Perseid meteor shower

A long exposure of the Perseid meteor shower, Aug. 11-12, 2016.

Every year in mid-August, the Earth intersects the orbit of the Swift-Tuttle comet, and the Perseid meteor shower occurs.

Swift-Tuttle is a short-period Halley-type comet with an orbital period of 133 years. Swift-Tuttle last visited the inner solar system in 1992, so it will be 107 years before it returns to the sun, but the comet provides the source materials for this annual meteor shower.

When Earth intersects the comet’s orbit, it enters a debris field created from the rocks and dust released by the comet as it melts when it passes near the sun. We entered this debris field in mid-July and will leave it by the end of August. This field is filled with rocky pieces of space debris called meteoroids, most of which range in size from grains of sand to small pebbles.

These pieces of space debris can enter the Earth’s atmosphere at very high speeds — 7 to 46 miles per second. Most are then vaporized by the heat of the air friction, forming the white-hot streaks we see as meteors.

If the pieces are large enough to survive the atmosphere and not be completely vaporized, they will fall to the Earth’s surface. These rocky fragments are then called meteorites.

However, not all meteoroids that enter the Earth’s atmosphere create meteors. Most of them are micrometeoroids, space debris particles only about the size of dust grains. Because these particles are very small, when they hit the Earth’s atmosphere the air resistance slows them so quickly that instead of burning up, they fall gently to the surface of the Earth. Although we cannot see them, it is estimated that 5,000 to 10,000 tons of this material falls to Earth every day.

The word meteor is derived from the Greek word meteoros, which means “high in the air.” Meteors become visible at very high altitudes — 50 to 70 miles — with the faster-moving particles visible at the greater heights. They lose their brightness when they are still 10 to 12 miles above the ground.

In meteor showers, the meteors are on parallel paths; as a result, they all appear to come from the same point in the sky, which is known as the radiant. Because we always intersect the comet’s orbit at the same place in the Earth’s orbit, the radiant is located in the same place in the sky every year.

The shower takes the name of the constellation where the radiant is located. In this case, the Perseid shower has its radiant in the constellation Perseus.

Perseid meteors can be seen any night during the period of the shower. Because of the direction of the Earth’s rotation and orbit, the best time to look for meteors is after midnight, with the very best time being one to two hours before dawn. At that time, you are facing the direction of the Earth’s motion and hitting the particles head-on, just as your car’s windshield hits more bugs than its rear window.

As the Earth passes through the edges of the debris field, you may see as many as six to eight meteors per hour. This rate will increase, with a predicted peak between the mornings of Aug. 12 and 13. On these mornings, rates may be as high as 100 per hour. A few days before and after Aug. 12 will also be good times to view.

This year the moon will be in the new phase just before the peak time, meaning the sky will be dark and viewing should be very good. The constellation Perseus will be in the northeast sky, and the Perseid meteors will come from that direction, but you could see them anywhere in the sky. Since the radiant is circumpolar, you can see meteors all night, although the best time will be between midnight and dawn.

To see the highest number of meteors, you will have to be at a dark site with a full view of the sky. Find a location with clear skies that is not near city lights; the darker the skies, the better. You will see two or three times the number of meteors by going just a few miles out of town.

Each night during the month of August, get a blanket or a chair and go outside for a few hours to search the sky for meteors. If it’s cool outside, you can bring some hot chocolate!

Marty Scott is a resident astronomer at Pacific Northwest Regional Observatory. He designs, builds and maintains astronomical equipment. He also formerly taught astronomy at  Walla Walla University. He can be reached at marty.scott@wallawalla.edu.

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