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.

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.

Composition of Comet Nuclei 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

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 In 1994 the Comet Shoemaker-Levy collided with Jupiter. Before it hit tidal forces broke it into pieces. Each hit separately.