15. Asteroids & Comets
• The discovery of the asteroid belt
• Jupiter’s gravity shapes the asteroid belt
• Asteroids occasionally hit one another
• Some asteroids orbit outside the asteroid belt
• Stony, stony iron & iron meteorites
• Some meteorites contain primordial materials
• The “dirty snowball” comet model
• Comets come from beyond Pluto
• Comet remnants produce meteor showers
Earth, the Moon & Ceres to Scale
The Discovery of the First Asteroid
• The Titius-Bode Law
– Not a “law;” just a mnemonic [memory] device
• Planetary distances rather accurately “predicted” but…
• Titius-Bode does not work for Neptune & Pluto and…
• There is a “missing planet” between Mars & Jupiter
• The “Celestial Police”
– Six German astronomers organized a search
– Sicilian astronomer Giuseppe Piazzi strikes first
•
•
•
•
•
1 January 1801: Sees an uncharted object moving nightly
Wrote to Bode, Director of the Berlin Observatory
Letter arrived in late March, at conjunction
Karl Friedrich Gauss calculates a future location
Ceres is re-discovered on 31 December 1801
An Enhanced HST View of Ceres
http://upload.wikimedia.org/wikipedia/commons/f/fc/Ceres_optimized.jpg
The Discovery of the Asteroid Belt
• Properties of Ceres
– Orbits the Sun at 2.77 AU once every 4.6 years
– Largest asteroid is 918 km (570 mi) in diameter
• Additional discoveries
– Heinrich Olbers discovers 2 Pallas 28 March 1802
• Orbits the Sun at 2.77 AU once every 4.6 years
• Only 522 km in diameter
– 3 Juno discovered in 1804
– 4 Vesta discovered in 1807
– Several hundred more in the mid-1800s
– Max Wolf used photography to discover asteroids
• Discovered 228 asteroids on long-exposure photos
• Requirements for official recognition
– Observed on 4 consecutive oppositions
Some Disappointing Facts
• Mass
– 1 Ceres contains ~ 30% the mass of all asteroids
• Diameter
– Only 1 Ceres, 2 Pallas & 4 Vesta are
• 1 Ceres
• 2 Pallas
• 4 Vesta
> 300 km
960 x 932
km
570 x 525 x 482 km
530
km
– 30 other asteroids
are > 200 km
– 200 other asteroids
are > 100 km
– Vast majority of asteroids
are < 1 km
– All asteroids combined would be ≅ 1,500 km
• ~ 43% the Moon’s diameter & ~ 8% the Moon’s volume
• Numbers
– ~ 500,000 asteroids are known
4 Vesta Rotation
http://upload.wikimedia.org/wikipedia/commons/f/ff/Vesta_Rotation.gif
The Asteroid Belt
Jupiter’s Gravity Formed Asteroid Belt
• Starting assumptions
– ~ 109 planetesimals
– Total mass that of the Earth
• Without Jupiter
– An Earth-sized planet forms
• With
Jupiter
– Jupiter’s gravity clears out this region
• Most planetesimals are ejected from the Solar System
• Some planetesimals are hurled in toward the Sun
– Jupiter’s gravity cannot explain some characteristics
• Wide variety of orbital periods, eccentricities & inclinations
– At least one Mars-sized planet probably formed
• Collision that formed the Moon
• Collision that formed the Mercury’s Caloris Basin
Jupiter’s Gravity Sculpts Asteroid Belt
• Basic physical process
– Orbital resonances
• Simple fractional relationships between orbital periods
– Examples
• 2:1 resonance
• 3:1 resonance
• 3:2 resonance
2 asteroid orbits for every 1 Jupiter orbit
3 asteroid orbits for every 1 Jupiter orbit
3 asteroid orbits for every 2 Jupiter orbits
• Basic observations
– Daniel Kirkwood found evidence in 1867
• Several regions in the asteroid belt with very few asteroids
• Current understanding
– Kirkwood gaps in the asteroid belt
– Comparable to the Cassini division in Saturn’s rings
Kirkwood Gaps: Orbital Resonance
Asteroids Sometimes Hit One Another
• Basic physical process
– All asteroid orbits
are
– All asteroid orbits
are inclined to each other
– Occasional impacts are
slightly elliptical
inevitable
• Basic observations
– The largest asteroids have some basaltic lava flows
• This implies chemical differentiation
– Only the largest asteroids are spherical in shape
– Most asteroids have highly irregular shapes
– All asteroid exhibit cratering
• Six asteroids have been visited by spacecraft
Asteroids Up-Close & Personal
• 951 Gaspra
Galileo spacecraft
1991
– Made of metal-rich silicates & blocks of pure metal
• 243 Ida
Galileo spacecraft
1993
– Discovered the first natural satellite of an asteroid
• 253 Mathilde
NEAR Shoemaker
1997
– As reflective as a charcoal briquette
– Very low average density; probably a “rubble pile”
• Probably the case for most asteroids
• 9969 Braille
Deep Space 1
1999
– May have collided with asteroid Vesta long ago
• 433 Eros
NEAR Shoemaker
2000
– First spacecraft to orbit an asteroid
• Approach speed of ~ 18 mph & orbital speed of ~ 12 mph
• Touched down on Eros after 1 year in orbit
Three Asteroids: Comparative View
Asteroid 951 Gaspra: Natural Color
Asteroid 243 Ida & Its Moon Dactyl
Asteroid 253 Mathilde
NEAR Shoemaker
Asteroid 9969 Braille
Deep Space 1,
1999
Various Views of Asteroid 433 Eros
Boulders
Asteroids Imaged Using Radar
• Asteroid 216 Kleopatra
– Imaged using the Arecibo radio telescope
• ~ 171 . 106 km (~ 106 . 106 mi) from Earth
• Accurate to within ~ 15 km (~ 9 mi)
– Distinctive dog-bone shape
• About the size of New Jersey
• Coloring suggests it contains metal
Asteroid 216 Kleopatra: Radar View
Arecibo Radio
Asteroid 216 Kleopatra: Radar Views
Arecibo Radio Telescope
Asteroid Itokawa: Winter of 2006
Itokawa Rotation
http://upload.wikimedia.org/wikipedia/en/b/b4/Itokawa4.jpg
Asteroid Itokawa: 21 Nov. 2005
http://apod.nasa.gov/apod/ap051121.html
The Five Lagrangian Points
• Basic properties
– Gravity precisely balanced between two objects
• Gravity saddles
Unstable
– Tendency to move away from
• Gravity valleys
– Tendency to
locations
these points
Stable
locations
stay at
these points
• The five locations
– Three unstable Lagrangian points
• L1
• L2
• L3
– Two
• L4
• L5
In line with the two masses
In line with the two masses
In line with the two masses
stable
&
between them
& beyond the smaller
& beyond the larger
Lagrangian points
Co-orbital with smaller mass &
Co-orbital with smaller mass &
60° ahead of it
60° behind it
Five Lagrangian Points: Diagram
http://www.paias.com/paias/home/Science/Newton/Newton_files/lagrpts.jpg
Earth’s Lagrangian Point Animation
Jupiter’s Trojan Asteroids
http://upload.wikimedia.org/wikipedia/commons/f/f3/InnerSolarSystem-en.png
Orbits of Jupiter’s Trojan Asteroids
4
5
More Trojan Asteroids
• Jupiter’s Trojan Asteroids
– Located at two Lagrangian points
– Co-orbital with Jupiter around the Sun
• Leading group
• Trailing group
Small orbits around L4
Small orbits around L5
Greeks
Trojans
– Possibly > 1,000,000 that are ≥ 1 km in diameter
• Other
Trojan Asteroids
– Earth
• 2010 TK7 confirmed in 2011 at Earth’s L4 point
– Mars
• 5261 Eureka, 1998 VF31, & 1999 UJ7 (2007 NS2?)
– Neptune
• Nine known Neptunian Trojans
Near-Earth Objects (NEO’s)
• Formal definition
– Asteroids whose orbits cross Mars’s orbit, or…
– Asteroids whose orbits lie inside Mars’s orbit
• Known asteroids
– ~ 300 asteroids are known to cross Earth’s orbit
– Several hundred thousand probably do so
– Anything < 10 m diameter would probably break up
• Chelyabinsk bolide of 15 February 2013
– Injured ~ 1,500, mostly by flying glass
– Caused ~ $30 million in physical damage
– Energy ~ 440 kilotons of TNT
• 20 to 30 times more than Hiroshima & Nagasaki bombs
Chelyabinsk Bolide: 15 Feb. 2013
http://www.space.com/19802-russian-meteor-blast-photos.html
NEO’s Occasionally Hit the Earth
• The geologic record
– ~ 100 impact craters 3 < Diameter < 150 km
– All are < 500 million years old
• Plate tectonics recycles Earth’s surface
• Barringer Crater
Winslow, Arizona
– Impact ~ 50,000 years ago
– Meteoroid was ~ 50 m in diameter
– Formed a crater ~ 1.2 km in diameter
• Equivalent to a 20 megaton nuclear weapon
• Crater is 24 times the diameter of the impacting object
Barringer Crater, Arizona
Humphreys Peak
(Flagstaff, AZ)
Extinction of the Dinosaurs
• The K-T Boundary Event
– Major extinction between the Cretaceous & Tertiary
• All
dinosaurs went extinct
• Most life forms went extinct
• Mammals survived & thrived
– Iridium-rich layer at many places around the Earth
• Very rare in Earth rocks & minerals
• Highly concentrated in some asteroids
• Possible impact site
– Chicxulub crater
Yucatan Peninsula, Mexico
• Recently dated at 64.98 million years old
Iridium-Rich Clay Sediment Layer
The Peekskill Meteorite
• The fireball
Peekskill Meteor--1
Peekskill Meteor--2
– Seen by many observers
• Traveled WSW to ENE over NY, PA, WV, VA, MD & NC
• Visible on video for at least 17 seconds
– Initially green and eventually orange in color
• Spalling of fragments common near the end
• The impact
– Right rear corner of Ms. Michelle Knapp’s car
– Sonic boom accompanied its arrival
• The meteorite
– Stony meteorite
• An L6 chondrite 30 x 18 x 11.5 cm in size
• One piece displayed at Smithsonian in Washington, DC
– Black fusion crust with red paint from the car it hit
Peekskill Meteorite (9 Oct 1992)
Stony, Stony Iron & Iron Meteorites
• Stony
meteorites
~ 95%
– Very difficult to distinguish from terrestrial rocks
• Fusion crust
• Streamlined shapes
• Stony iron meteorites
~ 1%
– Approximately equal amounts of stone & iron
• Pallasites are a common type of stony iron meteorite
• Iron
meteorites
~ 4%
– Range from almost pure iron to ~ 20% nickel
– ~ 75% of these exhibit Widmanstätten patterns
• Sure indicator that the metal came from an asteroid
– These crystals take millions of years to grow
• Network of elongated iron crystals in a matrix of nickel
A Stony Meteorite From Texas
Collection of R. A. Oriti
A Stony-Iron Meteorite From Chile
Chip Clark
An Iron Meteorite From Australia
Collection of R. A.
Oriti
Widmanstätten Patterns: Australia
Collection of R. A. Oriti
Widmanstätten Pallasite: Smithsonian
© 2009 Rev. Ronald J. Wasowski, C.S.C.
Some Important Terminology
• Meteoroids
– In orbit around the Sun
• Virtually invisible because of small size
• Meteors
– In Earth’s atmosphere
• Brilliant but extremely brief streaks of light
• Friction ionizes air molecules, much as lightning does
• Meteorites
– On Earth’s surface
• Stony meteorites are almost impossible to identify
• Stony iron & iron meteorites are easy to identify
Primordial Materials in Meteorites
• Carbonaceous chondrites
– No evidence of melting
• No chemical differentiation in a large asteroid
– Abundant carbon & complex organic molecules
• ~ 20% water in some types of molecules
– Some carbonaceous chondrites have amino acids
• The Allende meteorite
Chihuahua, Mexico
– Blue-white fireball just after midnight 8 Feb 1969
• Thousands of fragments fell to the ground
• Strewnfield extended 10 km x 50 km
– Evidence of a nearby supernova ~ 4.6 Bya
• 26Al which had decayed into 26Mg
• This may be the event that triggered the Sun’s formation
The “Dirty Snowball” Comet Model
• Solid objects beyond the condensation distance
– Rock & metal were able to condense & persist
– Ices also
were able to condense & persist
• H2O, CH4, NH3 & CO2
– “Rubble piles” were able to
form by gravity
• At great distances, these are comets, not asteroids
• Orbital characteristics
– Asteroid orbits are nearly circular in ecliptic plane
– Comet
orbits are highly elliptical in random planes
• Ices sublimate only when closer to the Sun than Saturn
Three Classes of Comets
• Jupiter-family
comets
– Orbital periods < 20 years
• Return repeatedly until all ices have sublimated
• These seldom last more than a few hundred years
• Intermediate-period comets
– Orbital periods between 20 & 200 years
• Can persist for several millennia
• Comet Halley is the classic intermediate-period comet
– Its orbital period is ~ 76 years
– Its last perihelion was in 1986/1987
• Long-period
comets
– Orbital periods > 200 years (up to 30 million years)
• Comet Hyakutake in 1996
• Comet Hale-Bopp in 1997
The Structure of a Comet
• Center
– Nucleus
Diameter of ~ 101 km
• The only solid part of a comet
– Coma
Diameter of ~ 106 km
• Highly visible fog cloud centered on the nucleus
– Hydrogen envelope
Diameter of ~ 107 km
• Emission from molecules such as CN & C2
• Exterior
– UV-visible ion tail
Distinctive blue color
• Reflection from
subatomic
particles
• Blown away by solar wind, usually very straight
– Dust tail
Distinctive white color
• Reflection from sand grain sized particles
• Blown away by solar wind, often slightly curved
Diagram of a Comet’s Structure
Follows orbital path
Away from the Sun
Comet Tails Point Away From Sun
Comet Jets Face the Sun
• Comets rotate about an axis
– Comets share this with all astronomical objects
• Differential heating
– The“night” side of a comet is intensely cold
• Ices are stable
and
do not
sublimate
– The “day” side of a comet is intensely hot
• Ices are unstable
and
rapidly
sublimate
– Gaseous jets originate from bare ices on the comet’s nucleus
– This activity can affect a comet’s rotation & orbit
– This gas is the source of the coma, hydrogen envelope & ion tail
– Dust in the sublimating ices is the source of the dust tail
• The solar wind forces the gases away from the nucleus
Nucleus of Comet Halley (1986)
15 km
Sun
The European Space Agency Giotto Spacecraft
Comet Halley’s Eccentric Orbit
Nucleus of Comet Hartley (2010)
http://upload.wikimedia.org/wikipedia/commons/b/b3/495296main_epoxi-1-full_full.jpg
Comet Hyakutake (25 March 1996)
http://encke.jpl.nasa.gov/images/96B2/96B2_960325_df2.gif
Comet Hyakutake’s Orbital Plane
Comet Hale-Bopp (1997)
Courtesy of Johnny
Horne
Comet Hale-Bopp: Two Tails (1997)
Tony & Daphne Hallas Astrophotos
Comets Come from Beyond Pluto
• The Kuiper belt
– Comet reservoir like
narrow belt
around the Sun
• Essentially in the plane of the ecliptic
• Begins ~
40 AU from the Sun
– Source of short- and intermediate-period comets
• The Öpik-Oort cloud
– Comet reservoir like spherical halo around the Sun
• Far outside the plane of the ecliptic
• Begins ~ 2,000 AU from the Sun
– Source of long-period comets
Comet Remnants ⇒ Meteor Showers
• Comets die hard
– Ices are very easily sublimated & quickly dissipate
• The ion tail is dispersed into interplanetary space
– Tiny dust particles are blown away by solar wind
• This dust is dispersed into interplanetary space
– Larger rock & metal fragments remain in solar orbit
•
•
•
•
They generally follow the comet’s original orbit
Each perihelion releases a cluster of fragments
Each fragment cluster is in a slightly different orbit
Comet fragment clusters sometimes enter Earth’s atmosphere
• Many annual meteor showers come from comets
–
–
–
–
Perseids
Draconids
Leonids
Ursids
August
October
November
December
Comet Swift-Tuttle
Comet Giacobini-Zinner
Comet Tempel-Tuttle
Comet 8P/Tuttle
Meteoritic Swarms: Comet Debris
Ten Major Annual Meteor Showers
The Tunguska Event
• Some details
– Huge explosion over Siberia on 30 June 1908
•
•
•
•
Explosion heard ~ 1,000 km away
Trees stripped & blown down 25 km in all directions
One person knocked off a porch ~ 60 km away
No crater at all
– Russia did not send scientists until 1927
• Initial
–A
conclusion
comet
exploded before reaching surface
• Revised conclusion
– A stony asteroid exploded before reaching surface
• Probably ~ 80 m in diameter
• Probably ~ 22 km . sec-1 (~ 50,000 mph)
Tunguska Blowdown Zone (1908)
Important Concepts: Asteroids
•
Discovery of the asteroid belt
•
– The Titius-Bode “law”
– Ceres discovered on 1 January 1801
• 2.77 AU, 4.6 years, 522 km diameter
• ~ 30% the mass of all asteroids
– All asteroids together ~ 1,500 km
– Cross or entirely inside Mars’s orbit
– ~ 300
known NEO’s
– ~ 300,000 possible NEO’s
•
• 1992
Properties of the asteroid belt
– Barringer crater
– Located between Mars & Jupiter
– Resonances create Kirkwood gaps
– Asteroids occasionally hit each other
• Cratering is very common
• Many asteroids are “rubble piles”
•
Lagrangian points
– 2 stable & 3 unstable
– Jupiter’s Trojan asteroids at L4 & L5
• Leading & trailing Trojan groups
Terrestrial impacts
– Peekskill meteorite
• 43% Moon’s diameter & 8% volume
•
Near-Earth Objects (NEO’s)
Arizona
• ~ 50 m object, ~ 1.2 km crater
• ~ 50,000
years ago
– Chicxulub crater
Yucatan
• ~ 64,980,000 years ago
•
Types of meteors
– Stony
– Stony iron
– Iron
• Widmanstätten patterns
~ 95%
~ 1%
~ 4%
Important Concepts: Comets
•
Basic properties
– The “dirty snowball” model
– Large & highly elliptical orbits
•
Structure of comets
– Central
• Nucleus, coma & hydrogen envelope
– Elongated
• Ion & dust tails point away from Sun
– Comet jets
• Solar heating sublimates ices
• May affect comet’s rotation & orbit
•
Comet sources
– Kuiper belt
• Ecliptic plane; short-period comets
– Oort cloud
• Spherical shell; long-period comets