Comets:  Structure and Tails

 
Early History: Comets - Bad Juju.
In early times, these "hairy stars" were thought to be mysterious omens of bad events.

Were they

  • clouds of fire in the upper air?

    • strange objects moving through the heavens?
Science gets involved
In 1577, Brahe and Mastlin used parallax to show that comets were very distant -- further than the Moon.

In 1704 Edmund Halley applied Newton's laws to the motion of comets to calculate orbits. He found that the comets which had appeared in 1456, 1531, 1607, and 1682 all had a similar orbit. Halley inferred that they were the same comet, reappearing every 76 years, and predicted that it would return in 1758. It did, and is now called Halley's comet.
 
 
But what are they?

Dirty Snowballs travelling on very elliptical orbits:

  • mostly water ice
  • ammonia
  • silicates (dust)
  • organic material (C, H, O, N)
What happens when a comet nears the Sun? 
It melts -- or more correctly, sublimates.
A picture of a comet during most of its life:

The structure of a comet

 
  • Nucleus: the dirty ice ball itself (a few km in size).



  • Coma: the heated cloud of sublimating gas and dust (10,000 km in size -- bigger than Earth!)

    •  
Halley's Comet Nucleus

  • Tail(s): streamers of dust and ionized gas, millions of km long (as big as an AU!)
    • Ion Tail: (ionized gases interacting with charged particles in the solar wind, along magnetic fields)
    • Dust Tail  (trailing dust particles pushed by radiation pressure and solar wind)

    What do you think would happen if the Earth went right through the tail of a comet?