A Sudden Arc of Brilliance

Last Friday night, I had the pleasure of observing with a group of children and their parents far up in Sherman, CT, right at the border with New York. The skies were similar in darkness to my home location in Southbury, but we were in a large open space with nearly clear horizons, a great treat for me, since my yard is surrounded by nearby trees.

After completing the setup and alignment of the telescope, I was beginning to describe the next object we were going to observe, when out of the corner of my eye I caught a great brilliant arc far off to the north, growing brighter as it approached the horizon. Before vanishing in the distance, the falling object broke apart into at least 3 pieces.. It was all over in a matter of a second or two. The brightness of this object outshone the quarter moon that was sinking toward the west. This was the brightest meteor I have ever seen in my many years of observing.

Most of us are familiar with meteors, romantically described as shooting stars. You may have had the luck to have seen one or more meteor showers in your lives, when dozens or more of these fast-moving streaks of light can be seen in an hour. Very bright meteors - that appear brighter than the brightest planet (Venus) are labeled fireballs. Most of the larger meteor showers include large numbers of fireballs.

What we saw that night, however, was a bollide. A bollide is significantly brighter than a fireball, can be seen to break up into smaller bodies near the end of its descent to Earth, and will be accompanied by an audible explosion if you are near enough to the falling object. Even bollides, however, rarely reach the ground as meteorites, having disintegrated into dust thousands of feet above the ground.

So of what are meteors made, and from where do they come? As we have discussed previously, the meteors we see during meteor showers have come from the gradual evaporation of comets as they pass near the Sun, leaving behind sand in their wake.  However, most of the brighter, larger, meteoric bodies are not associated with comets, but are independent rocks formed from the primordial cloud of gas and dust from which the Sun and planets were created over 4 billion years ago.

Meteors represent samples of the building blocks of the solar system, unchanged by the action of weather, volcanic melting, or geological compression and metamorphism.  From collections of meteorites - the meteors that actually reach the ground in pieces larger than dust - we can learn a great deal about the composition of the early solar system. 

By far the most common meteorites (chrondites) are similar in composition to the rocks in your backyard, found throughout the crust and mantle of the Earth.  A subgroup of chrondites resemble the composition of the Sun, without the gases present, of course.  About 5% of meteorites are composed of a dense mixture of iron and nickel - the same materials of which the cores of the Earth and other rocky planets consist.

Meteors enter Earth's atmosphere at over 20,000 miles per hour, and heat up to several thousand degrees in a matter of seconds.  They begin to glow between 70 and 35 miles above Earth, and usually dissipate several miles above the ground.  Despite common thinking, the meteor is very rapidly slowed by the lower atmosphere, and stops glowing more than 10 miles above the ground, and all but the most massive meteorites hit the ground at less than 100mph.  The burning off of the outer layers while the meteor is glowing followed by the much slower descent to the ground means that the average meteorite, if found immediately after impact, is only slightly warm, and can be easily handled.

Meteors are continually falling on the Earth.  Several hundred tons of meteor material  are deposited on the Earth every day, mostly in the form of dust.  Approximately 4,000 fireballs occur each day, though only a very few are observed by humans - half would occur during daylight hours, and another quarter in the deep night.  It is estimated that about 100 meteorites reach the ground daily, of which approximately 12 would be in areas man inhabits.

The American Meteor Society collects reports of fireballs and correlates them by time and location to attempt to identify individual events.  Last Saturday morning, after my observation the night before, I found that the AMS had over 70 sightings of this massive meteor, ranging from the Catskill area of New York up through Ottawa.  The event seemed to be centered in southern New Hampshire.