03 Oct 2000 ASTR103, GMU, Dr. Correll 1

Ch 8--Asteroids, Meteors, CometsCh 8--Asteroids, Meteors, Comets

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What do you think?What do you think?

• Are the asteroids a planet that was somehow destroyed?

• How far apart are the asteroids on average?• Why do comets have tails?• In which direction does a comet tail point?• What is a shooting star?

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Early Solar SystemEarly Solar System

• From studying cratering on the Moon and the planets– First half-billion

year had many collisions between planetesimals

– More recently the collisions are much less frequent, but do occur!

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AsteroidsAsteroids

• Asteroids:– remnant planetesimals

• also known as “minor planets”

– mainly concentrated in the asteroid belt between Mars and Jupiter--about 10 million miles average separation

– Mostly stony or metallic in composition

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AsteroidsAsteroids

• 1801--Piazza discovers Ceres--the first asteroid

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Asteroids--OriginsAsteroids--Origins

• A destroyed planet?– Probably not

• Jupiter’s gravity creates “resonances” which create bands of stable orbits and gaps of unstable orbits– Kirkwood gaps

discovered 1867

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Asteroids outside the beltAsteroids outside the belt

• Gravitational perturbations eject comets from the belt into elliptical orbits– Some approach the inner planets– “Apollo” asteroids cross the Earth’s orbit

• Near-Earth Objects (NEOs)

• Near-Earth Asteroids (NEAs)

• Potentially Hazardous Objects (PHOs)

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CometsComets

• Comets--best thought of as “dirty snowballs” orbiting the Sun which evaporate due to solar heating

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Comets OriginsComets Origins

• Primordial gas, dust, and ice frozen in clumps at the outer limits of the solar system– Kuiper belt--out to 500

AU– Oort Cloud--50,000

AU

• Occasionally perturbed into elliptical orbits approaching the Sun

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Parts of a CometParts of a Comet

• Coma--spherical cloud of vaporized gas around the frozen nucleus

• Tails--trail of debris– dust tail--dust

particles blown away by solar radiation

– ion tail--ions carried away by solar wind

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Parts of a CometParts of a Comet

• Tails always point away from the Sun!

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Comets DemiseComets Demise

• Comets eventually– breakup into smaller fragments (Comet West below)– evaporate– collide with the sun– collide with other planets

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Comets DemiseComets Demise

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Comets DemiseComets Demise

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Meteoroids, Meteors, MeteoritesMeteoroids, Meteors, Meteorites

• Meteoroids--small rocky or metallic particles throughout the solar system

• Meteors--upon entering Earth’s atmosphere, friction causes heat and disintegration--shooting stars

• Meteorites--remnants surviving on the ground

16ASTR103, GMU, Dr. Correll 03 Oct 2000UWO Meteor Group

Earth’sEarth’sLocationLocation

everyeveryNov 17Nov 17

Meteor Showers/Storms--Earth occasionally passes through cometary debris streams leading meteor showers!

Leonid Meteor Shower (Comet Temple-Tuttle stream)

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MeteoritesMeteorites

• Meteorites--remnants surviving on the ground– Mostly remnants of asteroids

(to have survived entry into Earth’s atmosphere)

• Stony meteorites

• Iron meteorites

– Show melting and wearing patterns from processing in asteroids

– Carbonaceous Chondrites show no melting

• may be primordial components of solar system

• contain organic material!

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Dino Killers?Dino Killers?

• Evidence for large impact 65 Million years ago

• Alvarez theory of K-T boundary

• Impact crater evidence found off north shore Yucatan at Chixulub

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More Recent EventsMore Recent Events

• Peekskill 1993– ~1m size

• Tunguska 1908– ~ 1/2 km size– 2200 sq km area

• Meteor Crater– 30,000 BCE– 1.2 km crater– 30 m meteor

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Planetary Defense?Planetary Defense?

Easy to study; expensive to carry out!

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What do you think?What do you think?

• Are the asteroids a planet that was somehow destroyed?– No, the gravitational pull of Jupiter prevented the planetesimal in the

asteroid belt to coalesce into planet

• How far apart are the asteroids on average?– The distance between asteroids averages about 10 million miles

• Why do comets have tails?– Gas and dust evaporated from the comet nucleus are blow away by

the solar wind and solar radiation pressure, respectively

• In which direction does a comet tail point?– Gas tails point directly away from the Sun; Dust tails point indirectly

away from the sun in arcs

• What is a shooting star?– A piece of space debris plunging white hot into the Earth’s

atmosphere--a meteor, not a star

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Questions for ThoughtQuestions for Thought

• Explain the origins of comets and describe the various features seen when near the Sun.