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Transcript
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 (set 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 3 types from spectra: – C: resemble carbonaceous chondrite meteorites – S: resemble stony meteorites – M: resemble metallic meteorites Minor Planets 1 Ceres R = 457 km 4 Vesta 573 x 557 x 446 km Minor Planets 21 Lutetia (M) 121 x 101 x 75 km 951 Gaspra (S) 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 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 acceleraFon 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 (a\er Ceres) – 9% of mass of asteroid belt • Second largest asteroid (a\er 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 chondriFc material, hydrated minerals • Last remaining rocky protoplanet? 1 Ceres 1 Ceres RotaFng Ceres viewed by Dawn Bright Spots – Ice? Ceres: 6 km mountain 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 chondriFc material, hydrated minerals – Outgassing H2O Water in Asteroid Spectra 2/12/01: NEAR Lands on Eros Surface of Eros Asteroid ProperFes • ComposiFon – 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 45 Eugenia é 243 Ida çDactyl 90 AnFope Period 16.5 hours; diameters about 80 km Historical Evidence 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 100 million km thick. • The volume of the asteroid belt is – V ~ π(R2out – R2in)h = 4.4 x 1025 km3 • The volume per asteroid is – v = V/N = 4.4 x 1019 km3 • So the distance between the asteroids is – D = v⅓ = 3.5 million km They are so far apart, there is not much to dodge! Do asteroids collide? SomeFmes 4 HST images over 5 months appear to show collision. Dust tail emanates from object 400 km across. Dust volume ~ 10m radius hrp://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 formaFon of the solar system Meteors “It is easier to believe that Yankee professors would lie than that stones would fall from heaven.” -‐-‐ arributed 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 a\er 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-‐Turle 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 ClassificaFon PrimiFve meteorites (chondrites) Unchanged since solar system formaFon • 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 differenFaFon 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 Size ~ 20 m Stony type meteorite Orbit derived from observaFons Originated in the Apollo group of asteroids Chelyabinsk Fragment 112 gm; cube is 1 cm 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 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 • 1632 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 Summary • Meteorites let us sample the primiFve and processed material elsewhere in the solar system • Most originate in the asteroid belt • Most are idenFfiable with an asteroid family • Large rocks will hit the Earth in the future • Study of near-‐Earth asteroids may someday help protect us against a major impact The Canyon Diablo Meteorite • Fragments of the meteorite that created Barringer Crater, Arizona • Iron metorite: 90% Fe, 7% Ni, 1% S, 1% C • Total known weight: 30 tons – Our fragment: 70 lb?