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Comets
The Oort Cloud and Kuiper Belt
The Oort cloud is a
“cloud” of icy
planetismals that
extends from
Neptune’s orbit to
150,000 AU
To the very edge of the
solar system.
Most of these orbit in
the same plane as
the planets forming
a “belt”
that extends from Neptune to about 50 AU called the Kuiper
Belt
( in green)
Composition
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
They are often called “Cometary Bodies”
The Origin of Comets
Occasionally the gravity of a passing star 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 and a cloud of
gas and dust called a coma which 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.
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.
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.