Meteors


Tiny particles of dust, usually no larger than grains of sand, enter our atmosphere from interplanetary space with spectacular results. Because they are moving at high speed, their passage through the air creates friction that produces extreme heat and excites atmospheric molecules, causing them to radiate. It also causes the dust particles themselves to vaporize into hot, glowing gas. These burning particles of space dust, known as meteors, are commonly called shooting stars or falling stars. The bright streak of light created by their luminous vapors seems to move quickly across the sky before fading out seconds later.

Primitive man probably thought the stars really were falling when he saw meteors streaking across the night sky. And when the spectacular Leonid meteor shower occurred on November 12, 1833, there were those who believed the world was coming to an end. Don’t laugh; you might have thought so, too, if you had been there to see hundreds of thousands of bright, fiery objects racing toward the earth and had not known you were watching a meteor shower.

When the meteors in a swarm are spread fairly evenly along their orbit, as in the Perseid shower, the earth encounters about the same number of meteors from it each year.

Fanciful people have been making wishes on falling stars for years, but meteors certainly are not rare. If you were able to see the entire sky from the dark half of the earth, you might observe as many as 10 million meteors in a single night. They also occur during the day, but we cannot see most of them when the sky is light. To be visible, a meteor must be within 100 to 150 miles of the observer. On a dark, moonless night an alert person usually can see one every ten or fifteen minutes.

In case you are wondering where this space dust comes from, it is the debris left behind by a passing comet or asteroid. Although most of it is no larger than a grain of sand, occasionally larger pieces find their way into our atmosphere.

As you would expect, the larger the meteor, the brighter it burns. One the size of a baseball, or larger, may reach the brilliance of the full moon and can be seen in the broad daylight. These extremely bright meteors are called fireballs. They sometimes explode in midair, shooting off sparks and making a noise like thunder.

When the meteors bunch up along their orbit, as in the Leonid shower, the earth passes through a denser portion of the swarm some years, and a more spectacular display results.

Most meteors burn up while still thirty to fifty miles above the earth, but a few manage to survive their fiery journey through the atmosphere. In the terminology of astronomers, a meteor that lands on the earth is called a meteorite. Only a thousand or so meteorites fall to earth each year, and most of them land in the oceans or on uninhabited areas. For this we can be very thankful since some of them are quite large and destructive.

Siberia has been the site of the two most spectacular falls in recent time. On June 30, 1908, an object of some kind landed in a forest in central Siberia, killing as many as fifteen hundred reindeer. Trees, with their branches seared from the heat, fell away from the impact point for twenty miles in every direction. The impact registered on seismographs in Europe, and it is reported that a man standing on his porch fifty miles from the site was knocked unconscious. Since no meteoric residue has been found at the site, some scientists have speculated that the impact may have been caused by a small comet. The second meteor, which fell near Vladivostock on February 12, 1947, produced 106 craters in sizes ranging up to thirty yards wide and ten yards deep. Trees fell radically around the larger craters and the damaged area covered nearly two square miles. More than five tons of iron meteorite fragments have been recovered at this site.

A colorful Perseid streaks over Half Dome in Yosemite National Park on August 12, 2004.
Photo © Dirk Obudzinski 2004
www.borealis2000.com

Probably the most famous crater is the Barringer Meteorite Crater near Winslow, Arizona. It is 4,200 feet across and 600 feet deep and has a rim that rises 150 above the level of the surrounding countryside. It is estimated to be 5,000 to 75,000 years old. More than thirty tons of meteorite fragments have been found, and sizable chunks of meteorite iron still are being discovered near the crater under a shallow layer of earth.

Micrometeroites also strike the earth each day, but they are so small they leave no obvious sign of their landing. Insignificant as they may seem, however, it is estimated that fifty to a hundred tons of these tiny meteorites accumulate on the earth’s surface each day.

The random meteors that bombard our atmosphere day and night from any and all directions are generally stray particles from the asteroid belt between Mars and Jupiter, but several times each year we come in contact with swarms of particles moving together through space. As these meteors enter our atmosphere, we experience a meteor shower. Because these meteor swarms follow definite orbits or trails around the sun, the earth encounters certain ones on specific days each year.

In August we have a chance to see the well-known Perseid shower. It lasts for a period of about two weeks, but the peak activity occurs on or about August 11 or 12, and the best viewing is during the hours after midnight until 5:30 A.M. Since the orbit of the Perseid shower is almost identical to that of Comet 1862 III, it is believed that the meteors in this swarm are the debris from that disintegrating comet. The dust particles in the Perseid shower are spread fairly evenly along its orbit, so the earth encounters about the same number of meteors from it each year.

Some swarms, such as the November Leonid shower mentioned earlier, tend to be more bunched up along their orbit. As a result, the earth passes through a denser portion of the swarm some years, and a more spectacular display than usual is seen. Such was the case on November 12, 1933, when as many as 200,000 meteors were seen from one place in a span of a few hours. The Leonid shower is observed e3ach year, but the last spectacular display occurred on November 17, 1966. At that time up to 140 meteors per second were observed in some southwestern states.

All of the meteors in a shower appear to be coming from one point in the sky, but in reality the meteors are traveling in parallel lines twenty to a hundred miles apart. The same optical illusion that makes it look as if railroad tracks come together in the distance causes the meteors in a shower to appear to come from one distant point. Astronomers call this point the radiant. Meteor showers are named for the constellation closest to this radiant. The Perseid shower gets its name from the constellation Perseus.

If you would like to watch the Perseid meteor shower this year, you will find the best viewing after midnight when the constellation Perseus moves up above the eastern horizon. Take this printed sky chart and hold it toward the east to locate the constellation Perseus.

Glare from the moon can lower the number of meteors per hour viewed some years. Since the combination of moonlight and city streetlights can overpower all but the brightest meteors, getting out in the country will improve your viewing. Many people combine viewing a meteor shower with a camp-out in a state park. If you are observing the shower just before dawn, Perseus will be high in the sky. Standing or sitting with your neck bent backward to view the sky can get uncomfortable in a hurry, but lying in a sleeping bag or relaxing in a chaise lounge out under the stars is a comfortable way to observe the meteor shower.

Falling asleep could be a problem. To guard against this, go to bed early and get your rest. Set your alarm for midnight and by the time you are awake, the sky show should begin. View the meteor shower with a friend and help keep each other awake.

Meteors in a shower travel in parallel lines 20 to 100 miles apart, but the same optical illusion that makes it look as if railroad tracks come together in the distance causes meteors to appear to come from one distant point. This point is called the radiant, and meteor showers are named for the constellation closest to the radiant.


Additional Information:

Ilo Hiller
1983 Meteors. Young Naturalist. The Louise Lindsey Merrick Texas Environment Series, No. 6, pp. 118-122. Texas A&M University Press, College Station.

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