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Asteroids and Comets
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Debris of the Solar System
Chapter 9
Asteroids
• Small, rocky objects
– like terrestrial planets
– size much smaller
• shape not round
• no atmosphere
– little volatile material
• Also called “Minor Planets”
• Early solar system remnants
• Four largest:
– Ceres, Pallas, Vesta, Hygeia
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Discovery of Asteroids
• Ceres discovered in 1801
– by Piazzi
– thought it was “missing planet”
– more discovered later
• 300 known by 1890
• today >10,000 known
• Most in asteroid belt
– between Mars and Jupiter
(2.2-3.3 AU)
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Composition & Classification
• Three main types:
• C-type (carbon-rich):
– silicates mixed with dark carbon compounds
– “primitive” (unchanged since formation of solar system)
• S-type (stony):
– mostly silicates, no dark carbon compounds
– “primitive”
• M-type (metallic):
– rare
– fragments from core of differentiated asteroid
– potential mining resource
Comets
• Observed from earliest recorded times
– frequently interpreted as “harbingers of doom”,
or bad omens
• Primordial remnants of early solar system
• Three main components:
– nucleus
– coma
– tail (often two parts)
• gas (ion) tail
• dust tail
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Comet Nucleus
• Small, solid body (few km)
– “dirty snowball” model
• nucleus made of ices
• mixed with rocks and dust
• When heated (near sun)
– ices vaporize
– release gas & dust
• Gas released in “jets”
– not steady
– like geysers
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Comet Atmosphere (Coma)
• Escaping gases
– forms “cloud” around nucleus
• Coma mostly water vapor
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Comet Tails
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Sunlight pushes gas & dust
Gas & dust flows away from nucleus
Forms tail
Tail points away from Sun
– gas (ion) tail straighter, bluer
– dust tail curves, yellower
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Comet Orbits
• Orbits highly elongated
– most have aphelion well beyond Pluto
• 50,000 AU!
– perihelion near sun
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Origin of Comets
• Comets come from large distances
• Jan Oort (1950)
– proposed “cloud” (reservoir) of ancient
icy bodies
• Oort Cloud
– extends half way to nearest star
– contains 10 trillion (1013) comets
• total mass: 1000 Earth masses
• more mass than all the planets
– Occasionally comets fall into inner
solar system
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The Kuiper Belt
• Second source of comets
– in plane of solar system
– beyond the orbit of Pluto
– proposed by Kuiper
• Pluto probably largest
Kuiper belt object
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Meteors
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• Rocks from space falling to Earth
– vaporized by air friction
– glow from heat
– produces “shooting stars”
• Typical size < 1 gram (pea-sized)
– 100 tons/day fall to Earth
• Occasional larger objects
– produce fireballs (bolides)
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Meteor Showers
• Comets leave debris in their orbits
• Meteor showers
– occur when Earth passes through
comet debris
– produces large number of meteors
– meteors appear to come from a point
in the sky (called the radiant)
• shower named for location of radiant
(Leonids come from Leo)
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Meteorites
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• Meteors that land on Earth
called meteorites
• Origin
– early solar system debris
(asteroids / comets)
– planets (e.g. Mars)
– the Moon
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Meteorites
• Three types:
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– iron (almost pure nickel-iron)
• clearly extraterrestrial
• easy to identify
– stony (silicate or rocky)
• difficult to find, look like rocks
– stony-iron (mixture)
• Age ~ 4.5 billion years
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