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 > 300 km 1 Ceres 960 x 932 km 2 Pallas 570 x 525 x 482 km 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 The Asteroid Belt http://upload.wikimedia.org/wikipedia/commons/f/ff/vesta_rotation.gif Jupiter s Gravity Formed Asteroid Belt Starting assumptions ~ 10 9 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 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 Asteroid 9969 Braille NEAR Shoemaker Spacecraft 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. 10 6 km (~ 106. 10 6 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 Asteroid 216 Kleopatra: Radar Views Arecibo Radio Telescope Arecibo Radio Telescope Asteroid Itokawa: Winter of 2006 Asteroid Itokawa: 21 Nov. 2005 Itokawa Rotation http://upload.wikimedia.org/wikipedia/en/b/b4/itokawa4.jpg http://apod.nasa.gov/apod/ap051121.html
The Five Lagrangian Points Basic properties Gravity precisely balanced between two objects Gravity saddles Unstable locations Tendency to move away from these points Gravity valleys Stable locations Tendency to stay at these points The five locations Three unstable Lagrangian points L 1 In line with the two masses & between them L 2 In line with the two masses & beyond the smaller L 3 In line with the two masses & beyond the larger Two stable Lagrangian points L 4 Co-orbital with smaller mass & 60 ahead of it L 5 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 L 4 Greeks Trailing group Small orbits around L 5 Trojans Possibly > 1,000,000 that are 1 km in diameter Other Trojan Asteroids Earth 2010 TK 7 confirmed in 2011 at Earth s L 4 point Mars 5261 Eureka, 1998 VF 31, & 1999 UJ 7 (2007 NS 2?) 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 Seen by many observers Peekskill Meteor--1 Peekskill Meteor--2 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 An Iron Meteorite From Australia Chip Clark Collection of R. A. Oriti
Widmanstätten Patterns: Australia Widmanstätten Pallasite: Smithsonian Collection of R. A. Oriti 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 26 Al which had decayed into 26 Mg 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 H 2 O, CH 4, NH 3 & CO 2 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 ~ 10 1 km The only solid part of a comet Coma Diameter of ~ 10 6 km Highly visible fog cloud centered on the nucleus Hydrogen envelope Diameter of ~ 10 7 km Emission from molecules such as CN & C 2 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) Comet Halley s Eccentric Orbit Sun 15 km The European Space Agency Giotto Spacecraft
Nucleus of Comet Hartley (2010) Comet Hyakutake (25 March 1996) http://encke.jpl.nasa.gov/images/96b2/96b2_960325_df2.gif http://upload.wikimedia.org/wikipedia/commons/b/b3/495296main_epoxi-1-full_full.jpg Comet Hyakutake s Orbital Plane Comet Hale-Bopp (1997) Courtesy of Johnny Horne Comet Hale-Bopp: Two Tails (1997) Blue Ion Tail White Dust Tail 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 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) 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 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 L 4 & L 5 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: 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