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Meteorites, Asteroids,
and Comets
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Meteorites
• Meteoroid = small body in space
Distinguish
between:
• 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
Meteoroids, Meteors and
Meteorites
• Meteoroids are small rocky bodies that
travel through space.
• A meteor is a bright streak that results
from a meteoroid burns up in Earth’s
atmosphere, what we call shooting stars.
• A meteorite is a meteoroid that reaches
the Earth’s surface without burning up.
Three Types of Meteorites
• Stony- Rocky material
• Metallic- Iron and Nickel
• Stony Metallic- Rocky material, iron and
nickel
Stony Meteorites
• Rocky material
Metallic Meteorite
• iron and nickel
Stony-Iron Meteorite
• rocky material, iron and nickel
A fragment of the meteorite that
created Meteorite Crater.
Metallic Meteorite
Meteorites
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 months.
Typically impact onto the atmosphere with 10 – 30 km/s
(≈ 30 times faster than a rifle bullet).
Meteor Showers
Most meteors appear in showers, peaking
periodically at specific dates of the year.
Meteoroid Orbits
Meteoroids contributing
to a meteor shower are
debris particles, orbiting
in the path of a comet.
Spread out all along the
orbit of the comet.
Comet may still exist or
have been destroyed.
Only few sporadic meteors are not associated with comet orbits.
Radiants of Meteor Showers
Tracing the tracks of meteors in a shower backwards,
they appear to come from a common origin, the radiant.
↔ Common
direction of
motion
through
space.
The Perseid Meteor Shower
The Leonid Meteor
Shower in 2002
Meteorite Crater-Winslow, Arizona
• Winslow, Arizona
This crater was formed approximately 50,000 years ago when an iron
mass, estimated to be about 80 feet in diameter and weighing over
60,000 tons entered the Earth's atmosphere over the American
Southwest. The resulting formation is about 4,000 feet (1,200 meters)
wide and 570 feet (175 meters) deep.
Meteorite Impacts on Earth
Over 150 impact craters found on Earth.
Famous
example:
Barringer
Crater near
Flagstaff, AZ:
Formed ~ 50,000 years ago by a
meteorite of ~ 80 – 100 m diameter
Weird Science!
• In 1954, Mrs. Hodge, of Alabama was
struck by a meteorite as she was taking
• her afternoon nap. Bruised , but not badly
injured, she is one of only two people
known to have been struck by a meteorite.
Asteroids
• Asteroids are small rocky bodies that revolve
around the sun.
• They range in size from a few meters to more
than 900 kilometers in diameter.
• Asteroids have irregular shapes, but some are
spherical, or round.
• Most asteroids orbit the sun in asteroid belt.
• The asteroid belt orbits between Mars and
Jupiter.
• Asteroids are thought to be left over from the
formation of the solar system.
Asteroids
Last remains of
planetesimals
that built the
planets 4.6
billion years
ago!
The Asteroid Belt
Most asteroids
orbit the sun in a
wide zone
between the
orbits of Mars
and Jupiter.
(Distances and times reproduced to scale)
The Asteroid Belt
Small, irregular
objects, mostly in the
apparent gap
between the orbits of
Mars and Jupiter.
Thousands of
asteroids with
accurately
determined orbits
are known today.
Sizes and shapes of the largest
asteroids, compared to the moon
Kirkwood Gaps
The asteroid orbits are not evenly distributed throughout
the asteroid belt between Mars and Jupiter.
There are several gaps where no asteroids are found:
Kirkwood gaps
These correspond
to resonances of
the orbits with the
orbit of Jupiter.
Example:
2:3 resonance
Non-Belt Asteroids
Not all asteroids orbit within the asteroid belt.
Apollo-Amor
Objects:
Asteroids
with elliptical
orbits,
reaching into
the inner
solar system.
Some
potentially
colliding with
Mars or Earth.
Trojans:
Sharing stable
orbits along
the orbit of
Jupiter.
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Comets
Comet Ikeya-Seki in
the dawn sky in 1965
Weird Science
• In 1908, an object thought to be a comet
about 60 m in diameter exploded less than
10 km above a remote part of Siberia. The
blast flattened trees in an area greater
than 2,000km2 and left no crater.
Throughout history, comets have been considered
as portants 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
Comets
•
•
A comet is a small body of ice, rock and
cosmic dust loosely packed together.
Scientists refer to them as dirty
snowballs.
Gas and Dust Tails of
Comet Mrkos in 1957
Comet HaleBopp in 1997
Fragmentation of Comet Nuclei
Comet nuclei are very fragile and are easily fragmented.
Animation 1
Animation 2
Comet Shoemaker-Levy was disrupted by tidal forces of Jupiter
Two chains of impact
craters on Earth’s
moon and on
Jupiter’s moon
Callisto may have
been caused by
fragments of a comet.
Fragmenting Comets
Comet Linear
apparently completely
vaporized during its
sun passage in 2000.
Only small rocky
fragments remained.
The comet's tail gets bigger as it gets closer to
the sun and then decreases as it moves away
from the sun.
Parts of a Comet
When they are near the Sun and active, comets have
several distinct parts:
Nucleus: relatively solid and stable, mostly ice and gas
with a small amount of dust and other solids;
Coma: dense cloud of water, carbon dioxide and other
neutral gases
Hydrogen cloud: huge (millions of km in diameter) but
very sparse envelope of neutral hydrogen;
Dust tail: up to 10 million km long composed of smokesized dust particles driven off the nucleus by escaping
gases; this is the most prominent part of a comet to the
unaided eye;
Ion tail: consists of electrically charged particles called
ions. It produces rays and streamers caused by
interactions with the solar wind.
Two Types of Tails
Ion 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
sun’s radiation
pressure. Lagging
behind the comet along
its trajectory
Comets Facts
•
•
•
•
•
A comets ion tail always points away from the
sun.
Solar winds blow the ion tail away from the
sun.
The dust tail follows the comet’s orbit and do
not always point away from the sun.
Scientists believe that comets come from the
Oort Cloud, a region that surrounds the solar
system.
Comets can also come from the Kuiper Belt
which exists outside of Neptune’s orbit.
Comet’s Orbit
• Comets orbit the Sun in highly elliptical orbits.
• Their velocity increases greatly when they are near the
Sun and slows down at the far reaches of the orbit.
• Since the comet is light only when it is near the Sun (and
is it vaporizing), comets are dark (virtually invisible)
throughout most of their orbit.
• The solar wind pushes the tail away from the Sun.
Comet facts continue:
• A comet develops a coma because of heat
from the sun.
• The comet develops a tail because of the
solar wind.
• After a comet has made several trips
around the Sun-the heat from the Sun
vaporizes most of the frozen ice and the
remaining small particles spread out in the
orbit of the comet.
The Geology of Comet Nuclei
Comet nuclei contain ices of water, carbon dioxide, methane, ammonia, etc.
(Materials that should have condensed from the outer solar nebula).
Not solid ice
balls, but fluffy
material with
significant
amounts of
empty space.
“Dirty
snowballs”
Shoemaker-Levy Comet
• On 1994 July 16-22, over twenty fragments of
comet Shoemaker-Levy 9 collided with the
planet Jupiter.
• The comet, discovered the previous year by
astronomers Carolyn and Eugene Shoemaker
and David Levy, was observed by astronomers
at hundreds of observatories around the world
as it crashed into Jupiter's southern hemisphere.
The Deep Impact Mission
Video 1
Video 2
Placing a probe into the path of Comet Tempel 1
and documenting the result of the impact
Impact: July 4, 2005
The Origin of Comets
Comets are believed to originate in the Oort cloud:
Spherical cloud of several trillion icy bodies,
~ 10,000 – 100,000 AU from the sun.
Gravitational influence
of occasional passing
stars may perturb some
orbits and draw them
towards the inner solar
system.
Oort Cloud
Interactions with
planets may perturb
orbits further,
capturing comets in
short-period orbits.
The Kuiper Belt
Second source of small, icy bodies in the outer solar system:
Kuiper Belt, at
~ 30 – 100 AU
from the sun.
Pluto and Charon
may be captured
Kuiper-Belt objects.
Beyond the Solar System