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Lecture #24: Comets The Main Point • Comets! •What are they? Comets are small icy objects from the outer solar system that evaporate spectacularly as they get heated when their orbits carry them closer to the Sun •Where do they come from? •Relationship to asteroids and planets... • Reading: – Chapter 12.2 Astro 102/104 1 Astro 102/104 2 Comet Lingo • "Comet" comes from the Greek word "kometes", meaning long-haired • The solid inner part of a comet is called the nucleus. Most comet nuclei are only a few km in diameter, and are composed of rock and ice. • The "fuzzy" bright region around the nucleus is called the "coma" ("hair"), and is composed of gas and dust. • Solar heating leads to the formation of an extended region of gas and dust, called the tail. A part of the Bayeux Tapestry, commemorating the Norman conquest of 1066 and also archiving the appearance of a "portentous star" in the sky. The star is now known to have been Halley's comet Astro 102/104 • An enormous hydrogen cloud surrounds the coma of most comets 3 Astro 102/104 4 1 Comet Tails The life of a Comet Gas Dust • Dust tails are small pieces (micron to mm sized) of dust, rock, and ice that are shedding off the nucleus and following Keplerian orbits • Ion tails (or plasma tails) are gases evaporated from the nucleus, ionized, and pushed in the anti-Sun direction by the solar wind Astro 102/104 5 A Model Comet... Astro 102/104 6 Other Cometary Properties • A comet is essentially a "dirty snowball" • Comet nuclei are irregularly-shaped, like asteroids, but smaller – Model proposed by astronomer Fred Whipple in 1950 • Cometary activity is caused by sublimation of ices • The activity appears to be confined to isolated jets on the nucleus • Comet nuclei are very dark, typically reflecting < 4% of the incident sunlight (as dark as a charcoal briquet) • Comet nuclei have very low density (0.1 to 0.25 g/cm3), and thus appear to be mostly made of loosely packed (porous or "fluffy") ice • Comets have more than just two types of tails; one is primarily dust and ice, but several others are composed of charged particles Astro 102/104 7 Astro 102/104 8 2 Cometary Composition Halley's Comet Up Close! Atoms, Molecules, and Ions Identified in Comets Atoms: H,C,O,S,Na,Fe,K,Ca,V,Cr,Mn,Co,Ni,Cu,Si,Mg,Al,Ti Molecules:OH,S2,OCS,H2S,H2O,HDO,H2CO,C2,C3,CH,CN,CO,CO2, CS,CH4,C2H2,C2H6,NH,NH2,HCN,CH3CN,N2,NH3 Ions: H2O+,OH+,H3O+,CO+,CO2+,CH+,CN+,N2+,C+,Ca+ • Images and other data from the VEGA and Giotto spacecraft (1986 flybys) Sketch map Astro 102/104 Giotto image 9 • The best measured is comet Halley, with a composition of nearly 80% water ice. But Halley is highly evolved, and may not be typical • Farther from the Sun, at temperatures below the evaporation temperature of water ice, evidence for CO, CO2, and other ices is seen • The dark surface material may be complex organic molecules formed from the residual cometary material after the ices are melted away (from direct spacecraft measurements, as well as spectroscopy from telescopes) Astro 102/104 Comet Orbits • Short-Period Comets There are two populations of comet orbits: • – short period comets orbit the Sun in < 200 years, often have low inclinations, and only travel out as far as the outer planets • • Examples: • – long period comets take thousands of years or more to orbit the Sun, and come from all directions in the sky • • Examples: – Hyakutake (32,000 years!) Astro 102/104 Recent discoveries of a disk of small bodies beyond Neptune, the Kuiper Belt, appears to provide a source for the short period comets They are brought closer to the Sun by frequent encounters with Jupiter and other giant planets – Fragments, remnants become the small short period comets – Halley (76 years) – Encke (3.3 years) • These two populations must have different sources 10 • 11 Only the largest and brightest Kuiper Belt Objects (KBOs) have been discovered so far There may be 100,000 or more KBOs larger than a few hundred km in size There may be hundreds of millions of KBOs that are comet-sized (few km) Astro 102/104 12 3 Long-Period Comets Comet McNaught • Orbits are traced back in time... • Most lead to a vast but unobserved spherical cloud of small objects: – This is the Oort Cloud – 20,000 to 100,000 AU • Stars passing "close" to the Sun can jostle these objects and cause them to fall inwards • There may be 1011 to 1012 comets in the outer Oort cloud, and perhaps ten times that many comets in the cloud as a whole! • Comet mass > 1000 Earths?? Astro 102/104 Comet McNaught swung by the Sun in mid-January 2007. Fierce solar heat turned it into the brightest comet in 40 years. Now it is receding back into the outer solar system. 13 Astro 102/104 14 Meteors: Pieces of Comets? Comet 17P/Holmes • The short period comets appear to be the sources of several meteor showers, as the Earth crosses through their debris •The Hubble image taken Nov. 4, 2007 shows the heart of the comet. •The composite color image, taken Nov. 1 by an amateur astronomer, shows the complex structure of the entire coma. •Comet Holmes brightened by nearly a millionfold in a 24-hour period beginning Oct. 23, 2007. Astro 102/104 15 Astro 102/104 16 4 Why are Comets Important? Spectacular Comet Deaths! • Comets can be thought of as big celestial chemistry sets! • Comets hitting the Sun – Heat, light, water, organic chemicals... – Aren't these the building blocks of life? – SOHO satellite data – "Sun grazers" – Do these events influence solar activity? • Comets must occasionally impact the Sun & planets... – They deliver volatiles and organic molecules (Bringers of Life!) – They can also cause massive ecological destructions (Bringers of Death!) Astro 102/104 17 Spectacular Comet Deaths! Astro 102/104 18 Current & Future Comet Missions • Comets smashing into Jupiter • Completed: Stardust (NASA) Stardust re-entry – Mission to study and return samples from comet Wild-2 – Launched in 1999, Comet encounter Jan. 2, 2004, Earth sample return Jan. 15, 2006 – Shoemaker-Levy 9, July 1994 • Completed: Deep Impact (NASA) – Mission to study and expose the subsurface of comet Tempel 1 – Launch Jan. 2005, encounter comet in July 2005 • Earth for scale Comet impact site Underway: Rosetta (ESA) – Mission to orbit and send landers to an active comet – Launched March 2, 2004; encounter Comet 67P/Churyumov-Gerasimenko in the year 2014 • Proposed: CONTOUR 2 (Cornell) – Mission to study diversity of comets – Reflight of Cornell’s CONTOUR mission lost in 2002 Astro 102/104 19 Astro 102/104 20 5 Deep Impact: A Smashing Success Summary • Comets are small, irregular, icy and rocky objects that evaporate spectacularly as they approach the Sun • Comets fall into two main classes: This movie taken by Deep Impact's flyby spacecraft shows the flash that occurred when comet Tempel 1 ran over the spacecraft's probe. It was taken by the flyby craft's high-resolution camera over a period of about 40 seconds. – Short period comets (P < 200 yr.); source = Kuiper Belt – Long period comets (P > 1000 yr.); source = Oort cloud Arrows point to a large, smooth region. The impact site is indicated by the large arrow at the bottom of this image. Astro 102/104 21 • The main parts of a comet are the nucleus, coma, tail, and extended hydrogen cloud • Comets are essentially "dirty snowballs" composed of water and other ices, silicate minerals, and organic compounds • Comet impacts may have brought substantial quantities of water, other volatiles, and organics to the planets... Astro 102/104 22 Next Lecture... Impacts! • Will asteroids and comets hit Earth? • What is the magnitude of the risk? • What would be the consequences? • What can be done about it? Reading: Chapter 12.4 Astro 102/104 23 6