Slide 1CometsSlide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34PYTS/ASTR 206 – Comets1AnnouncementsHW6 available today, due in a weekUse Kevin as the TA for this one2 In-class assignments left in 3 lecturesPYTS/ASTR 206 – Comets2PTYS/ASTR 206 – The Golden Age of Planetary ExplorationShane Byrne – [email protected]/ASTR 206 – Comets3In this lecture…In this lecture…Observations of cometsWhat are comets?Composition and structureCometary tailsIon and dust tailsWhere do comets come from?Orbits of cometsOort cloudScattered Kuiper BeltPYTS/ASTR 206 – Comets4Comets have been known from ancient timesThought to foreshadow disasters and major battlesPre-telescopes the known solar system was a pretty empty placeMoon and the SunMercury, Venus, Mars, Jupiter, Saturn And COMETSNo UranusNo NeptuneNo planetary Moons (except ours)No AsteroidsNo Kuiper Belt ObjectsObservations of cometsObservations of cometsPYTS/ASTR 206 – Comets5People have recorded comet sightings for millennia167 BC687 AD1986 ADPYTS/ASTR 206 – Comets6Ancient Greeks thought comets were atmospheric phenomenaIn the west this went unchallenged until telescopes came alongTycho Brahe’s parallax measurements proved this wrongComets were much further away than the MoonRenaissance astronomers thought comets moved in straight lines through the solar systemEven Kepler argued they shouldn’t follow elliptical orbits like the planetsIn the 1680s astronomers tracked a comet and showed it had an elliptical orbit Comets were solar system objects – just like planets☄PYTS/ASTR 206 – Comets7Newton finally settled this in his ‘Principia Mathematica’ (1687)Showed that comets moved in parabolic or elliptical orbits by the Sun’s gravitye = 1A parabolic orbitPYTS/ASTR 206 – Comets8If they have orbits… then they’re periodicThe same comet should come backIn 1705 Edmund Halley connected the dots…Used Newton’s laws to figure out the orbit of many cometsComets seen in 1531, 1607, and 1682 were the same objectPredicted a return in 1759Halley’s comet has been seen ~30 times687 AD1986 AD167 BCPYTS/ASTR 206 – Comets9Many telescopic observations of comets (including Halley’s comet)Even a few spacecraft missionsGiotto & VegaComet HalleyDeep-space 1Comet BorrellyStardustComet Wild 2Deep ImpactComet Tempel 1PYTS/ASTR 206 – Comets10Comets have several partsNucleus~10 kmComa~1,000,000 kmAlmost as big as the sun!•1,400,000 kmHydrogen envelope~10,000,000 kmTailIon tailDust tail~100,000,000 kmAbout 2/3 of 1AU!What are comets?What are comets?PYTS/ASTR 206 – Comets11Cometary nuclei are usually invisible from the EarthHidden by the comaSpacecraft missions can visit far from the Sun when the coma is inactive Comet Tempel 1Comet HolmesPYTS/ASTR 206 – Comets12Comet Nuclei are ‘dirty snowballs’Random mixtures of ices and dark stuffIcesMostly water iceA little CH4, CO, CO2 etcDark ‘stuff’Organic compounds (H,C,O)Rock-like material •Like asteroidsVery small objectsNot enough self-gravity fora round shapePYTS/ASTR 206 – Comets13When comets are close to the sunSurface heats upIce sublimates (turns to vapor)Dark organic stuff gets concentrated on the surfacePYTS/ASTR 206 – Comets14Comets are ice rich…but among the darkest objects in the solar systemAlbedo of 2-4%Like tar Comet nuclei are very hard to see without their comasPYTS/ASTR 206 – Comets15This thick crust builds up over many orbitsSublimating ice comes out in jetsCollapse pits form on the surface from removal of sub-surface iceJets act like rocket engines – can alter the orbits of cometsPYTS/ASTR 206 – Comets16Comet nuclei are typically small < 40km Mass estimates come from spacecraft flybysComets are very low densityContain significant internal voids From WikipediaWater ice~0.9 g/cm3PYTS/ASTR 206 – Comets17The Deep Impact mission370-kg (815-lb) copper impactorAnalysis of vapor plumeCrater 100m wide, 30m deepComposition was ‘dirtier’ than expectedData analysis still in progressPYTS/ASTR 206 – Comets18Sublimation jets produce cometary atmosphereMostly water ice crystals – some dustComet’s gravity can’t hold onto this materialOccasionally a big piece of the comets surface will break off exposing fresh iceComet Holmes brightened by a factor of 1 million within a few daysComa of Comet HolmesPYTS/ASTR 206 – Comets19What happens to the water ice crystals?UV solar radiation breaks up the water molecules H2OOH-H+We can see this using ultraviolet lightPYTS/ASTR 206 – Comets20Comets have two tailsIon tail of OH- and H+ Ions are swept up by the solar windIon tails point away from the SunBlue-ish in colorPYTS/ASTR 206 – Comets21Dust tailsAlso swept by the solar wind but less efficientlyDust tail is brighter and whiterTail direction affected by the comets motion and is curvedDirection of comet’s motionPYTS/ASTR 206 – Comets22Comets can appear to have a tail and an anti-tailEarthThis observer sees the this comet Ion Tail Dust TailPYTS/ASTR 206 – Comets23PYTS/ASTR 206 – Comets24Cometary orbits are very different from asteroidsComets have very elliptical orbitsComets have randomly inclined orbitsComets have very large orbits Where do comets come from? Where do comets come from?PYTS/ASTR 206 – Comets25Divided into short period (<200 years) and long period (>200 years)Short period cometsJupiter family comets (Periods <20 years)Orbits controlled by JupiterAll low inclinationHalley family comets (Periods 20-200 years)Come from the Kuiper BeltSpread in inclinationsEventually transition to Jupiter family cometsPYTS/ASTR 206 – Comets26Long-period comets Have totally random inclinationsHave very long periods/large orbitsMany of these appear to be on their first pass through the inner solar systemA body with a semi-major axis of 10,000 AU will orbit once every million yearsPYTS/ASTR 206 – Comets27The Oort cloudA
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