Artist’s conception

551

Guidepost

In Chapter 19 you began your study of planetary astronomy by considering evidence

about how our solar system formed. In the fi ve chapters that followed you surveyed the

planets and found more clues about the origin of the solar system. But most traces of

the early histories of the planets have been erased by geological activity or other pro-

cesses. Now you can study smaller, better-preserved objects that tell more about the era

of planet building.

Asteroids and comets are unevolved objects, leftover planet construction “bricks.” You

will fi nd them much as they were when they formed 4.6 billion years ago. Meteors and

meteorites are fragments of comets and asteroids that arrive at Earth and can give you

a close look at those ancient planetesimals. As you explore, you will fi nd answers to four

important questions:

Where do meteors and meteorites come from?

What are asteroids?

What are comets?

What happens when an asteroid or comet hits Earth?

As you fi nish this chapter, you will have acquired real insight into your place in nature.

You live on the surface of a planet. Are any other planets inhabited? That is the subject

of the next, and fi nal, chapter.

Meteorites, Asteroids,

25 and Comets

The Dawn spacecraft was

launched in 2007 on a

mission to explore the

large asteroid Vesta and

the dwarf planet Ceres

that circle the sun in the

asteroid belt between the

orbits of Mars and Jupiter.

This artist’s conception

shows Dawn’s approach to

Vesta (top right),

scheduled for 2011.