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 Orbitwhich 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 metallicsolids (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 SunTYPES:

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.

COMETS, METEORS AND METEORITESCarolyn Grande: BEEd IIIA General

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 CometsComet Period a(years) Closest Approach to Sun (in AU)

2P/Encke 3.3 0.3421P/Giacobini-Zinner 6.6 1.03

14P/Wolf 8.2 2.4155P/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.