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Transcript
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
• 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)
PTYS/ASTR 206
Meteorites/Comets
4/27/06
Semantics
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?
A. To study the formation of the solar system
B. To predict when a comet will appear again
C. To try to explain meteor showers
D. To know when God (or gods) was displeased with us
PTYS/ASTR 206
Meteorites/Comets
4/27/06
Why did we originally study comets?
A. To study the formation of the solar system
B. To predict when a comet will appear again
C. To try to explain meteor showers
D. 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 Meteorites/Comets
Comets
PTYS/ASTR 206
• Orbital period between 204/27/06
and 200 years
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
PTYS/ASTR 206
Meteorites/Comets
4/27/06
Deep Impact
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
• Due to Earth’s passage
through the remains of an
extinct comet
Meteor Showers
• 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
• 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
PTYS/ASTR 206
Meteorites/Comets
4/27/06
Pluto
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
• 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
PTYS/ASTR 206
Pluto: Basic Facts
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
• 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: Pluto’s Moon
• 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
as "Small Solar System Bodies".
PTYS/ASTR
206 to collectivelyMeteorites/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