Lecture 39. Asteroids/ Minor Planets In "Gap" between Mars and Jupiter: 20,000 observed small objects, 6000 with known orbits:

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1 Lecture 39 Interplanetary Matter Asteroids Meteorites Comets Oort Cloud Apr 28, 2006 Astro 100 Lecture 39 1 Asteroids/ Minor Planets In "Gap" between Mars and Jupiter: 20,000 observed small objects, 6000 with known orbits: Name 1-Ceres 2-Pallas 4-Vesta 433-Eros Dia (km) Orbit (AU) Gaspra 20 km Total Mass < Moon Density: 1-3 (rock/iron + vacuum) Orbits: for 99.8%, A is between Mars (1.5AU) and Jupiter (5.2AU) Apr 28, 2006 Astro 100 Lecture 39 2

2 Asteroids as collision Hazards Orbital evolution: biggest effect, perturbations with Jupiter followed by collisions with other asteroids and planets. (eg, With Earth: > 1 km dia every few 100 Myr. Cretaceous- Tertiary "K-T" extinction?) Orbits greatly affected by Jupiter: Kirkwood Gaps: P = 1/3, 2/5, 1/2 P(Jupiter) Trojan's: P = P(Jup), 60 leading and trailing Amor's: rare Mars crossers, perturbed into: Apollo's: Earth crossers (e.g. Icarus, Hermes) As break up, parent -> asteroid family -> meteoroids Near-Earth Asteroids biggest hazard eg433-eros. Visited (and landed upon) by NEAR spacecraft in Apr 28, 2006 Astro 100 Lecture 39 3 Meteorites Objects reaching ground: mostly from asteroid belt From these can infer origin of asteroids: Meteorite composition: Stony: 96% (but hard to find). Including Carbonaceous Chondrites Stony-Iron: 1% Iron: 3%. Easily recognized by "Widmanstätten" crystal patterns associated with slow cooling Material was in a "parent body" km dia, differentiated into iron core/ rocky mantle like terrestrial planet, then broken up by collisions. Meteorites include "siderophile" elements, some quite rare on Earth: E.g. Iridium: large enhancement in Iridium in K-T rock layer supports asteroid scenario. Current favorite K-T site: off Yucatan in Caribbean. Apr 28, 2006 Astro 100 Lecture 39 4

3 Comets Icy analog of asteroids in outer solar system. Most only seen when perturbed into elliptical orbits coming within Mars' orbit. Vaporizing of ices causes temporary atmosphere during perihelion passage: Nucleus: < 10 km ("dirty ice", gravity negligible) Coma: 10 6 km (constantly escaping water vapor, dust) Tail: up to km (1 A.U.!): Dust Tail: remains in orbit but is pushed away from sun by weak solar light pressure. Curved. Ion Tail: molecular gas, loses electron from solar UV, blown out of solar system by magnetic field in solar wind. Straight. Apr 28, 2006 Astro 100 Lecture 39 5 Comet Fate 4 comet flybies, direct knowledge of cometary nucleus: Very non-spherical, dark: albedo 0.04 (< asphalt) Density 0.3 gm/cm 3, "strength of talcum powder" (Deep Impact) Coma gas/ dust escaping in "jets" Comets eventually disintegrate into inactive husks + fragile dust, seen as: Meteor stream. "meteor showers", occurring on specific dates when Earth passes through orbit. Best after midnight when "looking forward" in Earth's orbit. Interplanetary dust. Seen by scattered light near ecliptic in "zodiacal light". Apr 28, 2006 Astro 100 Lecture 39 6

4 Comet Origin Some comets are new comets : entering solar system on near-parabolic orbits from all directions. Postulated Oort Cloud of small icy bodies AU from solar system: material that never made it into protoplanetary disk. Occasionally disturbed by nearby star into passing close enough to sun to be perturbed by outer planet (esp Jupiter) Others are Long-period Comets with periods years, most in plane of solar system (eg Hale- Bopp). Kuiper Belt of icy planetesimals beyond Neptune. Many become short-period comets (eg Halley) by encounters with planets, are swept up by planets or the Sun. Apr 28, 2006 Astro 100 Lecture 39 7 Asteroid Orbits Apr 28, 2006 Astro 100 Lecture 39 8

5 Eros NEAR Landing Apr 28, 2006 Astro 100 Lecture 39 9 Meteorites Chondrite Stony Apr 28, 2006 Astro 100 Lecture 39 10

6 Comet Hale-Bopp ion tail dust tail Apr 28, 2006 Astro 100 Lecture Comet Tails Figure 17.15, p530, Arny Apr 28, 2006 Astro 100 Lecture 39 12

7 Comet Tails Solar Wind Apr 28, 2006 Astro 100 Lecture Comet Nuclei Borrelly Deep Space 1 (2001) Halley Giotto (1986) Tempel 1 Deep Impact (2005) Apr 28, 2006 Astro 100 Lecture 39 14

8 Stardust: Wild 2 Launch: Feb 1999 Encounter: Jan 2004 Sample Return: Jan 2006 Apr 28, 2006 Astro 100 Lecture Leonid Meteor Shower Apr 28, 2006 Astro 100 Lecture 39 16

9 Interplanetary Dust: Zodiacal Light Apr 28, 2006 Astro 100 Lecture Comet Origins Figure 17.11, p527, Arny Apr 28, 2006 Astro 100 Lecture 39 18

10 Kuiper Belt Orbits Red Plutinos Blue CKBO s Black SKBO s Apr 28, 2006 Astro 100 Lecture 39 19