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What are comets?
Learning Target for 10/19
I can describe the make up of comets
I can model a comet’s structure
I can explain why astronomers use
models to understand celestial bodies
A little bit about Comets
Questions for the video:
1. What is the difference between long period
and short period comets?
2. Why are some comets visible and others not?
3. Why do some comets have different tails?
4. How are the two tails formed?
5. Why do scientists think that comets may have
been a cause for the extinction of dinosaurs
and might have imported water to the earth?
What does Comet mean?
from the Greek word
komē, meaning "hair of
the head“
Aristotle first used the
derivation komētēs to
depict comets as "stars
with hair."
The astronomical symbol
for comets (☄) consists
of a disc with a hair- like
Comets are small,
fragile, irregularly
shaped bodies
composed of a mixture
of non-volatile grains
and frozen gases.
have highly elliptical
orbits that bring them
very close to the Sun
and swing them deeply
into space, often beyond
the orbit of Pluto.
Comet Structure
Comet structures are diverse
Has a surrounding cloud of
diffuse material, called a coma
usually grows in size and
brightness as the comet
approaches the Sun.
Usually a small, bright nucleus
(less than 10 km in diameter)
is visible in the middle of the
coma. The coma and the
nucleus together constitute the
head of the comet.
Anatomy of a comet
Physical Characteristics
As comets approach the
Sun they develop
enormous tail that flow
away from the Sun.
When far from the Sun,
the nucleus is very cold.
These Comets are
sometimes referred to as
a "dirty snowball," since
over half of their material
is ice.
Why are comets important?
Comets may be an important part of the
recipe for making planets and may be material
left over from solar system formation.
Some comets may have crashed into forming
planets adding to their water and rock
Some believe that material from comets may
have brought water to Earth through impacts.
How many Comets are their?
The number of naked-eye comets averages to roughly
one per year, though many of these are faint and
When a historically bright or naked-eye comet is
witnessed by many, it is called a “Great Comet.”
Halley’s comet occurs every 75 years, last seen 1985
Comet McNaught, also known as the Great Comet of 2007
Comet Hale-Bopp seen in 1997
Where are comets found?
Today, most comets are located outside our
solar system in part of the original cloud of
dust and gas that has remained virtually
untouched for billions of years. These
regions are referred to as the Oort Cloud
and the Kuiper Belt.
Kuiper (Pronounced Kiper)Belt
The Kuiper Belt is a region of space
outside the orbit of Neptune.
 30 to 50 AU from the sun
 It contains countless comets and
Oort Cloud
The Oort cloud is the farthest from the Sun that
you can go and still be in the Solar System.
It is believed that most comets originate in the
Oort cloud before "falling" toward the Sun.
Comets with long orbital periods are not
confined to the plan of the
solar system.
Fate of Comets
Comets have a lifetime of
about 10,000 years or
1,000 revolutions
 Eventually most of the
volatile material
contained in a comet
nucleus evaporates away,
and the comet becomes a
small, dark, inert lump of
rock or rubble that can
resemble an asteroid.
Fate of Comets
Comets are also
known to break up
into large fragments.
Some comets meet a
more spectacular end
– either falling into the
Sun, or smashing into
a planet or other
body. These are
called sungrazers.
How scientist use models in space
Scientists make scientific questions based
on current scientific theory and
observation of models of phenomenon.
NASA missions are designed to answer
specific questions. To answer these
questions, scientists start with a model
which they compare with actual
observations in space. As new data is
gathered, the model is improved if needed
and re-tested against observations
Make up of your model
1 large trash bag
1/3 Liter of water
to help with formation of your comet
comets are made of mostly water
2/3 cup of dirt
1 teaspoon of corn starch
1 teaspoon of vinegar
1 teaspoon of alcohol
1 teaspoon of corn syrup
2 lbs of dry ice
Sand and dirt take the place of
interplanetary dust and silicates that
are trapped in the frozen ice of
helps hold the comet together
represents amino acids
represents methanol
represents organics. Organic material
means anything made up of four
elements carbon, hydrogen, nitrogen,
and oxygen
Comets are frozen gases
Making a model of a comet
Test Your Knowledge
What is the lifespan of a comet?
10,000 years or 1,000 revolutions
Test Your Knowledge
What is the cloud of diffuse material surrounding
a comet called?
A coma
Test Your Knowledge
What is the average number of comets that can
be seen by the naked eye every year?
One a year
Test Your Knowledge
Why do you think that your model is a good
representation of a comet?
Test your knowledge
Why do astronomers use models?
Test Your Knowledge
What is one name for a comet that is far away
from the sun and has a nucleus that is in a
frozen solid state?
"dirty snowball"
Turn in
Make sure you turn in your homework, lab
sheet and your constellation project. You
may turn in constellation projects next
class as well.
Please make sure that your table is cleaned
up, wiped down and all materials are
returned to the roll cart. Comets should be
in the orange basket in the sinks to