- Comets may provide us with a close-up look at Kuiper Belt objects and similar leftovers from the formation of the Solar System.
- Comets are "dirty-snowballs" in orbit about the Sun.
- Most are no larger than 10 km across.
- Unlike the planets, they tend to have very elliptical
- Halley's comet orbits the Sun once every 76 years.
- According to Kepler's 3rd
law, a=18 AU.
It's orbit carries it beyond Uranus.
- Most comets spend most of their time at vast distances
from the Sun.
- During their brief visits close to the Sun,
ices sublimate from their surface making them a spectacular sight.
- Because comets are most active when they are
close to the Sun they are usually best viewed in the morning and
- Anatomy of a Comet
- Nucleus - a less than 10 kilometer diameter ``dirty
snowball'' consisting of water, carbon dioxide, methane, and ammonia ices and dust grains.
- The nuclei are thought to be icy planetary "building blocks" preserved from the time of the origin of the Solar System.
- Encounters with the forming giant planets flung some of them halfway out to the nearest star.
- Now a small fraction of them are on orbits which carry them close to the Sun, making them "comets".
- Coma -- cloud of gas and dust grains produced by
the evaporation of the nucleus when warmed by the Sun. Diameter = 100,000 km.
- Tail -- coma material pushed back by the ``solar wind" and light pressure from sunlight.
- Comet tails point away from the Sun and can be up to 1AU long.
- Since gas and dust behave differently under the
influence of sunlight pressure and the solar
wind comets usually have separate gas and dust tails.
- The gas tail, often called an ion tail, points
directly away from the Sun.
- Dust particles are less strongly influenced
by the Sun and tend to produce a curved tail.
- Comets are icy planetesimals preserved from the
beginnings of the Solar System.
- Interaction with the forming giant planets ejected them into
the ``Oort cloud" -- a vast reservoir of comet nuclei surrounding
the solar system which extends nearly 1/2 way to the nearest
star (50,000 AU).
- The Kuiper Belt is also a source of comets and represents a region of the protoplanetary disk where accretion happened too slowly. Just planetesimals and partially built planets (e.g. Pluto) remain.
- The Oort Cloud may contain 1,000,000,000,000 (a trillion) cometary
nuclei, although the combined mass of these comets is less than
that of the Earth.
- Only a tiny fraction of these icy bodies are on highly elongated
orbits which will carry them close to the Sun.
- Long vs. Short period comets.
- Long period comets are comets falling in from
the distant reaches of the Oort Cloud. They may be passing near
the Sun for the first time or are on such elliptical orbits that
the visit the neighborhood of the Sun only every few million years.
- Short period comets are former long period comets trapped
in the Solar System when their orbital path was modified by
gravitational attraction when passing close to a large planet.
- Halley's Comet orbits the Sun once every 76 years.
Comet Encke orbits the Sun every 3.3 years.
- A comet loses about 1% of its
mass with each passage near the Sun. Short period comets
eventually become inactive and look more like asteroids
after several hundred orbits.