents in a surface caused by the electromagnetic light waves interacting with the same fields and causing a repulsion. One of the exercises I did as a Physics student was to prove that these two concepts produced equal results. I also had to calculate the approximate size of a particle for which the inverse-square law of gravitational attraction would exactly counterbalance the inverse-square law of repulsion due to radiation pressure. A particle of this size would travel in a straight line through our solar system. The critical size turned out to be less than a millimetre.

But I digress: the dust particles are all smaller than this critical size and so are pushed away by the pressure of sunlight to form the “dust tail”. Also, the gas is blown away by radiation pressure and is energised too, glowing with fluorescence induced by the sunlight. Usually there is a strong green tint from the emission of diatomic carbon.

Gas has much less mass than dust so it tends to be blown in a straight line away from the Sun, while the heavier dust tail typically has a curved shape as it moves slower and the comets orbital path is curved.So most comets have two tails. And they both generally point away from the Sun. On their inward path, the tails are naturally behind them, but after perihelion, when they are now receding from the Sun, the tails are ahead of them.

Periodic and Non-Periodic Comets
Some comets always stay in the relatively inner parts of the Solar System, and are well known, with orbits which bring them back every few years (where “few” can mean anything up to 200!). The best known is Halley’s Comet, which has the official designation 1P/Halley, where the P means it’s periodic, and 
1 means it was the first one to be so designated. The next periodic comet to be identified was the slightly less well-known 2P/Encke (with a period of 3.3 years), though it took more than a century after Halley’s achievement to figure it out (Encke succeeded in 1819).

Today there are hundreds of known periodic comets. But, on the whole (with the notable exception of Halley’s Comet) they are faint and unspectacular. They have had so many perihelion passages that most of the volatiles and loose dust has been dispersed long ago.

The unexpected comets which appear from time to time, and which are sometimes spectacular, are non-periodic comets. They are thought to come from the Oort Cloud, a swarm of icy bodies far beyond the orbits of the planets and extending as much as a couple of light-years. Although objects there have generally stable obits, the idea is that occasionally there are gravitational disturbances, be it from the giant planets in our solar system, passing stars, or maybe interactions between the objects themselves. These disturbances may upset the orbit of a potential comet and send it inwards towards the inner solar system.


The next bright comet to be visible in the northern hemisphere did not appear until October 1965. It was called Comet Ikeya-Seki and was only really visible for a few days near the time it passed around the Sun. But I didn’t see it: I wasn’t yet interested in astronomy – that would come the following year.


Comet Ikeya-Seki (another one)

In 1968 there was another comet called Ikeya-Seki (because it was discovered by the same pair of Japanese astronomers). By this time I was equipped with a 6-inch Newtonian reflector telescope. Having read about the previous, bright comet of the same name I made a point of observing this one. Here is a drawing I made at the time (aged 15), at 5:15 in the morning!


I was to be disappointed by this comet though. It stayed quite far from the Sun and never became visible to the naked eye nor developed much of a tail.


After that I concentrated on other aspects of astronomy, such as looking at the planets or drawing features of the Moon.


Another comet … and a change in pace

There was another bright comet in 1970: Comet Bennet, but it was not well placed for Northern hemisphere astronomers.


By the next year I was very preoccupied with major exams and the subsequent start of university life. I studied Physics at Sheffield University, and left my telescope back at home in Wales. Sheffield was much more light-polluted than Wales so there was no real possibility to continue my hobby. And frankly, the mix of studying and social life didn’t leave much time for it anyway.


But, in time for the Christmas break at the end of 1973 came news of a new comet. Comet Kohoutek was expected to be the “comet of the century” and provide a brilliant display at Christmas of that year. It was an utter disappointment. I made several attempts to see it and take photos, on black-and-white film using my new Praktica-LB camera, a 21st birthday present. I never found any sign of the comet on the photos (and I didn’t see it either). I’ve not been able to find either the original negatives or any scans for this blog, but they would not show much in any case.


In spring of 1976 there was another bright comet: Comet West. In the northern hemisphere it was an object to be seen in the dawn sky. Although by this time my interest in astronomy was picking up again (I was helping teach undergraduates in the new Physics and Astronomy course at the University, and I was President of the University’s Astronomy Society) the challenge of an early morning comet was not for me!


Fast-forward another 20 years: Comet Hyakutake

We now jump to 1996. In the meantime I finished at Sheffield and worked in the aerospace industry for 6 years before joining the European Space Agency at the main technical centre in The Netherlands. Throughout all of these moves I tried, from time to time, to make visual observations, and even to attempt astro-photography, usually with a camera on a fixed tripod. But the conditions were never very good. Holland, in particular, has a major problem with light pollution, which is almost impossible to escape.


Then came the news of Comet Hyakutake, expected to be a brilliant comet in Spring 1996. I was with friends for a ski weekend in the Austrian Alps from 21-24 March, at the time of the peak brightness. Fortunately we were staying in a remote cabin only accessible by chair-lift, high above the valley, with a splendid view to the North-East, which turned out to be just perfect. On the evening of 23 Mar 1996 we all went outside to view the comet, in total darkness. What a sight! The comet was right in front of us, above the valley, with its head twice the diameter of the full Moon and the tail very long. I tried taking some photos but, having only a pocket camera and no tripod, I wasn’t optimistic.


In those days, using film, the idea was to expose the whole roll and then get it developed. You would not find out the results until days (or weeks) later. Though hard to imagine now, that was what we were used to (unless you could process negatives at home, which I could for black-and-white, but not colour).


I decided on my return to The Netherlands to take the first opportunity to head out to the darkest site I knew (a small road leading through the extensive sand-dunes, deserted at night) with a tripod and my Olympus OM-2 camera, into which I had transferred the remainder of the colour negative film. I also took my 13-year old son Ben, who remembers the occasion mostly for how cold he was.


I was pleased with the results at the time, but I have now had a look at the old digitised images and decided I could probably improve them. I picked a half-dozen of the better ones and invoked the magic of PixInsight to register and integrate them, and then crop, remove gradients, reduce noise and sharpen a bit, to produce a final integrated image.


But I still didn’t know where it was in the sky, nor exactly which date I had taken the images.


That’s where a second bit of magic comes in, in the form of astrometry.net. This wonderful website will take an uploaded image and with no a-priori information at all, figure out exactly where it is, the orientation and the image scale. Feeding this information back into PixInsight allows me to make an annotated image, from which I can read off the exact position of the comet.


Then, using JPL’s HORIZONS website (https://ssd.jpl.nasa.gov/?horizons) I could generate the detailed ephemeris over the period of interest and identify exactly when I had taken the image – this is really impressive. It turns out that it was 00:10 CET on 27 Mar 1996.


No wonder Ben was cold!

Here is the image of Comet Hyakutake (C/1996 B2 (Hyakutake), after combining several digitised film frames in PixInsight.


This is the same image with annotation added by PixInsight. Note that the comet is very close to the celestial pole (and the pole star), and that the tail is more than 18 degrees long.


Comet Hale-Bopp

The following year, 1997, there was to be another great comet, and it was visible for much of the year: Comet Hale-Bopp. It was bright for a long time: I remember being at a meeting in Frascati, in the hills south of Rome, and after dinner we could plainly see the comet above the city lights of Rome. But I don’t remember taking any photos of it. A real pity, but this was a period when I was less interested in astronomy, and had quite a few other things to distract me.


Comet C/2019 Y4 (ATLAS)

Well there have been quite a few bright comets in recent years, but actually no really bright ones since we built our observatory in southern France in 2013. Until now, it seems. A comet called c/2019 Y4 (ATLAS) has arrived and recently I tried imaging it, using a telescope for the first time. At the time of writing it’s too faint to see with the naked eye, but it’s expected to get brighter.


The comet is going to be around for a while, so I’ll be updating this blog as I get more images. For now, here’s my first effort, made on 28 Mar 2020 (almost exactly 24 years after my image of Comet Hyakutake).


Comets are tricky to image because they're moving across the star background, and this one is going quite fast. As usual in astronomy you get best results if you take lots of images (sub-exposures) and then combine them, and that's essential for me as the camera is monochrome, so I need to take images with red, green, blue and luminance filters to be able to assemble a colour image.


So either you track the stars, in which case the comet is in a different position for each of the separate sub-exposures, or you track the comet, in which case the stars are different each time. I tried sub-exposures of 60 seconds, then switched to 300 seconds, for a total of about 4 hours, and I tracked the comet not the stars. With the 300s exposures the star images were noticeably streaked.


Of course, when combining images made with any individual filter there were gaps in the star images while the other filters were being used.


I managed an image of the comet alone by turning up the rejection threshold during image integration, even though there are some residual streaks visible (and many more before I cropped the image). Subtracting the luminance comet image from the subs didn't work very well unfortunately, so how to get the stars back?


Finally, about 2am last night, after several experiments, the wine must have inspired me and I decided to use a single 60s sub-exposure of each of the colour filters and combine them into a (noisy!) star image. Then I combined both using the maximum value of either image in the result.


So, here's my first comet image. I hope to refine the technique and get better results next time!


Image of C/2019 Y4 (ATLAS) made on 28 Mar 2020.


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