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 did not arrive until 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 but only 918 km 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 uses 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> 300 km• 1 Ceres 960 x 932 km• 2 Pallas 570 x 525 x 482km• 4 Vesta 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 2 asteroid orbits for every 1 Jupiter

orbit• 3:1 resonance 3 asteroid orbits for every 1 Jupiter

orbit• 3:2 resonance 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 slightly elliptical– All asteroid orbits are inclined to each other– Occasional impacts are 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 spacecraft1993

– 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 Shoemaker2000

– 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 Spacecraft

Asteroid 9969 Braille

Deep Space 1, 1999

Various Views of Asteroid 433 ErosBoulders

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 Telescope

Asteroid 216 Kleopatra: Radar Views

Arecibo Radio Telescope

http://upload.wikimedia.org/wikipedia/en/b/b4/Itokawa4.jpg

Asteroid Itokawa: Winter of 2006

Itokawa Rotation

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 Unstablelocation

– Tendency to move away from these points• Gravity valleys Stable location

– Tendency to stay at these points

• The five locations– Unstable Lagrangian points

• L1      In line with the two masses &between them• L2     In line with the two masses &beyond the smaller• L3     In line with the two masses &beyond the larger

– Stable Lagrangian points• L4     Co-orbital with smaller mass &60° ahead of it• L5     Co-orbital with smaller mass &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 Small orbits around L4 Greeks• Trailing group Small orbits around L5 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 exist– 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, ArizonaHumphreys 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

– 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 Meteor--1Peekskill Meteor--2

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)

The European Space Agency Giotto Spacecraft

15

km

Sun

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

Blue Ion Tail

White Dust Tail

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 August Comet Swift-Tuttle– Draconids October Comet Giacobini-Zinner– Leonids November Comet Tempel-Tuttle– Ursids December 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 conclusion

– A 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)

• 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• 43% Moon’s diameter & 8% volume

• Properties of the asteroid belt– 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

• Near-Earth Objects (NEO’s)– Cross or entirely inside Mars’s orbit– ~ 300 known NEO’s– ~ 300,000 possible NEO’s

• Terrestrial impacts– Peekskill meteorite

• 1992– Barringer crater

Arizona• ~ 50 m object, ~ 1.2 km crater• ~ 50,000 years ago

– Chicxulub crater

Yucatan• ~ 64,980,000 years ago

• Types of meteors– Stony

~ 95%– Stony iron

~ 1%– Iron

~ 4%• Widmanstätten patterns

Important Concepts: Asteroids

• 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

Important Concepts: Comets