Comets and Kuiper Belt Objects PTYS/ASTR 206 Meteorites/Comets 4/27/06

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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 Pluto: Basic Facts PTYS/ASTR 206 Meteorites/Comets 4/27/06

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

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

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

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

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

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

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

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