• Final in room B130 Van Vleck 12:25pm May 16
• cumulative
• Homework #7 on web
• Write up observing project
• Extra credit project posted on web

• Finish off Pluto and its moon Charon
• may be a Kuiper Belt object
• Kuiper Belt
• similar to asteroid belt but beyond Pluto
• source of short period comets
• Oort Cloud
• Large spherical cloud of ice chunks centered on Sun
• source of long period comets
• Solar system length scale review
• Comets
• The death of comets

• Clyde Tombaugh did found Pluto at Lowell observatory in 1930
• found needle in haystack
• Pluto's orbit eccentric compared to other planets
• sometimes even inside of Neptune
• Retrograde rotation
• what other planets have retrograde rotation
• Venus, Uranus
• Pluto has a moon that is almost as big as it is
• Charon rotates synchronously
• Pluto rotates once per Charon's orbit
• Staring at each other
• What causes synchronous rotation?
• tidal forces
• Our view of Pluto with Hubble Space telescope is like Lowell's view of Mars
• Pluto probably has Nitrogen polar ice caps
• Similar in size, density to Triton
• recall Triton was probably captured by Neptune
• Pluto got away
• Pluto may be part of the Kuiper Belt...

• Originally proposed by Gerard Kuiper in 1951
• First Kuiper belt object seen in 1992 (talk about going out on a limb)
• Kuiper belt thought to be a reservoir of chucks of ice that become comets if they fall toward the sun
• Disk shaped, in plane of solar system
• like asteroid belt
• 50 -- 500 AU
• What is the asteroid belt's extent
• 2 -- 3.5 AU
• Orbits in Kuiper Belt are ~200 years
• orbits can be disturbed by outer planets
• as ice chunck comes toward Sun, becomes a comet (more later)
• Comets from the Kuiper Belt (or closer) are short period comets
• Comet Halley orbits every 76 years
• most (>35,000) just stay out in Kuiper Belt

• Proposed by Jan Oort in 1950
• Oort cloud objects yet to be see in the Oort cloud
• See ~ 1 per month in the form of a long period comet
• periods of 10,000 to 30 million years!
• e.g. Hale-Bopp in 1997
• Oort cloud spherical
• otherwise long period comets would all be in plane of solar system
• extends from 500 AU to 50,000 AU
• nearest star is ~250,000 AU
• Oort cloud probably icy planetesimals that were ejected during formation of solar system
• Probably billions of objects in Oort cloud
• The solar system had a messy childhood

• Where do comets come from?
• Oort cloud or Kuiper Belt
• Are comets young or old?
• Very old
• Probably left-over icy planetesimals
• Comets are mostly ice
• What happens to ice when it is far from the sun?
• nothing
• When ice gets close to the sun it vaporizes
• Water vapor (H₂O) is broken up by sunlight into oxygen and hydrogen
• Sunlight does not push on hydrogen, expands in a ball 10 million km wide
• is this what we think of when we see a comet?
• other effects give the interesting shapes, colors
• Does a comet die when it comes close to the sun?
• Usually not: it looses 0.5% to 1% of its mass per orbit
• Death sequence described later

• Before it starts vaporizing, the comet is just a dirty snowball
• snowball part becomes nucleus
• Water vapor expanding off of nucleus that has not broken into H and O yet forms coma
• The coma is generally what we think of as the "head" of the comet
• Hard to see nucleus through coma
• Generally see jets of water vapor coming off of nucleus
• "Dirt" part of comet flys off with water, becomes dust tail
• dust is reflective, light actually pushes on it!
• radiation pressure
• Sunlight that breaks up H₂O also ionizes H₂O and other elements found in comet (e.g. CO)
• ions get swept back by solar wind
• Solar wind much more effective at pushing ions than radiation pressure is at pushing dust
• Two tails
• Ion tail (straight back)
• Dust tail (curved)
• a 3rd tail was discovered in Hale-Bopp due to a special interaction of sodium and sunlight, sticks even straighter back than ion tail
• Production of water, ions, etc. can vary from day to day, changes affect ion tail more (moving faster)
• Comet tails always point away from the Sun (at least as viewed from above

Nucleus of Comet Halley from Giotto spacecraft

Two tails of comet Mrkos (1957, two day intervals)

• I study oxygen in comets
• Like hydrogen, oxygen should expand out like a ball (nothing pushes on it)
• However, we observed something different in comet Hale-Bopp

• Comets loose 0.5 -- 1% of their material each orbit
• most of the material is water, but some dirt left behind too
• Eventually the comet hits something like Jupiter
• Recall comet Shoemaker-Levy 9
• OR the ice that holds the dirt together will completely give out
• Former comets become a string of pebbles that continues to orbit the Sun
• string of debris eventually spreads out on orbit
• If we pass through comet tail debris we see a meteor shower
• Perseids, Aug 12 (comet Swift-Tuttle)
• Leonids Nov 16 (comet Temple-Tuttle)
• If we are lucky, we see hit a dense spot in the tail debris and see a meteor storm
• 1833 Leonids -- ignited popular interest in astronomy

• Finish off Pluto and its moon Charon
• may be a Kuiper Belt object
• Kuiper Belt
• similar to asteroid belt but beyond Pluto
• source of short period comets
• Oort Cloud
• Large spherical cloud of ice chunks centered on Sun
• source of long period comets
• Solar system length scale review
• Comets
• The death of comets