Ice gone - dust remains Remnant spreads out in elliptical donut shape - a meteoroid swarm

1. Dead Comets • Ice gone - dust remains • Remnant spreads out in elliptical donut shape - a meteoroid swarm • If Earth gets in the way, meteor shower

2. Meteor Showers

3. Asteroids

4. 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!

5. Dedicated Asteroid Missions NEAR Dawn Hayabusa

6. 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

7. The Asteroid Belt • Region between Mars and Jupiter where most asteroids lie Q. 57: Composition of Asteroids vs. Comets

8. Asteroids We’ve Seen Close Up

9. Vesta, Third Largest Asteroid

10. Ceres • The largest asteroid • 952 km in diameter • Dawn went in orbit around it • Bright spots – salt deposits • Indicates briny ices just below

11. Ceres Composition

12. 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

13. 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

14. Meteorite Classification Stony Primitive Stony - Iron Iron

15. 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

16. Why Is There an Asteroid Belt? Normal planetary coalescence: • Planetismals in near circular orbits • Collisions are gentle • Easy for little pieces to stick together

17. Collisions in the Asteroid Belt • Jupiter disturbs orbits • Planetismals collide at high velocities • Pieces get smashed apart as often as they stick together. Jupiter

18. How We Got Different Meteorites Planetismal: Primitive w/ carbon Now smash the protoplanet! Stony - Iron Collided Planetismal: Primitive no carbon Stony Differentiated Protoplanet Iron

19. Large Meteor Craters on Earth

20. The Iridium Layer • Thin layer in Earth contains high fraction of iridium • Common in meteorites • Dates from demise of dinosaurs

21. Chicxulub Impact Site • 65 Myr old • Precursor about 10 km in diameter

22. Extrasolar Planets 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

23. Detection Methods Dynamical Methods Methods for finding them • Direct Imaging • Gravitational Microlensing • Astrometry • Pulsar Timing • Other Timing • Radial Velocity • Transit Method

24. Direct Imaging 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

25. Gravitational Microlensing Planet 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 • By measuring brightening carefully, can tell if there is just a star or star plus planet

26. 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

27. 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

28. Astrometry 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

29. Gaia Mission • 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