Meteors, Meteoroids, and Meteorites

Woodcut Depicting Leonid Meteor StormFalling stars, shooting stars, fireballs, bolides - they go by many different names, but they are all meteors. They are not stars at all, but little bits of matter plunging into the Earth's atmosphere and burning up in a fiery display.

As the bit of matter encounters friction with the atmosphere, it heats up and burns away, leaving the bright streak we call a meteor. The particle itself is referred to as the meteoroid. Actually, billions of meteoroids hit the Earth's atmosphere every day, but only a tiny fraction of those are large enough to be seen in the sky. Most meteoroids are very tiny. Even the ones that leave a visible trail are rarely larger than a pea.

Meteor Showers

At a number of specific times during the year, the number of meteors observed increases greatly. During such a meteor shower, it may be possible to see as many as 60 meteors per hour, or more. During a meteor shower, the Earth is passing through a relatively dense region of meteoroids in space. These meteoroids are bits of debris left behind by a comet. Comets are "dirty snowballs" composed of ices and dust. When the comet nears the Sun, some of the ices melt, leaving a trail of particles along its orbit. When the Earth crosses the orbit each year, it encounters the particles, and a meteor shower is seen.

Showers are generally named for the constellation which contains their radiant, the point in the sky from where the meteors appear to be originating. For instance, the Geminid meteor shower has its radiant in the constellation Gemini. You can also think of the radiant as the point in the sky the Earth is generally moving towards at the time of the shower. Imagine driving through a snowstorm, watching the path of the snowflakes as you move forward -- they will all seem to originate from ahead of you, but as they approach, some will pass to the left, some to the right, and some above or below your windshield.

The Geminid shower is generally the most reliable shower, while the Quadrantids are usually fairly sparse. The Perseid shower is the most popular, as it occurs during warmer weather in the Northern Hemisphere. Even though the Leonids are normally not very spectacular, about every 33 years the Leonids are a meteor storm, when for a limited time the rate may exceed 2,000 meteors per minute.

Seven Major Meteor Showers

Date

Name

Rate

Radiant

Associated Comet

January 4

Quadrantids

40/hr

Bootes

?

May 4

Eta Aquarids

20/hr

Aquarius

Halley

July 28

Delta Aquarids

20/hr

Aquarius

?

August 12

Perseids

50/hr

Perseus

Swift-Tuttle 1862 III

October 21

Orionids

25/hr

Orion/Gemini

Halley

November 16

Leonids

15/hr

Leo

Temple 1866 I

December 13

Geminids

50/hr

Gemini

3200 Phaethon *

*3200 Phaethon is actually an asteroid. It is possible the asteroid suffered a collision, leaving the debris responsible for the Geminid shower.

The best time to observe a meteor shower is usually between midnight and dawn. Imagine you're driving a car moving in the rain. You'll get more rain drops and bigger splashes on the front window than on the back window. As the Earth orbits the Sun and rotates, we are looking in the backward direction of Earth's orbit before midnight, and in the forward direction after midnight. (The exception to the rule is the Geminid meteor shower in December; due to the angle of the debris trail's orbit, the Geminid shower can usually be seen just as well before midnight as after.)

Meteorites

Sometimes, a meteoroid is big enough that it can survive its passage as a meteor through the Earth's atmosphere, and actually strike the ground. The part that reaches the ground as a solid chunk is called a meteorite Most of these originate from the asteroid belt, though some have been shown to be rocks from the Moon or Mars, flung into space during a collision with perhaps another large meteoroid.

Of those meteoroids large enough to survive the trip through Earth's atmosphere, there are three basic classifications -- stony, iron, and stony-iron. Although most meteorites are stony, irons are most commonly found because they more obviously differ from rocks naturally found on Earth. Irons are very dense, are magnetic, and appear to have been somewhat melted by their fiery fall. Irons are composed of about 90% iron and 9% nickel. Stony meteorites are composed of low-density materials similar to Earth's surface rocks, while stony-iron meteorites represent a blend between the stony and iron types.

Observing meteors

Don't use a telescope or binoculars when looking for meteors - they can only point to a small portion of the sky -- the next meteor will almost always appear in somewhere else in the sky. Meteors move so quickly, you would never be able to align your telescope with them before they disappeared.

Instead, it is best to lie close to flat on your back (say, on a blanket, a lounge chair, or the hood of your car) and generally gaze at a wide area of the sky to spot the meteors.

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