Saturday, November 21, 2015

33 | Free-form: Meteor Showers

For as long as I can remember, I've headed outside a few times a year to watch meteor showers. A few prominent showers come to mind -- the Perseids, Geminids, and the Leonids. These events are named after the constellations around which they are centered in the nighttime sky.

Geminid Meteor Shower (image:

Although I've watched these mesmerizing events, I never knew why they occurred. Sure, I knew that meteors or "shooting stars" were caused by chunks of rock falling through Earth's atmosphere and burning up along the way, but why do periodic meteor showers occur?

It turns out that meteor showers occur when Earth passes through debris left by a passing comet. Comets are large collections of rock and ice that have highly elliptical orbits around the Sun.


So-called "short-period" comets travel around the Sun in as few as twenty years, which is on the order of the period of planets within the Solar System. Other comets have much, much longer periods. For example, Comet West is predicted to have an orbital period of as high as 6 million years!

Comet West (image: WikiCommons | J. Linder)

As these comets approach the Sun, the ice and rock within the comet begin to break apart. This material streams off behind the comet, which has picked up speed as it is pulled towards the Sun. This material is what gives comets the characteristic "tail" that we are used to seeing.

Comet ISON with its exceptionally long visible tail--20 million kilometers long! (image:

The Earth's orbit occasionally passes through the orbit of a comet. Luckily for the Earth, it would be very unlikely for a comet to actually collide with the planet.


However, the debris left behind a comet remains long after the comet has passed. When the Earth passes through this collection of rocks left by a comet, some of this material hits the Earth's atmosphere and falls to the ground, thus creating meteors. Because there is a high concentration of this debris in a given spot, we observe thousands and thousands of meteors. This is a meteor shower.


Because the debris is at a single point along Earth's orbit, we encounter this material once a year. This results in a shower that occurs at the same time every year.


Because meteor showers are caused by comet remnants, these events are strongest soon after the comet passed near the Sun. This also means that, over time, the meteor showers we are most familiar with will likely fade until the next time the comet passes close to the Sun. Furthermore, it is possible that there are meteor showers that humans have not yet witnessed, as the comets have not passed through Earth's orbit within the last few millennia.


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