• A Perseid meteor on August 12, 1986.Meteors appear when
centimeter-sized meteoroids travelling at least 11 km/sec, but more
usually 30 to 55 km/sec, strike the Earth's atmosphere. The
maximum speed is 72 km/sec. The meteoroid's kinetic energy of
motion converts into heat, vaporizing the meteoroid at heights
above 60 km. The hot vapor trail is what we see as a meteor. The
vaporized material may reach temperatures of 1,000 to 2,000
Kelvins. The period during which the meteor flashes is
calledincandescent flight. The period after the light phenomena
cease is calleddark flight. Meteorites the size of a golf ball (two or
three centimeters) or larger vaporize in exceptionally brilliant
flashes called fireballs or bolides. These may also produce a variety
of sounds. Theodor Abrahamsen's photo at the right captures a
Perseid meteor on August 1
• Meteor Storms
• Leonid Storm of November, 1833Occasionally the Earth passes
through an unusually heavy concentration of cometary debris
resulting in a meteor storm. Hundreds or even thousands of
meteors may flash each hour. One of the historically most
prominent meteor storms, the Leonid storm, occurs at about thirtythree year intervals. The Leonid shower normally produces about
ten meteors per hour. When they storm, the Leonids can produce
the equivalent of over one hundred thousand meteors per hour for
a short period. The woodcut at the right by artist Adolf Vollmy,
based upon an original painting by the Swiss artist Karl Jauslin,
portrays the great Leonid meteor storm of November 12-13, 1833.
Victorian era astronomy writer Agnes Clerke described that storm
Meteoroids the size of a fist or larger may survive the trip through the atmosphere
to land on the Earth's surface. They are then known as meteorites. A meteorite
located after a witnessed descent is called a fall. A meteorite from an unwitnessed
descent is called a find. Meteorites are usually named for a post office or another
geographic landmark close to the place where the meteorite was found. The name
of the meteorite can refer to either a specimen of the meteorite itself or to the
locality in which it was found.
Meteorites include some of the oldest and most primitive solar system material.
Radiometric dates suggest some meteorites are as much as 4.54 billion years old.
Some even include cosmic material formed before the solar system itself was born.
Because many meteorites have changed so little in the intervening eons, they offer
a window into the early history of the solar system. Meteorites also represent
some of the rarest material on Earth. Until the advent of the space age meteorites
were the only extraterrestrial material available for study here on Earth. Both the
scarcity and the scientific importance of meteorites leads collectors and
researchers alike to seek them out.
Types Of Meteorites
Primarily iron and nickel.
Sikhote-Alin, Russia (IIB)
Similar to type M asteroids.
Mixture of iron and stony
material. Similar to type S
Vaca Muerta, Altacama,
Iron meteorites are probably what most people picture as "typical" meteorites.
Iron meteorites consist almost entirely of a mixture of metallic nickel and iron.
They are easier to spot on the ground because their highly unoxidized iron content
stands out from background rocks. The outer surface of iron meteorites often
melts during their passage through the atmosphere resulting in a dark fusion crust.
Primary fusion crust forms while the meteoroid is incandescent. Secondary fusion
crust forms on the broken surfaces of fragments which break free from the main
mass during incandescent flight. They may also exhibit flow markings and
interesting molten metal shapes. The interior of some iron meteorites displays a
criss-cross pattern of different iron-nickel minerals.
Iron meteorites may originate in the cores of differentiated parent bodies at least
100 km in diameter. The composition of some main-belt asteroids called M-type
asteroids resembles that of iron meteorites. These M-type asteroids may be the
source of iron meteorites. Iron meteorites with weights of 50 to 100 kg are not
uncommon. The Hoba meteorite, at 60 tons, is the largest known iron meteorite to
have landed without exploding. It still lies where it was found.
• Stony meteorites are the most common, making up about
94% of observed falls. They are composed of 75-90% rocky
silicates including familiar minerals such as pyroxene,
olivine, and plagioclase, and 10-25% nickel-iron metal and
iron sulfide. (Silicates are minerals containing silicon,
oxygen, and one or more metals.)
• Stony meteorites are difficult to find because they look like
terrestrial rocks. The best places to find stony meteorites
are in deserts or on the ice sheet of Antarctica. The
meteorites stand out against the background of ice or sand.
Like iron meteorites, stony meteorites often exhibit a dark
fusion crust. There are three major subgroups of stony
meteorites, Chondrites, Carbonaceous Chondrites,