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Solar System Debris Asteroids, Comets, Meteors, Oh My! 3B Asteroids • Asteroids are small, irregular, rocky bodies that orbit the sun • Most are located between he orbits of Mars and Jupiter in the asteroid belt • Some are located in the same orbit as Jupiter, but are separated by 60º - Trojans • Some asteroids are in highly elliptical orbits that pass earth – Earth-crossing asteroids, Near-earth asteroids (NEAs), Near-earth objects (NEOs) 3B What do asteroids look like? • Too small to be seen with the naked eye – need large telescopes • Appear as streaks because of their motion • Three fly-bys of asteroids: – Gaspra by Galileo in 1998 – Ida by Galileo in ???? – Mathilde by NEAR in 1999 3B What are asteroids made of? • Asteroids are classified into a number of types according to their spectra (and hence their chemical composition) and albedo: 1. C-type, includes more than 75% of known asteroids: – extremely dark (albedo 0.03) – approximately the same chemical composition as the Sun minus hydrogen, helium and other volatiles 2. S-type, 17% – relatively bright (albedo .10-.22) – metallic nickel-iron mixed with iron- and magnesiumsilicates 3. M-type, most of the rest – bright (albedo .10-.18) – pure nickel-iron 4. There are also a dozen or so other rare types 3B Asteroids • How many are there? – Some 5000 with well-determined orbits – Probably >500,000 larger than 1 km • How big are they? – – – – Largest (Ceres) is 940 km in diameter Three larger than 500 km About a dozen larger than 250 km Number increases rapidly with decreasing size • Total mass of all asteroids ~ 5% of the moon 3B Eros 3B More Eros 3B Eros Landing 3B Comets • Comets are small bodies of ice and dust that form tails as they near the sun. • What way is this comet moving? • Recall Kepler’s second law. Where do comets spend most of their time? 3B What are comets made of? • When they are near the Sun and active, comets have several distinct parts: – nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solids; – coma: dense cloud of water, carbon dioxide and other neutral gases sublimed from the nucleus; – hydrogen cloud: huge (millions of km in diameter) but very sparse envelope of neutral hydrogen; – dust tail: up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the unaided eye; – ion tail: as much as several hundred million km long composed of plasma and laced with rays and streamers caused by interactions with the solar wind. 3B Comet Parts 3B Where do comets live? • Short period comets come from the Kuiper belt • Long period comets from the Oort cloud 3B What happens when comets loose their tails? • Meteor showers 3B Meteorites Three main types: • Stony – Look like ordinary earth rocks at first glance. If you were to look closely, you would see that the outsides of the rocks look melted. Most meteorites (93%) striking the earth are stony • Irons – These dark grey, metallic, and very dense meteorites are unlike any earth rocks that you have ever seen. Five percent of meteorites striking the earth are irons • Stony-Irons – These meteorites have clumps of both iron and stone 3B Asteroid – Meteorite Connection • Iron – similar to type M asteroids • Stony Iron – like type S asteroids • Stony – Chondrite • by far the largest number of meteorites fall into this class; similar in composition to the mantles and crusts of the terrestrial planets – Carbonaceous Chondrite • similar to type C asteroids – Achondrite • similar to terrestrial basalts 3B Finding Meteorites • Finds – Found on earth, often long after they landed – usually noticed because of their unusual appearance or their unusual location • Falls – These meteorites are seen falling, making a fireball in the sky. By following the trail, the landing site and meteorite can be found. Most of these are stony meteorites Peekskill, NY 1994 3B Impacts • What happens when a meteor hits the ground? – Usually nothing – Occasionally, large pieces make it to the ground and create impact craters 3B Impact craters 3B Impact Hazards • No person has ever been killed by a meteorite hitting earth • However, a large impact killed the dinosaurs • A smaller impact in remote Russia in 1908 knocked down trees and could have caused severe damage in a populated area 3B Civilization Threatening Impact Mass Extinction Impact Earth Sterilizing Impact 3B Terrestrial Impact Frequency Hiroshima year Tunguska century Tsunami danger ten thousand yr. Global catastrophe million yr. K/T billion yr. 0.01 1 100 10,000 million 100 million TNT equivalent yield (MT) 3B