Newbury Astronomical Society

The Geminids this year will peak on the afternoon of 14th December but unfortunately it is only a few days after full moon, which will seriously interfere with the view. Weather prospects in the UK aren’t that good either but it will still be worth looking out if there is a good clear patch. The Geminids are one of the more reliable showers so although the moonlight will drown out the fainter meteors there are usually several fairly bright ones per hour that should be visible.

The sky chart shows the position of the radiant at midnight, with the moon not far away. Look well away from the moon to reduce the glare, don't forget that meteors can be seen anywhere in the sky.
The grey areas on the graph show when the moon is above the horizon and the yellow bars show the evenings when the greatest Geminid activity is expected. The lighter blue lines show the extent of the shower a few days either side of maximum. Sunset is about 4pm and sunrise about 8am.

Soon after asteroid numbered 3200 was discovered in 1983 the similarity of its orbit to the Geminid meteor stream was noticed. Meteor streams are usually associated with comets but, despite careful searches, this object has never shown any sign of recent cometary activity.

Subsequently called Phaethon it belongs to an important group of asteroids whose orbit can cross the Earth’s. It turns out to be an unusual member of that group with a different colour suggesting a different composition to most asteroids.

One of the curious things about the Geminid particles is that they are more solid than meteoroids known to come from comets. This is good for meteor watchers; giving us more bright meteors, but it also tends to support the idea of a broken asteroid rather than a typical comet.

Things got even more interesting in 2005 when a small asteroid called 2005UD was discovered in a similar orbit to Phaethon, it has the same unusual colour which points to a common origin. More recently another asteroid, 1999 YC, has been suggested as a member of the group. Phaethon is clearly the major member at about 5 km across, the other two are little more than a kilometre in diameter.

A collision is one way to break up an asteroid but that doesn’t seem particularly likely in this case. Far more plausible is the suggestion that Phaethon contained some deep pockets of ice that were heated enough to vapourise and fracture the body, breaking off large chunks in the process. Phaethon goes much closer to the Sun than Mercury so the heating is intense, any ice in the outer layers would soon disappear and the deeper layers would eventually warm up.

Small asteroids and comets are probably little more than piles of rubble and dirty ice anyway. Breakups can be messy affairs and there will be a vast number of small pieces and perhaps a few big ones left over.

The total mass of the meteor stream needed to explain the number of Geminids that we currently see is perhaps 10% of the mass of Phaethon. It wouldn’t have to be a complete break up, just breaking off one end might have been enough.

After many orbits the debris gradually separated and now, perhaps 1000 orbits later, it is scattered all around the original orbit. As they separate each particle will experience slightly different planetary encounters so the orbits can diverge significantly.

Fortunately for us the big chunks won’t come anywhere near the Earth, Phaethon’s closest approach this century is 3 million km in December 2093. 2005 UD s closest approach was at its discovery in October 2005 at the very safe distance of 13 million km.

The orbit of the meteor stream also changes over time, in the middle of the 19th century it started to drift across ours and a new but weak meteor shower was noticed. Observations by amateur and professional astronomers over decades have shown that rates have increased as we reach denser parts of the stream. In another hundred years or so it will move off again and we will lose the Geminid meteor shower completely.

There could well be many other chunks of rubble tens or hundreds of metres across left over from the original breakup. These would now be on slightly different orbits to the rest of the meteor stream and are well below current detection levels. It will be interesting to see how the Phaethon-Geminid family of objects grows in the coming years.

We don’t know exactly when Phaethon was deflected into its current orbit. If it was originally an active comet it would have taken many orbits for all the ices to have been lost, and would have been an impressive sight. However it may have been a stony asteroid with pockets of ice. On those first early passes close to the sun any pockets of ice near the surface would have vapourised giving at least some cometary activity. During the actual breakup Phaethon would certainly have looked like a comet, if only for a few days or weeks. It wouldn’t have needed a great deal of ice to fracture the rubble pile so it might not have been as spectacular as an icy comet breaking up.

The answer to the original question really depends on how you draw the line between comets and asteroids, but it does seem rather likely that Phaethon was more asteroid than comet. It may be a long time before we learn from a spacecraft what Phaethon really looks like but those little streaks of light we see each December tell an interesting story.

We talk about meteor ‘showers’, but the word shower is something of a misnomer as far as meteors are concerned. Throughout recorded history there have only been a handful of examples of the kind of display that many people would imagine as a shower.

Even at the peak of the Perseid meteors, if you expect to see at least one meteor a minute you will often be disappointed. Expecting to see a meteor every few minutes is far more realistic but, just like London buses, it's not unusual to wait ages and then find several come along at once! If you go out expecting to see 5 meteors an hour and manage to see 10 you'll be delighted; expect to see 100 but only see 10 and it’s a flop.

When people talk about meteor numbers what they are usually quoting is the Zenithal Hourly Rate (ZHR). This is an idealised figure which assumes excellent sky conditions with the radiant (the area from which meteors appear to come from) directly overhead. This allows useful comparisons of meteor activity from hour to hour, day to day and across the years.

Using the maximum ZHR we can see that some showers, such as the Geminids in December, have strengthened significantly over the last century; while others, such as the Leonids in November, show large variations in activity from year to year. The rate quoted in the headlines is usually the maximum for a shower but unfortunately the rate can vary by the hour so very often there isn't even a single number which applies to the whole night!

The visual rates are mostly calculated from data gathered by small bands of dedicated amateur observers who spend many hours meticulously recording the meteors they see, along with enough information to arrive at (amongst other things) the ZHR. These are the real heroes of #Meteorwatch because without their work we wouldn't know what to expect, and even the best models still need observations to check the results. If you would like to help with this work visit the BAA meteor section pages and the International Meteor Organization (IMO) website.

So how does all this translate into what the average suburban dweller might see at maximum?

Lets start with someone at a reasonably dark site. Typically they would see around 5 random, or sporadic, meteors per hour on any moonless night of the year. If we then get a shower with a predicted ZHR of (say) 100, you might think they would get to see around 20 times more meteors than on an average night.

However, this assumes the radiant is overhead. Unfortunately the lower the radiant is the fewer meteors will pass through the visible area of sky. For the Perseids, at midnight in the UK, the radiant is only 45 degrees up so this reduces the likely rate to about 70 meteors per hour - even under the darkest skies.

Now allow for the light pollution that we all live under which affects the limiting magnitude, that is the faintest stars we can see. There are generally many more faint meteors than bright ones, so orange-lit skies can rapidly reduce the numbers visible. Even in a semi-rural area where you can clearly see the Milky Way this can knock enough off the limiting magnitude, to reduce the rate to 30 per hour.

Move to a small town such as Newbury and you could lose almost another magnitude, so the rate would drop to around 15 per hour. By the time you get to the London suburbs only the brightest meteors are visible and the rate could be as low as 5 per hour - but the ZHR figure is still 100 per hour!

This is not intended to put anyone off but rather to emphasise that, if you go to a secure, dark location, you can dramatically improve the number of meteors you will see. The brightest meteors are the ones people enjoy most, and many of these will still show through light polluted skies - so all is not lost even in the suburbs.

Also don't forget that predictions are just that, we don't have a detailed enough knowledge of where all the debris that causes meteors is, so there is always room for pleasant surprises if we hit a denser patch.

So – if you want to see more meteors – go to the darkest spot you can find (away from street lights), on the nights around the predicted maximum, look up and see what there is to see!

A useful guide to sky brightness in your area is the night sky simulator on the Needless website.

In theory, imaging meteors could hardly be simpler, just point the camera at the sky and leave the shutter open for long enough. In practice you will have to take a lot of images to get more than a handful of meteors. Meteors are so brief and move so fast that there isn't time to expose much of the film or digital sensor, so it is usually only the brightest fireballs that produce impressive images. What appears to the eye as an obvious meteor barely shows up on an image, a good example of just how powerful the human eye can be.

One of the problems with imaging meteors today is that the sky is now so full of man-made objects that you are far more likely to record them than a meteor. To the eye a meteor is obvious because of its fast movement and short duration. On a photograph this shows up a streak of light, but so do satellites and it can be easy to mistake a satellite trail for a meteor, particularly if you weren't watching at the time.

Things like planes are easy to pick out, the red and green or flashing lights are a bit of a giveaway. Mostly satellites show as a steady track, across several frames if a series is taken, and are fairly easily recognised. Sometimes though it isn't quite so obvious so it pays to check, here are a few examples of what to look out for:

A bright Iridium flare This was an Iridium satellite flare with the full path showing how it brightened and faded. At its best this was much brighter than Venus and has been recorded more strongly than an equally bright meteor would have done because of the slower motion. A bright Iridium flare	Partial satellite flare This image from the Perseids last year seemed almost to good to be true, and it was. The trail points back to the radiant but there is something not quite right about the look of it. On closer examination it turned out to be a satellite flare truncated by the end of the exposure. Partial satellite flare	The trail of a small satellite This is a satellite trail through the same region. It wasn't as bright but has recorded more strongly than the meteor. The trail continued on the following image leaving no doubt about what caused it. The trail of a small satellite	ISS passing through Orion This was a pass of the International Space Station which can appear as bright as Venus. For the southern UK there will be some early morning passes of the ISS during the few days around Perseid maximum. ISS passing through Orion
ISS going into earth shadow Visually the ISS can't be mistaken for a meteor, taking several minutes to cross the sky. However catching the trail on camera as it enters earth shadow can give a deceptive image. ISS going into earth shadow	A Genuine Perseid This genuine Perseid was seen as an obvious 1st magnitude meteor and comparable to some of the brightest stars in the sky, but the trail on the image isn't very strong. The green colour is distinctive although it wasn't as prominent as this to the eye. A Genuine Perseid

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