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Chapter 9

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University of Toronto Mississauga
John Lester

Astronomy Ch. 9 - Asteroids, Comets and Dwarf Planets (Nov. 16) Smaller Objects - Solar System has many objects smaller than the major planets: - asteroids = minor planets - meteoroids - comets - dwarf planets (since 2006) What are Asteroids? - “aster” = star + “oid” = body - an object that looks like a star, but it orbits the Sun - Largest (known) = 1000 km in diameter - Smallest detectable ≈ 100 m - More than 400,000 catalogued asteroids, but many more have been detected - Probably more than 1 million asteroids larger than 1 km Where are the Asteroids? - Almost all have nearly circular orbits between Mars and Jupiter = “asteroid belt” - these orbits show the strong influence of Jupiter’s powerful gravity - A few orbit with Jupiter, ahead or following in its orbit - captured by Jupiter + Sun What are Meteoroids? - Tiny asteroids, too small to detect at a distance - sizes are m, cm, mm, ... dust - Some meteoroids (speed ≈ 30 km/s) hit Earth’s atmosphere, heat up (friction), give off a flash of light as they disintegrate ▯ = meteor or shooting star - Any remnant large enough to survive and reach the ground = meteorite = sample of solar system matter Families of Meteorites - “Primitive” meteorites - the matter is just clumped together - no signs of having been heated - “Processed” meteorites - the matter has been heated - has been hot enough to melt into pure metal, mostly iron What are Comets? - Comet = hairy star - Orbits are very elliptical - comets move from near the Sun out to the edge of the Solar System - Mostly “ice” = frozen water + frozen gases + small amount of dust/grit/sand/pebbles mixed into the ice = like a dirty snowball - Comet approaches the Sun - heats up - Note: comets are evaporating ▯ → the comets we see near the Sun cannot survive this loss of material for billions ▯ of years Where do Comets Live? - Comets break all the “rules” - Orbits are very elliptical → come from the outer Solar System where it is very cold, where they live most of the time (Kepler
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