Lec25-Apr19 - 4/19/11 Key Concepts Astro 109 Lecture...

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Unformatted text preview: 4/19/11 Key Concepts Astro 109 Lecture 25: Minor Bodies – Asteroids, Comets, Pluto, and the Kuiper Belt •  locaCons of minor bodies •  inferring size from brightness •  orbital dynamics: resonances, scaHering April 19 •  impacts on Earth Apr. 19 Thought QuesCon Finding asteroids What is the best way to determine the size of a distant planet that appears only as a dot in our best telescopes? (Assume we know the distance to the planet.) A.  use a high ­resoluCon radio interferometer B.  measure its mass and assume an average density C.  measure its brightness in opCcal light and assume an average albedo D.  measure its brightness in both opCcal and infrared light E.  it can’t be done Apr. 19 Wikipedia Apr. 19 Asteroid facts •  Rocky leYovers of planet formaCon Ceres Diameter (km) Number (>D) 900 1 500 3 •  Largest: Ceres, diameter ~1000 km 300 5 200 30 100 200 •  Most are too small to be round 50 600 30 1100 10 10,000 •  Total mass ~4% of Moon 5 90,000 Wikipedia Hubble Space Telescope (Wikipedia) Apr. 19 Apr. 19 1 4/19/11 Apr. 19 Apr. 19 Apr. 19 Apr. 19 Discussion QuesCon Where are they? What causes the Kirkwood gaps in the asteroid belt? A.  B.  C.  D.  E.  no asteroids formed there large asteroids clear certain regions shepherd moons orbital resonances with Jupiter Cdal forces from the Sun hHp://ssd.jpl.nasa.gov/?ss_inner Apr. 19 Apr. 19 2 4/19/11 “Greeks” “Lagrange points” “Trojans” hHp://ssd.jpl.nasa.gov/?ss_inner Apr. 19 hHp://ssd.jpl.nasa.gov/?histo_a_ast Apr. 19 Apr. 19 Apr. 19 Apr. 19 Apr. 19 3 4/19/11 Charon Pluto Wikipedia hHp://www.boulder.swri.edu/plutonews/ Apr. 19 Apr. 19 Physical properCes •  orbital moCon  ­ ­> mass: •  radius: M = 1.3×1022 kg R = 1153 km Planet Density (kg/m3) Mercury 5430 Venus 5243 Earth 5515 Mars 3934 Jupiter 1326 Saturn Apr. 19 1318 Neptune Apr. 19 687 Uranus 1638 Pluto 2033 cf. Lec 12 Example What is the largest angular size of Pluto as seen from Earth? •  •  •  •  radius: R = 1153 km perihelion distance: amin = 29.66 AU = 4.44×109 km closest to Earth: dmin = 28.66 AU = 4.29×109 km maximum angular size: θmax = 206265 arcsec × Apr. 19 R = 0.055 arcsec dmin cf. Lec 2 Apr. 19 4 4/19/11 Trans ­Neptunian Objects / Kuiper Belt Wikipedia known objects Apr. 19 Apr. 19 Eris moon – Dysnomia •  discovered in 2005 •  thought to be bigger than Pluto •  how do we know?? Apr. 19 Discussion QuesCon When we observe a distant planet with an opCcal telescope, what are we actually seeing? A.  B.  C.  D.  E.  reflected sunlight thermal emission from the planet itself emission lines from the planet’s atmosphere glow from the planet’s volcanoes streetlights in the planet’s ciCes Apr. 19 Discussion QuesCon total incident power : reflected power : Apr. 19 Pref = a × Pin emitted power : Which of the following affect(s) the reflected brightness of a planet? A.  B.  C.  D.  E.  Pin L⊙ 4π d2 = F × π R2 L⊙ R2 = 4d2 Pem = (1 − a) × Pin incident flux : size distance from Sun albedo both A and B all three A, B, and C Apr. 19 F = cf. Lec 19 5 4/19/11 Discussion QuesCon total incident power : reflected power : Pref = a × Pin emitted power : A.  B.  C.  D.  E.  Pin L⊙ 4π d2 = F × π R2 L⊙ R2 = 4d2 Pem = (1 − a) × Pin incident flux : What is the best way to determine the albedo of a distant, unresolved planet? assume an average albedo make a guess based on the planet’s density compare the planet’s reflected and emiHed brightnesses use the Doppler effect it can’t be done ratio = Apr. 19 Apr. 19 Pem 1−a = Pref a cf. Lec 19 What is the best way to determine the size of a distant planet that appears only as a dot in our best telescopes? (Assume we know the distance to the planet.) How can we measure both the reflected and emiHed brightnesses of a distant, cold planet? use an opCcal telescope use opCcal and infrared telescopes use opCcal and UV telescopes use opCcal and radio telescopes use infrared and radio telescopes Apr. 19 = Discussion QuesCon Discussion QuesCon A.  B.  C.  D.  E.  F A.  use a high ­resoluCon radio interferometer B.  measure its mass and assume an average density C.  measure its brightness in opCcal light and assume an average albedo D.  measure its brightness in both opCcal and infrared light E.  it can’t be done Apr. 19 cf. Lec 9 Known KBOs Wikipedia ar/st’s impressions Apr. 19 Wikipedia Apr. 19 6 4/19/11 Discussion QuesCon Neptune KBO orbits PluCnos all have the same semimajor axes, but different eccentriciCes. What about their orbital periods? A.  B.  C.  D.  E.  PluCnos All have the same orbital periods. All have orbital periods shorter than Pluto’s. All have orbital periods longer than Pluto’s. There are periods both longer and shorter than Pluto’s. We don’t know – their orbital periods are too long to measure. Kepler’s Third Law: P 2 = a3 Kuiper “belt” Apr. 19 hHp://www2.ess.ucla.edu/~jewiH/kb.html Apr. 19 What’s special about the PluCnos? •  Pluto’s orbital period: 248.1 yr •  Neptune’s orbital period: Pluto/Neptune = 1.5 KBO orbits 164.8 yr •  raCo: Neptune resonances 3:2 •  3 Neptune orbits for every 2 Pluto orbits – 3:2 orbital resonance 4:3 •  prevents Pluto from colliding with Neptune! 2:1 •  (similar for all PluCnos) 1:1 Apr. 19 Apr. 19 Impact rate hHp://www2.ess.ucla.edu/~jewiH/kb.html Chicxulub Demise of the dinosaurs sediment layer: •  iridium and other metals •  shocked quartz •  spherical rock droplets •  soot Apr. 19 Apr. 19 7 ...
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This note was uploaded on 01/19/2012 for the course ASTRO 109750 taught by Professor Keeting during the Fall '11 term at Rutgers.

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