This guest post was written by Laura Marseglia for an assignment in the Fall 2019 Foundations of Modern Astrophysics class taught by Professor Cara Battersby. As part of the course, students were tasked with writing an Astrobite-style summary of a topic in astronomy. Stay tuned for more bites in this series!
Laura is a sophomore at the University of Connecticut. She is studying physics, inspired by her experiences at Frosty Drew Observatory, and aspires to keep making astronomy more accessible to people outside of academia. Besides physics, she enjoys hiking and needlepoint.
The first thing that many people think of when someone mentions a meteor shower is hundreds of shooting stars streaking across the sky. However, if you asked for specific details, most people would probably draw a blank. Some people might think that meteor showers are like a cosmic fireworks display. They might also say that a meteor shower only lasts for one night, or maybe for two or three. People who don’t have a lot of experience observing meteor showers often have many misconceptions, so let’s confront them head-on.
Let’s start with some definitions. Many people confuse the terms asteroid, meteor, meteorite, and comet. An asteroid is a small rocky body that is in orbit around the Sun. A good example is the Asteroid Belt between Mars and Jupiter, which has rocks that range between the size of cars and houses, down to the size of a grain of rice. A meteor, on the other hand, is any inter-planetary rock that enters Earth’s atmosphere. These tend to be very small, usually the size of a grain of sand or even a dust particle. Every once in awhile the Earth passes through a particularly dense patch of particles, which causes a meteor shower. A meteorite is simply a meteor that reaches Earth’s surface; think of the rock that killed the dinosaurs, or the Chelyabinsk Event in Russia in 2013. Comets, though, are a little more complex.
The source of the aforementioned dense patches of dusty meteors is actually the debris left over from comets. Comets are made in part of smaller rocks clumped together, and ices such as water, ammonia, and methane. When comets get close to the Sun, the ice starts to sublimate due to the Sun’s heat. The debris forms a tail behind the comet, some of which is left behind as the comet travels closer toward the Sun. Meteor showers are caused by the Earth passing through the paths of comets and encountering the leftover ice and dust, which is then burnt up in the Earth’s atmosphere.
So, if the ice holding comets together evaporates each time the comet passes the Sun, why doesn’t the comet eventually disintegrate? In addition to the comet’s tail, a “halo” of ice forms around the comet (Fig. 1). The ices in the halo refreeze onto the comet as it moves away from the Sun again. To reiterate, some of the debris from the comet remains in the halo while the rest ends up in the tail. Only this ice and dust in the tail is responsible for the “shooting stars” seen in Earth’s atmosphere.
The reason this dust burns up in the atmosphere is because of its extreme velocity relative to the Earth (about 160,000 miles per hour). The friction between the dust particles and the molecules in Earth’s atmosphere cause the dust particles to heat up to the point of disintegration. Interestingly, most of these particles burn up 60 miles from the Earth’s surface, where the density of the atmosphere is almost zero. Then the few molecules of air that do exist this high up burn the dust from the comet within seconds, and these events are bright enough for us to see down at Earth’s surface. Due to the distance the particle would have to travel, the friction it would endure, and also the fact that it is very rare for a comet to leave behind particles much larger than grains of rice, it is very unlikely that a large meteor will fall to the surface of the Earth.
With this knowledge of comets we can more formally define meteor showers. They occur when the Earth passes through a cloud of debris left over from a comet. The meteors appear as bright lights (roughly the size of a star) that radiate outward from a specific point in the sky, usually in or near a constellation. The specific constellation where the radiant point is located is what gives a meteor shower its name. For example, the shooting stars of the Orionid meteor shower that occurs in October, seem to originate from the constellation Orion; similarly, the Perseid meteor shower, which happens in mid-August, seems to radiate from Perseus.
The first instance of a scientist determining the origins of a meteor shower was in 1867. In his book Notes Upon the Astronomical Theory of the Falling Stars, Italian astronomer Giovanni Schiaparelli hypothesized the relation between comets and meteors. The next advance in this regard was made by Donald Yeomans at NASA-JPL in 1981, who compared the orbits of the comet 55P/Tempel-Tuttle to the appearance of the Leonid meteor shower. Since then scientists have been able to correlate the orbits of different comets with the occurrence of meteor showers.
When a news article is written for the general public about meteor showers, the article tends to portray the duration of the whole meteor shower to be the length of the peak. In reality, a meteor shower can last for about two weeks. However, the peak of a meteor shower, a period during which the number of meteors per hour increases considerably, usually lasts two or three days. For example, the Perseid meteor shower (caused by the comet 109P/Swift-Tuttle), has a peak of about three days (August 11-13 of 2019). However, Earth passes through Comet Swift-Tuttle’s debris field between about July 17 and August 24, and a significant number of meteors were visible starting on August 8. In addition, many people also don’t recognize that meteors don’t occur with great frequency, even during the peak of a meteor shower. During the Orionids, you would only be able to see about 20 meteors per hour during the peak. Comparatively, for one of the largest meteor showers, the Perseids, you could see 200 meteors an hour in 2018, or about three meteors a minute.
By now, you should have a better understanding of the origins and characteristics of a meteor shower. The next time one occurs, try to watch one for yourself – and spread your newfound knowledge of the science behind it to whoever is watching with you.