1. Comets Astronomy 311 Professor Lee Carkner Lecture 22

2. Upcoming  No class Wednesday, Oct 19  Quiz #3 on Monday, Oct 24  Covers Gas Giants through The Sun  Observing project due next Friday, Oct 28  Final exam Monday, Oct 31, 3pm  Covers entire course

3. Ancient Comets  People throughout history have observed the passing of comets  “When beggars die, there are no comets seen; The heavens themselves blaze forth the death of princes.” --Julius Caesar, Act II, Scene II

4. Comet Halley  Around 1700 Edmund Halley was studying the records of a comet that seemed to reappear at regular intervals   In 1758 the comet returned just as Halley predicted   Comet Halley will return again in 2061

5. Finding Comets   Large observatories do not have time to spend looking for them   If you see a faint fuzzy patch in the sky with your telescope, that is a good candidate for a new comet  Comets are generally named after their discoverers, e. g. Comet Hale-Bopp  More and more comets are being found by automated observatories

6. Observing a Comet  When we look at a comet with our eyes (or a small telescope) we see:  Coma:  Tail: Long streamer of gas and particles that can be more than 100 million km long

7. Structure of a Comet

8. Comet Tails  The tail is the most visible and most dramatic part of a comet   Ion Tail (blue)   Always points away from the Sun  Dust Tail (yellow)   Points roughly away from the Sun, but is curved back towards the Sun by gravity

9. The Two Tails of a Comet

10. Angular Size  plate scale = arcminutes/cm  Note: 60 arcminutes = 1 degree  The object has three sizes   An angular size, q  A real size, s

11. Angular Size and Real Size  plate scale =  /l  If the distance to the object is d, and the real size (diameter) is s tan (½  ) = (½s)/d   Note that  must be in degrees to take the tan

12. The Heart of the Comet   This is what the comet looks like far from the Sun and is the source of the tail and the coma   Composed of rock and ice

13. Comet Jets   The heat from the Sun boils off material  The material of the comet is well mixed   These jets can change a comet’s orbit  Comet orbits cannot be strictly predicted by Newton’s laws

14. Comet Composition  A comet is a mixture of ice and rock   Comets are composed of:   Water (ice)   Ammonia   Comets contain many carbon compounds including C 2, CH, CN (cyanogen)

15. Comet Orbits  Comets have highly elliptical orbits that bring them close to the Sun and then back to the Kuiper Belt or Oort Cloud   Short period comet orbits are constantly being altered by Jupiter and Saturn

16. Meteor Showers  As the comet circles the Sun its orbit fills up with lost material   When the Earth passes through this material we get a meteor shower   Meteors are small dust particles and thus burn up before they reach the ground

17. Anatomy of a Meteor Shower

18. The Perseid Meteor Shower  Occur every year around August 12   Get about 50 meteors per hour  One of the best meteor showers   The Perseids are debris from Comet Swift- Tuttle

19. Comet Deliveries   Comets could be a source of volatiles, including water   Could the Earth’s water have come from comets?

20. Death of a Comet  At each passage, the comet loses material   Eventually all the volatiles will boil off   Comets can also hit a planet or be ejected from the solar system in a close encounter

21. Spacecraft Studying Comets  Imaging   Gathering  Stardust (1999) -- gathered (Jan 2004) and returned (2006) a sample of the coma of Comet Wild 2  Impacting   Landing  Rosetta (2004) -- will land a probe on the nucleus of Comet 67P/Churyumov-Gerasimenko (2014) 

22. Next Time  Read Chapter 16 for Friday

23. Summary  Comets are small (10 km) bodies that have highly elliptical orbits that originate in the Kuiper belt or Oort cloud  The Sun boils off material making the comet visible  Comets can produce meteor showers and large impacts

24. Summary: Comet Structure  Nucleus: small (10km) core that is the source of the comet material  Coma: large (~1 million km) cloud of gas around the nucleus  Tail: comets have two tails, both pointing away from the Sun:  Ion -- pushed by solar wind  Dust -- pushed by solar light pressure  Jets: gas expelled from the nucleus under pressure