Comets History of Comets. I. •
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Observed for millenia “hairy stars” Thought to be omens Thought to be atmospheric phenomena (Aristotle) –  But others (e.g., Seneca) disagreed History of Comets. II •  Tycho (1577): exo-­‐atmospheric •  Halley used Newton's laws to predict orbits –  Comets of 1531, 1607, and 1682 had similar orbits –  Predicted they were the same object –  Halley’s comet returned as predicted in 1758 Comet P/Halley 1910 Comet P/Halley 1986 Next AppariWon? Next AppariWon? •  Period = 76 years •  1986 + 76 = 2062 Orbit of Halley’s Comet P =75.3 years a= 17.8 AU (0.58 – 35.1 AU) i = 162.3 e = 0.967 Comet Structure •  Nucleus –  A dirty snowball •  Coma –  A cloud of evaporaWng material •  Tails –  Dust tail –  Plasma tail Comet Tails Cometary Tails ComposiWon Determined from spectroscopy •  Ice + dust: “dirty snowballs” •  Indicate formaWon in cold, outer SS •  Water, CO2, CO emission lines Comet Wild-­‐2 from Stardust Comets Close Up Halley from Gio`o Comets Close Up Borelly Comets Close Up Wild-­‐2 Comets Close Up Deep Impact Flyby of comet 9P/Tempel 1 in 2005 •  Crashed a probe into the comet •  Ejecta studied to determine composiWon Comet 9P/Tempel 1 •
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Orbital Period ~ 5.5 yr Perihelion ~ 1.6 AU Size ~ 11x4x4 km Albedo ~ 4% Spin period ~ 2 days Deep Impact Impact! New Crater on Tempel 1 Changes in Tempel 1 2005 (Deep Impact) to 2011 (Stardust) 20-­‐30 m of erosion on smooth surface Comet Holmes (Nov. 2007). The coma is bigger than the Sun. h`p://antwrp.gsfc.nasa.gov/apod/ap071121.html Nucleus •
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“dirty snowball”—carbon-­‐rich material + ice Albedo ~ 5% Size ~ 5 – 10 km Low density ( < 1 g cm-­‐3) Borelly (8 x 4 km) Halley (15 x 8 km) Rose`a and Philae at 67P Churyumov–Gerasimenko How Big? How Big? Landing on a Comet Philae The Comet 3 km from Philae Philae’s Descent 40 m above the surface First Touchdown The Landing •  A.) 15:33 UTC •  B.) 17:26 UTC •  C.) 17:33 UTC •  g ~ 10-­‐3 cm/s2 (Earth: 980) Where is Philae? Philae’s Fate •  Nov 12 2014: Failed to a`ach itself to comet •  PosiWon known to with 16 x 160m •  Operated 2.5 days unWl ba`ery ran down •  Plan: 7 hrs sunlight/12 hr “day) •  June 13 2015: contact re-­‐established (78 sec) •  Solar array charging inefficient. In shadow? Jets from Nucleus Comet Borrelly (2001) 67P Jets 67P Jets What Have We Learned? •  Comet 67P appears to be composite –  Formed by excess erosion of neck? –  Two lobes have different composiWon –  2 comets collided slowly an stuck •  Surface not uniform •  Albedo ~ 4% (dark as coal) •  Cometary Water and Terrestrial Water are different Coma •
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SublimaWon when comet nears Sun Dust + gas create atmosphere Can be ~ 1 million km in radius Very low density Comet SWAN Tail •  Swept away material –  generally poinWng away from Sun •  Can have lengths up to 1 AU •  Prominent ~ when comet passes orbit of Mars Two types of tails: –  plasma tail •
Blown by solar wind –  dust tail •
Tracks orbit •  Plasma tail –  UV ionized gas caught up in the solar wind –  Tells us about solar wind velocity •  Dust tail –  Dust parWcles –  Pushed back by radiaWon pressure –  Curves back in direcWon it came from Tail affected by Solar Wind •  Swept away material—generally poinWng away from Sun •  Can have lengths up to 1 AU •  Prominent ~ when comet passes orbit of Mars Two types of tails: plasma tail and dust tail Comet Ikeya-­‐Zhang-­‐Jones (2014) Comets and Meteor Showers Origin of Comets •  Most comets have random orbits –  Oort cloud comets –  Orbits show origin very distant—1/4 distance to nearest star! •  Oort cloud –  ~ 1012 comets –  D ~ 50,000 AU •  Some comets have ordered orbits –  Kuiper belt comets –  Aphelion closer than twice Neptune's orbit (30 AU) Origin of Comets: Oort Cloud Origin of Comets: Oort Cloud Comet Families •  Short Period •  Oort Cloud –  Formed among Jovian planets, ejected into Oort cloud –  Spherical cloud ~ 1012 comets –  ~ 50,000 AU from Sun –  Source of long-­‐period comets •  Kuiper Belt –  Smaller disk of comets beyond the orbit of Neptune –  Form close to where we find them now –  Source of short-­‐period comets History of a Comet Fate of Comets •  Solid material is carried away with evaporaWng ices •  Ices evaporate; dust lev behind blocks further gas escape –  Looks like asteroid? –  Stays together? –  Impact with a planet or the Sun On close examinaWon, some Earth-­‐crossing asteroids have outgassing and small tails when near the Sun Asteroid 62412 main belt Hygiea group Sco` Shepard, CIW/DTM ReconciliaWon •  Comets form from mostly icy material outside the frost line •  Asteroids form from mostly rocky material inside the frost line •  Comets can evolve into asteroids •  Both comets and asteroids, and their debris (meteoroids) can it Earth Comet Impacts: Tunguska 1908 Tunguska Event •  Air burst @ 5-­‐10 km •  Inferred size: 60-­‐200m •  Inferred energy: 15-­‐30 Mt –  Mag 5 earthquake •  Fla`ened 2000 km2 of forest •  Impactor: comet or stony asteroid Other Impacts Other Effects •  Can Comets carry volaWles to terrestrial planets? •  Comet Lovejoy: outgassing –  Ethyl alcohol –  Glycoaldehyde (a sugar) –  19 other organic molecules Take-­‐Aways •  Comets are primiWve material from the outer solar system •  Comets are dirty snowballs •  Comets can be the largest objects in the SS •  Comets are rich in volaWles •  Did comets return volaWles to the inner SS? •  Comets can evolve into asteroids Trans-Neptunian Objects
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90377 Sedna
a=518 AU e=0.85 Discovered at ~ 90 AU
Radius: 1200-1600 km
136199 Eris
Radius ~ 2330 km - same as Pluto