Comets and Kuiper Belt Objects

1. Comets and Kuiper Belt Objects Meteorites/Comets 4/27/06

2. Announcements • In-class activity today, course evaluations today • Movie night – Thursday, 4/27, 6:45PM, Room 312 • “Deep Impact” • Activity (hand this in to the preceptors after the movie) • Last lecture – Tuesday, May 2 • Astrobiology (given by Prof. D. Lauretta) • Brief exam review after the lecture • Final Exam – Tuesday May 9, 2PM-4PM • Preceptor-led study group (Thursday, May 4, 2-4PM) Meteorites/Comets 4/27/06

3. Semantics • Asteroids • Small rocky bodies in orbit about the Sun • Comets • Small bodies that orbit the Sun and (at least occasionally) exhibits a coma (or atmosphere) and/or a tail • Meteoroids • Small asteroids • Meteorites • the debris collected on Earth • Meteors • A brief flash of light (i.e. a shooting star) Meteorites/Comets 4/27/06

4. Asteroids and Comets • Asteroids • Mostly nearly-circular orbits • Mostly confined to the asteroid belt • Close to the ecliptic plane • Relatively short orbital periods • Comets • Highly elliptical orbits • Random inclinations • i.e. not-confined to the ecliptic • Very long orbital periods Meteorites/Comets 4/27/06

5. Why did we originally study comets? • To study the formation of the solar system • To predict when a comet will appear again • To try to explain meteor showers • To know when God (or gods) was displeased with us Meteorites/Comets 4/27/06

6. Why did we originally study comets? • To study the formation of the solar system • To predict when a comet will appear again • To try to explain meteor showers • To know when God (or gods) was displeased with us Meteorites/Comets 4/27/06

7. Harbinger’s of Doom • Rome: marked the assassination of Julius Ceaser • England: blamed for bringing the Black Death • Incan Empire: foreshadowed the brutal conquering by Francisco Pizarro Meteorites/Comets 4/27/06

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9. Comet Types (based on orbital characteristics) • Short-Period Comets • Comet with an orbital period of less than 200 years • Shortest lived • Breakup due to gravitational forces • Life expectancy of about 12,000 years • Long-Period Comets • Comets with orbital periods greater than 200 years (though typically around millions of years) • Other Definitions • Jupiter Family Comets (JFC) • Orbital period less than 20 years • Intermediate-Period Comets • Orbital period between 20 and 200 years Meteorites/Comets 4/27/06

10. Comet Origin • Kuiper Belt • Jupiter family and intermediate period comets • Formed outside Neptune’s orbit • These comets have low inclinations compared to long-period comets • Oort Cloud • Spherical distribution • Frequent observations of long- and intermediate-period comets indicate that the reservoir must be huge • Cannot be seen directly because it is so far away • Perturbed by passing stars or galactic tides • Formed in the region between Jupiter and Saturn Meteorites/Comets 4/27/06

11. Comet orbits are often highly inclined • Long-period comets, originating from the roughly spherically distributed Oort cloud have orbits with random inclinations • Intermediate and Jupiter-family comets tend to have orbits more confined to the ecliptic • Asteroids seldom have highly inclined orbits Meteorites/Comets 4/27/06

12. Appearance of Comets • When comets are far from the Sun, they are dark and hard to see • Low albedos, far away • Because of their large eccentricities, they occasionally come very close to the Sun • The body outgases as it heats and releases dust and cometary atoms • This produces a long visible tail • Directed away from the Sun Meteorites/Comets 4/27/06

13. Comets: Basic Structure • Dust tail • Away from the Sun, but curved slightly • Usually white • Scattered light • Ion tail • Directed away from the Sun • Blueish color • Charged water and carbon monoxide molecules “picked up” by the solar wind Meteorites/Comets 4/27/06

14. Comet Hale-Bopp Meteorites/Comets 4/27/06

15. Comets: Basic Structure • Dust tail • Away from the Sun, but curved slightly • Ion tail • Directed away from the Sun • Coma • The inner “fuzzy” region • Million km across • Nucleus • The actual surface (possibly a “dirty snowball”) • A few km across Meteorites/Comets 4/27/06

16. Orbit/Tail Configuration Meteorites/Comets 4/27/06

17. Nucleus of Comet Halley as seen by the Giotto spacecraft Meteorites/Comets 4/27/06

18. Structure and Composition • Solid object • Not a sandbank • Originally thought to be a dirty ice ball • Not much evidence for water ice (blue indicates water ice) • Icy dirt ball? Meteorites/Comets 4/27/06

19. Coma density and Jets • Coma is actually very thin, can only be seen because it’s very deep • Coma particles about the size of smoke particles • Jets have higher density, but would still appear transparent without overexposure • Still don’t fully understand what causes jets • Increased sublimation? • Rocket effect? Meteorites/Comets 4/27/06

20. Deep Impact • On July 4, 2005, a space probe was intentionally crashed into comet Tempel 1. • Impactor was about the size of a normal coffee table • The impact was monitored from a “mother” ship nearby • Flyby craft is about the size of a Volkswagon • The resulting impact could be seen by Hubble Space Telescope • Gave us exceptional closeup views of a comet • Fine powdery dust, like talcum powder • This was a big surprise Meteorites/Comets 4/27/06

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23. Approach Meteorites/Comets 4/27/06

24. Comets eventually break apart, and their fragments give rise to meteor showers Meteorites/Comets 4/27/06

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26. Meteor Showers • Due to Earth’s passage through the remains of an extinct comet • Named after the constellation they appear to come from (which depends on the time of year) • For example, the Leonids and Perseids • Sand-grain sized objects Meteorites/Comets 4/27/06

27. Sun Grazers • Not all comets are icy objects • Comets have a coma and a tail • Can be produced by vaporizing silicates • 70 times closer to the Sun than Mercury Meteorites/Comets 4/27/06

28. Pluto • Most distant “planet” • Most of the time, but not all of the time! Its orbit can bring it inside of Neptune’s (as it did from 1979-1999) • Only “planet” not visited by a spacecraft • New Horizons, launched in Jan. 2006, will reach Pluto in 2015. • Pluto can be seen with an amateur telescope, but it is not easy! • About 14th magnitude • 6th magnitude is the limit for the naked eye • Pluto is about 1600 times dimmer than this Meteorites/Comets 4/27/06

29. Pluto: Basic Facts • Avg. distance to the Sun • 39.5 AU • Orbital Period • About 250 years • Retrograde rotation • Like Venus and Uranus • Eccentricity • 0.25 • larger than all of the planets Meteorites/Comets 4/27/06

30. Pluto: Basic Facts • Avg. density • About 1900 kg/m3 • Rock and ice • Diameter • 0.18 Earth Diameters • Not that well known (+/- 1%) • Mass • 0.0021 Earth Masses • Also not well known, although the combined mass of Pluto and Charon is well known • Tenuous atmosphere of N2 that is probably not in equilibrium Meteorites/Comets 4/27/06

31. Neptune and Pluto are in a 3:2 orbital resonance • Pluto’s orbital period is 1.5 times Neptune’s • Neptune is about 164 years • Pluto is about 249 years • This is a stable configuration and they will never crash into each other • Pluto is like other objects in this sense – comprising a class of objects called “Plutino’s” Meteorites/Comets 4/27/06

32. Charon: Pluto’s Moon • Discovered in 1978 by Jim Christy. • Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together. • Charon is the largest moon with respect to its primary planet in the Solar System (a distinction once held by Earth's Moon). • Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon. Meteorites/Comets 4/27/06

33. Pluto and Charon are in an unusual resonance. • Charon has an orbital period equal to both its rotational period AND Pluto’s orbital period • From one side of Pluto, Charon just sits in the same place in the sky (never sets, never rises)! Meteorites/Comets 4/27/06

34. Is Pluto a planet ? • Since the discovery of thousands of objects orbiting the Sun, called Edgeworth-Kuiper objects (or Kuiper-belt objects, KBOs), it has been suggested that Pluto is simply one of these and is not a planet • Many KBO’s are in orbital resonances with Neptune (3:2 resonance  “Plutino”) • This is further complicated by the fact that the recently discovered Eris is larger than Pluto. • In 2006, Pluto was downgraded to a new class of objects called “dwarf planets” • But the debate rages on Meteorites/Comets 4/27/06

35. Kuiper Belt Meteorites/Comets 4/27/06

36. The IAU’s decision in August 2006 • The IAU...resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way: (1) A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. • (2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite. • (3) All other objects [3], except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies". Meteorites/Comets 4/27/06

37. Eris Meteorites/Comets 4/27/06

38. History of planets • Ancients recognized 7 planets • Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn • Earth was not a planet! • 1543, changed from 7 to 6 planets • Definition changed to an object orbiting the Sun • Sun and Moon removed, added Earth • 1852, changed from 23 to 8 planets • Asteroids were demoted since they did not have a resolvable disc • Uranus and Neptune had been discovered by now • 2006, changed from 9 to 8 planets • Pluto demoted Meteorites/Comets 4/27/06