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Dead Comets Ice gone - dust remains
Remnant spreads out in elliptical donut shape - a meteoroid swarm If Earth gets in the way, meteor shower
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Meteor Showers
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Asteroids
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Asteroids – How We Know What We Know
A few moons are probably captured asteroids A handful of asteroids were passed en route to elsewhere Gaspra (’91), Ida & Dactyl (’93), Braille (’99), Annefrank (’02), Steins (’08) and Lutetia (’10) A small number of dedicated asteroid missions NEAR flew past Mathilde (’97) and orbited Eros (’00 –’01) Hayabusa visited Itokawa (’05), returned dust particles Dawn orbited Vesta (’11–’12) and Ceres (’15–’16) Hayabusa 2 is at Ryugu (’14–’19) and will return sample to Earth (’20) First sample taken on Feb. 22 OSIRIS-REx sample return mission at Bennu (’16 – ’23) Meteorites are small fallen asteroids!
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Dedicated Asteroid Missions
NEAR Dawn Hayabusa
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Asteroids – General Characteristics
From microscopic up to nearly 1000 km Largest would be considered medium moons Shape Largest (Ceres) and perhaps a few others are spheres Most are irregular Orbits Most lie between Mars and Jupiter A few make it in near Earth Composition: Rock, metal, or rock mixed with metal Some of the largest may have some water
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The Asteroid Belt Q. 57: Composition of Asteroids vs. Comets
Region between Mars and Jupiter where most asteroids lie Q. 57: Composition of Asteroids vs. Comets
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Asteroids We’ve Seen Close Up
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Vesta, Third Largest Asteroid
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Ceres The largest asteroid 952 km in diameter
Dawn went in orbit around it Bright spots – salt deposits Indicates briny ices just below
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Ceres Composition
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Meteors and Meteorites
A meteor is the flash of light that occurs when a natural object falls to Earth from space A meteorite is the object we find on the Earth afterwards
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Meteorite Classification
Primitive Contain metal bits mixed in Some also contain carbon-rich materials! Non-primitive Stony Pure stone, no metal bits Iron Pure metal, mostly iron Stony-Iron Inhomogeneous mix of stony and iron meteorites
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Meteorite Classification
Stony Primitive Stony - Iron Iron
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Primitive Meteorites Primitive with Carbon Primitive no Carbon
The carbon-compound bearing meteorites have never been heated They come from the formation of the solar system We can find age of solar system from these Solar system is 4.56 billion years old Q. 58: Age of Solar System
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Why Is There an Asteroid Belt?
Normal planetary coalescence: Planetismals in near circular orbits Collisions are gentle Easy for little pieces to stick together
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Collisions in the Asteroid Belt
Jupiter disturbs orbits Planetismals collide at high velocities Pieces get smashed apart as often as they stick together. Jupiter
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How We Got Different Meteorites
Planetismal: Primitive w/ carbon Now smash the protoplanet! Stony - Iron Collided Planetismal: Primitive no carbon Stony Differentiated Protoplanet Iron
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Large Meteor Craters on Earth
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The Iridium Layer Thin layer in Earth contains high fraction of iridium Common in meteorites Dates from demise of dinosaurs
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Chicxulub Impact Site 65 Myr old Precursor about 10 km in diameter
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Extrasolar Planets Found = 3999, Systems Found = 2987
What’s an Extrasolar Planet? An extrasolar planet is a planet that goes around another star A star is something that is hot enough to fuse hydrogen More on this later A brown dwarf is too small to be a star, and too large to be a planet A planet is something that is large enough, but not too large Lower limit – irrelevant, we can’t see them that small Upper limit – about 13 times Jupiter’s mass Extrasolar Planets Found = 3999, Systems Found = 2987
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Detection Methods Methods for finding them Direct Imaging
Gravitational Microlensing Astrometry Pulsar Timing Other Timing Radial Velocity Transit Method Dynamical Methods
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Direct Imaging Score = 122 Disadvantages
Almost impossible to see planet next to bright star Must be done from space Advantages Can estimate size of planet Can study spectrum of planet Score = 122
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Gravitational Microlensing
The Technique Einstein – Light is bent by gravity Any object passing in front of a distant star can cause it to look brighter Foreground Star Distant Star Planet By measuring brightening carefully, can tell if there is just a star or star plus planet
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Gravitational Microlensing
Disadvantages Requires lucky alignment Tells little about the planet Cannot be repeated Advantages Can monitor many stars cheaply Sensitive to small masses Score = 89
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Dynamical Methods Astrometry Pulsar Timing Other Timing
Radial Velocity How they work Planet orbits star Star also goes in a small circle Easier to detect motion of star than of planet
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Astrometry Score = 6 How it works
Star moves left and right as viewed by us Disadvantages Insensitive Massive planets Large Orbits Patience Advantages Can see planets with large orbits Score = 6
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Gaia Mission Q. 59: Astrometry from Space
European spacecraft repeatedly measuring millions of stellar positions Will allow accurate parallax (distance) to countless stars Should discover thousands of planets Final data releases in 2020 and 2021 Q. 59: Astrometry from Space
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