ARMAGEDDON Astro 202 Spring 2008 COMETS and ASTEROIDS Small bodies in the solar system Impacts on Earth and other planets The NEO threat to Earth Lecture 1 Don Campbell DEEP IMPACT Last Days On Earth Part 1 History Channel/ABC http://www.youtube.com/watch?v=fuwryjzjsoe 1
The Cretaceous-Tertiary (K-T) impact and extinction of the dinosaurs (plus a lot of other species) EXTINCTION OF THE DINOSAURS Chicxulub crater ~ 180 km diameter - seismic data showing buried crater - dated to 65 million years ago (about a 10 km diameter impactor) Painting by Don Davis ASTRO 202 SPRING 2008 REFERENCE MATERIAL FOR NEAR-EARTH OBJECT DISCUSSION Near-Earth Object Survey and Definition Analysis of Alternatives Report to Congress March 2007 Summary document (790k) http://www.nasa.gov/pdf/171331main_neo_report_march07.pdf Background material and analysis (15 MB) http://www.hq.nasa.gov/office/pao/foia/neo_analysis_doc.pdf Near Earth Object Issues: Should we be worried? Finding all or most of them search techniques Determining precise orbits so know if there is a possibility of impacting Earth Characterizing general population and specific potentially hazardous objects (PHOs) Mitigation techniques i.e. doing something about a high probability PHO Political/policy issues Who is responsible? Cost Who is going to pay? Articles on NEO Threat in Science, March 7 Issue 2008 http://www.sciencemag.org/cgi/reprint/319/5868/1326.pdf http://www.sciencemag.org/cgi/reprint/319/5868/1329.pdf 2
COMETS: First known small bodies; Atmospheric phenomenon or further away from Earth? An Earth centered universe until the 16 th century Thought to be incorruptible and immutable nothing ever changes Ptolemaic (Greek) view of the universe - Sun and planets form the first 7 crystal spheres and the stars the 8 th. Outside that is the sphere of the heavens. Historically: Comets thought to be harbingers of bad things. Like the red star that from his flaming hair Shakes down disease, pestilence and war Homer, 900 BCE Comets were thought of as stars with flaming hair. This was a persistent theme until almost modern times because comets broke the regularity of the heavens which were supposed to be fixed and immutable. Tycho Brahe (Danish astronomer) challenged principle of celestial incorruptibility - he showed that the comet of 1577 was at least 4 times the distance of the Moon so could not be an atmospheric phenomenon. He used parallax several other astronomers measured positions from other observatories. Destroyed the idea of immutable crystalline spheres. Tycho Brahe (1546 1601) 3
The idea that the heavens change and that the Earth and other bodies in the solar system can be impacted by small bodies in the solar system was only slowly accepted. The New Solar System: Read; Chapter 24, Comets by John Brandt Chapter 25, Asteroids by Clark Chapman Affected the interpretation of craters on the Moon as late as 50 years ago volcanic or impact origin big fight. SMALL BODIES ASTEROIDS AND COMETS Asteroids - Most in Main Belt between Mars and Jupiter source region for NEAs Near Earth Asteroids (NEAs) - Inside orbit of Mars Potentially Hazardous Asteroids (PHAs) - NEAs that cross Earth s orbit Could hit Earth if orbits intersect Comets - Periodic comets There are about 200 comets in short period < 200 years orbits about the Sun. Another ~200 have been detected but have either been lost, destroyed or not completed at least two orbits New comets - Ones that have never been into the inner solar system before or have periods so long that this appears to be the case. N.B. Not always easy to distinguish between asteroids and comets old comets that have exhausted most of the volatiles near their surfaces i.e. no longer have a a coma or tail can look like an asteroid. COMETS: What is a comet? A small body orbiting the Sun with a composition that includes ices, primarily water ice. As the comet comes near the Sun in its orbit, ices on and below the surface sublimate into gases such as water vapor. The gas escapes through localized breaks in the surface carrying off large amounts of dust and small fragments forming the coma and tails that we see. Comets have two tails, a dust tail and a ionized gas tail (blue in the image). Some asteroids are thought to be gassed out comets. 4
Halley s comet in 1910 apparition Dirty snow ball model of nucleus by Fred Whipple (1950) Ion tail is blue because ions of carbon monoxide fluoresce under sunlight Comets: The nucleus! Halley s comet returned in 1986 its orbital period is 76 years. First spacecraft flyby of a comet European and Russian (Soviet Union) missions. COMETARY NUCLEI WERE THOUGHT TO BE COMPOSED MAINLY OF ICES AND SO VERY GOOD REFLECTORS OF SUNLIGHT WRONG Missions to Halley showed that they are very dark about the darkest bodies in the solar system. Therefore much larger than we thought See gas and dust jets on Halley Comet Borelly from Deep Space 1 spacecraft Halley s comet nucleus from ESO Giotto mission about 15 km in size. 5
DEEP IMPACT July 4, 2005 Comet Tempel 1 Impact speed 37,000 km/hr Only ~1% of surface covered in ice STARDUST Mission to Comet Wild 2. Flyby Jan 2, 2004 Returned to Earth, Jan 15, 2006 Not primitive material!! Comets: Where do they come from? Two reservoirs, Kuiper belt and Oort Cloud. Orbits disturbed by passing star, etc causing comets to come into the inner solar system. COMET ORBITS http://janus.astro.umd.edu/orbits/outerss.html Halley close approach to Sun = 0.587 AU; eccentricity = 0.967 Semi-major axis = 17.8 AU; Farthest distance from Sun = 35 AU 6
Comet velocities: Very high when close to the Sun i.e. at perihelion Very low for high eccentricity orbits far from the Sun - i.e. at aphelion At Perihelion how high? IMPACT ENERGIES: Kinetic energy = ½ M V 2 M = mass of the object, V = velocity Assume V = 40 km/sec impact velocity M = 4/3 π R 3 x density R = radius of (spherical) object Halley s comet: ~58 km/sec = 200,000 km /hr = ~130,000 miles/hr Assume R = 2 km and density = 1 g/cm 3 - i.e. density of water What is Earth s velocity in its orbit = 2π R/P = 2π 1.5 x 10 8 / 3 x 10 7 = 30 km/sec Relative velocity between 90 km/sec and 30 km/sec Assume typical comet impact velocities to be ~40 km/sec Kinetic energy = ½ x 1.7 10 10 x 1.6 10 9 Joules = 1.4 10 19 Joules How much is that? One ton of TNT (a common explosive) = 4 10 9 Joules THEREFORE COMET KINETIC ENERGY = 3 10 9 tons of TNT equivalent = 3,000 megatons of TNT Do we know of any comet impacts on Earth? Hard to distinguish between comet and asteroid impacts but: Tunguska a giant explosion occurred on June 30, 1908 in Russian Siberia Energy estimated at 5 to 10 megatons No expedition reached scene until about 20 years later Devastated area up to 40 km from the center. Originally thought to be a comet impact but now thought to be a stony asteroid about 50m in diameter traveling at about 20 km/sec. ½M V 2 = 10 megatons TNT A few minutes after the explosion The air explosion from 60 km away Paintings by William Hartmann artist and cratering expert 7
Comet Shoemaker-Levy Breakup and Impact into Jupiter July 1994 HST image just before last impacts showing impact scars Crater diameter approx. = 0.1 times the cube root of the energy E in kilotons Glenn Beck: The $53 trillion asteroid NEW YORK (CNN) -- Let's say a giant asteroid was headed toward Earth right now and experts say it has a good chance of ending civilization as we know it. Let's also say that we've known about this asteroid for years but even as it gets closer and closer our leaders do nothing. "Don't worry," they tell us, "The next administration will figure something out." Stardust STARDUST Mission to Comet Wild 2. Flyby Jan 2, 2004 Returned to Earth, Jan 15, 2006 Comet is 4.5 km in size. Originated in the Kuiper Belt 8
Stardust Stardust Stardust 9
Stardust landed gently Genesis did not 10
Further analysis found no primitive material i.e. material (interstellar dust) from which the Sun and solar system was made that was not heated by the forming Sun. Particle is ~10-5 of a meter in size, 3 components, ion sulphide, Magnesium silicate (Enstatite) and a fine grained material with a composition close to that of chrondites in meteors. The first two are the result of high temperature processing close to the forming Sun so particle a mix a materials that originated near and far from the Sun. Brownlee et al, Science, 314, 1711, 2006 Ishii et al, Science, 319, 447, Jan 2008 11
Deep Impact Deep Impact Impactor weighs 370 kg (about 800 lbs) and the impact velocity was ~10 km/sec. Kinetic energy (1/2 MV 2 ) was about 19 10 9 Joules = ~5 kilotons of TNT Deep Impact - January 12, 2005 12
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Final crater estimated to be about 100 m across and 30 m deep - implies very porous material with density about half that of water ice. Very fine grained particles reflected sunlight making it difficult to see the crater. 14
Mix of primitive material and material that underwent high temperature processing near the early sun as it formed. * Comet Churyumov-Gerasimenko 15
NEAR-EARTH ASTEROIDS Positions of all known asteroids projected onto the plane of the Earth s orbit on Jan 20, 2008 Red ones (Apollo and Alten) cross the orbit of Earth so are potentially hazardous. Yellow ones (Amors) don t cross Earth s orbit now but could in the future if their orbits are disturbed. 16