Comets

• Observer: Dianne Marshall
• Location: Fairbanks, Alaska
• Date: March 28, 1996 08:15 UT
• Notes: One minute exposure, 50 mm lens, Kodak 1000 Gold film, F4.5, lots of sky glow but no direct aurora. Time about 11:15 AST

• It looks like a fuzzy ball.
• The ``best'' comets are fairly bright, so a comet is not difficult to see if you get away from lights.
• It has a tail, but the tail is difficult to see.
• Bad: moonlight; city lights.
• Good: your eyes.
  • They are very sensitive when adapted to the dark.
  • The most sensitive part of your retina is away from its center.
  • Thus to see the tail, you should look a little above it or below it or to the side.

Here is an image of Halley's comet.
Where does it come from? What makes it shine? What is it made of? How big is it? What makes the tail?

Some basic information about comets

• Where does it come from?
  • Like the planets, comets are in orbits about the Sun.
  • Most of them come from very far away; their orbits are very eccentric.
  • A period of millions of years and a maximum distance from the sun of tens of thousands fo AU is typical, although there are many comets also with shorter periods.
  How could this be? If the comet were at one time in a more or less circular orbit around the sun, it would be moving very slowly. Suppose that a small perturbation (say from a passing star) almost stopped its motion. What would happen then?

  This kind of reasoning suggests that comets come from two sources:

  1. Oort cloud. A swarm of comets extending out in every direction from the sun, out to perhaps 100,000 AU.
  2. Kuiper belt. A swarm of comets in a disk that lies in the plane of the ecliptic and extends out to perhaps 1000 AU.
  If an astronomer finds the orbit of a comet and assumes that its orbit has not been much perturbed by passing near a planet, how could she guess whether the comet came from the Oort cloud or the Kuiper belt?

• What makes a comet shine?
  • We see it because it reflects sunlight.
    • This is the same reason that we see the Moon and the planets.
  • Also, the gases in a comet can absorb sunlight and then glow from the absorbed energy.
• What is it made of?
  • Ice and dust.
  • We can tell by analysing the light that comes from the comet.
• How big is it?
  • The nucleus is usually several km in diameter.
  • The best measurement is of Comet Halley. A spcecraft came nearby and photographed its nucleus. It measured 8 km by 15 km.
  • The fuzzy ball that you see, called the coma, is typically 1 million km in diameter.
  • The tail can be over 100 million km in length.
• Why does it have a tail?
  • Comets have tails only when they are near the Sun.
  • The ice evaporates in the warmth of the Sun.
  • This spreads gas and dust around the region near the nucleus.
  • The gas gets pushed away from the Sun by the solar wind -- a stream of particles coming from the Sun. This makes the 'plasma tail', which is rather blue in the picture.
  • The dust gets pushed away from the Sun by the (tiny) pressure of sunlight. This makes the 'dust' tail, which is more yellow.