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Comets
The Kuiper Belt
Many Plutoids and
smaller icy
planetismals or
“cometary bodies”
orbit close to the same
plane as the planets
forming a “belt”
that extends from
Neptune to about 50
AU called the Kuiper
Belt
In Green
The Oort Cloud
The Oort cloud is a
“cloud” of cometary
bodies, Plutoids
And perhaps planets!
that extends from
Neptune’s orbit to
100,000 AU
(or so..)
To the very edge of the
solar system.
Composition of Cometary Bodies
These planetismals are made of mostly
water ice with some CO₂ or Dry Ice, with
traces of other gas ices
They also contain small rocks and dust,
including carbon compounds
These are what become comets
The Origin of Comets
Occasionally the gravity of a passing star, planet etc.
changes the orbit of some of these icy planetismals
so that they go closer to the Sun.
When the icy planetismal, a comet nucleus,
usually about 10 km in diameter
reaches Jupiter’s orbit it begins to melt, creating
a cloud of gas and dust called a coma grows to as
much as 1,000,000 km across.
Sunlight breaks down water into hydrogen and oxygen
and forms a dim hydrogen envelope up to
10,000,000 km across.
Two tails
As it gets closer to the Sun the solar wind and
radiation push on the coma and create two
“tails.”
The solar wind pushes Ionized Gases directly away
from the Sun forming an blue Ion or Gas tail.
Radiation pressure pushes the dust forming a
yellow-white dust-tail that points between
directly away from the Sun and the comet’s
path.
The Tails grow longer as it approaches the Sun and
can be as much as 1 AU long!
Comets in History
Comets are spectacular celestial objects.
They appear suddenly are there for a few
weeks or months
And then disappear.
Until the 1700’s Comets were thought to be
unique events, often believed to be omens
of disaster….such as the death of Kings
For example the Normans used the
appearance of a comet to justify their
invasion of England (The comet meant God
wanted the English king to die!)
Halley’s Comet
In 1705 English Astronomer
Edmund Halley realized that a
comet seen in 1681 was the
same one that appeared in
1607 and 1531.
He calculated that it was on an
orbit with a period of 75-76
years and predicted its
reappearance in 1758
It has been recorded as early as
240 BCE,
and as recently as 1986 when a
probe imaged its nucleus
Short period comets
Comets that orbit with a period of less than
200 years are called short period comets.
They originate from the Kuiper belt
Each time a comet nears the Sun it loses
mass.
Gradually it evaporates leaving only dust
and rock…
a meteor shower
Long Period and Other Comets
Some comets take much longer….up to 5000
years or more to complete one orbit…
a “Long Period” Comet from the Oort cloud.
Some are “One-timers”
When knocked out of the Oort cloud they took
an hyperbolic path
thus passing close to the Sun and then heading
out into interstellar space…forever!
In 1994 the Comet Shoemaker-Levy collided with
Jupiter.
Before it hit tidal forces broke it into pieces.
Each hit separately.