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Meteorites, Asteroids, and Comets


The 1908 Tunguska event in Siberia destroyed an area the size of a large city. Here the area of destruction is superimposed on a map of Washington, D.C., ... – PowerPoint PPT presentation

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Title: Meteorites, Asteroids, and Comets

Meteorites, Asteroids, and Comets
  • Chapter 19

  • Meteoroid fragment of a comet or asteroid in
  • Meteor meteoroid colliding with Earth and
    producing a visible light trace in the sky
  • Meteorite meteor that survives the plunge
    through the atmosphere to strike the ground

Comets leave a trail of debris behind them as
they orbit the sun. Meteoroids contributing to a
meteor shower are debris particles, orbiting in
the path of a comet.
A meteor shower occurs when Earth passes through
the orbital path of a comet. The comet may still
exist or have been destroyed.
Meteor Showers
Most meteors appear in showers, peaking
periodically at specific dates of the year. All
of the meteors in a given shower have the same
Shower Date R.A. Dec. Associated
Comet Perseids Aug. 10-14 3h4m
58o 1982 III Leonids Nov. 14-19 10h12m 22o 18
66 I Temp Geminids Dec. 10-13 7h28m 32o
  • Most meteors we see, whether or not there is a
    shower, come from comets. Therefore, they are
    small specks of matter that burn up in the

Sizes from microscopic dust to a few centimeters.
About 2 meteorites large enough to produce
visible impacts strike the Earth every day.
Statistically, one meteorite is expected to
strike a building somewhere on Earth every 16
Typically impact onto the atmosphere with 10 30
km/s ( 30 times faster than a rifle bullet).
Analysis of Meteorites
3 broad categories
  • Iron meteorites
  • Stony meteorites
  • Stony-iron meteorites

  • Iron Meteorites
  • Dense and heavy
  • Dark rusted surfaces
  • When sliced, polished, and etched with nitric
    acid, they reveal Widmanstatten patterns caused
    by crystals of nickel-iron alloys that have grown
    large. This indicates that the meteorite cooled
  • Stony-iron meteorites are a mixture of iron and
    stone. They appear to have formed when a mixture
    of molten iron and rock cooled and solidified.

  • Stony Meteorites
  • Chondrites
  • Contain chondrules (rounded bits of glassy rock
    ranging from microscopic to pea size.)
  • They formed from droplets of molten rock that
    cooled and hardened rapidly when the solar system
    was young.
  • Their presence indicates that the meteorites have
    not melted since they formed.
  • Some chondrites only have a few volatiles
    indicating they were heated slightly, which
    caused them to lose their volatiles, but not
    heated enough to destroy the chondrules.
  • Carbonaceous chondrites contain both chondrules
    and volatile compounds including carbon. They
    have not been heated since the formation of the
    solar system.
  • Achondrites contain no chondrules and lack
    volatiles. They appear to have been heated.
    They are similar to Earths lavas.

The Origins of Meteorites
  • Probably formed in the solar nebula, 4.6
    billion years ago.
  • Almost certainly not from comets (in contrast to
    meteors in meteor showers!).
  • Probably fragments of stony-iron planetesimals

Small, irregular objects, mostly in the apparent
gap between the orbits of Mars and Jupiter.
Last remains of planetesimals that built the
planets 4.6 billion years ago!
Evidence for Collisions
Hirayama families Groups of asteroids sharing
the same orbits and spectroscopic characteristics
apparently result of common origin through
Radar images of asteroids reveal irregular
shapes, sometimes peanut-like shapes
Evidence for low-velocity collisions between
asteroids on very similar orbits.
  • Not all asteroids are in the asteroid belt.
  • A few thousand asteroids larger than 1 km cross
    Earths orbit.
  • Near Earth Objects (NEOs)
  • Searches are underway to find these NEOs.

The Origin of the Asteroids
  • Ray blasts from Death Stars are unlikely to cause
    planets to explode as in Star Wars.
  • Besides, the total mass of all the asteroids is
    only 1/20 that of the moon.
  • The asteroids probably are not the result of a
    planet exploding.
  • Asteroids are probably the remains of a planet
    that did not form at 2.8 Au from the sun due to
    Jupiters gravity.
  • Therefore, asteroids are probably fragments of
    left over planetesimals.
  • The ones in the outer belt formed where the solar
    nebula was cooler so carbon could condense.
    Thats why type C asteroids are in the outer belt
    and type S are in the inner belt.

Comet C/2001 Q4
Throughout history, comets have been considered
as portents of doom, even until very recently
Appearances of comet Kohoutek (1973), Halley
(1986), and Hale-Bopp (1997) caused great concern
among superstitious.
Comet Hyakutake in 1996
Comet Hale-Bopp in 1997
Comet NcNaught (2007) was visible in the southern
sky. It will never return.
  • When a comet is far from the sun, its just
    the nucleus. When it gets close enough to the
    sun, it begins to sublime and a coma and tail
  • The coma of a comet is the cloud of gas and dust
    that surrounds the nucleus. It can be over a
    million km in diameter, which is bigger than the

Two Types of Tails
gas tail Ionized gas pushed away from the comet
by the solar wind. Pointing straight away from
the sun.
Dust tail Dust set free from vaporizing ice in
the comet carried away from the comet by the
suns radiation pressure. Lagging behind the
comet along its trajectory
  • Comet tails point generally away from the sun,
    but their precise direction depends on the flow
    of the solar wind and the orbital motion of the

Comet Mrkos in 1957 shows how The gas tail
can change from night to night due to changes in
the magnetic field in the solar wind.
  • Comets cannot last more than 100 to 1000 orbits
    around the sun before all their ice is gone and
    there is nothing left but dust and rock.
  • The comets we see today cannot have been orbiting
    close to the sun for 4.6 billion years.
  • Where do new comets come from?

Impacts on Earth
  • Small meteorite impacts occur quite often.
  • Every few years a building is damaged by a
  • A few years ago, a car was hit by a meteorite and
    then auctioned off for 10,000,000.
  • Really large impacts are rare.

In 1954 Mrs. E. Hulitt Hodges of Sylacauga,
Alabama was hit by a meteorite while napping in
her living room. This is the only known person
to have been injured by a meteorite.
Over 150 impact craters found on Earth.
Famous example Barringer Crater near Flagstaff,
Formed 50,000 years ago by a meteorite of 80
100 m diameter
Barringer Crater 1.2 km diameter 200 m deep
  • Sediments from all over the Earth from 65 million
    years ago have an overabundance of iridium, an
    element common in meteorites but rare in the
    Earths crust.
  • The impact of a large meteorite at that time may
    have altered the atmosphere and climate on Earth,
    which caused the extinction of the dinosaurs and
    75 of the other species on the planet.

  • The biggest extinction we know of occurred 250
    million years ago The Great Dying.
  • 95 of life in the oceans died out.
  • 80 of life on land died out.
  • Data indicates that a large impact occurred off
    the shore of Australia 250 million years ago.

The 1908 Tunguska event in Siberia destroyed an
area the size of a large city. Here the area of
destruction is superimposed on a map of
Washington, D.C., and its surrounding beltway. In
the central area, trees were burned in the outer
area, trees were blown down pointing away from
the center of the blast for as far as 30 km.
The Effects of a Large Impact on Earth
  • If on land, the initial shock would be deadly.
  • If on sea, there would be tidal waves hundreds of
    meters high that would devastate coastal regions.
  • Lots of dust would be thrown into the atmosphere.
  • The hot dust falling back to Earth could start
  • The dust left in the atmosphere would block
    sunlight, making temperatures cooler for a time.

  • In 1998, newspaper headlines read Mile Wide
    Asteroid to Hit Earth in October 2028.
  • Rumors of Earths demise were greatly
    exaggerated. The asteroid will miss Earth by
    600,000 miles.
  • Now rumor is a 430 mile wide asteroid named
    Apophis will hit in 2029 or 2036.
  • Actually Apophis is not 430 miles in diameter but
    more like 250 METERS.
  • The future for Apophis on Friday, April 13 of
    2029 includes an approach to Earth no closer than
    29,470 km (18,300 miles, or 5.6 Earth radii from
    the center, or 4.6 Earth-radii from the surface)
    over the mid-Atlantic, appearing to the naked eye
    as a moderately bright point of light moving
    rapidly across the sky.
  • Updated computational techniques and newly
    available data indicate the probability of an
    Earth encounter on April 13, 2036, for Apophis
    has dropped from one-in-45,000 to about four-in-a
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