Lecture 3: Overview of Asteroids and MeteoritesAstro 202Prof. Jim Bell ([email protected])Spring 2008But first...A few words about Referencing in science writing...(see http://astrosun2.astro.cornell.edu/academics/courses/a202/referencing.html)Paper 2 is “Handed Out” onlineREMINDER: Paper 1 due at beginning of class this Thursday, Jan. 30Astro202 3Asteroids...Space junk...? Rosetta Stones...? or Harbingers of DOOM?Astro202 4Asteroids!"Asteroid" is Greek for "star-like"!Asteroids are small, rocky objects that look star-like in telescopes, except they move across the sky like the planets do!The first asteroid was discovered in 1801– Ceres, about 940 km diameter!There may be 1,000,000+ >1 km diameterAstro202 5Asteroids are discovered in telescopic images because they move at a different speed across the sky than the stars"Vermin of the skies" (!)Astro202 6The Largest Asteroids!1 Ceres: Diam. = 940 km, a = 2.77 AU– Discovered by Guiseppe Piazzi in 1801– Piazzi was searching for a "missing planet" between Mars and Jupiter!2 Pallas: D=540 km, a=2.77 AU, disc. 1802!4 Vesta: D=510 km, a=2.36 AU, disc. 1807!13 main belt asteroids have D > 250 kmAstro202 7Where are the Asteroids?!Asteroids can be found throughout the solar system, but there are two main populations:– The Main Belt between Mars and Jupiter– The Kuiper Belt beyond Neptune's orbit!Most of the asteroids with well-determined orbits are in the main belt!Other smaller populations exist too– Trojans, Apollos, Amors, Centaurs, ...Astro202 8The Main Asteroid Belt• The main belt extends from about 2.2 to 3.3 AU• Most of the orbits lie at or near (±10° to 20°) the plane of the ecliptic• More than 100,000 main belt asteroids have well-known orbital parameters• The number has been recently increasing by a few thousand per year because of new telescopic search programsEdge onAstro202 9A snapshot of the locations of all known inner solar system asteroids as of today(Jan. 29, 2008)(outer circle is the orbit of Jupiter)http://cfa-www.harvard.edu/iau/lists/InnerPlot.htmlAstro202 10Gaps in the Belt...• Astronomer Daniel Kirkwood (1886) noticed that the main belt is not uniformly populated• Asteroids "missing" from places where disturbances by Jupiter are strongest• These places are where resonances with Jupiter's orbit occur• May explain why no planet here: Jupiter only allowed small bodies to coalesce...Astro202 11Other Asteroid Groups!Near-Earth Asteroids (NEAs)– 436 Atens: orbits interior to Earth's– 2489 Apollos: orbits that cross Earth's– 2161 Amors: orbits exterior to Earth's (e.g. 433 Eros) In the news...Asteroid 2007 WD5: A Mars impactor?http://neo.jpl.nasa.gov/news/news151.htmlTomorrow...Astro202 13Other Asteroid Groups!Near-Earth Asteroids (NEAs)– 436 Atens: orbits interior to Earth's– 2489 Apollos: orbits that cross Earth's– 2161 Amors: orbits exterior to Earth's (e.g. 433 Eros) !Trojans– 2319 asteroids leading and trailing Jupiter by 60°– There are even a few Mars (4) and Neptune (5) Trojans!Centaurs– 67 asteroids found between Jupiter & Neptune!Trans-Neptunian or “Kuiper Belt” Objects– More than 1000 large asteroids beyond Neptune !All these populations are incompletely countedAstro202 14A snapshot of the locations of all known outer solar system asteroids as of today(outer circle is the orbit of Neptune)Astronomers believe that there may be 100,000 or more Kuiper Belt Objects still to be discovered...The solar system does not end at Pluto!http://cfa-www.harvard.edu/iau/lists/OuterPlot.htmlPlutoNeptuneAstro202 15How Many Asteroids are There?• The number of asteroids of a given diameter D is proportional to 1/D2: a collisional distribution• For example:! ! 3 > 500 km! ! 13 > 250 km! ! hundreds > 100 km! ! 10,000+ > 10 km (?)• Total: >1,000,000 > 1 km (?)• Most of the MASS is in the largest few asteroids• Total mass of all asteroids is only ~5-10% the mass of the MoonNumber ofNear-Earth Asteroids(a model)Astro202 16Discovering Asteroids!For many years, most discoveries were random!Within the past decade, dedicated surveys have begun in response to the possible threat of asteroid and comet impacts on the Earth– NASA Spacewatch Program– Air Force Near Earth Asteroid Tracking Program (NEAT)– Lowell Observatory Near Earth Object Survey (LONEOS)– Lincoln (MIT) Near Earth Asteroid Research Project (LINEAR)– Catalina (Arizona) Sky Survey!Hundreds of thousands of objects discovered, many are Near-Earth Asteroids (NEAs)...Astro202 17Astronomers believe that dedicated search programs started in the last decade have located about 80%-90% of the >1 km Potentially Hazardous Asteroids (PHAs) that could pose a threat to life on Earth...http://neo.jpl.nasa.gov/statsAstro202 18Asteroid Physical Properties!Many diagnostic properties can be determined by telescopes:– Albedo (% of reflected sunlight)• darkest are 1% to 2%, brightest are 40% to 50%– Spin period (length of an asteroid "day")– Shape (from radar & variations in the light curve)Some asteroid sizes have also been measured directly...(from NSS Chapter 25)Astro202 20Asteroid Composition!Spectroscopy and radar observations of asteroids reveal four main classes:– C Type: dark, "carbon rich", albedo ~5%– S Type: brighter, "stony", albedo ~20%– D Type: dark, very red spectra– M Type: rare "metallic" type, very radar bright !C,S,D Types probably all/mostly primitive (?)!M Types and other anomalous classes (like Vesta) from differentiated parent bodiesAstro202 21• Composition varies systematically with distance from the Sun• Implies that asteroids preserve the conditions of planetary formation in the early solar system• But primitive vs. differentiated issue is controversial!(controversial!)Astro202 22Asteroid Geology!4 asteroids visited up close by spacecraft– 951 Gaspra: Galileo flyby in 1991– 243 Ida: Galileo flyby in 1993– 253 Mathilde: NEAR flyby in 1997– 433 Eros: NEAR orbital mission in 2000-2001!Also: Spacecraft images of Martian moons Phobos and Deimos: captured asteroids?!Abundant evidence for impacts, and surprising evidence for erosion and tectonism on these bodiesAstro202 23951 GaspraS Type16 x 11 x 10 km 243 IdaS Type60 x 25 x 19 kmDactylIda's moon!D = 1.5 kmAstro202 24253 MathildeC Type66 x 48 x 46 km (large!)Many large cratersDensity only 1.3 g/cm3May be 50% "empty"433 ErosS Type31 x 13 x 13 kmMany large cratersDensity about 2.6
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