AY 101 Lecture Notes - Lecture 13: Meteor Crater, Perseids, Incandescence
Minor Objects: Asteroids, Comets, and More
Four basic categories of smaller materials exist in the solar system: meteoroids;
asteroids (or minor planets); comets; and dust and gas. These categories are
differentiated on the basis of chemistry, orbital characteristics, and their origins.
Meteoroids
Meteoroids are basically the smaller bodies in between the planets, defined as any
rocky metallic objects less than 100 meters, or alternatively 1 kilometer, in size. It is ‐
these objects that generally fall to Earth. While heated to incandescence by
atmospheric friction during their passage through the atmosphere, they are
termed meteors. A fragment that survives to hit the ground is known as a meteorite.
Astronomers distinguish two types of meteors: the sporadic, whose orbital paths
intersect that of Earth in random directions; and shower meteors,which are the
remains of old comets that have left lots of small particles and dust in a common orbit.
The material of sporadic meteors originates from the breakup of larger asteroids and old
comets and the scattering of the debris away from the original orbits. When the orbit of
shower meteors intersects that of Earth, numerous meteors may be viewed coming in
from the same point, or radiant, in the sky. The association of meteors with comets is
well known with the Leonids (observable around November 16 with a radiant in the
constellation of Leo), representing the debris of Comet 1866I, and the Perseids (about
August 11), which is the debris of Comet 1862III.
A typical meteor is only 0.25 grams and enters the atmosphere with a velocity of 30
km/s and a kinetic energy of approximately a 200,000 watt second, allowing frictional ‐
heating to produce an incandescence equivalent to a 20,000 watt light bulb burning for
10 seconds. Daily, 10,000,000 meteors enter the atmosphere, equivalent to about 20
tons of material. The smaller and more fragile material that doesn't survive passage
through the atmosphere is primarily from comets. Larger meteors, which are more solid,
less fragile, and of asteroidal origin, also hit Earth about 25 times a year (the largest
recovered meteorite is about 50 tons). Every 100 million years, an object 10 kilometers
in diameter can be expected to strike Earth producing an impact that resembles the
event that explains the demise of the dinosaurs at the end of the Cretaceous period.
Evidence of some 200 large meteor craters remain preserved (but mostly hidden by
erosion) on Earth's surface. One of the most recent and best known meteor crators that
is preserved, the Barringer Meteor Crater in northern Arizona, is 25,000 years old, 4,200
feet in diameter, and has a depth of 600 feet. It represents an impact due to a 50,000‐
ton object.
Chemically, meteorites are classified into three types: irons, composed of 90 percent
iron and 10 percent nickel), (representing about 5 percent of meteor falls), stony‐
irons, of mixed composition (1 percent of meteor falls), and stones (95 percent of
meteor falls). The latter are composed of various types of silicates but are not quite
chemically identical to Earth rocks. The majority of these stones
are chondrites, containing chondrules,microscopic spherules of elements that appear
to have condensed out of a gas. About 5 percent are carbonaceous chondites, high in
carbon and volatile elements, and are believed to be the most primitive and unaltered
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Document Summary
Four basic categories of smaller materials exist in the solar system: meteoroids; asteroids (or minor planets); comets; and dust and gas. These categories are differentiated on the basis of chemistry, orbital characteristics, and their origins. Meteoroids are basically the smaller bodies in between the planets, defined as any rocky metallic objects less than 100 meters, or alternatively 1 kilometer, in size. It is these objects that generally fall to earth. While heated to incandescence by atmospheric friction during their passage through the atmosphere, they are termed meteors. A fragment that survives to hit the ground is known as a meteorite. The material of sporadic meteors originates from the breakup of larger asteroids and old comets and the scattering of the debris away from the original orbits. When the orbit of shower meteors intersects that of earth, numerous meteors may be viewed coming in from the same point, or radiant, in the sky.
