Roger Freedman • Robert Geller • William Kaufmann III Universe Tenth Edition Chapter 15 Vagabonds of the Solar System Comet Hale-Bopp, the Great Comet of 1997 By reading this chapter, you will learn • 15-1 What led to the discovery of the asteroids • 15-2 Why the asteroids never formed into a planet • 15-3 What asteroids look like • 15-4 How an asteroid led to the demise of the dinosaurs • 15-5 What meteorites tell us about the nature of asteroids • 15-6 What meteorites may reveal about the origin of the solar system • 15-7 What comets are and why they have tails • 15-8 How comets originate from the outer solar system Ceres Compared with Earth and Moon 15-1: A search for a planet between Mars and Jupiter led to the discovery of asteroids The Trail of an Asteroid The Asteroid Belt The Asteroid Belt 15-2: Jupiter’s gravity helped shaped the asteroid belt The Kirkwood Gaps The Kirkwood Gaps A Hubble Space Telescope View of a Large Asteroid 15-3: Astronomers use a variety of techniques to study asteroids A Radar View of a Medium-Sized Asteroid A Spacecraft View of a Small Asteroid Sample Returned from Itokawa Vesta Impact on Vesta Impact on Vesta An Asteroid with Two Satellites Asteroid Collisions Asteroid Collisions Asteroids Outside the 15-4: Asteroids are found outside the asteroid belt – and have struck Earth The Barringer Crater Iridium-Rich Clay Confirming an Extinction-Level Impact Site Aftermath of the Tunguska Event Russian Asteroid Impact of 2013 Russian Asteroid Impact of 2013 Russian Asteroid Impact of 2013 A Meteor A Meteorite “Fender Bender” 15-5: Meteorites are classified as stones, stony irons or irons depending on their composition Stony Meteorites Stony Meteorites Stony Meteorites A Stony Iron Meteorite Iron Meteorites Iron Meteorites Iron Meteorites 15-6: Some meteorites retrain traces of the early solar system A Piece of the Allende Meteorite 15-7: A comet is a chunk of ice and dust that partially vaporizes as it passes near the Sun Comet Hyakutake The Structure of a Comet Comet Nuclei and Jets Comet Nuclei and Jets Comet Wild 2 Comet Wild 2 Comet Wild 2 Comet Tempel A Comet and Its Hydrogen Envelope A Comet and Its Hydrogen Envelope The Orbit and Tail of a Comet The Two Tails of Comet Hale-Bopp The Antitail of Comet Hale-Bopp Comet Halley’s Eccentric Orbit 15-8: Comets originate either from the Kuiper belt or from the Oort Cloud The Kuiper Belt and the Oort Transforming a Comet Orbit Transforming a Comet Orbit The Fragmentation of Comets The Fragmentation of Comets Comet Shoemaker-9 and Its Encounter with Jupiter Meteoritic Storms Meteoritic Storms Asteroid 243 Ida and Its Tiny Moon Dactyl Key Ideas • Discovery of the Asteroids: Astronomers first discovered the asteroids while searching for a “missing planet.” • Thousands of asteroids with diameters ranging from a few kilometers up to 1000 kilometers orbit within the asteroid belt between the orbits of Mars and Jupiter. • Origin of the Asteroids: The asteroids are the relics of planetesimals that failed to accrete into a full-sized planet, thanks to the effects of Jupiter and other Mars-sized objects. • Even today, gravitational perturbations by Jupiter deplete certain orbits within the asteroid belt. The resulting gaps, called Kirkwood gaps, occur at simple fractions of Jupiter’s orbital period. • Jupiter’s gravity also captures asteroids in two locations, called Lagrangian points, along Jupiter’s orbit. Key Ideas • Asteroid Collisions: Asteroids undergo collisions with each other, causing them to break up into smaller fragments. • Some asteroids, called near-Earth objects, move in elliptical orbits that cross the orbits of Mars and Earth. If such an asteroid strikes the Earth, it forms an impact crater whose diameter depends on both the mass and the speed of the asteroid. • An asteroid struck the Earth 65 million years ago, possibly causing the extinction of the dinosaurs and many other species. Key Ideas • Meteoroids, Meteors, and Meteorites: Small rocks in space are called meteoroids. If a meteoroid enters the Earth’s atmosphere, it produces a bright trail called a meteor (or shooting star). A rock that does not burn up in the atmosphere and reaches Earth’s surface is called a meteorite; most of the meteoroids are too small to make it to Earth before becoming completely vaporized. • Meteorites are grouped into three major classes, according to composition: iron, stony iron, and stony meteorites. Irons and stony irons are fragments of the core of an asteroid that was large enough and hot enough to have undergone chemical differentiation, just like a terrestrial planet. Some stony meteorites come from the crust of such differentiated meteorites, while others are fragments of small asteroids that never underwent differentiation. Key Ideas • Rare stony meteorites called carbonaceous chondrites may be relatively unmodified material from the solar nebula. These meteorites often contain organic material and may have played a role in the origin of life on Earth. • Analysis of isotopes in certain meteorites suggests that a nearby supernova may have triggered the formation of the solar system 4.56 billion years ago. Key Ideas • Comets: A comet is a chunk of ice with embedded rock fragments that generally moves in a highly elliptical orbit about the Sun. • As a comet approaches the Sun, its icy nucleus develops a luminous coma, surrounded by a vast hydrogen envelope. An ion tail and a dust tail extend from the comet, pushed away from the Sun by the solar wind and radiation pressure, respectively. • Origin and Fate of Comets: Comets are thought to originate from two regions, the Kuiper belt and the Oort cloud. Key Ideas • The Kuiper belt lies in the plane of the ecliptic at distances between 30 and 50 AU from the Sun. Its icy objects are thought to be the source of short-period comets, whose periods are less than 200 years. • The Oort cloud is thought to contain billions of icy objects in a spherical distribution that extends out to 50,000 AU from the Sun. The long period comets, with periods greater than 200 years are thought to originate in the Oort cloud. • Fragments of “burned out” comets produce meteoritic swarms. A meteor shower is seen when the Earth passes through a meteoritic swarm.