The Solar System

578 PART 4^ |^ THE SOLAR SYSTEM

one big impact. Astronomers point out that the biggest objects

are so rare they can be ignored. Th e real danger lies in the more

common smaller, yet still substantial, meteoroids, which are dif-

fi cult to detect with current telescopes. Th e future of our civiliza-

tion on Earth may depend on our doing an increasingly careful

job of tracking both large and small objects that cross our path

with surprising frequency.

very near miss indeed, yet DD45 was fi rst spotted only three days
before its closest approach.
Some people have argued that the danger from asteroid and
comet impacts is so great that governments should develop mas-
sive nuclear-tipped missiles, ready to blast a meteoroid to pieces
long before it can reach Earth. Other experts respond that lots of
small fragments slamming into Earth may be even worse than

What Are We? Sitting Ducks

Human civilization is spread out over Earth’s

surface and exposed to anything that falls

out of the sky. Meteorites, asteroids, and

comets bombard Earth, producing impacts

that vary from dust settling gently on

rooftops to disasters capable of destroying

all life. In this case, the scientifi c evidence

is conclusive and highly unwelcome.

Statistically we are quite safe. The

chance that a major impact will occur

during your lifetime is so small it is hard

to estimate. But the consequences of such

an impact are so severe that humanity

should be preparing. One way to prepare is

to fi nd those objects that could hit us,

map their orbits, and identify any that are

dangerous.

What we do next isn’t clear. Blowing up

a dangerous asteroid in space might make a

good movie, but converting one big

projectile into a thousand small ones might

not be very smart. Changing an asteroid’s

orbit could be diffi cult without a few

decades’ advance warning. Unlikely or not,

large impacts demand consideration and

preparation.

Throughout the universe there may be

two kinds of inhabited worlds. On one type

of world, intelligent creatures have devel-

oped ways to prevent asteroid and comet

impacts from altering their climates and

destroying their civilizations. But on other

worlds, including Earth, intelligent races

have not yet found ways to protect

themselves. Some of those civilizations

survive. Some don’t.

Summary

▶ (^) Review from Chapter 19: The term meteoroid refers to small solid
particles orbiting in the solar system. The term meteor refers to a vis-
ible streak of light from a meteoroid heated and glowing as it enters
Earth’s atmosphere. The term meteorite refers to space material that
has reached Earth’s surface.
▶ (^) Meteorites that are seen to hit Earth are called falls (p. 552). Finds
(p. 552 ) are meteorites discovered on the ground that fell unobserved,
perhaps thousands of years ago.
▶ (^) Iron meteorites (p. 552) are mostly iron and nickel; when sliced open,
polished, and etched, they show Widmanstätten patterns (p. 552).
These reveal that the metal cooled from a molten state very slowly.
▶ (^) Stony meteorites (p. 552) include chondrites (p. 553), which
contain small, glassy particles called chondrules (p. 553), solidifi ed
droplets of once-molten material that formed in the solar nebula by an
as-yet unknown mechanism.
▶ (^) Stony-iron meteorites (p. 552) are quite rare and, as the name
implies, consist of a mixture of stony and metallic material.
▶ (^) Selection effects (p. 552, 555) cause iron meteorites to be the most
common fi nds, even though stony meteorites are the most common
falls.
▶ (^) Stony meteorites that are rich in volatiles and carbon are called car-
bonaceous chondrites (p. 552). They are among the least modifi ed
meteorites. Some carbonaceous chondrites contain CAIs (p. 555),
calcium–aluminum-rich inclusions, which are understood to be the very
fi rst solid particles to condense in the cooling solar nebula.
▶ (^) An achondrite (p. 553) is a stony meteorite that contains no chon-
drules and no volatiles. Achondrites appear to have been melted after
they formed and, in some cases, resemble solidifi ed lavas.
▶ (^) Evidence from the orbits of meteorites seen to fall, and from the
composition of the meteorites, suggests that meteorites are fragments
of asteroids. Other evidence, including orbital paths indicated by the
radiant (p. 556) points of meteor showers (p. 556), indicates in
contrast that the vast majority of meteors, including both meteors in
meteor showers and isolated sporadic meteors (p. 557), appear to be
low-density, fragile bits of debris from comets.
▶ (^) Many meteorites appear to have formed as part of larger bodies that
melted, differentiated, and cooled very slowly. Later these bodies were
broken up, and fragments from the core became iron meteorites, frag-
ments from the outer layers became stony meteorites, and fragments
from intermediate layers became stony-irons.
▶ (^) Asteroids are irregular in shape and heavily cratered from collisions.
Their surfaces are covered by gray, pulverized rock, and some asteroids
have such low densities they must be fragmented rubble piles.