Comets Astronomy 311 Professor Lee Carkner Lecture 22

Upcoming  Quiz #3 on Monday, Nov 6  Covers Gas Giants through The Sun  Final exam Monday Nov 13, 3pm  Covers entire course  Observing project due next Friday, Nov 10

Comets Throughout History  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

Edmund Halley

Comet Halley  Around 1700 Edmund Halley was studying the records of a comet that seemed to reappear at regular intervals  He used Newton’s new laws to determine its orbit (P=76 years so A=18 AU)    Comet Halley will return again in 2062

Comet Halley in 1986

What is a Comet?   Comets have highly elliptical orbits which bring them into the inner solar system 

Finding Comets  Comets are quite faint and hard to see when they are far from the Sun   Comets are generally found by amateur astronomers   Comets are generally named after their discoverers, e. g. Comet Hale-Bopp 

Observing a Comet  When we look at a comet with our eyes (or a small telescope) we see:  Coma:  Tail:

Structure of a Comet

Comet Tails  The tail is the most visible and most dramatic part of a comet  A comet generally has 2 tails:  Ion Tail (blue)    Dust Tail (yellow)  

The 2 Tails of Comet Halley

The Two Tails of a Comet

The Heart of the Comet  At the center of the comet is the nucleus   Size: 

Comet Jets  When the comet is far from the Sun it has no tail or coma   The material of the comet is well mixed  Sometimes volatiles boil inside the comet and are released as a jet  

Jets on Hale-Bopp

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

Comet Orbits  Comets have highly elliptical orbits that bring them close to the Sun and then back to the Kuiper Belt or Oort Cloud  

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  

Anatomy of a Meteor Shower

The Perseid Meteor Shower  Occur every year around August 12   Get about 50 meteors per hour   Meteors appear to come from the direction of the constellation Perseus 

Comet Impacts  Comet impacts were probably common in the early solar system and still happen today   Many of the craters that we see on the Moon and other bodies may have been made by comets 

Comet Impact on Ganymede

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

Death of a Comet  At each passage, the comet loses material   Eventually all the volatiles will boil off  

Fragmentation of a Comet

Break-up of Comet LINEAR S4

Spacecraft Studying Comets  Imaging  Giotto (1985) --  Gathering  Stardust (1999) --  Impacting  Deep Impact (2004) –  Landing  Rosetta (2004) -- will land a probe on the nucleus of Comet 67P/Churyumov-Gerasimenko (2014) 

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

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