* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download The Main Points Asteroids
Survey
Document related concepts
Earth's rotation wikipedia , lookup
Planet Nine wikipedia , lookup
Exploration of Io wikipedia , lookup
Exploration of Jupiter wikipedia , lookup
Planets in astrology wikipedia , lookup
Juno (spacecraft) wikipedia , lookup
Comet Shoemaker–Levy 9 wikipedia , lookup
Definition of planet wikipedia , lookup
Sample-return mission wikipedia , lookup
Scattered disc wikipedia , lookup
Kuiper belt wikipedia , lookup
Jumping-Jupiter scenario wikipedia , lookup
Formation and evolution of the Solar System wikipedia , lookup
Asteroid impact avoidance wikipedia , lookup
Transcript
3/14/2008 The Main Points Lecture #22: Asteroids • Discovery/Observations Asteroids are small, rocky objects that exhibit surprisingly diverse compositions and surface geologies • Where are they? • How many are there? • What are they like? Most asteroids are found in the Main Belt between Mars and Jupiter or in the Kuiper Belt beyond Neptune, but many exist in near-Earth space too • Where did they come from? • Reading: Chapter 12.1 Astro 102/104 1 Astro 102/104 Asteroids 3 Asteroids are discovered in telescopic images because they move at a different speed across the sky than the stars • "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 "Vermin of the skies" (!) QuickTime™ and a GIF decompressor are needed to see this picture. – Ceres, about 940 km diameter • There may be 1,000,000+ >1 km diameter Astro 102/104 4 Astro 102/104 5 1 3/14/2008 The Largest Asteroids Where are the Asteroids? • Asteroids can be found throughout the solar system, but there are two main populations: • 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 – 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 • 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 Astro 102/104 – Trojans, Apollos, Amors, Centaurs, ... 6 Astro 102/104 The Main Asteroid Belt A snapshot of the locations of all known inner solar system asteroids as of March 2007 • 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 (outer circle is the orbit of Jupiter) • As of 2008, almost 200,000 main belt asteroids have well-known orbital parameters Astro 102/104 Edge on • The number has been recently increasing by a few thousand per year because of new telescopic9 search programs 8 Astro 102/104 http://cfa-www.harvard.edu/iau/lists/InnerPlot.html 10 2 3/14/2008 Gaps in the Belt... Astro 102/104 Other Asteroid Groups • Astronomer Daniel Kirkwood (1886) noticed that the main belt is not uniformaly populated • Asteroids "missing" f from places l where h 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... 11 A 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 by 100,000 or more Kuiper Belt Objects still to be discovered... The solar system does not end at Pluto! Astro 102/104 http://cfa-www.harvard.edu/iau/lists/OuterPlot.html 13 • Trojans – About 1000 asteroids leading and trailing Jupiter by 60° • Centaurs – About 100 asteroids found between Jupiter & Neptune • Kuiper Belt Objects – More than 1000 large asteroids beyond Neptune • Near-Earth Asteroids (NEAs) – Atens (6%): orbits interior to Earth's – Apollos (62%): orbits that cross Earth's – Amors (32%): orbits exterior to Earth's (e.g. 433 Eros) • All these populations are incompletely counted Astro 102/104 12 How Many Asteroids are There? Number of Near-Earth Asteroids (a model) Astro 102/104 • 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% mass of the 14 Moon 3 3/14/2008 Discovering 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 Dedicated search programs started in the last few years have located perhaps 70% of the >1 km Potentially Hazardous Asteroids (PHAs) that could pose a threat to life on Earth. • Tens of thousands of objects discovered, many NEAs... Astro 102/104 15 Astro 102/104 Asteroid Physical Properties • 16 Asteroid Composition • Spectroscopy and radar observations of asteroids reveal four main classes: Many diagnostic properties can be determined by telescopic remote sensing: – Albedo (% of reflected sunlight) – – – – • darkest are 1% to 2%, brightest are 40% to 50% – Spin period (length of an asteroid "day") – Shape Sh (f (from radar d & variations i ti in i the th light li ht curve)) 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 primitive (?) • M Types and other anomalous classes (like Vesta) from differentiated parent bodies Astro 102/104 17 Astro 102/104 19 4 3/14/2008 (controversial!) Asteroid Geology • 4 asteroids visited up close by spacecraft – – – – • Also: Spacecraft images of Martian moons Phobos and Deimos: captured asteroids? • Abundant evidence for impacts, and surprising evidence for erosion and tectonism on these bodies • 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! Astro 102/104 951 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 20 Astro 102/104 951 Gaspra S Type 16 x 11 x 10 km 253 Mathilde C Type 66 x 48 x 46 km (large!) Many large craters Density only 1.3 g/cm3 May be 50% "empty" 243 Ida S Type 60 x 25 x 19 km Dactyl Ida's moon! D = 1.5 km Astro 102/104 21 22 Astro 102/104 433 Eros S Type 31 x 13 x 13 km Many large craters Density about 2.6 g/cm3 Geology! 23 5 3/14/2008 Tectonic ridges: Eros appears to have been compressed and possibly even twisted during its history... Fine-grained regolith appears to be moving downhill on Eros: erosion! Last image from NEAR before its landing: very finegrained regolith! (Compare with basket ball) Eros movies from NEAR • craters, boulders, & more Astro 102/104 24 Astro 102/104 Summary Next Lecture… • Meteorites – Samples from the Early Solar System – Age, g , origin g and evolution – Classification – Parent Bodies: Asteroids & Planets • Reading • Asteroids are small, rocky, primitive objects thought to be remnants from the formation of the solar system • There are probably more than 1 million asteroids > 1 km in size in the solar system, but their total mass is only a small fraction of the mass of the Moon • Remote sensing shows that not all asteroids are alike – Different compositional classes (C, S, D, M) – Some asteroids have had complex internal histories • Spacecraft observations show that asteroids have interesting and complex geologic histories: impacts, tectonics, erosion Astro 102/104 25 26 – 12.1 Astro 102/104 27 6