The Solar System

556 PART 4^ |^ THE SOLAR SYSTEM

typical meteoroid has roughly the mass of a paper clip and

vaporizes at an altitude of about 80 km (50 mi) above Earth’s

surface. Th e meteor trail points back along the path of the

meteoroid, so if you study the direction and speed of meteors,

you can get clues to their orbits in the solar system before they

encountered Earth.

One way to backtrack meteor trails is to observe meteor

showers. On any clear night, you can see 3 to 15 meteors per

hour, but on some nights you can see a shower of hundreds of

meteors per hour that are obviously related to each other. To

confi rm this, try observing a meteor shower. Pick a shower

from ■ Table 25-2 and on the appropriate night stretch out in

a lawn chair and watch a large area of the sky. When you see a

meteor, sketch its path on the appropriate sky chart from the

back of this book. In just an hour or so you will discover that

all or almost all of the meteors you see seem to come from a

single area of the sky, called the radiant of the shower (■

Figure 25-5a). Meteor showers are named after the constella-

tion from which they seem to radiate; for example, the Perseid

shower seen in mid-August radiates from the constellation

Perseus.

Observing a meteor shower is a natural fi reworks show, but

it is even more exciting when you understand what a meteor

shower tells you. Th e fact that the meteors in a shower appear to

come from a single point in the sky means that the meteoroids

were traveling through space along parallel paths. When they

encounter Earth and are vaporized in the upper atmosphere, you

see their fi ery tracks in perspective, so they appear to come from

a single radiant point, just as railroad tracks seem to come from

a single point on the horizon (Figure 25-5b).

carbonaceous chondrites, most of which have avoided being
heated or modifi ed, to other chondrites in which the material
was slightly heated and somewhat altered from the form in
which it fi rst solidifi ed. Chondrites in general off er us the best
direct information about conditions and processes occurring
in the earliest days of the solar nebula when planetesimals and
planets were forming.
Stony meteorites called achondrites contain no chondrules
and also lack volatiles. Th ese rocks appear to have been sub-
jected to intense heat that melted the chondrules and com-
pletely drove off the volatiles, leaving behind rock with
composition similar to Earth’s basalts.
Th e diff erent types of meteorites evidently had a wide
variety of histories. Some achondrites seem like pieces of lava
fl ows, whereas stony-iron and iron meteorites apparently were
once deep inside the molten interiors of diff erentiated objects.
Th e diff erences between various chondrites are thought to
result from: (1) meteoritic material solidifying at diff erent
distances from the sun with diff erent chemical compositions
due to the temperature-dependent condensation sequence,
(2) each location in the nebula having some material trans-
ported and mixed in from other locations, and (3) processes
that altered meteoritic material after condensation. Meteorites
provide evidence that the early history of the solar system was
complex.

Orbits of Meteors and Meteorites

Meteoroids are much too small to be visible through even the
largest telescope. Th ey are visible only when they fall into
Earth’s atmosphere and are heated by friction with the air. A

■ Table 25-2 ❙ Meteor Showers

Shower Dates

Hourly

Rate

Radiant*

R. A. Dec.

Associated

Comet

Quadrantids Jan. 2–4 30 15h 24m 50°

Lyrids April 20–22 8 18h 4m 33° 1861 I

 Aquarids May 2–7^10 22h 24m 0° Halley?

 Aquarids July 26–31^15 22h 36m 10°

Perseids Aug. 10–14 40 3h 4m 58° Swift-Tuttle

Orionids Oct. 18–23 15 6h 20m 15° Halley?

Taurids Nov. 1–7 8 3h 40m 17° Encke

Leonids Nov. 14–19 6 10h 12m 22° 1866 I Temp

Geminids Dec. 10–13 50 7h 28m 32°

*R. A. and Dec. give the celestial coordinates (right ascension and declination) of the radiant for each shower.