December 16^th. The day that the comet that could have been famous will fly by Earth at only 30.4 times the distance of the Moon.

Could have been famous? Yes, had things worked out differently, a generation of space fans would have been learning to say “Wirtanen” (it is Vir-tanen) instead of “Churyumov-Gerasimenko” (Chury to his friends). Comet 46P/Wirtanen was the original choice for ESA’s Rosetta mission until an issue with the launcher caused a delay in the mission and obliged a change of target.

In recent years it has become more and more common to observe asteroids passing inside the Moon’s orbit. In fact, since the first was discovered in 1991 (1991 BA), no fewer than 391 asteroids have been observed passing closer than the Moon, two of which (2008 TC3 and 2014 AA) were then observed to enter the Earth’s atmosphere and impact.

Asteroid (308635) 2005 YU55 zipping through the field of view of the Herschel Space Observatory on November 8^th 2011. Some clever work from the Operations Team at the Herschel Science Centre allowed the telescope to observe the asteroid even though it was moving far too rapidly to be tracked by the observatory. Multiple scans were made through the asteroid’s position that caught it as it crossed the field of view.

In the world of Near Earth Objects, which includes asteroids, comets and returning space junk, the two numbers that are most used are the size of the object – the largest was (308635) 2005 YU55, at 360±40 metres diameter, although the majority are under 10 metres across – and the distance of approach to the centre of the Earth in Lunar Distances (LD). 1 LD is 384 401km. A large object passing even at several Lunar Distances can get bright enough to observe with binoculars: the champion though will be the asteroid 99842 Apophis, which, on April 13^th 2029, will pass just 31 200km over the centre of the Earth (that is, 0.081LD), closer than a geostationary satellite and is expected to reach about magnitude 3.4, becoming easily visible to the naked eye as it crosses the sky sedately from east to west setting, for an observer in Madrid, at 23:15UT.

In contrast, no comet has ever been seen that has approached the Earth to within 5LD, although the orbits of comets observed before about 1700 are often so poorly known that their distance from the Earth is often no more than guesswork. A case in point is C/1491 B1, which, if one believes the extremely uncertain orbit available, approached the Earth to just 3.7LD on February 20^th 1491. Of the reliable cases, where the orbit is known accurately, the three closest historical approaches by comets are:

• D/1770 L1 (Lexell), to 5.9LD, in 1770.
• 55P/Tempel-Tuttle, to 8.9LD, in 1366.
• P/2016 BA14 (PanSTARRS), to 9.4LD, in 2016.

Of course, these are the calculated distance to the nucleus of the comet. The Earth has passed through the tail of a comet on various occasions, most famously in 1910, when the Earth’s path brushed the tail of Comet Halley.

Overall, only twenty-three times in history has a comet approached the Earth to within 0.1AU, or 39LD. Such close approaches by comets are quite rare events and, because of that, more anticipated. Even the famous close approach by Comet Hyakutake in 1996, which was so stunningly beautiful seen in a dark sky, did not get inside the 0.1AU threshold[1].

However, in the all-time league table of close comet approaches, Wirtanen comes in a very respectable sixteenth place. Not all close approaches by comets are in any way spectacular. We know about the close approach of Tempel-Tuttle in 1366 because it was observed and Comet Lexell reached magnitude 2. Similarly, the comets with the fourth and fifth closest approaches – Comet IRAS-Iraki-Alcock, in 1983 and Comet Halley, in 837 – we both easily visible to the naked eye: in the year 837, Halley was estimated to reach about magnitude -4, while I well remember seeing Comet IRAS-Iraki-Alcock in May 1983, walking to the train station, out in the countryside, late one evening at East Malling (Kent, England) where I was working at the time. In contrast, P/2016 BA14 (PanSTARRS) never got brighter than magnitude 13, despite approaching the Earth so closely.

Although Comet Wirtanen is not a great comet, despite being a short-period comet, which returns every 5.44 years and rather small – the estimated diameter of the nucleus is 1.2km – it is not a tired old comet of very low activity and should, hopefully, be fairly easily visible with the naked eye for people with reasonably dark skies. Wirtanen is actually quite significantly more active than Comet Churyumov-Gerasimenko, which, if it passed at the same distance as Wirtanen is going to, would not even be visible to the naked eye at all. In the same way, in 2011, Comet 45P/Honda-Mrkos-Pajdusakova passed at 23.5LD and only reached magnitude 7.

This was Comet Wirtanen at its best in 2008. Image taken on February 5^th 2008, by José Francisco Hernández from Altamira Observatory (Fasnia, Tenerife), when the comet was about magnitude eight and a half and almost exactly at perihelion. There is a hint of a tail towards the left. This was the combination of 8 exposures of 10 minutes, guided on the comet, so the stars appear as trails behind it.

So, how bright will Comet Wirtanen be in December?

If the comet had repeated its activity from its pass in 2008, it would have got up to something between magnitude two and a half and three.

Now that sounds really bright. After all, one can see magnitude 2 stars even from the centre of a city. However, that is stars. Points of light. The light of Comet Wirtanen will be spread over a substantial area of the sky. Current estimates of the diameter of Comet Wirtanen are that, observed in a dark sky, with large binoculars, it is as large as half the diameter of the Moon. In December, it will be a third of the distance from the Earth that it is now and closer to the Sun and more active so, it may be as large as ONE DEGREE across: twice the diameter of the Moon. Of course, most people will not see it that large, as they will not be observing in ideal conditions but, even so, the comet’s light will be spread out over a substantial diameter in the sky.

The second word of warning is that we can compare the estimated brightness of the comet in previous appearances to the brightness that is being observed now. For most short period comets, the activity repeats time and time again, without significant changes. In the case of Comet Wirtanen, that does not appear to be happening necessarily.

Have a look at this light curve where the visual observations of Juan José González and Carlos Labordena, two of the most experienced Spanish visual comet observers are plotted for 2002, 2008 and 2018 – the comet was not observed in 2013 because it was behind the Sun when brightest. Here, the horizontal axis is how many days the comet was from perihelion, while in the vertical we plot the estimated magnitude, corrected for the changing distance to the Earth, so that we see only how the brightness changes as the comet approaches and recedes from the Sun. The points allow us to compare the true brightness and activity of the comet was on each of the three occasions that it has been observed since  the year 2000.

There are not many observations from 2002 and those were taken just after a quite significant outburst of the comet, but the estimates so far, taken in 2018, seem to be about a magnitude fainter than at the corresponding time in 2008. We can assume that the same observers observe in the same way each time, so that the data can be compared directly, so the difference would seem to be due to the comet, suggesting that it is less active in 2018 than in 2008. However, if I make the same type of plot with CCD observations, there appears to be no difference between 2008 and 2018, so I wonder which is right.

Anyway, the visual observations suggest that the comet will reach magnitude 4 in mid-December: bright enough to see with the naked eye, but not obvious without looking for it.

Even so, the comet should be brighter than magnitude 5 all through December, so not hard to locate with binoculars in a minimally decent sky, but that is a long way from being a spectacular object.

Where will it be? The comet comes up from below the Earth’s orbit, until it tracks almost over the North Pole at closest. It starts the month in Cetus, at a quite southerly declination of -20⁰ and, through the month climbs almost 75⁰ northwards, crossing successively the constellations of: Eridanus, Cetus (again), Taurus, Perseus, Auriga and Lynx. Around December 16^th, the comet will pass close to the Pleiades, which is a great moment to locate it easily.

So, what will it look like? If you are not familiar with observing comets, the best comparison is an object like Messier 13, the globular cluster in Hercules, observed in binoculars, which, visually looks very similar to a tailless comet such as Wirtanen: quite diffuse, but somewhat brighter in the centre. Probably most people have observed this famous object at some time. Comet Wirtanen will be nearly two magnitudes – say a factor of six – brighter, but will have about four times the diameter, so will appear probably to be dimmer visually than Messier 13.

[1] I saw this comet from a light-polluted sky, in which it was still easily visible to the naked eye and I saw it also in really dark skies in Tenerife. To see the ghostly tail trailing fifty degrees across the sky was just stunning, but such views are now only available to the privileged few who live in the countryside, far from bright, city lights. Where are the Great Comets now? They are lost in the light pollution of our modern world!