404 PART 4^ |^ THE SOLAR SYSTEM
(6 mi) in diameter. Th ere are probably a million or more that are
larger than 1 km (0.6 mi) and billions that are smaller. Because
the largest are only a few hundred kilometers in size, Earth-based
telescopes can detect no details on their surfaces, and even the
Hubble Space Telescope can image only the largest features.
Spectroscopic observations indicate that asteroid surfaces are
a variety of rocky and metallic materials. Photographs returned
by robotic spacecraft show that asteroids are generally irregular in
shape and covered with craters (■ Figure 19-4). Th ese observa-
tions will be discussed in detail in Chapter 25, but in this quick
survey of the solar system you have enough information to con-
clude that the solar nebula included elements that compose rock
and metals and that collisions have played an important role in
the solar system’s history.
Since 1992, astronomers have discovered more than a thou-
sand small, dark, icy bodies orbiting in the outer fringes of the
solar system beyond Neptune. Th is collection of objects is called
the Kuiper belt after astronomer Gerard Kuiper (KYE-per), who
predicted their existence in the 1950s. Th ere are probably 100
million bodies larger than 1 km in the Kuiper belt, many more
than in the asteroid belt, and a successful theory should also
explain how they came to be where they are. You will fi nd out
more about the origin of the Kuiper belt in Chapter 24.
Visual-wavelength imagesabIn contrast to the rocky asteroids and dark Kuiper belt
objects, the brightest comets can be seen with the naked eye and
are impressively beautiful objects (■ Figure 19-5). Most comets
are faint, however, and are diffi cult to locate even at their bright-
est. A comet may take months to sweep through the inner solar
system, during which time it appears as a glowing ball with an
extended tail of gas and dust.
Th e nuclei of comets are ice-rich bodies, a few kilometers or
tens of kilometers in diameter (similar in size to asteroids), that
are sometimes described as “dirty snowballs” or “icy mudballs,”
left over from the origin of the planets. From this you can con-
clude that at least some parts of the solar nebula had abundant
icy material. You will see more evidence about the composition
and history of comets in Chapter 25.
A comet nucleus remains frozen and inactive while it is far
from the sun. As the nucleus moves along its elliptical orbit into the
inner solar system, the sun’s heat begins to vaporize the ices, releas-
ing gas and dust. Th e pressure of sunlight and the solar wind push
the gas and dust away, forming a long tail. Th e motion of the
nucleus along its orbit, the eff ects of sunlight, and the outward fl ow
of the solar wind can create comet tails that are long and straight
or gently curved, but in any case the tail of a comet always points
approximately away from the sun (Figure 19-5b), no matter what
direction the comet itself is moving. Th e beautiful tail of a comet
can be longer than an AU, but, again, note that it is produced by a
relatively tiny nucleus only a few kilometers in diameter.
Unlike the stately comets, meteors fl ash across the sky in
momentary streaks of light (■ Figure 19-6). Th ey are commonly
called “shooting stars.” Of course, they are not stars but small bits
of rock and metal colliding with Earth’s atmosphere and bursting
into incandescent vapor because of friction with the air about
80 km (50 mi) above the ground. Th is vapor condenses to form
dust that settles slowly to Earth, adding about 40,000 tons per
year to the planet’s mass.■ Figure 19-4
(a) Over a period of three weeks, the NEAR
spacecraft approached the asteroid Eros and
recorded a series of images arranged here in an
entertaining pattern showing the irregular shape
and 5-hour rotation period of the asteroid. Eros
is 34 km (21 mi) long. (b) This close-up of
the surface of Eros shows an area about 11 km
(7 mi) from top to bottom. (Johns Hopkins
University, Applied Physics Laboratory, NASA)