Astronomy 161
Introduction to Solar System Astronomy
Prof. Paul Martini

Lecture 41: Comets

Key Ideas:

History: Sightings and Impact; Famous Comets
Orbital Information: Families of Comets; Origin; Connection to Meteor Showers
Physical Properties: Nucleus, Coma, and Tail

Halley's Comet

Earliest record dates from 260 BC (China)
Babylonian tables record the 164 BC visit
Bayeux Tapestry records the 1066 visit, the year of the Battle of Hastings (Norman conquest of England)
Often comets were considered good or bad omens
Halley returns every 75 to 76 years
Its orbital ellipticity is 0.967
Next visit will be in 2061

Orbital Families

Short period comets: Periods of 20-200 years (includes Halley's comet); Orbits near the ecliptic; Often influenced by the jovian planets
Long period comets: Periods of 200 to millions of years; Large range of orbital inclinations; Aphelia in the Kuiper Belt, Scattered Disk, or Oort Cloud

Meteor Showers

Comets frequently leave debris trails behind them
When the Earth passes through this debris, we see meteor showers
Examples are:: Perseids (August) are from Swift-Tuttle; Leonids (November) are from Tempel-Tuttle; Orionids (October) are from Halley

Comets

Comets are "dirty snowballs" of ices and dust that have fallen in from the outer solar system
As they approach the Sun:: The ices sublimate into gas; Gas and dust are swept by sunlight into a luminous tail
Many faint comets per year: Bright, naked-eye comets are visible every 10 years or so

Structure of Comets

Nucleus:: Dirty snowball of ices and dust a few kilometers across; Contains >99% the mass of the comet; Source of the gas and dust in the coma and tails; Very low density ~0.2 g/cc; Dark and heavily cratered
Coma:: Low-density cloud of gas and dust sublimed off the nucleus; >100,000 km in size

Comet Tails

Dust Tail:: Dust particles swept back by sunlight pressure; 1-10 million km long; White color is reflected sunlight
Ion Tail:: Ionized atoms and molecules swept back by the solar wind; 100 million km long; Blue color is from CO⁺ emission lines

Comet Sample Return

STARDUST - NASA Mission to return material from a comet: Launched February 7, 1999; Collected samples from the tail of "Wild 2"; Returned to Earth on January 15, 2006; Discovered many organic compounds and silicates
One big surprise: Many minerals that only form at high temperatures
Even bigger surprise: The amino acid glycine!

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.