Asteroids Sun’s Planets
•  Earth
•  Historical planets: (ρλανετ, or wanderer)
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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:
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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:
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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 •
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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 çDactyl 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 •
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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?