Linda T. Elkins-Tanton, Ph.D., produced this book in 2006. It discusses the solar system bodies that are not one of the eight planets or their moons. This field is moving so quickly that it has affected the whole of planetary science: The great numbers and significant sizes of the bodies being found in the distant Kuiper belt have forced a redefinition of what a planet is, effectively demoting Pluto, and are changing the theories of solar system formation, as well as inspiring a new generation of space missions to the outer solar system.
This revised 2010 edition covers the exciting new science concerning the massive collection of smaller bodies orbiting the Sun, including asteroids in the main asteroid belt as well as elsewhere throughout the solar system, comets both from the Kuiper belt and from the much-more-distant Oort cloud, and the interplanetary dust left in their wakes. Because for so many centuries mankind could only differentiate the largest of the planets from the background of stars, a culture has developed that thinks of the solar system as a Sun orbited by nine (now eight, with Pluto’s demotion) planets. Increasingly, as observers’ abilities to see smaller and smaller bodies in the solar system improves because of better instrumentation, and as scientists continue trying to catalog the number of large asteroidal bodies that might someday collide with Earth, the solar system is viewed as a collection of objects with a whole continuum of sizes.
The Sun is orbited by material that ranges in size from grains of interplanetary dust to the giant Jupiter, and the planets in their turn are orbited by moons from less than a kilometer in radius to the mammoth Ganymede with a radius of 1,645 miles (2,631 km). The new data and images from the astonishingly successful Deep Impact and Stardust missions are one reason for adding the great asset of full-color photos to this edition. The feats of these missions—sending an impactor into a comet to discover the physical and chemical characteristics of its crust and interior and capturing particles from a comet’s tail and returning them safely to Earth—have changed scientists’ views on solar system formation and the nature of comets. Stardust brought us images of tiny particles that made up our solar system, the Antarctic Meteorite Search brings back hundreds or even thousands of meteorites from Antarctica every year, and continuing surveys of the skies discover new asteroids, comets, and Kuiper belt and Oort cloud objects. All of these small bodies inspire new knowledge of the solar system formation and also offer spectacular images that we have used for this volume.
Both the small size of some of these objects and the extreme distance of others prevented their discovery for centuries, but their inventory grows almost daily as new sightings are made. In the 19th century when asteroids were first discovered, the continuum of sizes in the solar system was not understood. Scientists and the populace widely believed that stable planetary orbits precluded the possibility of swarms of smaller orbiting objects, and, when Ceres and the other early asteroids were discovered, they were at first classified as planets. When scientists realized with a shock that these first large asteroids were just a part of a huge population they were demoted from planet status. Asteroids now are commonly known as minor planets. In chapter 1 the discoveries of the asteroids are described, along with the long centuries of argument over the nature of meteorites and impact craters.
The process of discovering and understanding these small bodies is an important part of their story. The centuries of disbelief that rocks (meteorites) could fall from the sky prevented their accurate description as well as any understanding of the process of meteorite impacts. Understanding the immense damage possible from a large meteorite impact is new to the latter part of the 20th century. The current state of understanding of impacts is discussed in chapter 2. Most of Earth’s impact craters are heavily weathered and therefore difficult to study, while fresher impact craters are found on the Moon and to a lesser extent on Mars. The fresh craters on these and other bodies (such as Saturn’s moon Dione, shown in the photo at right) form a great natural laboratory for studying the processes and effects of impacts. Because large impacts are the only natural disaster known that can cause global extinctions, the possibility of future impacts on Earth is a significant concern.
Meteorites fall to Earth at an astonishing rate, thousands per year, with a combined weight of at least several tons per year. Few are found, because at first glance meteorites look much like Earth rocks. Over time, though, enough meteorites have been collected so that their compositions have been grouped into classes. At the same time, discoveries of asteroids have accelerated, and some information about their compositions has been obtained by remote sensing as well as by a few space missions. In chapter 3, the orbits, sizes, and compositions of asteroids are discussed, along with the correlations scientists have been able to make between meteorite classes and asteroids. Meteorites are now known to be small fragments of asteroids that have fallen into Earth’s gravity field. Though the majority of asteroids orbit in the main asteroid belt between Mars and Jupiter, scientists have found a number of additional populations elsewhere in the solar system.
A number of astronomers and planetary scientists dedicate their time to finding and cataloging asteroids that might in future strike the Earth. Other astronomers are focusing on the outer solar system, building the database on the Kuiper belt. For example, Saturn’s moon Dione shows many impact craters in this mosaic image of Voyager 1 photographs. The largest crater is about 60 miles (100 km) in diameter and shows a central peak.
In early 2004, one such search found a still more distant object: the first body within the Oort cloud, what had until then been a theoretical population of icy bodies orbiting from 70 to 30,000 times as far from the Sun as is the Earth. The new farthest-known object in the solar system has been named Sedna, after an Inuit goddess. These distant populations of icy bodies are thought to be the sources of comets. The distinctive bright tail of a comet consists of gases that are boiled from their frozen state by the heat of the Sun as the comet travels the part of its orbit closest to the Sun. Comets are the subject of chapter 4, which covers what is known about these enigmatic bodies, including hypotheses about where they originate in the solar system, how long they last, and what happens when their orbiting ends.
The study of the smaller bodies in the solar system is among the most active of planetary sciences. Their study has only been possible as increasingly good instrumentation provides new information. Information from recent space missions to asteroids and comets has shown the basic similarities between the two kinds of bodies, in particular the unexpected rocky strength of a cometary body and the ability of some asteroids to develop cometary tails during parts of their orbits. Beyond their great importance as possible Earth impactors, all the small bodies provide information about the early formation processes of the solar system, since they have gone through little later processing. This volume describes a continually changing body of information about the primordial matter that the solar system formed from, the great quantity of small bodies that orbit among and threaten the larger planets, from inner-solar-system asteroids to the most remote icy bodies at the outer edges of the solar system, partway to the nearest stars.
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