Is 2013 really the year of the comet?

21 November 2013

IOP North East Branch Chair Richard Hornby writes about comet hunting, and why he hasn't had much sleep recently.

If 2007 was the year of the financial crisis, and 2012 the year of “thunder Thursday”, when Newcastle was brought to a grinding halt by a freak weather system, 2013 certainly looked like it was going to be the year of the comet. Back in April, scientists were writing in the media, predicting big things for a then little-known comet called ISON. This comet was to become visible in November; coming from the depths of the solar system, to sweep around the sun, and fly out the other side. It was predicted to be a very, very bright, amazing sight in the evening and early morning skies.

That was what was forecast at Easter time. Come September, a group of astronomers and myself decided to have a look at the comet, and try to work out when was the best time to look at it. When we started to look, it wasn't just ISON we saw. Good old favourite Comet Encke, discovered in 1786 would be close by. Also, there was a fainter comet, X1-LINEAR, which was a more recent discovery in 2012 by the Lincoln Near Earth Asteroid Laboratory; but it too was going to be in the Eastern sky during mornings in November. So, that was three comets. Then, to top it all, comet hunter Terry Lovejoy found another – R1-Lovejoy. So then we had four comets, all lined up in the early morning sky.  We then set ourselves, a probably – too – ambitious, target: to find them all.

For those of you who don't know what a comet is, it's often described as a “dirty snowball”. It is a lump of frozen ice and rock from the distant edges of our solar system. Most comets originate from a place called the Oort Cloud – a place which is affected by our Sun's gravity, but doesn't get much heat and light from it at all. We're not talking near the orbit of Pluto here – we're talking way, way, way beyond it; the sort of areas that the Voyager spacecraft have only recently passed through. These comets, because they feel the gravitational attraction to the Sun, fly around it in very eccentric orbits; moving slowly when they are a distance from the Sun, and moving quickly when they are much closer with tight, elliptical orbits. I mentioned “dirty snowballs” earlier, which makes it sound as if they are mainly ice, and not very much of anything else. I suspect that actually a better description, although less poetic, would be “snowy dirtballs”. They are thought to be predominantly rock, but they have a large proportion of ice on them. Where they live, out in the Oort cloud, it's very cold, around 10K (-260C), and so the ice remains frozen. However, when they come into the inner solar system, they are exposed to much higher temperatures; and, as anyone who's stared at an ice cube knows, when you expose ice to high temperatures it melts. This melting is what you are observing when you see the “tail” behind a comet – the melting and decomposing of the surface.

Fast forward to the 17th October – a Thursday. It's 4.30am – on the first clear night we've had, so we decided to get up and look for comets. We find... nothing. Yes, we were looking in the right place, but the problem was that the “right place” was the direction of Middlesbrough and Stockton, and as everyone knows, light pollution is an astronomer's worst enemy. So, we were fighting a losing battle with it: all of the comets were too dim to see.   Of course, we knew they were moving towards the sun, and there would be plenty more opportunities in the coming months.

We didn't realise that the first opportunity would be two days later. The astronomy news was buzzing: comet LINEAR has exploded! It had got brighter. So, we got up again to take a look, again at 4am. There it was: comet LINEAR. We got a picture of it: a fuzzy blob – albeit a diffuse one. Let me just take you for a minute to the observation site: the top of a hill at 4am. Unsurprisingly, it's quite chilly up there... actually, more than “quite” chilly, it's really chilly. Hats, gloves – those are essential – as are three pairs of trousers and four jumpers! So, we're up there, jumping on the spot to keep warm, as we take it in turns to set up the telescope: screwing everything down, aligning the telescope, mounting the camera, and then taking some images and looking at them. It's quite a slow process, but when you find what you're looking for, it's really rewarding. After LINEAR had exploded, it had a short period of brightness, before fading back to being a dim comet, and being eaten by the dawn.

The next week, we tried for comet Encke, however unfortunately, it was cloudy, so we had to wait another few days. Which we did, and with another 4.30am start, on another really cold, crisp morning... and what did we see? We saw yet another fuzzy blob. Now comet Encke isn't like the other three comets, which were fresh out of the Oort cloud: it is instead periodic, in a stable orbit, going around the Sun every three years. Think of it a bit like Halley's Comet, regular and predictable – whereas other comets are often small, and don't survive their first passage close to the sun. We think we may have seen a slight tail to the comet, but it was fairly insignificant.

The next comet took a lot more effort: we got up in the morning; we imaged and saw nothing on four separate occasions. And so it came to our “one last chance” on the 19th November. We did a two-star align on the telescopes. We did it again, just to be doubly sure, and once the telescope was pointing in the right direction, we had a look. We could see and could see  small, green fuzzy blob. No doubt about it. About magnitude 4, just fading into a mixture of the dawn light and Stockton, was comet ISON complete with a long tail. Its head was all together, shining bright green into our camera. There was a general celebration among the four of us (we had started off as seven), once we had found the illusive comet, including a small bit of singing, and a fair amount of jumping up and down, although whether that was due to the cold and there being ice around, I'm not entirely sure. But eight mornings, eight starts between 4am and 5am, and we'd done it – we'd imaged three of the four comets of 2013.

Hang on though, you're thinking – what about Lovejoy? Well, Lovejoy is now circumpolar, meaning it can be seen at any time of the night, so an early morning probably isn't called for... however, it reaches its maximum brightness in three days time... so if it's not cloudy, I'll be out there looking for it.

So, was 2013 the “Year of the Comet”? Well in my eyes, no. The three (or four) comets of 2013 haven't yet beaten the 1997 passing of Comet Hale-Bopp, one of the defining moments in my life, which persuaded me to go and study astronomy. But was it  the “Year of the early morning comets”? Yes, that works – it could also be called the year of the lack of sleep! Was it worth it? I'd say yes. It's nice to be able to see these long distance travellers as they come to either the ends of, or to pay us a visit during their journeys. And who knows how many more there might be in future years, waiting to come and say “Hello”?

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This is the first of a series of “features” articles on northeast.iop.org.  If you’d be interested in writing a future article, please contact r.j.l.hornby@phyiscs.org.



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