Mrs. Painter’s Introduction to Comets
A comet is a small, icy celestial body that orbits around the sun. It is made up
of a nucleus (solid, frozen ice, gas and dust), a gaseous coma (water vapor,
CO2, and other gases) and a long tail (made of dust and ionized gases). The tail
develops when the comet is near the Sun. The long ion tail always points away
from the sun, because of the force of the solar wind. The tail can be up to 250
million km long, and is most of what we see. Comets are only visible when
they're near the sun in their highly eccentric orbits.
PARTS OF A COMET
Nucleus: The nucleus is the frozen center of a comet’s
head. It is made of ice, gas, and dust. The nucleus
contains most of the comet's mass but is very small
(about 1 to 10 km across - or more).
Coma: The coma is a sphere of gas that surrounds the
nucleus of a comet; it is about a million km across. The coma is made of water
vapor, carbon dioxide gas, ammonia, dust, and neutral gases that have sublimed
from the solid nucleus. The coma and the nucleus form the head of a comet.
Ion Tail: A tail of charged gases (ions) always faces away from the sun
because the solar wind (ions streaming from the sun at high velocities) pushes
it away (called the plasma tail). When the comet is approaching the Sun, the ion
tail trails the comet: when the comet is leaving of the Sun, the ion tail leads.
The tail fades as the comet moves far from the Sun. The ion tail can be well
over 100 million km long.
Dust Tail: The dust tail is a long, wide tail made of microscopic dust particles
that are surrounded by photons emitted from the Sun; the tail curves slightly
due to the comet's motion. The tail fades as the comet moves far from the Sun.
Hydrogen Envelope: Hydrogen gas surrounds the coma of the comet and trails
along for millions of miles (it is usually between the ion tail and the dust tail).
The hydrogen envelope is about 10 million km across at the nucleus of the
comet and about 100 million km long. It is bigger when the comet is near the
Sun.
A COMET'S ORBIT
Comets orbit the Sun in highly elliptical orbits. Their velocity increases greatly
when they are near the Sun and slows down at the far reaches of the orbit.
Since the comet is light only when it is near the Sun (and is it vaporizing),
comets are dark (virtually invisible) throughout most of their orbit. The solar
wind pushes the tail away from the Sun.
Some comets crash into the Sun or get so close that they burn up; these comets
are called sungrazers.
COMET EXPLORATION
List of Comets Visited by Spacecraft
List of spacecraft visits:
Date
Comet
1985 Sep 11
1986 Mar 6
1986 Mar 8
1986 Mar 9
1986 Mar 11
1986 Mar 14
1992 Jul 10
2001 Jan
2001 Sep 22
2004 Jan 2
2005 Jul 4
2010 Nov 4
2011 Feb 5
2014 Aug
21P/Giacobini-Zinner
1P/Halley
1P/Halley
1P/Halley
1P/Halley
1P/Halley
26P/Grigg-Skjellerup
107P/Wilson-Harrington
19P/Borelly
81P/Wild 2
9P/Tempel 1
103P/Hartley 2
9P/Tempel 1
67P/Churyumov-Gerasimenko
Mission
ISEE 3/ICE
Vega 1
Suisei
Vega 2
Sakigake
Giotto
Giotto
Deep Space 1
Deep Space 1
Stardust
Deep Impact
EPOXI
Stardust-NExT
Rosetta
Type
fly-by
fly-by
fly-by
fly-by
fly-by
fly-by
fly-by
fly-by
fly-by
sample-return
impact/fly-by
fly-by
fly-by
rendezvous/landing
COSMIC SNOWBALLS
There is a new and very controversial theory that comets (composed of frozen
water) are constantly bombarding the Earth. These "cosmic snowballs" have
(perhaps) been seen by the visible imaging system of the Polar Satellite. In
theory, these frozen comets vaporize in the atmosphere, adding water vapor to
the environment.
Use the section on the comets to answer the following
questions.
1. What celestial body do comets orbit?_________________
2. What is the nucleus of a comet made of?_______________________
3. Is a comet's coma solid, liquid, or gaseous?_______________________
4. When is the tail of a comet visible?_______________________
5. Are comets in circular or elliptical orbits? _____________________
6. Name a well-known comet whose visit was recorded in ancient times
_______________________
7. What is the name for the process in which a solid matter (like a comet's
nucleus) goes directly to a gaseous state?_______________________
8. Does a comet's tail ever face the Sun? _______________________
9. What happens when the Earth crosses a comet's orbit and encounters comet
debris? ______________________
10. What is the name of a comet that crashes into the sun (or gets so close that
it burns up)? ______________________