CHAPTER 25 | METEORITES, ASTEROIDS, AND COMETS 579
▶ Most asteroids lie in a belt between Mars and Jupiter. Kirkwood gaps
(p. 562) in asteroid belt orbits are caused by orbital resonances with
Jupiter.
▶ Two groups of asteroids called the Trojan asteroids (p. 563) are
caught in the Lagrangian points along Jupiter’s orbit 60° ahead
and 60° behind the planet. Apollo-Amor objects (p. 562) have
orbits that cross into the inner solar system. If they pass near
Earth, they are called Near-Earth Objects (NEOs) (p. 562).
Centaurs (p. 563) are asteroids that orbit among the planets of
the outer solar system.
▶ The asteroids formed as rocky planetesimals between Mars and Jupiter,
but Jupiter prevented them from accumulating into a planet. Collisions
have fragmented all but the largest of the asteroids. Most of the
material inferred to have been originally in the asteroid belt has been
gravitationally perturbed and swept up by the planets or tossed out of
the solar system.
▶ Hirayama families (p. 564) of asteroids follow very similar orbits and
have similar spectra. They appear to be fragments produced in past
collisions of asteroids.
▶ C-type (p. 564) asteroids are more common in the outer asteroid
belt where the solar nebula was cooler. They are darker and may
be carbonaceous. S-type (p. 564) asteroids are the most common
and may be the source of the more common kind of meteorites, the
chondrites. S-type asteroids are more frequently found in the inner
belt. M-type (p. 564) asteroids appear to have nickel-iron composi-
tions and may be the cores of differentiated asteroids shattered by
collisions.
▶ A visible comet is produced by a lump of ices and rock usually between
1 and 100 km in diameter, referred to as the comet nucleus. In long,
elliptical orbits, the icy nucleus stays frozen until it nears the sun.
Then, some of the ices vaporize and release dust and gas that is blown
away to form a prominent head and tail.
▶ (^) A type I (gas) (p. 568) comet tail is ionized gas carried away by the
solar wind. A type II (dust) (p. 568) tail is solid debris released from
the nucleus and blown outward by the pressure of sunlight. A comet’s
tail always points away from the sun, no matter in what direction the
comet is moving.
▶ (^) The coma (p. 568), or head, of a comet can be up to a million
kilometers in diameter.
▶ (^) Spacecraft fl ying past comets have revealed that they have very dark,
rocky crusts. Jets of vapor and dust issue from active regions on the
sunlit side.
▶ (^) The low density of comet nuclei shows that they are irregular mixtures
of ices and silicates, probably containing large voids. At least one
comet nucleus has surface features showing the material has a surpris-
ing amount of strength.
▶ (^) Comets are believed to have formed as icy planetesimals in the outer
solar system, and some were ejected to form the Oort cloud (p. 571).
Comets perturbed inward from the Oort cloud become long-period
comets.
▶ (^) Other icy bodies formed in the outer solar system and now make up
the Kuiper belt beyond Neptune. Objects from the Kuiper belt that
are perturbed into the inner solar system can become short-period
comets.
▶ (^) A major impact on Earth can trigger extinctions. These are due to
global fi res caused by heated material falling back into the atmo-
sphere, tsunamis inundating coastal regions around the world, acid
rain resulting from large amounts of carbon dioxide released into the
atmosphere, and climate change caused by the atmosphere fi lling with
dust, plunging the entire Earth into darkness for years.
▶ (^) An impact at Chicxulub (p. 576) in Mexico’s Yucatán region
65 million years ago appears to have triggered the extinction of
75 percent of the species then on Earth, including the dinosaurs.
Review Questions
- What do Widmanstätten patterns indicate about the history of iron
meteorites? - What do chondrules tell you about the history of chondrites?
- Why are there no chondrules in achondritic meteorites?
- Why do astronomers refer to carbonaceous chondrites as unmodifi ed
or “primitive” material? - How do observations of meteor showers reveal one of the sources of
meteoroids? - How can most meteors be cometary if all meteorites are asteroidal?
- Why do astronomers think the asteroids were never part of a full-sized
planet? - What evidence indicates that the asteroids are mostly fragments of
larger bodies? - What evidence indicates that some asteroids have differentiated?
- What evidence indicates that some asteroids have had geologically
active surfaces? - How is the composition of meteorites related to the formation and
evolution of asteroids? - What is the difference between a type I comet tail and a type II tail?
- What evidence indicates that cometary nuclei are rich in ices?
- Why do short-period comets tend to have orbits near the plane of the
solar system? - What are the hypotheses for how the bodies in the Kuiper belt and
the Oort cloud formed? - How Do We Know? How would studying the chemical composition
of only the largest, brightest, and most easily observed asteroids
possibly yield misleading information about asteroids in general?
Discussion Questions
- It has been suggested that humans may someday mine the asteroids
for materials to build and supply space colonies. What kinds of
materials could Earthlings get from asteroids? (Hint: What are S-, M-,
and C-type asteroids made of, respectively?) - If cometary nuclei were heated during the formation of the solar
system by internal radioactive decay rather than by solar radiation,
how would comets differ from what is observed? - Do you think the government should spend money to fi nd near-Earth
asteroids? How serious is the risk?
Problems
- Large meteorites are hardly slowed by Earth’s atmosphere. Assuming
the atmosphere is 100 km thick and that a large meteorite falls
perpendicular to the surface, how long does it take to reach the
ground? (Hint: Refer to the text for typical speeds of meteoroids.) - If a single asteroid 1 km in diameter were to be fragmented into
meteoroids 1 m in diameter, how many would it yield? (Hint: The
volume of a sphere 4 __ 3 πr^3 .) - If a trillion (10^12 ) asteroids, each 1 km in diameter, were assembled
into one body, how large would it be? (Hint: The volume of a sphere
__ 34 πr^3 .) Compare that to the size of Earth. - The asteroid Vesta has a mass of 2 1020 kg and a radius of about
250 km. What is its escape velocity? Could you jump off the asteroid?
(Hint: See Chapter 5.) - What is the maximum angular diameter of the largest asteroid, Ceres,
as seen from Earth? Could Earth-based telescopes detect surface
features? Could the Hubble Space Telescope? (Hints: Use the small-
angle formula, Chapter 3. The angular resolution of Earth-based