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Neil F. Comins • William J. Kaufmann III
Discovering the Universe
Ninth Edition
CHAPTER 9
Vagabonds of the Solar System
WHAT DO YOU THINK?
1.
2.
3.
4.
5.
Are the asteroids a former planet that was
somehow destroyed? Why or why not?
How far apart are the asteroids on average?
How are comet tails formed? Of what are they
made?
In which directions do a comet’s tails point?
What is a shooting star?
In this chapter you will discover…
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the properties of dwarf planets and small solar system
bodies
asteroids and meteoroids—pieces of interplanetary rock
and metal
comets—objects containing large amounts of ice and
rocky debris
space debris that falls through Earth’s atmosphere
the asteroid belt and the Kuiper belt, both filled with a
variety of debris, including orbiting pairs of objects
the impacts from space 250 million and 65 million years
ago that caused mass extinctions of life on Earth
wayward asteroids that could again threaten life on Earth
Different Classifications of Solar System Objects
Some of the definitions of the
different types of objects in the
solar system overlap. For
example, the largest asteroids
are also being classified as
dwarf planets; various transNeptunian objects (TNOs) are
asteroids or comets; some
comets are satellites of Jupiter;
some Kuiper belt objects
(KBOs) are satellites of other
KBOs. Furthermore, TNOs
exist in two groups: Kuiper belt
objects and Oort comet cloud
bodies.
Pluto
These three Hubble Space Telescope images of
Pluto show little detail but indicate that the major
features of Pluto’s surface each cover large
amounts of its area. Comparing these observations
to previous ones reveals that the surface changes
in color and brightness seasonally.
Discovery of Pluto
Pluto was discovered in 1930 by searching for a dim,
starlike object that slowly moved against the background
stars. These two photographs were taken 1 day apart.
Orbit of Pluto
(a) The high-eccentricity orbit of dwarf planet (and KBO) Pluto stands
out compared to the orbits of the outer three planets. Notice how many
significant events occurred on Earth during Pluto’s present orbit of the
Sun. (b) Details of Pluto’s passage inside the orbit of Neptune. The two
bodies will never collide.
Orbit of Pluto
(c) A nearly edge-on view of the ecliptic and Pluto’s orbit compared to it.
Discovery of Charon
Long ignored as just a
defect in the
photographic emulsion,
the bump on the upper
left side of this image of
Pluto led astronomer
James Christy to
discover the moon
Charon.
Pluto’s Moons Nix and Hydra
Observations by the Hubble Space Telescope in 2005 revealed
two small moons, each about 5000 times dimmer than Pluto.
Named Nix and Hydra, they are between 2 and 3 times farther
from Pluto than is its moon Charon. The lines radiating from Pluto
and Charon are artifacts of the exposure.
Comparison of Ceres with the Moon and Earth
Ceres, the Moon, and Earth are shown here to scale. Dwarf planet
Ceres, shown in this infrared photo (Earth and the Moon appear in
visible light), is the largest asteroid but is so small that it is not
considered a planet. Because it does not orbit a body other than the
Sun, it is also not classified as a moon. This image of Ceres suggests it
has regions of ice and rock on its surface. The asteroid will be visited by
the Dawn spacecraft in 2015.
Dwarf Planet Eris
Three perpendicular views of the orbit of Eris and Dysnomia
are compared to the planets and Pluto. Eris and Dysnomia’s
orbit around the Sun ranges from 38 to 98 AU, with orbital
eccentricity, e = 0.44, and an orbital inclination of 44°.
Dwarf Planet Eris
This is a Keck Telescope image of dwarf planet
Eris and its moon Dysnomia.
Asteroid Orbits
(a) The orbits of belt asteroids Ceres, Pallas, and Juno are indicated to
scale in this diagram. (b) These are the actual positions of all known
asteroids at Jupiter’s orbit or closer. The locations of the belt asteroids
are indicated by green dots. Objects passing closer than 1.3 AU to the
Sun are shown by red circles. Objects observed at least twice are
indicated by filled circles, and objects seen only once are indicated by
outline circles. Jupiter’s Trojan asteroids are deep blue squares.
Comets are filled and unfilled light-blue squares.
Discovering Asteroids
In 1998, the Hubble Space Telescope found this asteroid
while observing objects in the constellation Centaurus. The
exposure, tracking stars, shows the asteroid as a 19-arcsec
streak. This asteroid is about 2 km in diameter and was
located about 140 million km (87 million mi) from Earth.
The Kirkwood Gaps
This graph displays the number of asteroids at various distances from the
Sun. Note that few asteroids have orbital periods that correspond to such
simple fractions as 1⁄3, 2⁄5, 3⁄7, and 1⁄2 of Jupiter’s orbital period. Resonant
orbits with Jupiter have deflected asteroids away from these orbits. The
Trojan asteroids accompany Jupiter as it orbits the Sun.
Collision Between Two Asteroids
Observed in 2010, this X-shaped “object” (inset) is believed to be the
collision of two small asteroids. The event created dust that was pushed
away from the Sun, which is to the left and below this image. The collision
occurred 2 AU from the Sun and 1 AU from Earth.
Ida and Its Satellite
The 55-km-long rocky asteroid Ida, shown here with its
satellite Dactyl, is about twice the size of the younger
asteroid Gaspra (see Figure 5-7). Inset: Dactyl is also
heavily cratered.
Jupiter’s Trojan Asteroids
Groups of asteroids orbit at
the two stable Lagrange
points along Jupiter’s orbit,
trapped by the combined
gravitational forces of Jupiter
and the Sun.
Asteroid 1994 XM1
This image was obtained on December 9, 1994, shortly
before the near Earth asteroid arrived in Earth’s
vicinity. When it passed by Earth just 12 h later,
asteroid 1994 XM1 was less than half the distance
from Earth to the Moon.
Asteroids
(a) Reflecting only half as much light as
a charcoal briquette, Mathilde is half as
dense as typical stony asteroids. Slightly
larger than Ida, irregularly shaped
Mathilde measures 66 km x 48 km x 46
km, rotates once every 17.4 days, and
has a mass equivalent to 110 trillion
tons. The part of the asteroid shown is
about 59 km x 47 km. The large crater in
shadow is about 20 km across.
(b) The near-Earth asteroid
Itokawa was visited by the
Japanese space probe
Hayabusa. Samples of dust
particles from it are now being
analyzed.
Asteroid Eros
The Near-Earth Asteroid Rendezvous (NEAR) Shoemaker spacecraft took
these images of asteroid Eros in February 1999. (a) The top of the figure is
the asteroid’s north polar region. Eros’s dimensions are 33 km x 13 km x 13
km (21 mi x 8 mi x 8 mi) and it rotates every 5¼ h. Its density is 2700 kg/m3,
close to the average density of Earth’s crust and twice as dense as asteroid
Mathilde. (b) This is an image taken while looking into the large crater near
the top of (a), which is 5.3 km (3.3 mi) across. (c) This is the penultimate
image taken by NEAR Shoemaker before it gently landed on Eros. Taken
from an altitude of 250 m (820 ft), the image is only 12 m across. You can
see rocks and boulders buried to different depths in the regolith.
Current Positions of Known Dwarf Planets
and SSBs in the Outer Solar System
Objects with unusually high-eccentricity orbits are shown as cyan
triangles. Objects roaming among the outer planets, called Centaur
objects, are orange triangles. Plutinos are white circles. Miscellaneous
objects are magenta circles, and classical KBOs are red circles. Objects
observed only once are denoted by open symbols; objects with two
separate observations are denoted by filled symbols. Comets are filled
and unfilled light-blue squares.
Kuiper Belt Objects
(a, b) These 1993 images show the discovery
(white lines) of 1 of more than 1524 known
KBOs. These two images of KBO 1993 SC
were taken 4.6 h apart, during which time the
object moved against the background stars.
(c) The KBO 1998
WW31 and its moon
(lower left).
Sedna’s Orbit
(a) The farthest known body in the solar system is in a highly
elliptical orbit (b) that ranges from the outer reaches of the Kuiper
belt and possibly extends to the inner Oort comet cloud.
Comet Nuclei
(a) The nucleus of Comet Halley. This image, taken by the Giotto
spacecraft, shows the potato-shaped nucleus of the comet. Its dark
nucleus measures 15 km in its longest dimension and about 8 km in its
shortest. The numerous bright areas on the nucleus are icy
outcroppings that reflect more sunlight than surrounding areas of the
comet. Two jets of gas can be seen emanating from the left side of the
nucleus. (b) This is the nucleus of Comet Borrelly, in an image taken
by Deep Space 1. The nucleus is 8 km (5 mi) long.
Comet Wild 2
This picture shows two images combined. One is a highresolution photograph showing the surprisingly heavily
cratered comet. The other image is a longer photograph
showing gas and dust jetting from the comet. Its tails are
millions of kilometers long.
Comet Wild 2
A substance called aerogel was used to capture particles from
Comet Wild 2’s dust tail. A piece of space debris pierced the
aluminum foil holding the aerogel and embedded in it, along with
pieces of the foil.
Comet Wild 2
A 2-µm piece of comet dust, composed of a mineral
called forsterite. On Earth this mineral is used to
make gems called peridot.
Comet Hale-Bopp
In 1997, Comet Hale-Bopp had a hydrogen
envelope 1 AU in diameter (blue ovals). This gas
was observed in the ultraviolet. The visible light inset
shows the scale of the visible tails (see also the
image at the opening of this chapter).
Comet West
Astronomer Michael M. West first noticed this comet on a
photograph taken with a telescope in 1975. After passing
near the Sun, Comet West became one of the brightest
comets of the 1970s. This photograph shows the comet in
the predawn sky in March 1976.
The Orbit and Tails of a Comet
The sunlight and solar wind blow a comet’s
dust particles and ionized atoms away from
the Sun. Consequently, comets’ tails
always point away from the Sun.
The Two Tails of Comet Mrkos
(a) Comet Mrkos dominated the evening sky in August
1957. These three views, taken at 2-day intervals, show
dramatic changes in the comet’s gas tail. In contrast, the
slightly curved dust tail remained fuzzy and featureless.
The Two Tails of Comet Mrkos
(b) Wind blowing smoke from this forest fire causes
the smoke column to change shape and direction,
just like the solar wind and sunlight cause the tails of
comets to change shape and direction.
The Structure of a Comet
The solid part of a typical comet (the nucleus) is roughly 10 km
in diameter. The coma can be as large as 105 to 106 km across,
and the hydrogen envelope is typically 107 km in diameter. A
comet’s tail can be enormous—even longer than 1 AU.
The Head of Comet Brooks
This comet had an exceptionally large, bright coma.
Named after its discoverer, William R. Brooks, it
dominated the night skies in October 1911.
The Tail of Comet Ikeya-Seki
Named after its codiscoverers in Japan, this comet dominated
the predawn skies in late October 1965. The yellow in the tail
comes from emission by sodium atoms in the dust that was
released by the comet. Although its coma was tiny, its tail
spanned over 1 AU.
Comet Tempel 1
(a) This composite image of Comet Tempel 1 has higher
resolution at the bottom, as the projectile from Deep Impact
headed in that direction. The smooth regions on the comet
have yet to be explained. (b) This image was taken 30 s
before the projectile struck the comet.
Comet Tempel 1
(c) Seconds after impact, hot debris explodes away from the comet
nucleus. The white horizontal half-ellipses are areas where the
CCDs were overloaded with light from the event. (d) Moments later,
the gases and dust were expanding outward. (e) This is an image
taken 67 s after impact. Within minutes, the cloud of debris
eventually became much larger than the entire nucleus.
Transformation and
Evolution of a LongPeriod Comet
(a) The gravitational force of a
giant planet can change a
comet’s orbit. Comets initially
on highly elliptical orbits are
sometimes deflected into
more circular paths that keep
them in the inner solar
system. (b–d) These figures
show the evolution of a comet
into gas, dust, and rubble,
and why debris from some of
these comets strikes Earth.
The Fragmentation of Comet Schwassmann-Wachmann-3
This comet, with a 5.4-year orbit, has been coming apart
for decades. In 2006, it further fragmented after passing
perihelion. One piece, Fragment B, shed at least 30
smaller pieces, shown here.
Comet Hale-Bopp
Discovered on July 23, 1995, this comet was at its breathtaking best
in mid-1997. Inset: Jets of gas and debris were observed shooting out
from Comet Hale-Bopp several times. This image shows the comet
nucleus (lower bright region), an ejected piece of the comet’s surface
(upper bright region), and a spiral tail. The ejected piece eventually
disintegrated, following the same spiral pattern as the tail.
Sungrazing Comet
Comet SOHO LASCO C3 is shown in the smaller box and
magnified in the larger one. Discovered in March 2004, it was
the 750th sungrazing comet discovered from the SOHO data. It
completely sublimated near perihelion.
Meteor
This brilliant meteor is seen lighting up the dark
skies of the California desert area of Joshua Tree
National Park. Just to the right of the meteor trail
are the Pleiades.
Meteorite Impact,
Poughkeepsie, NY, 1992
Meteor Crater
An iron meteor measuring 50 m across struck the
ground in Arizona 50,000 years ago. The result
was this beautifully symmetric impact crater.
The Origin of Meteor Showers
As comets dissipate, they leave debris behind that spreads out
along their orbits. When Earth plows through such material,
many meteors can be seen emanating from the same place
within a very short time—a meteor shower. As shown in this
diagram, many comets have high orbital inclinations.
Meteor Shower
This time exposure, taken in 1998, shows meteors streaking
away from the constellation Leo Major. They are part of the
Leonid meteor shower. This shower occurs because Earth is
moving through debris left by comet Temple-Tuttle.
Recent Impacts on the Moon
The locations A–F are places on the Moon where impacts were
observed from Earth in 1999 during the Leonid meteor shower. The
impacting bodies hit the Moon at around 260,000 km/hr (160,000 mph)
and had masses of between 1 and 10 kg. Each impact created a shortlived cloud that momentarily heated to between 5 x 104 and 10 x 104 K,
much hotter than the surface of the Sun.
The Mass of Impacts on Earth
The Vatican Obelisk is about 300 tons, the amount of mass
that strikes Earth daily. As a result, Earth’s mass increases
by this amount every day.
Stony Meteorites
(a) Most meteorites that fall to
Earth are stones. Many freshly
discovered specimens, like the
one shown here, are coated with
thin, dark crusts. This stony
meteorite fell in Morocco.
(b) Some stony meteorites
contain tiny specks of iron, which
can be seen when the stones are
cut and polished. This specimen
was discovered in Ohio.
Iron Meteorites
(a) Irons are composed almost
entirely of iron-nickel minerals.
The surface of a typical iron is
covered with thumbprint-like
depressions created as the
meteorite’s outer layers vaporized
during its high-speed descent
through the atmosphere. This
specimen was found in Argentina.
(b) When cut, polished, and
etched with a weak acid solution,
most iron meteorites exhibit
interlocking crystals in designs,
called Widmanstätten patterns.
This meteorite was found in
Australia.
Stony-Iron Meteorite
Stony-irons account for about 1% of all meteorites that fall to
Earth. This specimen, a variety of stony-iron called a pallasite,
was found in Antarctica. This specimen is thinly cut and
appears to glow because of a light located behind it.
Pieces of the Allende Meteorite
(a) This carbonaceous chondrite
fell near Chihuahua, Mexico, in
February 1969. Note the
meteorite’s dark color, caused
by a high abundance of carbon.
Geologists believe that this
meteorite is a specimen of
primitive planetary material. The
ruler is 15 cm long.
(b) Sliced open, the
Allende meteorite shows
round, rocky inclusions
called chondrules in a
matrix of dark rock.
Finding a Meteorite in Antarctica
Good places to find meteorites include deserts and icecovered regions, such as Antarctica. By surveying such
areas, astronomers and geologists can accurately
determine the correct percentage of each of the different
types of meteorites.
Aftermath of the Tunguska Event
In 1908, a stony asteroid traveling at supersonic
speed struck Earth’s atmosphere and exploded
over the Tunguska region of Siberia. Trees were
blown down for many kilometers in all directions
from the impact site.
Iridium-Rich Layer of Clay
This photograph of strata in the Apennine Mountains of
Italy shows a dark-colored layer of iridium-rich clay
sandwiched between white limestone (bottom) from the
late Mesozoic era and grayish limestone (top) from the
early Cenozoic era. The coin is the size of a U.S. quarter.
Confirming an Extinction-Level Impact Site
By measuring slight variations in the
gravitational attraction of different
materials under Earth’s surface,
geologists create images of
underground features. Concentric rings
of the underground Chicxulub Crater
(right inset) lie under a portion of the
Yucatán Peninsula. This crater has
been dated to 65 million years ago and
is believed to be the site of the impact
that led to the extinction of the
dinosaurs. A piece of 65-million-year-old
meteorite discovered in the middle of
the Pacific Ocean in 1998 is believed to
be a fragment of that meteorite. The
fragment, about 0.3 cm (0.1 in.) long,
was cut into two pieces for study (left
inset).
Summary of Key Ideas
Asteroids
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Pieces of solar system debris larger than 10 m and
composed primarily of rock and metal are called
asteroids.
Tens of thousands of belt asteroids with diameters larger
than a kilometer are known to orbit the Sun between the
orbits of Mars and Jupiter. The gravitational attraction of
Jupiter depletes certain orbits within the asteroid belt.
The resulting Kirkwood gaps occur at simple fractions of
Jupiter’s orbital period.
Jupiter’s and the Sun’s gravity combine to capture Trojan
asteroids in two locations, called stable Lagrange points,
along Jupiter’s orbit.
Asteroids
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The Apollo asteroids move in highly elliptical orbits that
cross the orbit of Earth. Many of these asteroids will
eventually strike the inner planets.
A belt asteroid, Ceres, along with four KBOs (Pluto, Eris,
Haumea, and Makemake) are classified as dwarf
planets.
Pluto, a KBO and dwarf planet, is an icy world that may
well resemble the moon Triton.
Comets
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Comet nuclei are fragments of ice and rock often orbiting
at a great inclination to the plane of the ecliptic. In the
Kuiper belt and Oort cloud, comets have fairly circular
orbits. When close to the Sun, they generally move in
highly elliptical orbits.
Many comet nuclei orbit the Sun in the Kuiper belt, a
doughnut-shaped region beyond Pluto. Billions of
cometary nuclei are also believed to exist in the
spherical Oort cloud located far beyond the Kuiper belt.
As an icy comet nucleus approaches the Sun, it
develops a luminous coma surrounded by a vast
hydrogen envelope. A gas (or ion) tail and a dust tail
extend from the comet, pushed away from the Sun by
the solar wind and radiation pressure.
Meteoroids, Meteors, and Meteorites
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Boulder-sized and smaller pieces of rock and metal in
space are called meteoroids. When a meteoroid enters
Earth’s atmosphere, it produces a fiery trail, and it is then
called a meteor. If part of the object survives the fall, the
fragment that reaches Earth’s surface is called a
meteorite.
Meteorites are grouped in three major classes according
to their composition: iron, stony-iron, and stony meteorites.
Rare stony meteorites, called carbonaceous chondrites,
may be relatively unmodified material from the primordial
solar nebula. These meteorites often contain organic
hydrocarbon compounds, including amino acids.
Fragments of rock from “burned-out” comets produce
meteor showers.
Meteoroids, Meteors, and Meteorites
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An analysis of the Allende meteorite suggests that a
nearby supernova explosion may have been involved in
the formation of the solar system some 4.6 billion years
ago.
An asteroid that struck Earth 65 million years ago probably
contributed to the extinction of the dinosaurs and many
other species. Another impact may have caused the
“Great Dying” of life 250 million years ago. Such
devastating impacts occur on average every 100 million
years.
Key Terms
amino acid
Apollo asteroid
asteroid belt
belt asteroid
carbonaceous chondrite
chondrites
coma (of a comet)
dust tail
dwarf planet
gas (ion) tail
hydrogen envelope
impact crater
iron meteorite
Kirkwood gaps
long-period comet
meteor
meteor shower
meteorite
meteoroid
nucleus (of a comet)
Oort cloud
planet
radiation (photon)
pressure
short-period comet
small solar-system
bodies (SSSBs)
stable Lagrange
points
stony meteorite
stony-iron
meteorite
Trojan asteroid
Widmanstätten
Patterns
WHAT DID YOU THINK?
Are the asteroids a former planet that was
somehow destroyed? Why or why not?
 No. The gravitational pull from Jupiter
prevented a planet from ever forming in
the asteroid belt. Also, the total mass of
the asteroids is much less than even the
mass of tiny Pluto, a dwarf planet.
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WHAT DID YOU THINK?
How far apart are the asteroids on
average?
 The distance between asteroids averages
10 million km.
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WHAT DID YOU THINK?
How are comet tails formed? Of what are
they made?
 Ices in comet nuclei are turned into gas by
absorbing energy from the Sun. Debris is
released in this process. Sunlight and the
solar wind push on the gas and dust,
creating the tails.
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WHAT DID YOU THINK?
In what directions do a comet’s tails point?
 Comets’ gas tails point directly away from
the Sun; their dust tails make arcs pointing
away from the Sun.
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WHAT DID YOU THINK?
What is a shooting star?
 A shooting star is a piece of space debris
plunging through Earth’s atmosphere—a
meteor. It is not a star.
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