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Oort Cloud
• The Oort cloud surrounds
the Sun beyond the
Kuiper Belt.
– 10,000 to 100,000 AU
– 1/5 the way to the
nearest star
– Spherical shell of icy
Cometary Orbits
• Comets that are pulled toward the sun enter new orbits.
– Nudged by gravity
– Most pass only once – never return
– A few enter elliptical orbits
Comet Nuclei
• Comet Halley has a
nucleus with a lot of
carbon soot and grains of
sand mixed with ice.
• Comet Borrelly shows
gases coming from the
As a comet is heated near the sun the ice vaporizes.
The vapors form a coma around the nucleus.
Part of the gas streams away from the sun and forms a tail.
Comet tails can be millions of kilometers long.
Meteor Showers
• Many times each year
meteors will arrive in a
– Up to hundreds per hour
– Same direction from
• This is called a meteor
Gary W. Cronks Comets
and Meteor Showers
• In the 1800’s astronomers realized that meteors from a
single shower came from a single point among the stars.
• The point origin is called the radiant.
– Named for the constellation in which it appears to originate
Repeating Showers
• Some meteor showers recur at regular intervals.
– Leonids match comet Tempel-Tuttle
• Astronomers recognized that this was consistent with a
predictable orbit.
Orbit of the
Meteor Storms
• As comets pass the sun
they can leave a large
amount of debris.
• If the earth hits this debris
it can generate 1000’s of
meteors per hour.
• This is called a meteor