Lecture 13 - Seattle Central College

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Edmond Halley
Edmond Halley

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ASTR 100
Lecture 13: Comets, Kuiper Belt,
Oort Cloud
Today: The rest of the solar system and the formation
of the solar system
Start Ex. 5
Friday: The formation of the Solar system
…Ch. 6
And our Sun …Ch. 10
Kuiper belt, Oort cloud,
and the comets whom they hath begat
Comets: structure, orbits, and origin,
Meteor showers
Kuiper belt: An outer, icier, version of
the Asteroid belt
Oort cloud: An outer, icier, version of
the Kuiper belt
Comets observed throughout history, but usually
attributed to supernatural origins
1577 Tycho Brahe showed they’re in “the heavens”
1705 Sir Edmund Halley showed one obeyed
Newton’s/Kepler’s laws
Some regular visitors, some one-hit wonders
SOHO spacecraft has discovered 1600 comets in the
last couple of years
…what are they made of and why are they so pretty
Comet composition: Dirty snowballs which melt as they
get close to the Sun. “Evaporating volatiles and dust”
Comets have a (tiny) nucleus and three (giant) tails:
1) Plasma tail - evaporating gas, ionized by solar wind
2) Dust tail - unstuck as comet melts
3) Debris tail - bigger particles left in wake of the comet
~ 1AU
Earth passes through the debris tail = Meteor shower
Named after the constellation
the meteors appear to be
coming out of
Where do they come from?
The Story of Pluto
…Pluto’s a little weird.
1990’s: More Plutos….
1930: A new planet!
2006: Pluto reclassified as a “dwarf planet”
Dwarf planets have squished themselves spherical
but have not cleared their orbital paths of stuff
The Kuiper belt: The stuff in Pluto’s path
Icy (frozen volatiles) bodies
arranged in a doughnut
shape from 30-(100) AU
The Kuiper Belt can’t be the source of all comets
Some comets have
really high inclinations,
up to 90 degrees from
the ecliptic
Oort cloud possibly formed from material ejected
from the formation of the solar system
Like 1 Trillion snowballs thrown out of the SS, being
pulled back in by gravity
Sounds like there should be a
lot of comets. Where do they
1) They can totally evaporate:
Quadrantids (in Jan)
Delta Aquarids (in July/August)
Have no associated comet
2) They hit stuff:
More mass, more gravity, more
likely that you’ll get hit
We have seen a Jovian planet, capture and rip
apart a comet, Comet Shoemaker-Levy 9, in 1994
“Capture” comets part of the story
of planetary rings.
Must be universal.
Once collisions or tides make a debris
field, it maintains itself by “grinding
down” through more frequent
collisions and “catching” new stuff
Features of rings: Gaps,
“ringlets”, gap and
shepherd moons
Metallic and rocky meteorites leave behind debris
1908 Tuguska Event had no debris…
Comets may be responsible for Earth’s abundant water.
Thank you, comets.
How do we know?
1) Sending probes
2) Earth-based telescopes (Spectroscopy)
3) Historical sources
Giotto 1986
Stardust 2004
Deep Impact 2005
Deep Impact 2005
Comet Tempel 1
Key terms:
Comet, dust tail, plasma tail, Kuiper Belt, Oort cloud, Meteor
shower, Centaur, Kuiper Belt Object (KBO),
Key Ideas:
What are the features and basic composition of a comet?
How do the Jovian planets and Sun protect us from comet
What are the two main sources of comets?
The properties of Kuiper belt and Pluto
Properties of the (alleged) Oort cloud
What causes meteor showers?
How do we know?

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