Vb Ch

This book is printed on acid-free paper.

Contents

Preface v

Acknowledgments x

Introduction xi

1. Definitions and Discoveries of Small Solar System Bodies 1

What Are Asteroids, Comets, Meteorites, and Meteors? 1

Discovering the Asteroids 5

Discovering Planets:The Titius-Bode Law 6

The Long Debate over the Nature of Meteorites 16

The Slow History of Recognizing Impact Craters 20

2. Processes and Effects of Impacts on Planets 35

The Process of Small Meteorite Falls 35

Terminal Velocity 36

The Late Heavy Bombardment 40

The Process of Giant Meteorite Falls 42

Hypervelocity Impacts on Earth 53

3. Asteroids and Meteorites 61

Where Do Asteroids Orbit? 63

Asteroids in the Inner Solar System 63

Describing Orbits 66

Inner Planet-Crossing Asteroids: The Aten, Amor, and Apollo Families . . . .69

The Main Asteroid Belt 74

Asteroids in the Outer Solar System 79

How Are Meteorites and Asteroids Related? 84

Chondrites: Primordial Material 97

Presolar Grains 104

Achondrites 106

Iron Meteorites 112

Stony-Iron Meteorites 115

Compositions of Asteroids 117

Spectrophotometry and Mineral Absorption Bands 118

4. Comets and Other Distant Bodies 123

What Are Their Orbits? 132

Short-Period Comets 135

Long-Period Comets 148

5. Meteor Showers 153

6. Missions to Comets and Asteroids 155

7. Conclusions: The Known and the Unknown 163

Appendix 1: Units and Measurements 167

Fundamental Units 167

Comparisons among Kelvin, Celsius, and Fahrenheit 169

Useful Measures of Distance 171

Definitions for Electricity and Magnetism 175

Prefixes 178

Appendix 2: Light,Wavelength, and Radiation 179

Appendix 3: A List of All Known Moons 188

Glossary 190

Bibliography and Further Reading 198

Internet Resources 199

Organizations of Interest 201

Index 203

Preface

The planets Mercury,Venus, Mars, Jupiter, and Saturn—all visible to the naked eye—were known to ancient peoples. In fact, the Romans gave these planets their names as they are known today. Mercury was named after their god Mercury, the fleet-footed messenger of the gods, because the planet seems especially fast moving when viewed from Earth.Venus was named for the beautiful goddess Venus, brighter than anything in the sky except the Sun and Moon. The planet Mars appears red even from Earth and so was named after Mars, the god of war. Jupiter was named for the king of the gods, the biggest and most powerful of all, and Saturn was named for Jupiter's father. The ancient Chinese and the ancient Jews recognized the planets as well, and the Maya (250—900 c.e., Mexico and environs) and Aztec (ca. 1100—1700 c.e., Mexico and environs) called the planet Venus "Quetzalcoatl," after their god of good and light.

These planets, small and sometimes faint in the night sky, commanded such importance that days were named after them. The seven-day week originated in Mesopotamia, which was perhaps the world's first organized civilization (beginning around 3500 B.C.E. in modern-day Iraq). The Romans adopted the seven-day week almost 4,000 years later, around 321 c.e., and the concept spread throughout western Europe. Though there are centuries of translations between their original names and current names, Sunday is still named for the Sun, Monday for the Moon, Tuesday for Mars, Wednesday for Mercury,Thursday for Jupiter, Friday for Venus, and Saturday for Saturn. The Germanic peoples substituted Germanic equivalents for the names of four of the Roman gods: For Tuesday, Tiw, the god of war, replaced Mars; for Wednesday,Woden, the god of wisdom, replaced Mercury; for Thursday, Thor, the god of thunder, replaced Jupiter; and for Friday, Frigg, the goddess of love, replaced Venus.

More planets, of course, have been discovered by modern man, thanks to advances in technology. Science is often driven forward by the development of new technology, allowing researchers to make measurements that were previously impossible.The dawn of the new age in astronomy, the study of the solar system, occurred in 1608, when Hans Lippershey, a Dutch eyeglass-maker, attached a lens to each end of a hollow tube, creating the first telescope. Galileo Galilei, born in Pisa, Italy, in 1564, made his first telescope in 1609 from Lippershey's model. Galileo soon had noticed that Venus has phases like the Moon and that Saturn appeared to have "handles." These of course were the edges of Saturn's rings, though the telescope was not strong enough to resolve the rings correctly. In 1610, Galileo discovered four of Jupiter's moons, which are still called the Galilean satellites. These four moons were proof that not every heavenly body orbited the Earth, as Ptolemy, a Greek philosopher, had asserted around 140 c.e. Galileo's discovery was the beginning of the end of the strongly held belief that the Earth is the center of the solar system, as well as a beautiful example of a case where improved technology drove science forward.

Most of the science presented in this set comes from the startling-ly rapid developments of the last hundred years, brought about by technological development. The concept of the Earth-centered solar system is long gone, as is the notion that the "heavenly spheres" are unchanging and perfect. Looking down on the solar system from above the Sun's North Pole, the planets orbiting the Sun can be seen to be orbiting counterclockwise, in the manner of the original proto-planetary disk of material from which they formed. (This is called prograde rotation.) This simple statement, though, is almost the end of generalities about the solar system.The notion of planets spinning on their axes and orbiting around the Sun in an orderly way is incorrect: Some planets spin backward compared to the Earth, others planets are tipped over, and others orbit outside the ecliptic plane (the imaginary plane that contains the Earth's orbit) by substantial angles, the dwarf planet Pluto in particular (see the accompanying figure on obliquity and orbital inclination). Some planets and moons are hot enough to be volcanic, and some produce silicate lava (for example, Jupiter's moon Io), while others have exotic lavas made of molten ices (for example, Neptune's moon Triton). Some planets and even moons have atmospheres, with magnetic fields to protect them from the solar wind (for example, Venus, Earth, Mars, Io, Triton, and Saturn's moon Titan), while other planets have lost both their magnetic fields and their atmospheres and orbit the Sun fully exposed to its radiation and supersonic particles (for example, Mercury).

Size can be unexpected in the solar system: Saturn's moon Titan is larger than the planet Mercury, and Charon, Pluto's moon, is almost as big as Pluto itself. The figure on page viii shows the number of moons each planet has; large planets have far more than small planets, and every year scientists discover new celestial bodies orbiting the gas giant planets. Many large bodies orbit in the asteroid belt, or the Kuiper belt, and many sizable asteroids cross the orbits of planets as they make their way around the Sun. Some planets' moons are unstable and will make new ring systems as they crash into their hosts. Many moons, like Neptune's giant Triton, orbit their planets backward (clockwise when viewed from the North Pole, the opposite way that the planets orbit the Sun).Triton also has the coldest surface temperature of any moon or planet, including Pluto, which is much farther from the Sun.The solar system is made of bodies in a continuum of sizes and ages, and every rule has an exception.

Obliquity, orbital inclination, and rotational direction are three physical measurements used to describe a rotating, orbiting body.

Obliquity, Inclination, Rotation

Rotation:

Looking down on the north pole of a planet or moon, rotation in this direction is called direct, or prograde. Rotation in the opposite direction is called indirect, or retrograde; Venus, Uranus, and Pluto all have retrograde rotation.

Pluto

Pluto

inclination:

Pluto's orbit is inclined to the ecliptic (the plane of Earth's orbit) by 17.14 degrees; all the other planets have inclinations less than 7 degrees.

Obliquity: / The angle between the planet's equator and its orbital plane is called its obliquity. Pluto's obliquity is 122.5 degrees, Venus's is 177.3 degrees, and Mercury's is 0 degrees,

Obliquity: / The angle between the planet's equator and its orbital plane is called its obliquity. Pluto's obliquity is 122.5 degrees, Venus's is 177.3 degrees, and Mercury's is 0 degrees, inclination:

Pluto's orbit is inclined to the ecliptic (the plane of Earth's orbit) by 17.14 degrees; all the other planets have inclinations less than 7 degrees.

Number of Moons v. AU

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