It's meteor shower season once again - but what actually are they?

Anyone who has spent time lying back and casually looking up at the sky has likely seen the fiery trail of a meteor which streak across the sky every few minutes on a typical night. During meteor showers, the rate can increase dramatically and all of the meteors appear to originate near a point called the radiant. This week Justin Kerr discusses the source of these fascinating features of the night sky.

(Image source: NASA/Bill Dunford)

by Justin Kerr

With another year of the Perseid meteor shower drawing to a close next week, many of us have been lucky enough to see quite the show while outside of the city. For those who haven’t, you still have until approximately August 24th to catch a glimpse of it – and if you can’t get a good view by then, there will be more opportunities to see a major meteor shower later in the fall. But what actually are these meteor showers, and why is it that each one appears at the same time each year like clockwork?

Meteor showers are events in which a large quantity of meteors are visible in the sky and all appear to be originating from a single point in the sky. This apparent singular origin is how each of the recurring showers derives it’s name, with the shower taking on a name based upon the constellation which contains that apparent origin of the meteors. The meteors themselves are caused when small pieces of rock (meteoroids) enter the Earth’s atmosphere while traveling at tens of thousands of km/h (tens of km/s) relative to the Earth and begin to burn up. Since the rocks involved in meteor showers are typically only around the size of sand grains, they completely burn up in the atmosphere and never impact the Earth as a meteorite. Even though they are so small, we can still see such bright light as they burn up thanks to the intense heating by friction with the air resulting from their high velocities. Some meteors even leave trails of ionized gases in their wake, giving us a glowing trail to see for a few seconds after the meteor has burned up. The reason they all originate from the same apparent location along with why they occur on a yearly schedule is linked to the origin of these small space rocks.

These large groups of meteoroids striking the Earth are not just bits of rock leftover from the birth of the solar system or visitors from the asteroid belt. They stay in very specific orbits, which gives astronomers a clear clue to their origin. The meteors we see during meteor showers are in fact the remains of comets, which fill their orbit with debris as the ice holding them together melts away each time they pass the sun until eventually they are all that is left. Due to conservation of momentum, the small rocks contained in the comets stay in the same orbit as the comet once breaking free. At this point, we then have an orbit filled with meteoroids ready for Earth to strike instead of one large comet.

The yearly recurrence of meteor showers is simply due to the astronomical definition of the year itself – the (approximate) time it takes for the Earth to complete a full revolution around the Sun. The orbit of the Earth crosses the orbit of each dead or dying comet only at one point. Every time the Earth reaches that point in the orbit each year, it swings through the cloud of meteoroids and gives us a beautiful show in the night sky. Each of the different meteor showers we are familiar with come from the remains of a different comet, and so occur at a different time of the year when the Earth reaches that intersection location in it’s orbit around the Sun. The currently occurring Perseid meteor shower comes from the comet Swift-Tuttle, while the upcoming Orionids are leftovers from the famous Halley’s Comet. There are two meteor showers which are thought to be caused by the remains of asteroids instead of comets, most notably the Geminids originating from 3200 Phaethon, but all others we know of are the result of comets.

While many of us may fear the impact of a whole comet or asteroid, the tiny pieces of them hitting us during meteor showers are an entirely different story. To see for yourself, keep an eye on the sky during the night up until August 24th to catch the Perseids. The best time for viewing meteor showers is typically just before dawn, but any time after dark when the constellation the shower is named after is visible will do. While you will have a much better chance of seeing a meteor if you are outside of the city, it is even possible to catch some here in Toronto – I have even been lucky enough to spot a few while taking my dog for a walk in the cooler weather after dark! For a much better show, you can check out various areas outside of the city with a much darker sky – for areas relatively near Toronto, I can suggest the Torrance Barrens Dark Sky Preserve or a camping trip to Long Point Provincial Park (pandemic restrictions permitting). While the Perseids are nearly finished for the year, some of the other best opportunities for viewing a meteor shower this year are yet to come with the Orionids peaking on the night of October 21st, the Leonids on Novermber 16th, and the Geminids on December 13th. Make sure to keep your eyes on the sky this fall and catch a glimpse of the fiery end of some tiny pieces of comet!