Meteor Showers: What Are They, How Often Do They Happen, and Why?
Have you ever looked up at the night sky and seen a shooting star fly across it? Perhaps you've had the pleasure of witnessing a meteor shower light up the night. These shooting balls of rock are meteors, and beyond just being a pretty cool natural phenomenon, they actually have a lot of science behind them — and you might have some questions.
What actually are meteors? How often can you expect to see meteors? Why are meteors even a thing? All these questions are valid, and the answers to them are more complex than you might imagine.
What causes meteor showers?
Meteor showers are essentially space debris that falls through the Earth's atmosphere, burning upon its entry to our atmosphere. As the Earth orbits around the Sun, it often passes through debris left over from the disintegration of comets.
Comets themselves are generally not made up of solid pieces of rock. A comet's nucleus is made up of a combination of icy materials and loosely consolidated 'rocks' or 'dirt.' When a comet is heated by passing close to the Sun, it slowly disintegrates. This disintegration and trailing off of matter is what produces the visible tail of comets.
To give you an idea of their sizes, the core part of a comet can be tens of kilometers large and the tail itself can stretch out for millions of kilometers.
The rocky debris from the comet, which consists of mostly sand-sized particles, continues in an orbit around the Sun close to that of its parent comet. When the Earth intersects this orbit, it can run into this debris, which burns upon entry into the Earth's atmosphere, producing a visible shower of meteors.
Because many comets have regular orbits, you can actually check their schedules and watch out for the best times to see them, and their associated meteor showers, in the sky. But back to meteors!
Meteor showers are generally made up of the debris that is left behind from comets. When this is the case, each meteor shower will have a specific time associated with it throughout the year in correlation to the comet that is passing. At times when a given comet passes close to the Earth, the meteor showers may be particularly common.
How often do meteor showers occur?
Just like comets have schedules, meteor showers generally do too. Meteor showers that are associated with particular comet orbits occur at about the same time each year since the comet crosses the Earth's orbit at around the same time each year.
In particular, the month of November is known as one of the best months of the year to observe meteor showers in the sky, because this is when the Earth's orbit intersects with that of the comet Tempel-Tuttle, resulting in the Leonid meteor shower.
However, because some parts of the comet's path are richer in debris than others, the strength of a meteor shower from a particular comet tends to vary from one year to the next. Meteor showers tend to be strongest when the Earth crosses the comet's path shortly after the parent comet has passed.
Meteor storms are the most intense type of meteor showers and are defined as having at least 1,000 meteors per hour.
Meteor storms are generally caused by young meteor streams, in which the bulk of the streams’ mass is concentrated along the portion of the orbit occupied by the parent comet. Meteor storms occur when the Earth crosses the orbit of the meteor stream at the same time that the main mass of the meteor stream is crossing the orbit of the Earth. These streams tend to be very narrow, so the storms often only last a few hours. To see one, viewers need to be in just the right place at the right time with the right conditions.
Because the streams of young meteors also happen to be spaced incredibly close together in space, meteor storms are generally only visible in particular spots around the globe. This means that seeing these events can involve a decent amount of luck.
There are two meteor showers that commonly occur in November. These are known as the Andromedid and the Leonid shower, although the Andromedid shower is no longer generally visible to the naked eye. While the Leonids tend to produce a meteor storm every 33 years or so, the last time that a storm was produced by the Andromedid stream was in November of 1885, which produced an estimated 13,000 meteors per hour at the peak of the storm.
That said, the Leonid meteor stream tends to produce storms much more frequently. In 1966, this meteor stream created one of the highest known rates of any meteor storm ever recorded. The rate was thousands of meteors per minute, although the storm only lasted for around 15 minutes.
Perhaps the best known of all meteor showers is the Perseid shower, which is generally visible from mid-July to mid-August and produces relatively high meteor speeds, brightness, and a high proportion of trains, although there has been a gradual decline in activity from a peak in 2004.
As for other meteor showers and storms, you can actually look at schedules for where and when you can see them. The American Meteor Society, for example, does an incredible job of scheduling each meteor shower along with when and where you can best view them from here.
How fast do meteors travel?
Believe it or not, the meteors that you see in a meteor shower that light up the sky are mostly just particles that are the size of a pebble or grain of sand. You can see them so well due to the immense heat and effects of entering the Earth's atmosphere.
These particles weigh just a few grams but are often composed of dense metallic materials that will produce bright lights when heated upon entry.
It's clear then that meteors have to reach incredible speeds if the reason that we can see them is because of them spontaneously bursting into flames upon re-entry. But what speeds do they travel at? Most travel at very high speeds, ranging from 25,000 mph to 160,000 mph (11 km/sec to 72 km/sec).
This amount of speed causes the particles on and around the meteor to rapidly ionize, creating a visible light trail behind it, which is what we see. The ionized paths that these tiny meteoroid particles create are pretty incredible too. The length of a meteor's ionized path can be upwards of 6 miles (10 kilometers).
The speed of the meteoroid can change drastically. If it enters the Earth's atmosphere on the trailing edge, the meteoroid has to catch up to the atmosphere, resulting in a rather slow relative speed. On the other hand, when the meteoroid enters the atmosphere on the leading edge, the speed of the Earth and the meteor combine to produce exponentially faster speeds.
What are the different types of meteors?
The last question you might be asking is, is there just one kind of meteor? The short answer is no.
Meteoroids are the smallest members of the meteor family and can be just tiny grains of sand all the way up to larger fragments. The meteoroids on the smaller end of that spectrum are called micrometeoroids.
The meteoroids are the actual particles, but as the particle moves across the sky creating a light, this is then called a meteor.
When a meteoroid survives the trip through the atmosphere and makes its way to the Earth's surface, it then becomes a meteorite. Meteorites can be found in many places around the Earth, and depending upon size and quality can be considered quite valuable.
Fireballs are larger explosions of light and color that can last longer than an average meteor streak. Unlike meteors, which are made up of small pieces of debris, fireballs originate from larger particles of cometary material. They are also brighter. Earthgrazers are meteors that streak close to the horizon and are known for their long and colorful tails.
It's the scale in these matters of space that often may astound you. Here on earth, a comet passing by may just seem like a tiny blip in the sky, but in actuality, it's anything but.