CHAPTER 25 | METEORITES, ASTEROIDS, AND COMETS 565
the smaller asteroids are fragments produced by 4.6 billion years
of collisions.SCIENTIFIC ARGUMENT
What is the evidence that asteroids have been fragmented?
First, your argument might note that the solar nebula theory pre-
dicts that planetesimals collided and either stuck together or frag-
mented. This is suggestive, but it is not evidence. A theory can
never be used as evidence to support some other theory or hypoth-
esis. Evidence means observations or the results of experiments, so
your argument must cite observations. The spacecraft photographs
of asteroids show irregularly shaped little worlds heavily scarred
by impact craters. Further evidence indicates some asteroids may
be pairs of bodies split apart but still in contact, and images of
asteroid Ida reveal a small satellite, Dactyl. Other asteroids with
moons have been found. These double asteroids and asteroids
with moons probably reveal the results of fragmenting collisions
between asteroids. Furthermore, meteorites appear to have come
from the asteroid belt astronomically recently, so fragmentation
must be a continuing process there.
Now build an argument to combine what you know of meteor-
ites with your experience with asteroids. What evidence could you
cite to show what the fi rst planetesimals were like?Comets
Few sights in astronomy are more beautiful than a bright
comet hanging in the night sky (■ Figure 25-12). It is a
Common Misconception that comets whiz rapidly across
the sky like meteors. Actually, comets move with the stately grace
of great ships at sea, their motion hardly apparent; night by night
they shift position slightly against the background stars, and they
may remain visible for weeks.
Th roughout history, comets have been considered omens of
doom. Comets may be beautiful, but are also so strange in
appearance that they can create some instinctive alarm. Even
recent appearances of bright comets have caused predictions of
the end of the world. In 1910, Comet Halley was spectacular,
and it was frightening to some people. Comet Kohoutek in
1973, Comet Halley returning in 1986, and Comet Hale–Bopp
in 1997 also caused concern among the superstitious.*
Faint comets are common; several dozen are discovered
each year. Truly bright comets appear about once per decade.
Comet McNaught in 2007 was bright enough to be classed25-3
fragments from iron cores of diff erentiated asteroids. A few other
types of asteroids are known, and a number of individual aster-
oids have been found that are unique, but these three classes
include most of the known asteroids.
Although S-type asteroids are very common in the inner
asteroid belt, their colors and albedos are diff erent from those of
chondrites, the most common kind of meteorite. Th is repre-
sented a puzzle to astronomers; shouldn’t the common type of
asteroid nearest Earth be the source of the most common type
of meteorite that hit Earth? New evidence from the analysis of
moon rocks and from observations of Eros, an S-type asteroid,
shows that bombardment by micrometeorites and solar wind
particles can redden and darken rocky materials until they have
the colors and albedos of S-type asteroids. It therefore seems
likely that chondrite meteorites are in fact fragments of S-type
asteroids.
In March 2009 a small asteroid (2–3 meters in diameter,
about the size of a small truck) was spotted by the NEO detection
network on a collision course with Earth. Astronomers were able
to observe it in space before impact and discovered that its colors
and albedo matched the fairly rare F-type asteroids that are mostly
in the outer belt. Th e asteroid entered Earth’s atmosphere over the
desert of northern Sudan and was witnessed exploding. Scientists
Peter Jenniskens from the SETI Institute and Muawia Shaddad of
the University of Khartoum organized a team of local faculty and
students to search for pieces of the object. Th ey ultimately found
about 4 kilograms (9 pounds) of fragments corresponding to the
rate ureilite meteorite type. For the fi rst time, planetary scientists
were able to make a defi nite connection between an asteroid
observed in space and meteorites with properties measured in an
Earth laboratory.
As you saw in the case of Vesta, a few asteroids may once
have been geologically active, with lava fl owing on their surfaces
when they were young. Perhaps they incorporated especially
large amounts of short-lived radioactive elements such as alumi-
num-26. Th ose radioactive elements were probably produced by
a supernova explosion that could also have been the trigger for
the formation of the sun and planets while seeding the young
solar system with its nucleosynthesis products (see Chapter 13
and Figure 11-2).
Not all large asteroids have been active. Ceres, 900 km in
diameter, is almost twice as big as Vesta, but it shows no spectro-
scopic sign of past activity and evidently has an ice-rich mantle.
Th e puzzling diff erences between those two large asteroids will be
investigated by the Dawn spacecraft that is on its way to orbit
Vesta in 2011 and Ceres in 2015.
Although there are still mysteries to solve, you can under-
stand the story of the asteroids. Th ey are fragments of planetesi-
mals, some of which diff erentiated, developed molten metal
cores, in a few cases even had lava fl ows on their surfaces, and
then cooled slowly. Th e largest asteroids astronomers see today
may be nearly unbroken examples of original planetesimals, but
*Comets are now named after the person or persons who discover them.
Comet Halley, in contrast, which has been seen occasionally for at least the last
2,250 years, has no known “discoverer.” It is named after Edmund Halley, an
English astronomer who was a friend of Isaac Newton and who was the fi rst to
realize that certain comets appearing at 76-year intervals were actually the
same comet on a repeating orbit. Halley correctly predicted that the comet
would be seen again in a certain year.