CHAPTER 25 | METEORITES, ASTEROIDS, AND COMETS 557
of rock that become spread along its entire orbit (■ Figure 25-6).
When Earth passes through this stream of material, you see a
meteor shower. In some cases the comet has wasted away and is
no longer visible, but in other cases the comet is still prominent,
although located somewhere else along its orbit. For example,
each May, Earth comes near the orbit of Comet Halley, and you
can see the Eta Aquarid shower. Each October, Earth passes near
the other side of the orbit of Comet Halley, and the Orionid
shower appears.
Even when there is no shower, you still can occasionally see
meteors, which are called sporadic meteors because they are not
part of specifi c showers. Many of these are produced by stray bits
of matter that were released long ago by comets. Such comet
debris gradually get spread throughout the inner solar system,
away from their original tracks, and bits collide with Earth even
when there is no shower.
Another way to backtrack meteor trails is to photograph
the same meteor from two locations on Earth a few miles apart.Studies of meteor shower radiants reveal that those meteor-
oids are orbiting the sun along the paths of comets. As you
learned in Chapter 19, the vaporizing head of a comet releases bits
■ Figure 25-5
(a) Meteors in a meteor shower enter Earth’s atmosphere along parallel
paths, but perspective makes them appear to diverge from a single point in
the sky. (b) Similarly, parallel railroad tracks appear to diverge from a point
on the horizon.
a bOrbit of cometOrbit of cometInfrared imageEarth SunFresh material
ejected from the
comet forms a fan.■ Figure 25-6
(a) Meteors in a shower are debris left behind as a comet’s icy nucleus
vaporizes. The rocky and metallic bits of matter spread along the comet’s
orbit. If Earth passes through such material, you can see a meteor shower.
(b) In this infrared image of Comet Encke, the millimeter-sized bits of rock
along its orbit glow as they are warmed by sunlight. The Taurid meteor
shower occurs every October when Earth crosses Encke’s orbit. (NASA/JPL-
Caltech/M. Kelley, Univ. of Minnesota)