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LECTURE 13, OCTOBER 7, 2010

ASTR 101, SECTION 3INSTRUCTOR, JACK BRANDT

jcbrandt@unm.edu

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HALLEY’S COMET

• First comet to have its return predicted. Occurred in 1759 AD, a triumph of celestial mechanics

• Seen at nearly every apparition since 239 BC• Returns on average every 76 years• Last return was in 1986 AD & 5 spacecraft were

sent to its vicinity• Next return is in 2061 AD

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PROCESSES IN COMETS-1

• The basic process is sublimation, the direct transition from solid water ice to vapor

• Produces water vapor and releases dust particles and other gases trapped in the ice

• All cometary phenomena ultimately originate with material in the nucleus

• Energy source is solar radiation as the nucleus approaches the sun

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PROCESSES IN COMETS-2

• Gas and dust emission comes from localized areas on the surface of the nucleus

• After sublimation occurs from the surface layers, a loose, dust crust remains. Confirmed by Deep Impact

• Dust and gas (atoms & molecules) flow outward to form the coma and hydrogen cloud

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PROCESSES IN COMETS-3

• Solar radiation pressure pushes dust particles anti-sunward to make the dust tail

• Some molecules are ionized (electrically charged) and are carried anti-sunward by the solar wind to form the ion/plasma tail

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TEXT PROBLEM, PAGE 111, ETC.

• The text statement: “Both types of tails are in all cases directed away from the Sun by the solar wind, an invisible steam of matter and radiation of escaping from the Sun.”

• This is wrong. See previous slide. Also, the solar wind is composed of particles and magnetic fields, but not radiation.

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Question 9

Compared to asteroids, comets show all of these properties EXCEPT

a) their densities are higher.b) their orbits tend to be more elliptical.c) they tend to be made of ice.d) they can look fuzzy, whereas asteroids

appear as moving points of light.e) their average distances from the Sun

are far greater.

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Question 9

Compared to asteroids, comets show all of these properties EXCEPT

Comets have densities much lower than asteroids or

planets.

a) their densities are higher.b) their orbits tend to be more elliptical.c) they tend to be made of ice.d) they can look fuzzy, whereas asteroids

appear as moving points of light.e) their average distances from the Sun

are far greater.

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DISCONNECTION EVENTS

• Occasionally, the entire ion/plasma tail disconnects from the head region

• A new ion/plasma tail is formed• Produced by a specific feature in the solar

wind

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What causes a meteor shower?

Question 10a) A comet and an asteroid collide.b) Earth runs into a stray swarm of asteroids.c) Earth runs into the debris of an old comet littering

its orbit. d) Meteorites are ejected from the Moon.e) Debris from a supernova enters Earth’s

atmosphere

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What causes a meteor shower?

Question 10

Meteor showers can generate a few shooting stars, to hundreds of thousands, seen in an hour.

a) A comet and an asteroid collide.b) Earth runs into a stray swarm of asteroids.c) Earth runs into the debris of an old comet littering

its orbit. d) Meteorites are ejected from the Moon.e) Debris from a supernova enters Earth’s

atmosphere

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Question 12

a) a cold terrestrial planet.b) a small jovian planet.c) a large Kuiper belt object.d) a wandering moon.e) a captured comet.

Many astronomers believe Pluto is perhaps best classified as

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Many astronomers believe Pluto is perhaps best classified as

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a) a cold terrestrial planet.b) a small jovian planet.c) a large Kuiper belt object.d) a wandering moon.e) a captured comet.

Several other large Kuiper belt objects have been

discovered, including the largest known, called Eris.

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