The Perseids

Each August, the Perseid meteor shower lights up the night skies, but what causes this spectacular display and why is it so predictable? Let’s take a closer look at one of the year’s most spectacular astronomical events.

Why are they called the Perseids?

When observed from the Earth, meteor showers seem to arise from a radiant, a single point of origin in the night sky. The position of the radiant is different for each meteor shower; for the Perseid shower it lies near the constellation Perseus.

What causes the Perseids?

The Perseid meteors are actually bits of dust, rock, and ice left behind by the comet Swift-Tuttle as it orbits the sun. Each year, the Earth’s own orbit carries it through this debris trail. Although Swift-Tuttle only passes us once every 133 years, the debris trail can linger for millennia, creating a semi-permanent band that follows the comet’s orbit.

Why do the meteors glow?

As debris enters the Earth’s atmosphere, it moves at speeds of up to 160,000 miles per hour and reaches temperatures of up to 3,000 degrees Fahrenheit. This great speed and temperature causes two things to happen: First, the meteor begins to burn, giving off heat and light, and second the air around it ionizes, causing a brief glow.

Do meteors ever reach the ground?

Most meteors burn up in the Earth’s atmosphere. Occasionally, however, a meteor will be large enough that a piece of it will survive and strike the ground where it becomes known as a meteorite (meteoroids are in space, they become meteors when they enter the atmosphere, and become meteorites if they make it to the ground). About 100 tons of debris enter our atmosphere each day as meteors, but only a small percentage of those will reach the ground. Large meteorites, like the one that struck Tunguska, Russia in 1908, occur only once every 300-500 years. Catastrophic meteorites, like the Chicxulub impact, which is believed to have wiped out the dinosaurs, have happened only a few times in our planets 4.5 billion year history.

What are meteorites made of?

There are three basic types of meteors: stony, iron, and stony-iron. Stony meteors are the most common and are made of silicon-based minerals. Iron meteorites are made mostly of iron and nickel. Stony-iron meteors are a combination of metal and minerals. Stony meteorites can also be classified as either chondrite or achondrite. Chondrules are small, spherical bits of rock or metal that were heated and then coalesced into a larger body from which the meteor is derived. Achondrite meteors were heated after the parent body coalesced, causing the distinctive shape of the chondrules to be lost.

What’s the best way to see the Perseids?

Around the second week in August, find a spot with low light pollution and look toward the northeast. For those of us in the Americas, the best views will be predawn as the planet’s rotation brings us around to face the debris field. For the best viewing, lie on your back and give your eyes plenty of time – at least 40 minutes – to adjust to the darkness. Enjoy the show!


Photo: Perseid meteor by Noriaki Tanaka