1. Comets and Kuiper Belt Objects
PTYS/ASTR 206
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)
PTYS/ASTR 206
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)
Semantics
PTYS/ASTR 206
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
PTYS/ASTR 206
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
PTYS/ASTR 206
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
PTYS/ASTR 206
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
PTYS/ASTR 206
Meteorites/Comets
4/27/06

8. PTYS/ASTR 206
Meteorites/Comets
4/27/06

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
PTYS/ASTR 206
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
PTYS/ASTR 206
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
PTYS/ASTR 206
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
PTYS/ASTR 206
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
PTYS/ASTR 206
Meteorites/Comets
4/27/06

14. Comet Hale-Bopp
PTYS/ASTR 206
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
PTYS/ASTR 206
Meteorites/Comets
4/27/06

16. Orbit/Tail Configuration
PTYS/ASTR 206
Meteorites/Comets
4/27/06

17. Nucleus of Comet Halley as seen by the Giotto spacecraft
PTYS/ASTR 206
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?
PTYS/ASTR 206
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?
PTYS/ASTR 206
Meteorites/Comets
4/27/06

20. 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
Deep Impact
PTYS/ASTR 206
Meteorites/Comets
4/27/06

21. PTYS/ASTR 206
Meteorites/Comets
4/27/06

22. PTYS/ASTR 206
Meteorites/Comets
4/27/06

23. Approach
PTYS/ASTR 206
Meteorites/Comets
4/27/06

24. Comets eventually break apart, and their fragments give rise to meteor showers
PTYS/ASTR 206
Meteorites/Comets
4/27/06

25. PTYS/ASTR 206
Meteorites/Comets
4/27/06

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
PTYS/ASTR 206
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
PTYS/ASTR 206
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 )
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
Pluto
PTYS/ASTR 206
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
PTYS/ASTR 206
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
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
Pluto: Basic Facts
PTYS/ASTR 206
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”
PTYS/ASTR 206
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.
PTYS/ASTR 206
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)!
PTYS/ASTR 206
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
PTYS/ASTR 206
Meteorites/Comets
4/27/06

35. Kuiper Belt
PTYS/ASTR 206
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".
PTYS/ASTR 206
Meteorites/Comets
4/27/06

37. Eris
PTYS/ASTR 206
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
PTYS/ASTR 206
Meteorites/Comets
4/27/06