main page SPACE
Jiri W a g n e r , Jr., firstname.lastname@example.org
- 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.
- 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 (15 finds) or Martian (13)
- 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.
||primarily iron and nickel; similar to type M asteroids
||mixtures of iron and stony material like type S asteroids
||by far the largest number of meteorites fall into this class; similar in
composition to the mantles and crusts of the terrestrial planets
||very similar in composition to the Sun less volatiles; similar to type C asteroids
||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).
- A very large number of meteoroids enter the Earth's atmosphere each day amounting to
several 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
- 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
- A more recent impact occured 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 300,000 years on average. Larger
ones are less numerous and impacts are less frequent, but they do sometimes happen and
with disasterous consequences.
- The impact of a comet or asteroid about the
size of Hephaistos or Shoemaker-Levy 9 comet 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
- 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
- Here are educated guesses about the consequences of impacts of various sizes:
|Impactor Diameter (meters)
||meteors in upper atmosphere most don't reach surface
||10 - 100
||irons make craters like Meteor Crater; stones produce airbursts like
Tunguska; land impacts destroy area size of city
||100 - 1000
||irons,stones hit ground; comets produce airbursts; land impacts destroy
area size of large urban area (New York, Tokyo)
||1000 - 10,000
||land impacts destroy area size of small state; ocean impact produces mild
||10,000 - 100,000
||land impacts destroy area size of moderate state (Virginia) ocean impact
makes big tsunamis
||100,000 - 1,000,000
||land impact raises dust with global implication; destroys area size of
large state (California, France)
- (from 'The Impact Hazard', by Morrison, Chapman and Slovic, published in Hazards due to
Comets and Asteroids)
- Source: The Nine Planets
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