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As shown in this graph of number of moons versus planets, the large outer planets have far more moons than the smaller, inner planets or the dwarf planet, Pluto.

Every day new data are streaming back to Earth from space missions to Mars. Early in 2004, scientists proved that there was once standing liquid water on Mars. Another unmanned mission, this time to a comet, determined that the material in a comet's nucleus is as strong as some rocks and not the loose pile of ice and dust expected. Information streams in from space observations and Earth-based experiments, and scientists attempt to explain what they see, producing an equivalent stream of hypotheses about the formation and evolution of the solar system and all its parts.

In this age of constant space missions and discoveries, how can a printed book on the solar system be produced that is not instantly outdated? New hypotheses are typically not accepted immediately by the scientific community. The choice of a leading hypothesis among competing ideas is really a matter of opinion, and arguments can go on for decades. Even when one idea has reached prominence in the scientific community, there will be researchers who disagree with it. At every point along the way, though, there are people writing books about science. Once an explanation reaches the popular press, it is often frozen as perpetual truth and persists for decades, even if the scientific community has long since abandoned that theory.

In this set, some statements will be given as facts: the gravitational acceleration of the Earth, the radius of Mars, the height of prominences from the Sun, for instance. Almost everything else is open to argumentation and change. The numbers of moons known to be orbiting Jupiter and Saturn, for example, are increasing every year as observers are able to detect smaller and dimmer objects. These volumes will present some of the thought processes that have brought people to their conclusions (for example, why scientists state that the Sun is fueled by nuclear reactions), as well as observations of the solar system for which no one has a satisfactory explanation (such as why there is no detectable heat flow out of the gas giant planet Uranus). Science is often taught as a series of facts for memorization—in fact, not until the second half of a doctoral degree do many scientists learn to question all aspects of science, from the accepted theory to the data itself. Readers should feel empowered to question every statement.

The Solar System set explores the vast and enigmatic Sun at the center of the solar system and also covers the planets, examining each and comparing them from the point of view of a planetary scientist. Space missions that produced critical data for the understanding of solar system bodies are introduced in each volume, and their data and images shown and discussed.The volumes The Sun, Mercury, andVenus;The Earth and the Moon; and Mars place emphasis on the areas of unknowns and the results of new space missions. The important fact that the solar system consists of a continuum of sizes and types of bodies is stressed in Asteroids, Meteorites, and Comets. This book discusses the roles of these small bodies as recorders of the formation of the solar system, as well as their threat as impactors of planets. In Jupiter and Saturn, the two largest planets are described and compared. In the final volume, Uranus, Neptune, Pluto, and the Outer Solar System, Pluto is presented not as the final lonely dwarf planet but as the largest known of a extensive population of icy bodies that reach far out toward the closest stars, in effect linking the solar system to the galaxy itself.

In this set we hope to change the familiar litany Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto into a more complex understanding of the many sizes and types of bodies that orbit the Sun. Even a cursory study of each planet shows its uniqueness along with the great areas of knowledge that are unknown.These titles seek to make the familiar strange again.

Acknowledgments

Pore,,«, profound thanks to the following organizations for the great science and adventure they provide for humankind and, on a more prosaic note, for allowing the use of their images for these books: the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA), in conjunction with the Jet Propulsion Laboratory (JPL) and Malin Space Science Systems (MSSS). A large number of missions and their teams have provided invaluable data and images, including the Solar and Heliospheric Observer (SOHO), Mars Global Surveyor (MGS), Mars Odyssey, the Mars Exploration Rovers (MERs), Galileo, Stardust, Near-Earth Asteroid Rendezvous (NEAR), and Cassini. Special thanks to Steele Hill, SOHO Media Specialist at NASA, who prepared a number of images from the SOHO mission, to the astronauts who took the photos found at Astronaut Photography of the Earth, and to the providers of the National Space Science Data Center, Great Images in NASA, and the NASA/JPL Planetary Photojournal, all available on the Web (addresses given in the reference section).

Many thanks also to Frank K. Darmstadt, executive editor at Chelsea House; to Amy L. Conver, copy editor; to Jodie Rhodes, literary agent; and to E. Marc Parmentier at Brown University for his generous support.

Introduction ecause for so many centuries humankind 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 planets. Increasingly, as observers' abilities to see smaller and smaller bodies in the solar system improves because of better instrumentation, and as scientists continue cataloging the number of large asteroidal bodies that could potentially 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).

Asteroids, Meteorites, and Comets discusses the solar system bodies that are not one of the eight planets or their moons. This massive collection of smaller bodies orbiting the Sun includes 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 wake, as shown in the figure on page xii.

Both the small size of some of these objects and the extreme distance of others prevented their discovery for centuries, and their inventory grows almost daily with new sightings. 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 that these first large asteroids were just a part of a huge population, their classification was changed. Asteroids are now commonly known as minor planets or in some cases, dwarf planets. In chapter 1, the discoveries of the asteroids

All Orbits: Asteroid Belt, Kuiper Belt, Oort Cloud

Asteroid belt Mercury Venus Earth Mars

Asteroid belt Mercury Venus Earth Mars

Orbits of some of the major concentrations of small bodies: the asteroid belt, Kuiper belt, and Oort cloud. All the orbits are far closer to circular than shown in this oblique view, which was chosen to show the inclination of Pluto's orbit to the ecliptic.

Saturn's moon Dione shows many impact craters in this mosaic image of Voyager 1 photographs. The largest crater is about 60 miles (ioo km) in diameter and shows a central peak.. (NASA/JPL/Voyager i)

are described, along with the 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 fresh 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 here) form a great natural laboratory for studying the processes and effects of impact. 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.

Saturn's moon Dione shows many impact craters in this mosaic image of Voyager 1 photographs. The largest crater is about 60 miles (ioo km) in diameter and shows a central peak.. (NASA/JPL/Voyager i)

Meteorites fall to Earth at an astonishing rate, probably thousands per year. Few of those that fall are found because to a first approximation, meteorites look much like Earth rocks. Over time, enough meteorites have been collected 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 large number of astronomers and planetary scientists dedicate their time to finding and cataloging asteroids that might in the future strike the Earth. Other astronomers are focusing on the outer solar system, building the database on the Kuiper belt. 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—30,000 times as far from the Sun as the Earth is. 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.

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 undergone little change.This volume describes a continually changing body of information about the primordial matter from which the solar system formed, the great quantity of small bodies that orbit among and threaten the larger planets, from inner solar system asteroids to icy bodies at the outer edges of the solar system, partway to the nearest stars.

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