Lecture 3: Overview of But first... Asteroids and Meteorites Astro 202 A few words about Referencing in science writing... Prof. Jim Bell ([email protected]) (see http://astrosun2.astro.cornell.edu/academics/courses/a202/referencing.html) Spring 2008 Paper 2 is “Handed Out” online REMINDER: Paper 1 due at beginning of class this Thursday, Jan. 30 Asteroids... Asteroids !"Asteroid" is Greek for "star-like" Space junk...? !Asteroids are small, rocky objects that look star-like in telescopes, except they move Rosetta Stones...? across the sky like the planets do !The first asteroid was discovered in 1801 – Ceres, about 940 km diameter or Harbingers of DOOM? !There may be 1,000,000+ >1 km diameter Astro202 3 Astro202 4 Asteroids are discovered in telescopic images because they move at a different The Largest Asteroids speed across the sky than the stars !1 Ceres: Diam. = 940 km, a = 2.77 AU – Discovered by Guiseppe Piazzi in 1801 "Vermin of the skies" (!) – 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 km Astro202 5 Astro202 6 Where are the Asteroids? The Main Asteroid Belt • The main belt extends from !Asteroids can be found throughout the solar about 2.2 to 3.3 AU system, but there are two main populations: • Most of the orbits lie at or near – The Main Belt between Mars and Jupiter (±10° to 20°) the plane of the – The Kuiper Belt beyond Neptune's orbit ecliptic !Most of the asteroids with well-determined • More than 100,000 main belt asteroids have well-known orbits are in the main belt orbital parameters !Other smaller populations exist too • The number has been recently – Trojans, Apollos, Amors, Centaurs, ... increasing by a few thousand per year because of new telescopic search programs Astro202 7 Astro202 Edge on 8 A snapshot of the locations of all known inner solar Gaps in the Belt... system asteroids as of today • Astronomer Daniel (Jan. 29, 2008) Kirkwood (1886) noticed that the main belt is not uniformly (outer circle is the populated orbit of Jupiter) • 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 http://cfa-www.harvard.edu/iau/lists/InnerPlot.html 9 Astro202 10 In the news... Other Asteroid Groups Asteroid 2007 WD5: A Mars impactor? ! 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) Astro202 11 http://neo.jpl.nasa.gov/news/news151.html Tomorrow... A snapshot of the locations of all Other Asteroid Groups known outer solar system asteroids ! Near-Earth Asteroids (NEAs) as of today – 436 Atens: orbits interior to Earth's (outer circle is the – 2489 Apollos: orbits that cross Earth's orbit of Neptune) – 2161 Amors: orbits exterior to Earth's (e.g. 433 Eros) Astronomers !Trojans believe that there – 2319 asteroids leading and trailing Jupiter by 60° may be 100,000 – There are even a few Mars (4) and Neptune (5) Trojans or more Kuiper ! Centaurs Belt Objects still – 67 asteroids found between Jupiter & Neptune to be discovered... ! Trans-Neptunian or “Kuiper Belt” Objects The solar system – More than 1000 large asteroids beyond Neptune Pluto Neptune does not end at ! All these populations are incompletely counted Pluto! Astro202 13 Astro202 http://cfa-www.harvard.edu/iau/lists/OuterPlot.html 14 How Many Asteroids are There? Discovering Asteroids • The number of asteroids of a given ! For many years, most discoveries were random diameter D is proportional to 1/D2: ! Within the past decade, dedicated surveys have a collisional distribution begun in response to the possible threat of asteroid • For example: ! ! 3 > 500 km and comet impacts on the Earth ! ! 13 > 250 km – NASA Spacewatch Program ! ! hundreds > 100 km – Air Force Near Earth Asteroid Tracking Program (NEAT) ! ! 10,000+ > 10 km (?) – Lowell Observatory Near Earth Object Survey (LONEOS) Number of • Total: >1,000,000 > 1 km (?) – Lincoln (MIT) Near Earth Asteroid Research Project (LINEAR) Near-Earth Asteroids • Most of the MASS is in the largest – Catalina (Arizona) Sky Survey (a model) few asteroids ! Hundreds of thousands of objects discovered, many are • Total mass of all asteroids is only Near-Earth Asteroids (NEAs)... ~5-10% the mass of the Moon Astro202 15 Astro202 16.