Download Final exam

Final exam
In SRC C, noon, Mon. Dec 11
2.5 hours in length, don't be late!
Same rules as midterm Formula sheet and data given
You need a scientific calculator
Have 2 pieces of photo identification
If you have 3 or more midterms in 24 hours, you must inform all your instructors by Nov 29th (Wednesday!)
Comets! Omens of disaster or pretty to look at?
Probes of planet formation in the outer S.S.
Omens of disaster
Battle of Hastings, 1066, in the Bayou tapestry
Actually Halley's comet
Became first predicted comet in 1705
Comets vs Asteroids
Both are minor planets Can think of them as `leftover planetesimals'
Asteroids Circular orbits, between 2 ­4 AU
Rocky planetesimals
Comets
Big, eccentric orbits : some get inside Earth's orbit (with aphelion at or outside Jupiter, some got out as far as 50,000 AU
"Dirty snowballs" ­ mostly ice and some rock
Orbits
Big elliptical orbits (unlike asteroids)
Composition
"Dirty snowball"
Mostly ice (water ice, carbon dioxide ice, methane ice, ammonia ice)
Some rocky materials mixed in
See http://www.solarviews.com/eng/edu/comets.htm for instructions on how to create a comet nucleus for yourself! Ice => condensed in low temperature regions
Leftover planetesimals which formed outside Jupiter's orbit
Comet
Morphology
Most well­known feature : the tail
Gas and dust evaporated from comet nucleus ­ very diffuse
Dust tail (white)
Ion tail (blue)
Comet
Morphology
Most well­known feature : the tail
Dust tail
Hydrogen envelope
Ion tail Comet tails
Always point away from the Sun due to radiation pressure Don't always point "behind" comet's orbit
Ion tail made of smaller particles, pushed harder ­ make straighter tail
Comet pills
Many strange gases detected spectroscopically including cyanide
1910 : doctors sold "comet pills" to ward off ill effects of Earth passing through tail of Comet Halley
Actually tails very diffuse ­ but small particles are kicked off surface of comet and left behind its path
When Earth passes through these ­ get meteor showers
Meteor showers appear to come from a 'radiant'
Meteors appear to come from a single point
This is an 'illusion'; it just appears to do so because the meteors are coming from a single direction in space (see text)
The annual Leonid meteor shower ❑
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Leonid shower has on occasion HUGE output (>500/minute!)
Woodcut (left): shows spectacular 1833 display over U.S.A.
Peak: mid­November each year (search on web). 2005 was poor (full Moon).
Look easterly. Need dark sky, dress warm!
Another Leonid storm observation
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Woodcut from 1833
Too bad storms are rare.
Rest of the Morphology
Tail
Coma
Nucleus
Hydrogen envelope
Coma
Shell of evaporating gas and dust
~106 km across
A closer look... (nucleus)
Closest look : Giotto (ESA) flew within 600 km of Comet Halley nucleus (1986)
~10 km across
Future missions ­ Stardust, Deep Impact, Rosetta ­ will fly by comets and even capture material from surface. Or hit it! (July 4/2005)
Comets populations
Split into different groups.
The ACTIVE comets come close to Earth.
Jupiter Family comets (JFCs)
Orbit the Sun in less than 20 years; tied to Jupiter
That means (P2=a3 ! ) that a<7.3 AU Halley­type comets
Orbit with periods between 20 and 200 years Includes Halley (P=76 years), with retrograde orbit
Long period comets (LPCs)
Orbits greater than 200 years ­ can travel 105 AU from the Sun
Orbits
Different groups have different orbits
JFC's : small inclinations and small periods
LPC's : random inclinations and long periods
This means they came from different sources!
JFC's from the Kuiper Belt
Region from 30­1000 AU
flattened disk provides low inclination JFCs
LPC's from Oort Cloud
Extends out from 1000 AU to 50,000 AU
Where are these structures?
Oort cloud extends a reasonable fraction of the distance to the nearest star
Life of a comet
After the formation of the Solar system :
live in Kuiper Belt or Oort Cloud for > 4 Gyr
Perturbed from peaceful orbit into elliptical orbit
Many are ejected from the Solar System, but...
...a few enter the inner solar system and become "active comets"
How do they die?
Can't keep evaporating ice forever! Some get ices evaporated away and become 'extinct'
Most eventually ejected by Jupiter
But, the solar system is full of things to run into ...
Shoemaker­Levy 9
Illustrates both tides and fragmentation due to fragility of the cometary nucleus