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Key Concepts: Lecture 17: Outer Solar System; Asteroids, Impacts
Pluto
Comets and the Kuiper Belt
Kuiper Belt Objects and Dwarf Planets
The Oort Cloud
Asteroids
Impacts
Feb 18, 1930
Pluto
Pluto
• Global Properties
• Discovered 1930 by
Tombaugh
• Orbit
– Diameter - 2300 km
– Low density - 2000 kg/m3
Clyde
– higher inclination to the ecliptic than any
planet
– most elliptical orbit
– orbit crosses with the orbit of Neptune
– Orbital period = 248.6 years
– very tilted axis of rotation like Uranus
• Has 1 major satellite - Charon
January 29, 1930
– Diameter - 1200 km
– Low density - 1200 kg/m3
• Several other smaller satellites
• Light and dark areas
– Not clear what causes them.
Pluto is very hard to study because of its great distance: like seeing a
soccer ball in Key West from Gainesville
Origin of Pluto
• Different from Jovian planets
– Not a gas giant (too dense). Probably icy
– Very elliptical and inclined orbit
• Origin
– A true planet? - No!
• Why no large amount of H & He?
• Why such an odd orbit?
– Escaped moon of Neptune? - No!
• Properties like a Jovian moon
• BUT could not escape with satellite
– Therefore, we think that Pluto is related to Kuiper Belt
Objects (see below), that formed independently in the
outer solar system.
Pluto
New Horizons
On July 14, 2015, the
New Horizons
spacecraft became
the first spacecraft to
fly by Pluto.
What is the Kuiper Belt? Inferred from properties of
some (short period) comets.
Comets
• Comets have been
observed and recorded in
the histories of all ancient
civilizations
• Both feared & revered
Comets
Comets are named after their discoverers
Composition
Comet Halley
Comet Hyakutake
Comet Hale-Bopp
Anatomy of a comet
• Head
–
–
–
–
Nucleus
1-10 km in diameter
Coma
~ 100,000 km in diameter
• Tail
– ~ 100 million km long
– always points away from
the Sun
Comet West
• Nucleus - dirty snowball
– solid
– ices of H2O, CO2, NH3, & CH4
– dust
• Coma
– cloud of gas
– mostly H2O, CO2, CO
• Tail
– Dust tail & gas (ion) tail
Evolution of Comets
• As comet approaches Sun:
– solar radiation vaporizes icy nucleus
• dust is released
• forms larger tenuous coma
– radiation pressure pushes on dust
• forms dust tail
– solar wind blows on gas
• forms gas (ion) tail
– forces from the solar wind &
radiation pressure are directed away
from Sun
• tails point away from Sun
Possible Fates of Comets
• Crash into Sun
• Come too close to Sun & be destroyed
• Survive initial passage near Sun &
orbit
– each approach to Sun, it loses material
• Be influenced by gravity of planets
• impact the planet: e.g. Comet Shoemaker-Levy
hit Jupiter in 1994; perhaps comets were
source of water on Earth.
• be speeded up & ejected to outer solar system
or even from solar system
• be perturbed into an orbit with a shorter period
– each time it approaches Sun it loses some
material
Halley’s Comet
History of discovery [not examined]
Halley's Comet was the first comet to be
recognized as periodic. Perceiving that the
observed characteristics of the comet of 1682
were nearly the same as those of two comets
which had appeared in 1531 and 1607 (the
latter observed by Johannes Kepler in
Prague), Halley concluded that all three
comets were in fact the same object returning
every 76 years (a period that has since been
amended to every 75–76 years). After a
rough estimate of the perturbations the comet
would sustain from the attraction of the
planets, he predicted its return for 1758.
Halley's prediction of the comet's return
proved to be correct, although it was not seen
until 25 December 1758 by Johann Georg
Palitzsch, a German farmer and amateur
astronomer. Halley did not live to see the
comet's return, having died in 1742.
Halley’s Comet
• Twenty nine recorded
passages
• Earliest recorded passage
239 B.C.
• Returns every 75-76
years
• Highly elliptical orbit
• P2=a3; can infer a= 18AU
from P. Comet spends
most of its time nearer 2a
= 36AU
2a
Last appeared in 1986. Will be back in 2061.
Rosetta and
Comet 67P
First landing of a probe (Philae) on the surface of a comet (11/12/2015)
Two kinds of Comets: short and long period orbits
Origin of Comets (1)
• Kuiper Belt
– source of short
period comets
• orbital periods
about 100 years or so
The Kuiper Belt Objects and the “Tenth Planet”…
The discovery of 2003UB313,
first known as the “10th
planet”, now Eris, and
classified as a dwarf planet.
– just beyond orbit of
Neptune
– 1st Kuiper Belt
object discovered in
1992
Kuiper Belt Objects
• Some of the largest objects discovered in
the Kuiper Belt: (about 1000 objects are
now known).
Size of Eris
• Similar to Pluto
• It has a moon! - discovered
Sept 10th 2005
• This allows more accurate
estimate of mass (Newton’s
version of Kepler’s Law) - it
has a mass similar to Pluto
Discovery of Eris and similar objects in the Kuiper belt led to a
reclassification of “Planets” and “Dwarf Planets”
Origin of Comets (2)
• Oort Cloud: Long period
comets
– sphere of trillions of
comets
– 50,000 A.U.
– Comets disturbed by
passing massive object
• e.g. another star
– some ejected from solar
system
– some fall towards the
Sun in highly elliptical
orbits
What are “Planets”?
• It is probably best to describe the Solar System as
having
- 4 Terrestrial Planets and 4 Gas Giant (Jovian)
Planets. [Sometimes Uranus and Neptune are
referred to as “Ice Giants”]. To be a “Planet” an
object needs to be massive enough so that gravity
makes it round AND so that it dominates the
region in which it orbits.
- Many Asteroids (rocky minor planets)
- Many Kuiper Belt Objects (KBOs): icy minor
planets, including Pluto. The large KBOs (and some
of the large asteroids) are “Dwarf Planets” if
gravity makes them round.
Asteroids
• Ceres - first discovered in 1801
by Giuseppe Piazzi.
• Irregular, rocky chunk of
material smaller in size &
mass compared to the planets
– Originally called “planets”, now
called “Minor planets” or
“Asteroids”
• Large number of asteroids in
solar system
– >100,000 have known orbits
Ida flyby
Orbits of Asteroids
Properties
• Most follow elliptical
orbits between the orbits of
Mars & Jupiter in asteroid
belt
• Trojan asteroids orbit Sun
at the distance of Jupiter
• Some asteroids have orbits
that cross the orbit of Earth
– Aten, Apollo & 10% of
Amor asteroids
– source of meteorites
– collision with an
asteroid could lead to
mass extinctions - e.g.
dinosaurs
• Sizes: 1 km - 300 km
– largest ~ 950 km across
• Ceres
– ~ 6 known asteroids with
sizes > 300 km
– ~ 200 known asteroids
larger than 100 km
• All asteroids combined
< 1/1000 of Earth’s
mass
Ceres and the Dawn Probe
Entered orbit on 3/6/2015
Ceres appears to be
differentiated.
Highly cratered.
Surface has two distinct
bright spots - highly
reflective ice or salts?
Major Impacts on Earth
• “Large” asteroid bodies
strike the Earth every
~100 million years
• Global catastrophe
– Dust fills atmosphere several months of
darkness
– Global fire storms
– Acid rain
• The majority of life on
the planet becomes
extinct
End of Cretaceous Age
Tunguska, Siberia, 1908
• 10-20 Megatons
• 65 million years ago
• Mass extinction
– >90% of life forms disappeared
– Dinosaurs - disappeared
– Mammals emerged
• Evidence for impact
– Iridium layer
• Global layer of dust & ash with
cosmic iridium abundance
Chicxulub Crater, Yucatan, Mexico
•
•
•
•
On the tip of Yucatan
200 km across
Same age as CT
Filled with sediment now
• 5 billion Atom-bombs
• 100 trillion tons of dust
• Made hole in Earth’s crust
See Links section of class website
for full story.
Meteoroids, Meteors & Meteorites
• Meteoroids - small chunks of rock
in space
– smaller than asteroids
• Meteors - track of light in the sky
from rocks or dust burning up as it
falls through the Earth’s
atmosphere
• Meteorites - chunk of
interplanetary rock after it impacts
on a planet, Moon or Earth
Meteorites
Chelyabinsk Video
• Most meteors are small and burn up
in our atmosphere
• Some are large enough that they land
on Earth - create impact crater
• Chance of hitting a house or car is
small
– every 3 years or so
• Chance of hitting a person is smaller
– every 180 years in North America
• Nov 30, 1954 Alabama - injured
Mrs. Hodges
Chelyabinsk,
Russia, Feb. 15 2013
Meteor Showers
• Many meteors appearing to
come from a common point
in the sky
• Reoccur every year
• Can have 100’s meteors per hour
Airburst at ~30-50 km
1500 people injured (broken glass)
Initial meteor diameter ~20m, mass of ~10,000 tons
Leonids 1966
Meteor Showers
• Caused by the passage of the
Earth’s atmosphere through a
swarm of solid particles, usually
from comet debris orbiting Sun
–
–
–
–
Perseid - August 11 - Comet Swift-Tutle
Draconid/Draco - Oct. 9 - Giacobini-Zimmer
Taurid - Oct 31 - Comet Enke
Leonids - November 16 - Comet Tempel-Tutle
1833 Wood
Cutting