1. Comets Comets Hale-Bopp and the Andromeda galaxy

2. Comet miscellaneous Discovered as faint, fuzzy patches Discovered as faint, fuzzy patches Travel in highly elliptical orbits Travel in highly elliptical orbits Brightens and develops a tail as it nears the Sun Brightens and develops a tail as it nears the Sun Emit no visible light of their own Emit no visible light of their own A few dozen are discovered each year, more go by unnoticed A few dozen are discovered each year, more go by unnoticed

3. Comet miscellaneous They are essentially ice left over from the formation of the gas giants They are essentially ice left over from the formation of the gas giants Are named after their discoverers – the first 2 people to report it Are named after their discoverers – the first 2 people to report it About half of all discoveries are by amateur astronomers About half of all discoveries are by amateur astronomers The name comet comes from the Greek word kometes, meaning “wearing long hair” The name comet comes from the Greek word kometes, meaning “wearing long hair”

4. This comet appeared to pass in front of the Ring Nebula

5. An earlier view

6. Parts of a Comet Nucleus: main solid body Nucleus: main solid body –Typically only a few kilometers wide –The only part that remains when the comet is far away from the sun Coma: the halo around the nucleus Coma: the halo around the nucleus –Forms and becomes larger as the comet comes toward the sun –Can become almost as large as Jupiter or Saturn

7. Parts of a Comet Hydrogen envelope: Surrounds the comet Hydrogen envelope: Surrounds the comet –Invisible, stretches millions of km in space Ion tail: straight, makes glowing linear streamers Ion tail: straight, makes glowing linear streamers Dust tail: broad and gently curved Dust tail: broad and gently curved –Rich in dust that reflects sunlight

8. Comets tails, cont. Always point away from the sun, due to solar wind Always point away from the sun, due to solar wind Lies outside the comets orbit Lies outside the comets orbit Is in front of the comet, once it goes around the sun Is in front of the comet, once it goes around the sun

9. This picture of Hale-Bopp shows the white dust tail and the blue ion tail

10. Dust and ion tails of Comet NEAT

11. Physical Properties Its mass decreases with time (it loses material as it goes around the sun) Its mass decreases with time (it loses material as it goes around the sun) Made of dust particles, methane, ammonia, carbon monoxide, and water ice Made of dust particles, methane, ammonia, carbon monoxide, and water ice

12. Long Period Comets Comets may go as far as 50,000 A.U. from the sun Comets may go as far as 50,000 A.U. from the sun With very elliptical orbits, they slow down, according to Kepler’s 2 nd law With very elliptical orbits, they slow down, according to Kepler’s 2 nd law Most take hundreds of thousands (maybe millions) of years to complete an orbit. Most take hundreds of thousands (maybe millions) of years to complete an orbit. May be part of a huge cloud (Oort Cloud) orbiting far from the sun May be part of a huge cloud (Oort Cloud) orbiting far from the sun

13. How many comets do you see? Comet Bradford is to the left and Comet LINEAR to the right.

14. Short Period Comets Fewer in number Fewer in number Have orbital periods of less than 200 years Have orbital periods of less than 200 years Do not venture far beyond the orbit of Pluto Do not venture far beyond the orbit of Pluto Originate in the Kuiper belt Originate in the Kuiper belt

15. Short v’s Long Period Comets Short period comets orbit in roughly the same plane as the planets (ecliptic) Short period comets orbit in roughly the same plane as the planets (ecliptic) Long period comets orbit in all directions Long period comets orbit in all directions

17. Comet Encke returns every 3.3 years. Its orbit goes just outside of Mars and back!

18. Halley’s Comet British astronomer Edmund Halley realized that a comet seen in 1682 was not a one time event. British astronomer Edmund Halley realized that a comet seen in 1682 was not a one time event. He calculated its path and found that it had an orbital period of 76 years He calculated its path and found that it had an orbital period of 76 years He predicted that it would reappear in 1758 He predicted that it would reappear in 1758 He did not live to see it, but he was correct He did not live to see it, but he was correct

19. Halley’s Comet Ancient cultures record Halley’s comet in every pass since 240 B.C. Ancient cultures record Halley’s comet in every pass since 240 B.C. Was photographed in 1910 and again in 1986 Was photographed in 1910 and again in 1986 In 1986, a Soviet craft, Vega 2, traveled through its coma and was only 8000 km away from its nucleus In 1986, a Soviet craft, Vega 2, traveled through its coma and was only 8000 km away from its nucleus

20. Halley’s Comet A European spacecraft, Giotto, came within 600 km of its nucleus to take pictures A European spacecraft, Giotto, came within 600 km of its nucleus to take pictures Found that the nucleus was potato shaped (15 km x 10 km) and is almost black Found that the nucleus was potato shaped (15 km x 10 km) and is almost black Will next be near us in 2061 Will next be near us in 2061 When it passed in 837 AD, it was as bright as the crescent moon When it passed in 837 AD, it was as bright as the crescent moon

21. Comet Halley’s nucleus

22. Hale-Bopp Biggest comet to visit the solar system in more than 200 years Biggest comet to visit the solar system in more than 200 years Was visible to the naked eye for more than a year Was visible to the naked eye for more than a year Will return…in 2,380 years…sorry! Will return…in 2,380 years…sorry!

23. Comet Hale-Bopp early after its discovery

24. Comet Shoemaker-Levy 9 Discovered in 1993 orbiting Jupiter Discovered in 1993 orbiting Jupiter Jupiter’s gravity ripped it into 21 pieces Jupiter’s gravity ripped it into 21 pieces These pieces crashed into Jupiter a year later These pieces crashed into Jupiter a year later

25. Shoemaker-Levy 9 impacting Jupiter

26. Comet Ikeya- Shang and the Northern Lights!

27. Comet Hyakutake and a cactus

28. Comet McNaught – 2007

29. Comet Holmes – Fall 2008

30. 2010 – Comet Hartley2 and the Perseus Double Star Cluster