1. Comets
25 October 2018

2. Introduction
The history of comet watching dates back to 1000 BC from the Chinese records and Chaldea, a place in present Iraq.
Comets have been regarded as omens, even as recently as 1986.
Battle of Hastings – 1066 – Bayeau Tapestry
Today Astronomers study Comets from scientific perspectives, and our understanding of these fascinating objects have grown tremendously.

3. Dirty Snowballs Comets are dusty chunk of ice
During each orbit around the sun they partially vaporize
Have elliptical orbits
Courtesy: Calvin J. Hamilton

6. Orbits of Comets Elliptical in Shape Randomly oriented
Aphelion distance
Comet
Sun
Earth
Perihelion distance

7. Comet Hunters
Comet are named by International Astronomical Union (IAU) after the person who first discovers them.
Many comets are discovered by amateur astronomers.
Charles Messier, E. E. Bernard, Shoemaker and Levy, Hale and Bopp, Ikeya, Seki and Hayakutake are popular comet hunters.

8. Origins of Comets
Comets are thought to be the left over debris from during the time of formation of the solar system.
The elliptical orbits of comets suggest that they underwent gravitational pull from the giant planets.
This all lead us to infer two possible locations where comets could start their journey towards the sun: Oort Cloud for long period comets; Kuiper Belt for short period.

9. Comets Tails
Ludwig Biermann propose the idea of solar wind to explain comet tails. Mariner 2 spacecraft captured the one such event in 1962.

11. Comet Collisions
Courtesy: NASA/JPL

12. Kohoutek

13. Comet West

14. Shoemaker-Levy 9

15. Shoemaker-Levy hits Jupiter

16. Hale-Bopp
Ion tail &
Dust tail

17. Halley from Giotto

18. Hyakutake

19. Deep Impact Tempel 1

20. Comet ISON
4 October 2013

21. Comet ISON
10 April 2013

22. Comet ISON
8 October 2013

23. ISON’s path through the sky

24. ISON approaches the Sun

25. ISON is only debris cloud after passing Sun

26. Comet
Hartley
jets

27. Comet McNaught 2007

28. Siding Spring at Mars October 2014

29. MER
PanCam

30. MAVEN
IUVS
Siding Spring

31. Latest Rosetta Images:
Landing, 12 Nov 2014
Perihelion: 13 August 2015
Orbiter crashed: 30 Sep 2016

32. Rosetta with Philae lander, before separation

33. Rosetta Target: 67P Churyumov-Gerasimenko

37. Boulder ‘Cheops’

44. Approaching Perihelion

45. Outburst in Action

46. View through Camera 4

47. P67
Rotation and
Regions

48. Active Pits

49. Comet 67P seen from 10 km

50. Reconstructed last image from Rosetta

51. Key Concepts
Kuiper Belt, exterior to Neptune is the primary source of short period (P<200yr) comets
Oort Cloud, 10,000 AU from Sun, reservoir of long period comets, stored there billions of years
Small objects are much more abundant
Cometary activity is triggered by sunlight
Comet tails: dust, shaped by solar radiation; ion or plasma tail shaped by solar wind
Comet grains: CHON + refractory matter
Comet nucleus: dirty snowball