Comets……

What are they???

Comets are small bodies made out of dust and ices ("dirty snowballs").

The term "comet" derives from the Greek aster kometes, which means "long-haired star.

What do meteors and comets have in common???

Comet Hyakutake (1996)

Comet West (1976)

Comet Halley (1984)

The Nature of Comets Since the observations of Tycho Brahe, we have know that

they are parts of the Solar System well beyond Earth's atmosphere.

Most are on long elliptical orbits that take them from the outer reaches of the Solar System to the vicinity of the Sun.

When they are near the Sun and active, comets have several distinct parts:

Nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solids;

The nucleus of Comet Halley is approximately 16x8x8 kilometers.

The coma is formed from gas and dust ejected from the nucleus as it is heated by the Sun. The coma is bright both because it reflects sunlight and because its gases are excited by sunlight and emit electromagnetic radiation.

The Tail… The tails of bright comets can be 150 million

kilometers in length. Many comets have two tails, a gas tail (also called

the ion tail) composed of and a dust tail composed of dust particles that come from the nucleus as the ices are vaporized. The dust particles are left behind in the comet's orbit.

Comet Halley (1984)

THE TAIL ALWAYS POINTS AWAY FROM THE SUN

Hale-Bopp

A long-period comet that was discovered in 1995 and that reached perihelion in Spring, 1997.

Halley…

The best known comet of all time! Has a 74-79 year period since 240 B.C.

OUCH!!!!! When comets attack!!! In July of 1994, fragments of Comet

Shoemaker-Levy 9 impacted the planet Jupiter.

HALLEY’S COMETIn 1705 Edmond Halley predicted, using Newton's newly formulated laws of motion, that the comet seen in 1531, 1607, and 1682 would return in 1758 (which was, alas, after his death). The comet did indeed return as predicted and was later named in his honor.

Discovery: Edmond Halley was the first person to recognize that this comet was periodic. He computed parabolic orbits for 24 comets observed from 1337 to 1698. His analysis of the list revealed the comets of 1531, 1607, and 1682 moved in almost identical orbits and were separated by intervals of roughly 75 years. From this information, he predicted the comet that now bears his name. It was rediscovered on December 25, 1758 by Johann Georg Palitzsch), a German farmer and amateur astronomer

The density of Halley's nucleus is very low: about 0.1 gm/cm3 indicating that it isprobably porous,perhaps because it is largely dust remaining afterthe ices have sublimed away.

The average period of Halley's orbit is 76 years but you cannot calculate the dates of its reappearances by simply subtracting multiples of 76 years from 1986. The gravitational pull of the major planets alters the orbital period from revolution to revolution. Nongravitational effects (such as the reaction from gasses boiled off during its passage near the Sun) also play an important, but smaller, role in altering the orbit.

Comet Halley was visible in 1910 and again in 1986. It's next perihelion passage will be about 2061.

PATH OF HALLEY’S COMET

HALLEY’S COMET 1910

HALLEY’S COMET 1910

IMPACT OF SHOEMAKER-LEVY

Between 16 July 1994 and 22 July 1994 the fragments impacted the upper atmosphere of Jupiter. This was the first time that scientists had an opportunity to witness the collision of two extraterrestrial bodies.

Comet Shoemaker-Levy 9 was discovered by Eugene and Carolyn Shoemaker and David Levy in 1993. Shortly after its discovery it was determined to be in a highly elliptical path near Jupiter and on a collision course. It was difficult to calculate its orbit prior to its 1992 pass near the giant planet.

COMET SHOEMAKER-LEVY

HALE-BOPPEarth Closest Approach: March 22, 1997 (1.315 AU)

Sun Closest Approach: April 1, 1997 03:14 UT (0.914 AU)

1 AU = 93 Million Miles = 150 Million

Kilometers

HALE-BOPP FROM YELLOWSTONE

COMET WEST

HYAKUTAKE1996

Experiments on-board the interplanetary Ulysses probe, which lasted for several hours on May 1, 1996,indicate the probepassed throughcomet Hyakutake'stail.