Meteors

Zooming across the sky in a twinkling of a eye, "falling stars" are usually tiny, sand grain-sized bits of material being burnt into incandescent vapors by the friction of the Earth's atmosphere. Bright meteors are thrilling even to the most jaded sky watchers. What do we know about meteors? Where do they come from?

While the Sun and the planets are the objects of the Solar System that get most of our attention, most of the objects in the Solar System are the satellites, the minor planets (asteroids), and meteoroids. Meteoroids become meteorites when they enter the Earth's atmosphere. Since they have speeds of five to 25 kilometers per second (up to 50,000 miles per hour), they are rapidly heated by the friction of the upper atmosphere and leave a trail of incandescent hot gas. They first become luminous at altitudes of 60 to 120 kilometers and travel tens of kilometers before burning out. The typical visible meteor is probably caused by a particle about the size of a grain of sand.

On any night, a patient observer should be able to see several meteors per hour. You must be away from bright lights (including the Moon). The rate of observed meteors increases after midnight as then the part of the Earth on which you're sitting has turned into the direction of travel of the Earth around the Sun and you see the material as the Earth sweeps it up.

Some meteorites, called the carbonaceous condrites, have almost the identical abundance of the elements as the atmosphere of the Sun. Since they are uniform in composition (undifferentiated), they have not been melted, which can cause various elements to segregate. Thus they seem to be little changed from the early Solar System when the planets were first forming. Various dating techniques indicates these objects are about 4.6 billion years old; confirming geological estimates of the age of the Solar System.

About 90% of chondrites are called ordinary condrites and they have been changed by heating and differentiation. Thus they are thought to have been formed deep inside much larger bodies which compressed and heated them. These parent bodies were then broken up, probably by impacts and the debris floated around the Solar System until, by pure chance, they were swept up into the Earth's atmosphere.

Finally, there is a group of chondrites called achondrites, that are made up of a common kind of lava. These too must have come from larger bodies that compressed enough under the force of their own gravity to melt rocks.

So, most meteors come from larger Solar System bodies that have broken into smaller pieces. For many of these, they are probably the result of impacts over billions of years of the smaller bodies of our Solar System.

Several times a year, many more meteors are observed than usual. These meteor showers, which last from a few hours to a few days, occur each year on the same date and the extra meteors seem to come from the same location in the sky, the radiant of the shower.

Here is a list of the most prominent meteor showers:

For the most clumpy orbits, such as the one left by Comet Temple 1886, every 33 years the rate of meteors rises from about 15 per hour to more than 100,000 per hour! Truly, a once in a lifetime event!

The older sections of the Moon are heavily cratered by meteor impacts, indicating that they were far more numerous near the beginning of the Solar System. Still, 50 million tons of new material strike the Earth every year.

The meteorites that are typically seen in museums are the black, heavily-pitted, iron meteorites. They are easy to recognize as un-earthly material that has gone through traumatic heating after it was formed. Such meteorites, although popular in museums, represent only about five per cent of the meteorites. about 95% are stony meteorites, composed of the same materials found in ordinary terrestrial rocks: iron and magnesium silicates.

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