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ing star” is a meteor. Most meteors burn up or evaporate com-
pletely within seconds after reaching an altitude of about 100 km
(about 60 mi) because they are nothing more than specks of dust.
A meteor shower occurs when Earth passes through a stream of
particles left by a comet in its orbit. Earth might meet the stream
of particles concentrated in such an orbit on a regular basis as
it travels around the Sun, resulting in predictable meteor show-
ers (Table 15.5). In the third week of October, for example, Earth
crosses the orbital path of Halley’s comet, resulting in a shower of
some 10 to 15 meteors per hour. Meteor showers are named for
the constellation in which they appear to originate. The October
meteor shower resulting from an encounter with the orbit of Hal-
ley’s comet, for example, is called the Orionid shower because it
appears to come from the constellation Orion.
Did you know that atom-bomb-sized meteoroid explosions
FIGURE 15.17 Most of the asteroids in the asteroid belt are often occur high in Earth’s atmosphere? Most smaller meteors
about halfway between the Sun and Jupiter.
melt into the familiar trail of light and smoke. Larger meteors
may fragment upon entering the atmosphere, and the smaller
fragments will melt into multiple light trails. Still larger me-
600,000 mi) or so apart, but there is evidence of collisions teors may actually explode at altitudes of about 32 km (about
occur ring in the past. Most asteroids larger than 50 km (about 20 mi) or so. Military satellites that watch Earth for signs of
30 mi) have been studied by analyzing the sunlight reflected rockets blasting off or nuclear explosions record an average of
from their surfaces. These spectra provide information about eight meteor explosions a year. These are big explosions, with
the composition of the asteroids. Asteroids on the inside of the an energy equivalent estimated to be similar to that of a small
belt, toward the Sun, are made of stony materials, and those nuclear bomb. Actual explosions, however, may be 10 times
on the outside of the belt are dark with carbon minerals. Still larger than the estimation. Based on statistical data, scientists
other asteroids are metallic, containing iron and nickel. These have estimated that every 10 million years, Earth should be hit
spectral composition studies, analyses of the orbits of asteroids, by a very, very large meteor. The catastrophic explosion and af-
and studies of meteorites that have fallen to Earth all indicate termath would dev astate life over much of the planet, much like
that the asteroids are not the remains of a planet or planets that the theoretical dinosaur-killing impact of 65 million years ago.
were broken up. The asteroids are now believed to have formed If a meteoroid survives its fiery trip through the atmo-
some 4.6 billion years ago from the original solar nebula. Dur- sphere to strike the surface of Earth, it is called a meteorite.
ing their formation, or shortly thereafter, their interiors were Most meteors are from fragments of comets, but most meteor-
partly melted, perhaps from the heat of short-lived radioactive ites generally come from particles that resulted from collisions
decay reactions. Their location close to Jupiter, with its gigantic between asteroids that occurred long ago. Meteorites are clas-
gravitational field, prevented the slow gravitational clumping sified into three basic groups according to their composition:
together process that would have formed a planet. (1) iron meteorites, (2) stony meteorites, and (3) stony-iron me-
Jupiter’s gigantic gravitational field also captured some of teorites (Figure 15.18). The most common meteorites are stony,
the asteroids, pulling them into its orbit. Today, there are two composed of the same minerals that make up rocks on Earth.
groups of asteroids, called the Trojan asteroids, that lead and The stony meteorites are further subdivided into two groups ac-
follow Jupiter in its orbit. They lead and follow at a distance cording to their structure, the chondrites and the achondrites.
where the gravitational forces of Jupiter and the Sun balance to Chondrites have a structure of small, spherical lumps of silicate
keep them in the orbit. A third group of asteroids, called the
Apollo asteroids, has orbits that cross the orbit of Earth.
TABLE 15.5
METEORS AND METEORITES Some annual meteor showers
Comets leave trails of dust and rock particles after encountering Name Date of Maximum Hour Rate
the heat of the Sun, and collisions between asteroids in the past
Quadrantid January 3 30
have ejected fragments of rock particles into space. In space, the
Aquarid May 4 5
remnants of comets and asteroids are called meteoroids. When a
meteoroid encounters Earth moving through space, it accelerates Perseid August 12 40
toward the surface with a speed that depends on its direction of Orionid October 22 15
travel and the relative direction that Earth is moving. It soon be- Taurids November 1, 16 5
gins to heat from air compression in the upper atmo sphere, melt- Leonid November 17 5
ing into a visible trail of light and smoke. The streak of light and Geminid December 12 55
smoke in the sky is called a meteor. The “falling star” or “shoot-

392 CHAPTER 15 The Solar System 15-16