15. Asteroids & Comets The discovery of the asteroid belt Jupiter s gravity shapes the asteroid belt Asteroids occasionally hit one another Some asteroids orbit the Sun outside the asteroid belt Stony, stony iron & iron meteorites Primordial materials are found in some meteorites 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 but rather 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 The Sicilian astronomer Giuseppe Piazzi strikes first Sees an uncharted object moving nightly on 1 January 1801 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 on 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 Vestadiscovered in 1807 Several hundred more in the mid-1800s Max Wolf begins using photography to find asteroids Wolf discovered 228 asteroids as lines on long-exposure photos Requirements 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 larger than 300 km 1 Ceres 960 x 932 km 2 Pallas 570 x 525 x 482 km 4 Vesta 530 km 30 other asteroids are larger than 200 km 200 other asteroids are larger than 100 km The vast majority of asteroids are smaller than 1 km All asteroids in one object would be ~ 1,500 km ~ 43% the Moon s diameter & ~ 8% the Moon s volume Numbers ~ 100,000 asteroids might be discovered from Earth
The Asteroid Belt Jupiter s Gravity Formed the Asteroid Belt Computer models of planetary formation 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 asteroid orbital periods, eccentricities & inclinations At least one Mars-sized planet probably formed Collision that formed the Moon Collision that formed the Caloris Basin on Mercury Jupiter s Gravity Sculpts the 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 The Kirkwood Gaps: Orbital Resonance Asteroids Occasionally Hit One Another Basic physical process All asteroid orbits are at least slightly elliptical All asteroid orbits are slightly 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 Several 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 spacecraft 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 spacecraft 1999 May have collided with asteroid Vesta in the distant past 433 Eros NEAR Shoemaker spacecraft 2000 First spacecraft to orbit an asteroid Approach speed of ~ 18 mph & orbital speed of ~ 12 mph Touched down on Eros after 1 year of orbital observations
Three Asteroids: A Comparative View Asteroid 951 Gaspra in Natural Color Asteroid 243 Ida & Its Moon Dactyl Asteroid 253 Mathilde NEAR Shoemaker Spacecraft Asteroid 9969 Braille Various Views of Asteroid 433 Eros Boulders Deep Space 1, 1999
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 Seen By Radar Arecibo Radio Telescope Asteroid 216 Kleopatra: 8 Radar Views Asteroid Itokawa: Winter of 2006 Arecibo Radio Telescope http://www.newscientistspace.com/data/images/ns/cms/dn8259/dn8259-3_506.jpg Asteroid Itokawa: 21 November 2005 http://apod.nasa.gov/apod/ap051121.html The Five Lagrangian Points Basic properties Gravity precisely balances between two celestial objects Gravity saddles Unstable location Tendency to move away from these points Gravity valleys Stable location Tendency to stay at these points The five locations 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 Stable Lagrangian points L 4 Co-orbital with smaller mass & 60 ahead of it L 5 Co-orbital with smaller mass & 60 behind it
The Five Lagrangian Points: Diagram Earth s Lagrangian Points: Animation http://www.paias.com/paias/home/science/newton/newton_files/lagrpts.jpg Orbits of Jupiter s Trojan Asteroids 4 5 Some Asteroids Orbit Outside the Belt Jupiter s Trojan Asteroids Located at two Lagrangian points Co-orbital with Jupiter around the Sun Leading group Small orbits around L 4 Trailing group Small orbits around L 5 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 NEO s Occasionally Hit the Earth The geologic record ~ 100 impact craters such that 3.0 < 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 The crater is 24 times the diameter of the impacting object Barringer Crater, Arizona
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 a car owned by Ms. Michelle Knapp Sonic boom accompanied its arrival The meteorite Stony meteorite An L6 chondrite 30 x 18 x 11.5 cm in size One piece is on display at the Smithsonian in Washington, DC Black fusion crust with red paint from the car it hit The 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 pure iron to ~ 20% nickel ~ 75% of these exhibit Widmanstätten patterns Sure indicator that the metal came from an asteroid These 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 From Australia Widmanstätten Pallasite (Smithsonian) Collection of R. A. Oriti C 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 Some 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 Amino acids are found in some carbonaceous chondrites The Allende meteorite Chihuahua, Mexico Blue-white fireball just after midnight 8 February 1969 Thousands of fragments fell to the ground Strewnfield extended 10 km x 50 km Evidence of a nearby supernova ~ 4.6 billion years ago 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 At great distances, these are comets rather than asteroids Orbit 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 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 UV-visible cloud of H atoms dissociated from H 2 O molecules Periphery Ion tail Distinctive blue color Emission from molecules such as CN & C 2 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 The Structure of a Comet: A Diagram Comet Tails Point Away From the Sun Comet Jets Face the Sun Comets rotate about an axis Comets share this property 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 both the rotation & orbit of the comet This gas is the source of the coma, hydrogen envelope & ion tail Dust imbedded in the sublimating ices is the source of the dust tail The solar wind forces the gases away from the nucleus The Nucleus of Comet Halley (1986) Sun 15 km The European Space Agency Giotto Spacecraft
Comet Hyakutake (25 March 1996) Comet Hyakutake s Orbital Plane http://encke.jpl.nasa.gov/images/96b2/96b2_960325_df2.gif Comet Hale-Bopp (1997) Two Tails of Comet Hale-Bopp (1997) Courtesy of Johnny Horne Tony & Daphne Hallas Astrophotos Comet Halley s Eccentric Orbit Comets Come from Beyond Pluto The Kuiper belt A comet reservoir like a 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 Oort cloud A comet reservoir like a spherical halo around the Sun
Comet Remnants Make Meteor Showers Comets die hard Ices are very easily sublimated & quickly dissipate The ion tail is dispersed into interplanetary space Small dust particles are easily blown away by solar wind The dust tail 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 these fragments Each cluster of fragments is in a very slightly different orbit These comet fragment clusters sometimes encounter the Earth 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 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 (as much as 30 million years) Comet Hyakutake in 1996 Comet Hale-Bopp in 1997 Meteoritic Swarms: Comet Debris Ten Major Annual Meteor Showers 2002 Leonids Predictions for Portland The Tunguska Event Some details Tremendous explosion over Siberia on 30 June 1908 No crater at all Trees stripped of leaves & blown down 25 km in all directions A person knocked off a porch ~ 60 km away Explosion heard ~ 1,000 km away Russia did not send scientists until 1927 Tentative conclusion A comet exploded before reaching the surface Revised conclusion A stony asteroid exploded before reaching the surface Probably ~ 80 m in diameter Probably ~ 22 km. sec -1 (~ 50,000 mph)
Blowdown Zone: Tunguska (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