1. Comets Death by Photoshop From archives.org

2. Comets’ Appearance Dirty snowballs Do not streak across the sky! Move slowly from night to night (fast enough to have to track a long camera exposure such as Comet West, 1976, shown here) Named after discoverer(s)

3. Comet Orbits Orbits highly eccentric— some parabolic or hyperbolic Period years to millions of years By Kepler’s 2 nd law, spend most of their time far from the Sun Also range in inclination— the tilt of the orbit with respect to the ecliptic

4. Fuzzy Discoveries Slow background motion Charles Messier made his list of objects (M-numbers…) that are not comets

5. Components solid ice/rock nucleus as approaches sun, heats up and gaseous coma expands solar wind blows it into tail …

6. A Tale of Two Tails… A dust tail arced due to motion of comet (like water from a hose), and shines by reflection. A dust tail arced due to motion of comet (like water from a hose), and shines by reflection. Ion or gas tail points away from Sun, and glows by line emission. We went through the tail of Halley’s comet in 1910. Ion or gas tail points away from Sun, and glows by line emission. We went through the tail of Halley’s comet in 1910. Hale-Bopp 1995

7. Tail stays pointed away from the Sun

8. Ice Sublimates A 1-km nucleus loses ~3 meters of depth per passage by the sun

9. Sun-grazing comets

10. Most famous: Halley’s Comet

11. Seen since 684 AD, earlier by Chinese

12. Bayeux Tapestry Tells of William of Normand (France), who defeated the Saxon king Harold II of England in 1066 (Battle of Hastings) Dates to at least 1476

13. The Adoration of the Magi, Giotto, 1305

14. Hevelius, c. 1661

15. Edmund Halley Predicted the return of the comet – did return after his death. Post-Halley painting

16. Still feared! 1857 woodcut

17. Still feared?

18. Last passage: recovered in ‘82

19. Spacecraft encounters

20. Lick Obs. January 1986

21. January 1986 Disconnection Event (When solar wind reverses polarity)

22. Soviet Vega-2 Image and a Model Why not round??

23. Comet Hyakutake (1996) Many comets discovered every year, as I have told my students in the past…

24. Shoemaker-Levy Eugene and Carolyn Shoemaker, David Levy discovered a comet orbiting Jupiter that would hit it in 1994

25. String of Pearls…

26. Impact!

27. Deep Impact! – NASA probe Launched an impact probe Launched an impact probe Impacted Comet Temple 1 on July 2005 Impacted Comet Temple 1 on July 2005 800 lbs of mostly copper, hit at 10 km/s (Cu to control emission spectrum) 800 lbs of mostly copper, hit at 10 km/s (Cu to control emission spectrum)

28. Asteroids Also called “minor planets” Discovered accidentally or with surveys Few thousand have determined orbits

29. Gaspra a representative Asteroids a few to a few hundred kilometers in size (Gaspra 12x20x11) Note the craters

30. Ida and its ‘moon’ Dactyl Dactyl 1.5 km diameter; image by Galileo probe Dactyl

31. Asteroid belt Belt asteroids Trojans-a solution to the “restricted 3- body problem” in celestial mechanics….

32. … Bode’s Law Take the series 0.0, 0.3, 0.6, 1.2 … and add 0.4 to each to reproduce the spacing of the planets Missing: 2.8 Just a fluke?

33. NEO’s Earth-crossing “Near Earth Objects” are of great concern Their sizes…

34. NEO Sizes

35. Near Earth Asteroid Rendezvous (NEAR-Shoemaker)

36. Mathilde

37. Eros in stereo…

38. Actual dates later…

39. Eros Orbit and Final Descent Final DescentFinal Descent Descent/impact was on February 12, 2001