Wednesday, December 20, 2017

What causes shooting stars?

One of my first introductions to astronomy was being woken up by my mom in the middle of the night to go lie in the middle of the driveway and watch meteor showers. It requires no telescope or special equipment, just enough willpower to drag yourself out of bed if you're interested in catching the peak viewing time.

A representation of me at 2am
However, you don't need to watch at the peak of a meteor shower to see meteors (or shooting stars, as they're sometimes called). Why is this? And what IS a shooting star anyway? Can you find one that has fallen to the ground? Why do meteor showers only happen on specific dates?

A shooting star isn't actually a star at all. It's a meteor. Meteors are anything from space that falls through the Earth's atmosphere, burning up as it falls in. Most meteors are only as big as a grain of sand! They burn up as they enter the atmosphere because they're going SUPER fast. Like thousands of miles per hour fast. When they hit air it creates a lot of friction and that heats the grain of space rock up so hot that it disintegrates!

Some impressive dust specks. 
Jeff Dai/Arizona State University
Some space rocks wandering across the path of Earth's orbit are bigger than a grain of sand. They might be anywhere from pebble sized to larger than a car. The one that fell in Chelyabinsk, Russia in 2013 was estimated to be 20 meters (65 ft) wide before it started disintegrating in the atmosphere!

The Chelyabinsk meteor's trail
Alex Alishevskikh - FlickrMeteor trace

That one made it to the ground and was recorded as it fell and broke apart- a very rare event. It created a shockwave when it exploded in mid-air that shattered windows for miles around. When a meteor reaches the ground it is then called a meteorite, and can be collected and studied to find out what kind of minerals it's made out of, which can tell scientists more about when it formed and under what conditions.

Many of our meteor showers come from the tails of comets. Comets have very long period orbits, so they swing in close to the sun, begin melting, then swing way out into the outer solar system again and re-freeze. The melted dusty "tail" of the comet is what the Earth flies through when we experience a meteor shower. If the comet has passed by recently then we get an extra-dense dose of dust resulting in a better meteor shower. The Orionids, for example, are caused by the dust trail of Halley's Comet which passes by Earth every 74-79 years. We fly through part of the dust trail every October. However, since meteors are caused by any object burning up in Earth's atmosphere, shooting stars can haphazardly appear at any time. It just has to be a dark enough sky that they show up against the background!

A comet that could produce a meteor shower

The next few meteor showers coming up aren't very strong ones- the Ursids which peak on December 22nd don't produce a high rate of meteors, though you might see the odd shooting star if you happen to be out and about. The Quadrantid meteor shower peaks from January 2nd to January 3rd, but there will be a full moon for that one which will make the sky too bright to see some of the dimmer meteors. However, there are several meteor showers every year! Check them out and don't forget the hot chocolate and warm blankets while you're lying in the driveway. ;)

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