Unit 47

Comets

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The Structure of Comets

• Comets have two primary parts, the head and the tail

• The head consists of– The nucleus, a lump of

frozen gas mixed with loose rock and dust

• Only about 10 km across

• Dark in color, probably from dust and other materials

– The coma, the cloud of evaporated ices and gases streaming from the surface of the nucleus

• May be 100,000 km wide!

• The tail can be hundreds of millions of km long, and streams directly away from the Sun

Visiting Comets

Comet Halley, visited by Giotto

Comet Wild 2, visited by Stardust

Comet Tempel 1, visited by Deep Impact

The Origin of Comets

• Comets may originate in either the Oort Cloud or the Kuiper Belt– Oort cloud is a cloud of

comet-like planetesimals more than 100,000 AU from the Sun

– Oort cloud objects may have formed near the giant planets and then were tossed outwards by gravitational forces

• Passing stars or other gravitational influences nudge the comets into the inner Solar System

How a comet becomes visible

• As a comet moves into the inner solar system, it is warmed by the sun

– Ices on the surface sublimate (go from solid to gas) and stream away from the comet nucleus

– The sublimated gases form the coma

– Escaping gas carries dust particles outward

• Solar photons strike the dust particles, pushing them away

– Process is called radiation pressure

– This forms the dust tail!

• Gas and ions in the coma are pushed away from the nucleus by the solar wind

– This forms the ion tail, and usually points directly away from the Sun

• Gas in the coma and tail are lit up by the Sun, making them visible (fluorescence)

The Two Tails of a Comet

Another View of the Process

Meteor Showers

• As a comet orbits the sun, it leaves a trail of dust behind it.

• Occasionally, the Earth passes through one of these dust trails– Dust particles enter

Earth’s atmosphere and burn up

– We see them as meteors, in a meteor shower

• The meteors all appear to be coming from the same point in the sky called the radiant

The Names of Meteor Showers

• We name the meteor shower after what constellation the radiant is located in

• Perseus – Perseids

• Leo - Leonids

and so on