A truly great comet—one of the brightest of the twentieth century—illuminated the skies of the northern hemisphere in early 1997. Comet Hale-Bopp is shown here in this wide-angle photograph, taken at the moment of perihelion, or closest approach to the Sun, yet still some 0.9 A.U. away. Note how the gas (glowing bluish light) boiling off the comet extends straight away from the Sun, while the comet's heavier dust particles (reflected whitish light) lag behind, forming a more gently curved tail. The full extent of the tail measured nearly 40 degrees, sweeping a huge arc across the nighttime sky. Almost surely, Hale-Bopp was the most-photographed comet in history.
Studying this chapter will enable you to:
Describe the orbital properties of the major groups of asteroids.
Summarize the composition and physical properties of a typical asteroid.
Explain the effect of orbital resonances on the structure of the asteroid belt.
Detail the composition and structure of a typical comet, and explain the formation and appearance of its tail.
Discuss the characteristics of cometary orbits and what they tell us about the probable origin of comets.
Distinguish among the terms meteor, meteoroid, and meteorite.
Summarize the orbital and physical properties of meteoroids, and explain what these suggest about their probable origin.
According to classical definitions, there are only nine planets in the solar system. But several thousand other celestial bodies are also known to revolve around the Sun. These minor objects—the asteroids and comets—are small and of negligible mass compared with the planets and their major moons. Yet each is a separate world, with its own story to tell about the early solar system. On the basis of statistical deductions, astronomers estimate that there are more than a billion such objects still to be discovered. They may seem to be only rocky and icy "debris," but more than the planets themselves, they hold a record of the formative stages of our planetary system. Many are nearly pristine, unevolved bodies with much to teach us about our local origins.
