1. Ch. 13 Comets and Asteroids Debris of the Solar System
There are several kinds of objects in our Solar System
Terrestrial planets and Jovian planets, with satellites (moons)
Dwarf planets (which can also have moons) and
“small solar system bodies” – asteroids, comets and meteoroids
Objects are still being classified: Kuiper Belt Objects, Plutoids, Plutinos, Trans-Neptunian Objects (TNOs), Oort cloud objects

2. From largest to smallest, the objects in the Solar System
can be classified
into categories
Notice that there
is some overlap
among objects
of the same size.
FIGURE 9-1 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 trans-Neptunian 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.

3. Asteroids: three major groups The Asteroid belt Trojan Asteroids Near-Earth Objects (NEOs)

4. The Inner Solar System (sizes NOT to scale)

5. This is a plot
of actual
positions of
known comets
and asteroids
(April 1, 2005).
This shows the
inner solar
system, out
to Jupiter.
Notice most of
these are in
the asteroid
belt between
Mars and Jupiter.

6. Expanding
the first
plot, we get
a plot of
known comets
and asteroids
in the region
around the
Earth.
Fortunately,
there aren’t
very many
of these
Near Earth
Objects
(NEOs).

7. This is the
view from the
side, i.e., in
the ecliptic
plane, of the
plot of the
objects in
the inner
solar system.

8. FIGURE 9-10 Asteroid Orbits
(a) The orbits of belt asteroids
Ceres, Pallas, and Juno are indicated to scale in this diagram.
Some asteroids, such as Apollo and Icarus, have highly eccentric
paths that cross Earth’s orbit. Others, called the Trojan
asteroids, follow the same orbit as Jupiter. (b) 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. Although the
asteroids appear packed together in this drawing, they are
typically millions of kilometers apart.

9. FIGURE 9-10 Asteroid Orbits
(a) The orbits of belt asteroids
Ceres, Pallas, and Juno are indicated to scale in this diagram.
Some asteroids, such as Apollo and Icarus, have highly eccentric
paths that cross Earth’s orbit. Others, called the Trojan
asteroids, follow the same orbit as Jupiter. (b) 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. Although the
asteroids appear packed together in this drawing, they are
typically millions of kilometers apart.

10. Ceres, the largest asteroid, is also a dwarf planet.
FIGURE 9-8 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. (NASA)

11. Asteroid Icarus comes close to Earth and to the Sun

12. Asteroids are often seen as streaks in photos.
FIGURE 9-11 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. (R. Evans and K. Stapelfeldt, Jet
Propulsion Laboratory and NASA)

13. The distribution of asteroids is affected by Jupiter.
FIGURE 9-12 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.
Repeated alignments with Jupiter have deflected asteroids
away from these orbits. The Trojan asteroids accompany
Jupiter as it orbits the Sun.

14. Ida and its satellite Dactyl
FIGURE 9-13 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 5-8).
Inset: Dactyl is also heavily cratered. (NASA)

15. Asteroids and meteoroids are small “rocky” objects The main difference between the two is size.

16. Some asteroids have been studied up close.
Asteroid Eros

17. Matilde is darker than a briquette of charcoal.
FIGURE 9-16 Asteroids
(a) Mathilde 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 x 48 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 x 47 km. The large crater in shadow is
about 20 km across. (b) Itokawa A near Earth asteroid visited
by the Japanese space probe Hayabusa. Samples were taken,
but it is not known how much of the asteroid was captured by
the spacecraft, which will return to Earth in (a: Johns
Hopkins University, Applied Physics Laboratory; b: ISAS, JAXA)

18. Itokawa was visited by the Japanese spacecraft Hayabusa.
FIGURE 9-16 Asteroids
(a) Mathilde 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 x 48 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 x 47 km. The large crater in shadow is
about 20 km across. (b) Itokawa A near Earth asteroid visited
by the Japanese space probe Hayabusa. Samples were taken,
but it is not known how much of the asteroid was captured by
the spacecraft, which will return to Earth in (a: Johns
Hopkins University, Applied Physics Laboratory; b: ISAS, JAXA)

19. Asteroid Eros was imaged by a spacecraft which then landed on it and sent back data on its composition. NEAR spacecraft site: For a simulation of the orbit of Eros see:

20. Close-up pictures of Eros crater and surface.
FIGURE 9-17 Asteroid Eros
(a) The Near-Earth
Asteroid Rendezvous (NEAR) Shoemaker spacecraft
took this image of asteroid Eros in February
1999. The top of the figure is the asteroid’s north polar
region. Eros’s dimensions are 33 x 13 x13 km (21 x 8 x
8 mi) and it rotates every 51⁄4 h. Its density is 2700 kg/m3,
close to the average density of Earth’s crust and twice as
dense as asteroid Mathilde. (b) Looking into the large crater
near the top of (a), which is 5.3 km (3.3 mi) across. (c) 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. (Johns Hopkins
Applied Physics Laboratory)
Crater about 3 mi across Photo of area 12 m across

21. The DAWN spacecraft was in orbit around Vesta, the second-largest of the asteroids in the Asteroid Belt, for almost a year, and is now orbiting Ceres.
Ceres movie:
First mapping orbit:
Latest news about the Dawn spacecraft:
4 Vesta; see next slide
FIGURE 9-17 Asteroid Eros
(a) The Near-Earth
Asteroid Rendezvous (NEAR) Shoemaker spacecraft
took this image of asteroid Eros in February
1999. The top of the figure is the asteroid’s north polar
region. Eros’s dimensions are 33 x 13 x13 km (21 x 8 x
8 mi) and it rotates every 51⁄4 h. Its density is 2700 kg/m3,
close to the average density of Earth’s crust and twice as
dense as asteroid Mathilde. (b) Looking into the large crater
near the top of (a), which is 5.3 km (3.3 mi) across. (c) 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. (Johns Hopkins
Applied Physics Laboratory)

22. Vesta

23. Further out from the asteroid belt, the Trojan Asteroids are
clumped in the
orbit of Jupiter.
Lagrange points
are places where
asteroids will be
trapped in the
On the next
slide, the Trojans
are in yellow.
FIGURE 9-14 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.

24. This is another
plot of actual
positions of
known comets
and asteroids
(April 1, 2005).
This shows the
outer solar
system, with
the orbit of
Jupiter and its
Trojan asteroids.
Notice there is
another belt
of objects out
past Neptune.

25. This shows the
outer solar
system, past
the orbit of
Jupiter.
This is the view
from the side,
i.e., in the
ecliptic plane.
Notice that the
comets are
coming from
all directions,
but the other
objects are in
the ecliptic plane.

26. The larger orbits of some new objects hint at the existence of another planet far from the Sun

27. Summary Classifying the Solar System objects
Astronomical objects smaller than the eight planets are classified as dwarf planets or small solar-system bodies (SSSBs).
A variety of other names, including asteroids, comets, meteoroids, trans-Neptunian objects, Kuiper belt objects (KBOs), and Oort cloud objects, overlap with “dwarf planet” and “SSSB.”
KBOs and Oort cloud objects are trans-Neptunian objects—they orbit farther from the Sun than the outermost planet Neptune.

28. 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.
The Apollo asteroids move in highly elliptical orbits that cross the orbit of Earth. Many of these asteroids will eventually strike the inner planets. They are also called Near Earth Objects (NEOs).

29. Comets
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 Pluto.
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 they have fairly circular orbits. When close to the Sun, they generally move in highly elliptical orbits.
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.

30. Meteoroids, Meteors, and Meteorites
Boulders and smaller rocks 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 primitive solar nebula. These meteorites often contain organic hydrocarbon compounds, including amino acids.
Fragments of rock from “burned-out” comets produce meteor showers. 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 caused the “Great Dying” of life 250 million years ago. Such devastating impacts occur on average every 100 million years.

31. Key Terms iron meteorite amino acid Small Solar-System Kirkwood gaps
Apollo asteroid
asteroid belt
belt asteroid
carbonaceous chondrite
chondrites
coma (of a comet)
dust tail (of a comet)
dwarf planet
gas (ion) tai
hydrogen envelope
impact crater
iron meteorite
Kirkwood gaps
Kuiper belt
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