No, not Bonfire Night. We’re talking celestial firework displays! It’s October, and once again we’re coming up on the time for the annual Orionid meteor shower.

There’s been a trend over recent years of various parts of the media getting a bit hysterical about various astronomical phenomena, and in some cases hyping them up waaaaaay beyond any sensible justification and raising expectations to totally unrealistic levels. So if you’ve arrived here having heard stories about how spectacular the Orionids will be on October 21st, should you believe the hype? TLDR: no, but…

What is a meteor, anyway?

A meteor is actually quite mundane: a small piece of rock, generally smaller than a grain of rice that disintegrates as it flies through our upper atmosphere while travelling at many kilometres per second.

You can see meteors on any clear night of the year, all you have to do is find somewhere dark, look up, and be patient. These are called sporadic meteors and are just the detritus left over from the formation of the solar system, or debris from collisions between rocky bodies in the solar system. In the absence of something like a planet to crash into, they just float gently around in space not doing very much.

Usually you have to wait a while before you see a sporadic meteor, although because they are distributed randomly in space they don’t turn up a regular intervals and you may see a handful close together if you’re lucky.

Meteor showers, on the other hand, can be much more spectacular, and come round predictably at the same time each year. In the case of the Orionids, that time is October 21st – or thereabouts.

Origins of the Orionids

All meteor showers are the result of the Earth passing through regions of space with a higher than average concentration of these dust and rock particles. This happens because stuff gets left behind when comets (and some asteroids) go about their normal business on elliptical orbits around the Sun.

Comets are a bit like giant, dirty snowballs, containing large quantities of both ice, frozen gases, dust and rocks, and other volatile substances. We currently know of more than 3800 comets in the solar system, some of which we have actually visited giving us a much better idea of their chemical composition. Samples collected by the Stardust mission even finding the presence of the amino acid glycine, one of the fundamental building blocks of all life as we know it.

Comets spend most of their lives in the far reaches of the solar system where conditions are very cold indeed. Since comet orbits are elliptical, and centred on the Sun, their orbits also take them into the inner solar system for some of the time.

As a comet approaches the inner solar system it get closer to the Sun and so absorbs more solar radiation, heating the nucleus and causing some of the ice to sublimate – that is it turns directly from a solid ice into a gaseous vapour. As the ice turns to gas, the dust and rock particles embedded in it are released and float away, leaving a trail of debris behind the comet as it travels around the Sun.

 When the path of the Earth happens to cross one of these debris trails, we see an increase in meteors coming through our atmosphere.  This is the origin of a meteor shower, and explains why they are regular with predictable dates of activity.

The Orionid meteor shower is the result of debris left behind by Halley’s comet, one of the most famous comets in our solar system. The comet only returns to our skies once every 76 years (and is not due back until 2061), but the Earth travels through the debris trail each year, giving us the regular Orionid shower in October, and also the less well-known Eta-Aquariid shower in April/May.

Variations

A given meteor shower may not have the same level of activity, year to year. Some years a shower might be fairly unimpressive, with peak rates of only a few per hour. Other years we might have a much larger spike in activity and see rates of several hundred an hour. Rarely we might see rates of more than 1000 per hour – rarely seen meteor storm.

Imagine an aeroplane passing through the sky on a sunny day, leaving a contrail behind it in the sky. If you sit and watch that trail, it slowly expands, becomes less dense, and eventually disappears. Comet debris trails are a little like that (although not as easy to see, being made up of tiny dark particles of rock!).

As that trail ages, it gets less well-confined, the particles move apart slowly. Each time the comet comes round on its orbit, it deposits a fresh trail of denser debris along its orbital path. Add in the fact that the gravitational influence of the likes of Jupiter (and any other planet the comet comes relatively close to) can alter the trajectory of the comet, and you start to get a sense of why the number of meteors we see varies from year to year as the Earth passes though denser or less dense parts of the debris trail.

Very clever folks (such as the IMCCE meteoroids and meteors group) take observational data on the number of meteors observed each year (collected by seasoned observers – and you can help!) and build models of the debris trails for each meteor shower, and use those to make predictions about the number of meteor showers we are likely to see the following year. They are usually pretty accurate, but sometimes we see unexpected spikes in rates that were not possible to predict from past data alone – so it’s always worth a look.

What to expect in 2023

This year we are expecting good observing conditions on the peak night of October 21st, with the Moon at 48% illumination but setting before midnight.  The best time to observe will be after midnight when the Moon sets and the constellation of Orion will have risen – this is the location of the radiant of this shower, the location on the sky the meteors appear to come from, and what gives the shower its name.

Some showers are particularly spectacular with more than one a minute on average.  We’re not expecting that this year for this shower, with predictions of around 20 or so per hour.  However, any shower has the potential to be spectacular, so it’s always worth a look!

The best way to observe is to find somewhere away from street lighting, wrap up warm, and look up!  Obviously, you also need clear skies, but don’t worry if October 21st is cloudy as the streams of debris that cause most meteor showers are wide enough to provide activity over more than one night.  Catching meteors takes patience, but can be worth the effort, and can contribute to citizen science projects.

Clouded out? I expect a lot of us (here in the UK, anyway) will be, thanks to Storm Babet. You might have more luck in other parts of the world. The nice thing about the Orionids is that they are equatorial, meaning you can see them from both the northern and southern hemispheres.

If you do miss them, don’t despair. There are plenty more meteor showers in the calendar!

In December we will see the return of the Geminid meteor shower with a predicted peak of ~150/hour! Much more spectacular! The Moon will also be favourable as it will be very close to the Sun and not up during most of the night, aiding dark sky conditions which help you see the fainter meteors.

So, if you miss out this weekend, make a note in your calendar of December 14th, and make sure you remember to take a look!