Meteors, Meteroids, and Meteorites

A meteor is a bright streak of light in the sky (a "shooting star" or a "falling star") produced by the entry of a small meteoroid into the Earth's atmosphere. If you have a dark clear sky you will probably see a few per hour on an average night; during one of the annual meteor showers you may see as many as 100/hour. Very bright meteors are known as fireballs; if you see one please report it. Here are a few short definitions of terms:

Meteoroid: An object floating in space

Meteor: That same object falling toward Earth and lighting up to incandescence from the friction of Earth's atmosphere. This is the "shooting star" event that you can see on almost any given evening ... if the clouds are gone.

Meteorite: That same object that is large enough to survive the frictional heating and vaporization such that it ends on the Earth somewhere for you to find.


Meteorites are bits of the solar system that have fallen to the Earth. Most come from asteroids, including few are believed to have come specifically from 4 Vesta; a few probably come from comets. A small number of meteorites have been shown to be of Lunar (23 finds) or Martian (22) origin.

One of the Martian meteorites, known as ALH84001 (left), is believed to show evidence of early life on Mars.

Though meteorites may appear to be just boring rocks, they are extremely important in that we can analyze them carefully in our labs. Aside from the few kilos of moon rocks brought back by the Apollo and Luna missions, meteorites are our only material evidence of the universe beyond the Earth.

 

 


Meteorite Types

Iron

Primarily iron and nickel. Similar to type M asteroids

Stony Iron

Mixtures of iron and stony material. Similar to type S asteroids

Chondrite

Most common meteorite. Similar composition to mantles and crusts of the terrestrial planets.

Carbonaceous Chondrite

Similar in composition to the Sun without the volatiles. Found in type C asteroids

Achondrite

Similar to terrestrial basalts. The meteorites believed to have originated on the Moon and Mars are achondrites..

 

A "fall" means the meteorite was witnessed by someone as it fell from the sky. A "find" means the meteorite was not witnessed and the meteorite was found after the fact. About 33% of the meteorites are witnessed falls. The following table is from a book by Vagn F. Buchwald. Included are all known meteorites (4660 in all, weighing a total of 494625 kg) in the period 1740-1990 (excluding meteorites found in Antarctica).

Meteorite Statistics

Type

Fall%

Find%

Fall Weight

Find Weight

Stony

95.0

790.8

15200

8300

Stony-Iron

1.0

1.6

525

8600

Iron

4.0

18.6

27000

435000

 

A very large number of meteoroids enter the Earth's atmosphere each day amounting to more than a hundred tons of material. But they are almost all very small, just a few milligrams each. Only the largest ones ever reach the surface to become meteorites. The largest found meteorite (Hoba, in Namibia) weighs 60 tons.

The average meteoroid enters the atmosphere at between 10 and 70 km/sec. But all but the very largest are quickly decelerated to a few hundred km/hour by atmospheric friction and hit the Earth's surface with very little fanfare. However meteoroids larger than a few hundred tons are slowed very little; only these large (and fortunately rare) ones make craters.

A good example of what happens when a small asteroid hits the Earth is Barringer Crater (a.k.a. Meteor Crater) near Winslow, Arizona. It was formed about 50,000 years ago by an iron meteor about 30-50 meters in diameter. The crater is 1200 meters in diameter and 200 meters deep. About 120 impact craters have been identified on the Earth, so far (see below).

 

 

 

 

A more recent impact occurred in 1908 in a remote uninhabited region of western Siberia known as Tunguska. The impactor was about 60 meters in diameter and probably consisting of many loosely bound pieces. In contrast to the Barringer Crater event, the Tunguska object completely disintegrated before hitting the ground and so no crater was formed. Nevertheless, all the trees were flattened in an area 50 kilometers across. The sound of the explosion was heard half-way around the world in London.

 

There are probably at least 1000 asteroids larger than 1 km in diameter that cross the orbit of Earth. One of these hits the Earth about once in a million years or so on the average. Larger ones are less numerous and impacts are less frequent, but they do sometimes happen and with disastrous consequences. Below are two crater images from Quebec, Canada. On the right is the Manicouagan Crater, and to the left is the Clearwater Crater. I had a real thrill in spring, 2000 when my flight took me over the Clearwater impact site. To see these two craters right next to each other personally was very cool.

The impact of a comet or asteroid about the size of Hephaistos or SL9 hitting the Earth was probably responsible for the extinction of the dinosaurs 65 million years ago. It left a 180 km crater now buried below the jungle near Chicxulub in the Yucatan Peninsula (left).

 

 

 

Calculations based on the observed number of asteroids suggest that we should expect about 3 craters 10 km or more across to be formed on the Earth every million years. This is in good agreement with the geologic record. It is more difficult to compute the frequency of larger impacts like Chicxulub but once per 100 million years seems like a reasonable guess.

Here are educated guesses about the consequences of impacts of various sizes:

Impactor Diameter (m)

Yield (megatons)

Interval (years)

Consequences

<50

<10

<1

meteors in upper atomsphere, most don't reach surface

75

10-100

1000

irons make craters like Meteor Crater; stones produce airbursts like Tunguska; land impacts destroy area size of city

160

100-1000

5000

irons, stones hit ground; comets produce airbursts; land impacts destroy area size of large urban area (New York, Tokyo)

350

1000-10,000

15,000

land impacts destroy area of small state; ocean impacts produce mild tsunamis

700

10,000-100,000

63,000

land impacts destroy area size of moderate state (Virginia); ocean impacts make big tsunamis

1700

100,000-1,000,000

250,000

land impact raises dust with global implication; destroys area of large state (California, France)

Data from 'The Impact Hazard', by Morrison, Chapman and Slovic, published in Hazards due to Comets and Asteroids

More recent studies indicate a slightly lower frequency.

Giant Impacts

There is much evidence of major impacts in Earth's geological history called in the movie "Deep Impact," "Extinction Level Events." These events are rare, happening about once every 100 million years or so. However, the destruction of such an event is life altering for any organisms currently inhabiting the planet, and land altering for the site of the impact. To learn more about these giant impacts, check out the website devoted entirely to Giant Impacts.

Some interesting sites about meteors can be found at:

Meteorite Market which gives you an opportunity to purchase meteorites for yourself. I attended at Astronomy conference in New Mexico a few years ago and bought a pair of Moldovian Meteorite earrings for my wife. I was hoping that all kinds of people would ask questions about her unusual earrings, but women do not ask each other about jewlery most of the time, so my wife has had very few opportunities to talk about her thoughtful husband.

Meteorite and Impact Advisory Committee is a nice place to visit and learn about meteors in general and how to report seeing an actual fall.

Earth Impact Database is really cool because it has this map that you can find with every major impact site around the world. You can click on a crater site on the map and see an image of the crater.

If you have not already moved to the Impact page, then do so now, or else return to Asteroids, or the Planet Introduction.


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