METEOR AND METEORITE

A meteor, also known as a shooting or falling star, is the
streak of light produced by the vaporization of interplanetary
particles as they enter the Earth's atmosphere.  While still in 
space the particles are called meteoroids.  A large and
abnormally bright meteor is referred to as a fireball, and an
exploding fireball is called a bolide.  Larger meteors are not
completely vaporized, and the particles that reach the Earth's
surface are called meteorites.  They may form METEORITE
CRATERS, such as those found on the Moon, the inner planets,
and the satellites of Mars and Jupiter.
 
Meteor Showers
 
Although a few meteors can be seen on any night, especially
after midnight, during certain times of the year so many are
visible that they are termed meteor showers.  Records of meteor 
showers predate the 11th century.  Until the present century,
when the origin of such showers began to be understood, the
phenomenon was usually the cause of considerable fear.  Most
meteor showers are believed to be produced by the debris of
comets, which leave a trail of particles behind them as they
orbit the Sun.  When the Earth intercepts this stream of
cometary debris, the small, low-density particles strike the
atmosphere at speeds between 35 and 95 km/sec (22 and 60
mi/sec), causing them to vaporize and creating the visible
meteor display.  The average size of a meteor, which can be
estimated from its brightness, height, and distance, is about
the size of a grain of sand.
 
During a shower, meteors appear to radiate from a point in the
sky, called the radiant, which can be associated with a
particular constellation.  These radiant points give each
shower its name;  thus the Perseids (July 25-August 18) appear
to radiate from the constellation Perseus, and the Leonids
(November 15-19) from Leo.  During the height of the heaviest
showers, from 30 to 70 meteors may normally be seen every hour, 
but on rare occasions in a spectacular display that number may
be visible every second.
 
Meteorites
 
It has been estimated that micrometeorites, so small that they
drift to the ground without vaporizing, may add 1,000 tons to
the mass of the Earth each day.  Micrometeorites are not
associated with meteor showers, nor are the larger meteorites
that form craters.  Although 500 large meteorites may fall on
the Earth each year, only about 1% of these are recovered.
Somewhat more than 120 impact sites have been identified.
 
Meteorites may be classified according to composition as irons
(siderites), stony irons (siderolites), and stones (aerolites). 
Iron meteorites are extremely dense and consist of 90% iron and 
10% nickel.  Stone meteorites have only about 10% nickel-iron
and consist chiefly of silicates.  Stony iron meteorites, which 
are relatively rare, are composed of iron and silicates in
about equal proportions.  Most stone meteorites contain small,
spherical particles, or chondrules, composed of the silicates
olivine, pyroxene, or orthopyroxene.  These are called
CHONDRITES to distinguish them from the few stones without
chondrules, known as achondrites.  While chondrites may contain 
50 different minerals along with nickel-iron, achondrites
contain mainly pyroxene and plagioclase.  Nickel-iron is only
rarely found in achondrites.  It is not known if the glassy
fragments found in certain regions of the Earth, known as
TEKTITES, are related to meteorites.  They have never been
observed to fall.
 
Iron meteorites are the type most often found, because of their 
unusual appearance.  Stone meteorites are actually more
numerous, but since they resemble ordinary rocks, they are not
often found unless a fall is actually observed.  Of all
observed meteorite falls, 85% are chondrites, as are 95% of all 
stone falls.
 
A special class of chondrites of particular interest are the
carbonaceous chondrites, which consist largely of the mineral
serpentine and contain amino acids and other organic compounds
believed to be of extraterrestrial origin.  Most of the
evidence points to the nonbiological origin of these compounds, 
although it is possible that primitive life might arise from
them--one of the theories of the origin of life in the solar
system.  Very dark in color, they are extremely rare, and
constitute only about 2% of all stone falls.
 
Meteorites have never been observed to fall during a meteor
shower, which indicates that they are not remnants of comets.
A very few meteorites have been identified as lunar in origin,
but most are believed to originate from large parent bodies
that formed at the time of the formation of the solar system
and later broke up.  ASTEROIDS would have had a similar origin
and meteorites resemble them in general range of composition.
 

Bibliography:  Burke, J. G., Cosmic Debris (1991);  Dodd, R.
T., Thunderstones and Shooting Stars (1986);  Kerridge, J., and 
Matthews, M., eds., Meteorites and the Early Solar System
(1988);  Kronk, G. W., Meteor Showers (1988);  Kronk, G. W.,
Meteor Showers (1988);  McSween, H. Y., Jr., Meteorites and
Their Parent Planets (1987);  Wasson, J. T., Meteorites, 2
vols. (1974, 1985).

IMPORTANT ANNUAL METEOR SHOWERS IN THE NORTHERN HEMISPHERE
--------------------------------------------------------------- 
Name of Shower           Constellation of         Date of Peak
                         Radiant                  Activity
--------------------------------------------------------------- 
Quadrantids              Bootes                   Jan. 3
Alpha Aurigids           Auriga                   Feb. 8
Virginids                Virgo                    Mar. 20
Lyrids                   Lyra                     Apr. 21
Eta Aquarids             Aquarius                 May 4
Ophiuchids               Ophiuchus                June 20
Delta Aquarids           Aquarius                 July 29
Perseids                 Perseus                  Aug. 12
Kappa Cygnids            Cygnus                   Aug. 20
Alpha Aurigids           Auriga                   Sept. 22
Draconids                Draco                    Oct. 9
Orionids                 Orion                    Oct. 21
Taurids                  Taurus                   Nov. 1
Leonids                  Leo                      Nov. 17
Geminids                 Gemini                   Dec. 13
Ursids                   Ursa Major               Dec. 22
 
Name of Shower   Normal          Expected           Associated
                 Duration        Hourly Rates       Comet
--------------------------------------------------------------- 
Quadrantids      Jan. 1-4        50                 _
Alpha Aurigids   Feb. 5-10       5                  _
Virginids        Mar. 5-Apr. 2   5                  _
Lyrids           Apr. 19-23      10-15              1861 I
Eta Aquarids     May 1-8         20                 Halley
Ophiuchids       June 17-25      20                 _
Delta Aquarids   July 26-31      25                 _
Perseids         Aug. 10-14      60                 1862 III
Kappa Cygnids    Aug. 18-22      5-10               _
Alpha Aurigids   Sept. 20-24     5                  _


METEOR AND METEORITE

Draconids        Oct. 7-11       Varies (low to     Giacobini-
                                   high)            Zinner
Orionids         Oct. 18-23      25                 Halley
Taurids          Oct. 15-Dec. 1  15                 Encke
Leonids          Nov. 14-20      Varies (low to     1866 I
                                   high)
Geminids         Dec. 10-15      50                 -
Ursids           Dec. 17-24      15                 Tuttle
--------------------------------------------------------------- 

General Astronomy The Galaxy Space Exploration Gravity
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