Intro to Comets

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)

A picture of a comet during most of its life:

What happens when a comet nears the Sun? It melts -- or more correctly, sublimates.

Comet structure:

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

Halley's Comet Nucleus

Coma: the heated cloud of sublimating gas and dust (10,000 km in size -- bigger than Earth!)
Tail(s): streamers of dust and ionized gas, millions of km long (as big as an AU!)

Ion Tail (ionized gases)
Dust Tail (trailing dust particles)

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

What causes a tail to stream? Do the two tails point in the same direction?