Comets are icy members of the solar system that originate from either the Oort cloud or Kuiper belt. They travel in elliptical orbits around the sun and are composed of a nucleus of ice and dust surrounded by a coma of evaporated gases and ions. As comets near the sun, they develop two tails - an ion tail pushed away by solar wind and a dust tail curved by sunlight. Periodic comets like Halley's comet have orbits lasting less than 200 years, while long-period comets may take over 200 years or be non-repeating. Comets undergo changes in appearance as they travel from their icy state far from the sun to developing comas and tails when close to the sun.

1. COMETS, METEORS AND METEORITES
Carolyn Grande: BEEd IIIA General
Comet
 is an icy member of our solar system
 were named for their appearance. Both the Greek word kometes and
the Latin word cometa mean “long-haired.”.
 Comets travel in elliptical orbits around
the Sun and follow the basic laws of physics.
They are not supernatural signs at all.
SIGNIFICANCE OF COMET
 They are probably the only objects
left that are made out of the original material from Figure 1: Comet’s Orbit
which the whole solar system formed about 5 billion years ago.
 Earth, Moon, and other celestial bodies have all been changed by tectonicprocesses, erosion, or
numerous collisions. Only comets remain basically asthey were in the beginning.
1. Nucleus
 The nucleus contains most of the comet's mass but is
very small (about 1 to 10 km across - or more).
 made of mostly water ice and other frozen gases (the
“snow”) loosely mixed with stony or metallic
solids (the “dirt”).
 It has very low density and surface gravity.
2. Coma
 Cloud of evaporated ices and ions may be 100,000 km in
diameter
 the roughly spherical blob of gas that surrounds the
nucleus of a comet.
Figure 2: Structure of a Comet
3.Tail
 Always points away from Sun
TYPES:
a. 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 (it is also called the plasma
tail)
b. Dust tail: a long, wide tail composed of microscopic dust particles that are buffeted by photons
emitted from the Sun; this tail curves slightly due to the comet's motion. The tail fades as
the comet moves far from the Sun.
ORIGIN OF COMETS
 Comets are usually have 2 PERIODS:
1. LONG-PERIOD COMET
 are those that take more than 200 years to revolve once around the sun or may be a tangent and
never come back again. Usually, these comets come from the Oort cloud.
 This expansive cloud of maybe over 3 trillion comets encompasses our solar system 18 trillions
miles away from the sun and is believed to be the remains during the creation of the solar
system.

2. 2. SHORT-PERIOD COMET:
 comets that have a revolution around the sun in less than 200 years – are believed to have come
from the Kuiper Belt.
 This belt, located beyond the orbit of Neptune, is also believed to be remnants during the
creation of the universe and acts as a reservoir for short term comets. One such comet is the
Halley's Comet. After numerous perihelion passages, periodic comets finally lose all their
volatile material. only fragments of nonvolatile solids may survive.
PERIODIC COMETS
 Astronomers have catalogued about 150 short-period, or periodic comets, that have periods of
revolution around the Sun of a few years or decades up to 200 years.
 They shine periodically in the sky every time they come close to the Sun.
 The most consistently bright and most famous is Comet Halley, with 30 consecutive perihelion
passages recorded since 240 B.C.
 Sighted telescopically for over three years before and after its February 9, 1986, perihelion
passage, Comet Halley is also the best-analyzed comet so far.
TABLE 1: Some Periodic Comets
Comet Period a(years) Closest Approach to Sun (in AU)
2P/Encke 3.3 0.34
21P/Giacobini-Zinner 6.6 1.03
14P/Wolf 8.2 2.41
55P/Tempel-Tuttle 33.2 0.98
1P/Halley 76.0 0.59
FIVE CHANGES OF APPEARANCE THAT A COMET UNDERGOES AS IT TRAVELS IN ITS ORBIT
AROUND THE SUN
(1) (2) (3) (4) (5)
Figure 3: Comet Halley on seven different days as it receded from the Sun after its apparition in 1910.
(1) Far from the Sun, a comet consists of a nucleus of frozen gases and dust.
(2) Coma forms as a comet approaches the Sun.
(3) Close to the Sun, tails form.
(4) After going around the Sun, much cometary material refreezes.
(5) Far from the Sun again, coma and tails are gone.