Asteroids Sun’s Planets • Earth • Historical planets: (ρλανετ, or wanderer) – Mercury – Venus – Mars – Jupiter – Saturn • Later discoveries – Uranus (1781) – Neptune (1846) Titius-Bode Law A mathematical relation published by J.E. Bode in 1772 a = (2n x 3 + 4) / 10 • a is the semimajor axis of the orbit in au • n is an index: – Mercury: -1 (define 2-1 = 0) – Venus: 0 – Earth: 1 – Mars: 2 – Jupiter: 4 – Saturn: 5 a matches observation to within a few %. The Titius-Bode law is empirical: there is no physical reason why it should hold, but it has proven of some use as a predictor. Titius-Bode Law. II a = (2n x 3 + 4) / 10 “Missing” values of n: • 3: corresponds to the distance of Ceres, discovered in 1801 by Piazzi. • 6: corresponds to Uranus • 7: a=40 au. Neptune is at 30 au Why does the Titius-Bode Law appear to work? Simulations show planets cannot be too close together. Simulated stable planetary separations can often be approximated as a geometric series Asteroids means star-like • 1 Ceres discovered 1801 • 10 known by 1850 – Brightest: Ceres, Pallas, Juno, Vesta • Today: – Over 1 million known – Over 600,000 with orbits Asteroid ProperFes • Largest: Ceres, radius= 473 km (1/3 lunar) • Over 1 million with radius >1 km • Total mass < lunar mass • Most are aspherical – Too liRle mass to overcome the strength of the rock • 3 types from spectra: – C: resemble carbonaceous chondrite meteorites – S: resemble stony meteorites – M: resemble metallic meteorites Minor Planets 1 Ceres 4 Vesta R = 457 km 573 x 557 x 446 km Minor Planets 21 Lutetia (M) 951 Gaspra (S) 121 x 101 x 75 km 19 x 12 x 11 km Minor Planets 253 Mathilde (C) 66 x 48 x 46 km Minor Planets 25143 Itokawa (S) 535 x 294 x 209 m Minor Planet 443 Eros 40 x 14 x 14 km NEAR flyby 12/23/98 Bennu Minor Planets up close: the Dawn Mission • Launched 9/27/2007 – Encountered 4 Vesta 2012 – Arrived at 1 Ceres March 2015 • Uses ion drive for conFnuous acceleraon 4 Vesta 4 Vesta 4 Vesta 4 Vesta South Pole 4 Vesta Surface: Snowman Craters 4 Vesta Interior History of Vesta 4 Vesta • Brightest asteroid. – Distance = 2.4au • Second most massive asteroid (aer Ceres) – 9% of mass of asteroid belt • Second largest asteroid (aer Ceres) – Oblate spheroid (<r>=260 km) • Rocky: ρ = 3.4 g/cm3 • DifferenFated with metallic core – Surface composiFon matches 1200 “Vestan achondrite” meteorites – Evidence for chondric material, hydrated minerals • Last remaining rocky protoplanet? 1 Ceres 1 Ceres • First asteroid discovered – Distance = 2.8 au • Most massive asteroid – 30% of mass of asteroid belt • Largest asteroid – <r>=473 km • Icy: ρ = 2.6 g/cm3 • DifferenFated: rocky core with icy mantle – Evidence for chondric material, hydrated minerals – Outgassing H2O • Probably formed outside frost line 1 Ceres Rotang Ceres viewed by Dawn Bright Spots – Ice? Occator Crater (91 km diameter) Kerwan Crater 280 km diameter Ceres: 6 km mountain Ahuna Mons 5000 meter high cryo-volcano AnFpodal from 280km Kerwan crater Landslides on Ceres Type I: similar to rock glaciers Type II: resembles avalanches. Type III: related to craters. and icy landslides on Earth Most common type on Ceres Impact melFng? Dawn Mission • Nominal mission end July 2015 • Actual mission end Nov 1 2018 – Hydrazine fuel for poinFng ran out • Legacy: – Explored 2 very different worlds, Ceres and Vesta – Demonstrated ion engine technology Water in Asteroid Spectra 2/12/01: NEAR Lands on Eros Surface of Eros Asteroid ProperFes • Composion – M: metals. ρ ~ 3-5 g/cm3 – S: rocks. ρ ~ 2-3 g/cm3 – C: rocks/carbonaceous materials. ρ ~ 1-2 g/cm3 – General trend with distance from Sun • Structure: differenFated • Families: – Outer edge of belt: evidence for water & ices Binary Asteroids é 243 Ida 45 Eugenia çDactyl 90 AnFope Period 16.5 hours; diameters about 80 km Historical Evidence Lac à l'Eau Claire, Quebec 26, 36 km diameters Asteroid Orbits Asteroid Orbits Green: Main belt Red: Earth Crossers Blue: Trojans Near-Earth Asteroids Asteroid Orbits Resonances in the Asteroid Belt What is Wrong with this Picture? Do asteroids collide? • Assume there are: – 1 million asteroids, with – orbits between 2.2 and 3.3 AU, – the belt is 1 million km thick (h/r = 0.2%). • The volume of the asteroid belt is 2 2 23 3 – V ~ π(R out – R in)h = 4.3 x 10 km • The volume per asteroid is – v = V/N = 4.4 x 1017 km3 • So the distance between the asteroids is – D ~ v⅓ = 750,000 km They are so far apart, there is not much to dodge! Do asteroids collide? Somemes 4 HST images over 5 months appear to show collision. Dust tail emanates from object 400 km across. Dust volume ~ 10m radius hp://solarsystem.nasa.gov/mulFmedia/ display.cfm?IM_ID=11223 Asteroid Families • Based on – Spectral similariFes – Orbital similariFes • Most asteroids can be classified as a member of a small number of families • Most probably are fragments of larger asteroids • A 100 km radius asteroid can produce 106 1 km fragments Meteors, Meteorites, and Meteoroids • Meteor: the streak of light seen in the sky • Meteorite: the rock found on the ground • Meteoroid: the rock before it hits Earth Meteorites are easily-studied remnants of the formaon of the solar system Meteors “It is easier to believe that Yankee professors would lie than that stones would fall from heaven.” -- aributed to Thomas Jefferson From below … Meteors … and above Meteors • Most are the size of a grain of sand • They vaporize about 75-100 km up when they hit the atmosphere • Impact velociFes >20 km/s • The trails are ionized gas • Best viewed aer midnight Meteor Showers Occur when Earth passes through the orbit of a comet. Examples: • Orionids: comet 1P/Halley Oct 21-22 • Leonids: comet 109P/Tempel-Tule Nov 17-18 • Geminids: asteroid 3200 Phaeton Dec 13-14 • Perseids: comet 55P/Swi]-TuRle Aug 12-13 • Lyrids: comet Thatcher Apr 22-23 Orbits of Meteor Showers Fireballs and Bolides • Very bright meteors • May leave a persistent trail • Due to impacFng object bigger than about 1m Geminid Fireball 12/9/2010. source: S. Korotkiy, Russian Academy of Sciences Grand Tetons Meteor 8/10/72. 3-14m Apollo asteroid. V=15 km/s; 15km alFtude Peekskill Meteorite 10/9/92 12 kg Stony-Iron Classificaon PrimiFve meteorites (chondrites) Unchanged since solar system formaon • Stony: rocky minerals + small fracFon of metal flakes • Carbonaceous (Carbon-rich): like stony, with large amounts of carbon compounds PrimiFve meteorites (chondrites) Majority of meteors • Accreted from solar nebula – Chondrules: droplets formed during accreFon • Stony/Carbon-rich – > 3 AU, carbon compounds condense – Carbon-rich formed at outer edge • More stony hit Earth than carbon-rich Processed meteorites (achondrites) Fragment of larger, differenFated object • Metal rich: mostly iron/nickel • Stony-Iron: composiFon resembles terrestrial planet crust/mantle; some with basalts Processed meteorites (achondrites) Fragment of large asteroid that differenFated • Rocky – Made from lava flows – Surface material • Metal-rich – Proof of differenFaon EsFmate: ~10 geologically acFve asteroids iniFally • Last remaining: the asteroid Vesta Biases • Irons most likely to survive impact • Stony most likely to be overlooked C type Marília Meteorite: chondrite H4. Marília, Brazil, 10/5/1971 M type WillameRe - AMNH Pallasite Chelyabinsk 2/15/2013 Chelyabinsk • Incoming speed ~ 19 km/s • Shallow entry angle • Mass ~ 10,000 tons • Radius ~ 20 m • Stony (S) type meteorite – LL Chondrite (low Fe and total metal abundance) • Orbit derived from observaons • Originated in the Apollo group of asteroids Chelyabinsk KineCc Energy • 200-990 kT of TNT – kT = 4x1012 Joules = 4x1019 ergs – Hiroshima atom bomb: 15 kT • EsFmate from – a.) airburst shock waves (38-23 km alFtude) – b.) radiated energy • Damage as far as 120 km from impact • Largest fragment: 1.5m diameter Chelyabinsk Fragment 112 gm; cube is 1 cm Origin of Chelyabinsk Meteorite Dates to 4.6 Gya • Parent body affected by impacts at 30 Myr and 100 Myr • Argon isotopes suggest impact 29 Mya • Surface exposed to solar radiaon 1.2 Mya Dates from radioisotopic analysis Meteors and Asteroids • Most meteors originate in the asteroid belt • Meteors and asteroids – Have similar spectra – Have similar orbits – Differ primarily in size Orbits of Meteors Tunguska Event • June 30 1908 • 2150 km2 of forest flaened • No crater • Probably air burst at 5-10 km • EsFmated diameter 60-190 km • Equivalent of 3-5 MT explosion • Pressure wave equivalent of mag 5 earthquake • Comet or stony asteroid? Tunguska aermath Tunguska Event • Expect one about every 300 years • Since 2/3 of Earth is ocean, expect one on land every millenium. Yesterday’s Fireballs Nov 12 2018 UT 19 fireballs : 17 sporadics, Northern Taurids Yellow: 30 km/s Green: 45 km/s Blue: 60 km/s Source: spaceweather.com/ Near Earth Asteroids Near Earth Asteroids • PotenFally Hazardous Asteroids – Earth Minimum Orbit IntersecFon Distance (MOID) of 0.05 AU or less – diameter larger than 150 m • 1936 known • Not all will hit Earth Halloween Asteroid 2015 TB 145 Passed at 0.003AU on 10/31/15 Near Earth Asteroids Near Earth Asteroids Near Earth Asteroids Near Earth Asteroids NEAs this month Asteroid Date(UT) Miss Distance km/s D(m) • 2018 VY4 2018-Nov-11 5.5 LD 12.6 14 • 2018 VH5 2018-Nov-11 6 LD 6 13 • 2018 VA4 2018-Nov-11 3.1 LD 7.4 8 • 2018 VZ6 2018-Nov-12 9.5 LD 10.3 27 • 2018 VN6 2018-Nov-12 1.9 LD 14.2 8 • 2018 VC7 2018-Nov-13 0.9 LD 4.5 12 • 2018 VR6 2018-Nov-13 8 LD 9.5 12 • 2018 VF5 2018-Nov-13 9.7 LD 6.8 11 • 2018 UQ1 2018-Nov-13 9.4 LD 12.3 146 • 2018 VO3 2018-Nov-14 4.1 LD 7.7 14 • 2018 VX5 2018-Nov-14 3.6 LD 8.3