1. Comets……

2. 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???

3. Comet West (1976)
Comet Hyakutake (1996)
Comet Halley (1984)

4. 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.

5. What are they made of and why are they important???
They are a mixture of ices (both water and frozen gases) and dust that for some reason didn't get incorporated into planets when the solar system was formed.
Comet Halley (1984)

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

7. 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.

8. The Head and the Coma
The center of a comet's head is called its nucleus. The nucleus is a few kilometers across and is surrounded by a bright region called the coma that may be a million kilometers in diameter

9. 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.

10. 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)

11. THE TAIL ALWAYS POINTS AWAY FROM THE SUN

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

13. Halley… The best known comet of all time!
Has a year period since 240 B.C.

14. OUCH!!!!! When comets attack!!!
In July of 1994, fragments of Comet Shoemaker-Levy 9 impacted the planet Jupiter.

16. Are we next????
On the morning of June 30, 1908, in a remote region of central Siberia, a great blue-white fireball brighter than the Sun streaked through the sky and exploded while still in the air with a blinding flash and intense pulse of heat. The explosion was heard 1000 km away, and it flattened trees radially 30 kilometers out from a central point in the Stony Tunguska River valley.
The resulting pulse of air pressure circled the Earth twice, and astronomers observed for several nights afterwards a glowing red haze in the upper atmosphere, though they were not aware at the time of the cause.

17. It is estimated that the explosion had the force of a megaton hydrogen bomb and detonated in the atmosphere about 6-8 km above the surface, which would explain why no crater has ever been found.
The region is so remote that there were few witnesses and presumably little loss of life. As a result, news of the event filtered only slowly to the outside world. Because of the remoteness and the political turmoil of the early part of this century, it was only 1927 when a scientific expedition finally went to investigate the event. Though various fantastic theories have been proposed (the crash of an alien spaceship), the simplest explanation that is consistent with all the data is that the Earth was struck by the head of a small comet or a small rocky asteroid maybe 100 meters in diameter that exploded before striking the ground.

18. HALLEY’S COMET
In 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.

19. 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 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

20. The density of Halley's nucleus is very low: about 0
The density of Halley's nucleus is very low: about 0.1 gm/cm3 indicating that it is probably porous, perhaps because it is largely dust remaining after the ices have sublimed away.

21. 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 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 It's next perihelion passage will be about

22. PATH OF HALLEY’S COMET

23. HALLEY’S COMET 1910

24. HALLEY’S COMET 1910

25. 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.

26. Comet Shoemaker-Levy 9 was discovered by Eugene and Carolyn Shoemaker and David Levy in 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.

27. COMET SHOEMAKER-LEVY

28. HALE-BOPP Earth Closest Approach: March 22, 1997 (1.315 AU)
Sun Closest Approach: April 1, :14 UT (0.914 AU)
1 AU = 93 Million Miles = 150 Million
Kilometers

29. HALE-BOPP FROM YELLOWSTONE

30. COMET WEST

31. HYAKUTAKE 1996

32. Experiments on-board the interplanetary Ulysses probe, which lasted for several hours on May 1, 1996, indicate the probe passed through comet Hyakutake's tail.