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Chapter 15 - Solar System Debris
1. Meteors
If you feel so inclined, this might be a good place to insert a discussion of UFOs. On April
10, 1972 a house-size chunk of rock entered the Earth’s atmosphere at a grazing angle and,
after penetrating to an altitude of about 50 km, skipped back into space. The meteor it
produced was bright enough to be visible in the daytime. It spent a few minutes in the
atmosphere on a path that carried it northward over Utah, Idaho, Montana, and Alberta.
The object was very extensively observed, even though it appeared without warning.
There were many eyewitness reports, numerous still photographs (one of which was
featured in Life magazine), and movie footage. It was also detected by an Air Force
satellite designed to look for the infrared emission from enemy missiles. In all of the years
that UFO sightings have been reported, there isn’t a single instance of a sighting anywhere
near as well documented as the April 1972 meteor. I find it hard to believe that if UFOs are
physical objects in the Earth’s atmosphere we have so little observational evidence of their
Incidentally, if the body that skimmed the Earth’s atmosphere had entered at a slightly
steeper angle it would have penetrated to the ground rather than skipping back into space.
It would have struck southern Alberta and produced an explosion comparable to a nuclear
2. Meteorites
Some students may be curious how astronomers have been able to tell that the amino acids
found in some carbonaceous chondrites are extraterrestrial rather than terrestrial
contamination. After all, nearly all of the meteorites that have been recovered have been on
Earth long enough for significant contamination to occur. The reason to reject the
contamination hypothesis is that the amino acids in meteorites are an equal mixture of
right- and left-handed stereoisomers. Those in terrestrial organisms are exclusively lefthanded.
3. Asteroids
Figure 15.11, showing the location of 7000 asteroids on March 7, 1997, clearly
demonstrates that the majority of asteroids are in the asteroid belt. This figure, however,
may reinforce the impression students get from many movies, including the first Star Wars
movie, that asteroid belts are filled with asteroids only a few miles from one another. We
have now sent numerous spacecraft through the asteroid belt to get to the outer planets.
While there was some concern about the safety of the spacecraft, the danger was from
pinhead size bodies (which are also more numerous there than elsewhere) rather than large
chunks of rock.
4. Comets
It isn’t hard to predict that there will continue to be discoveries of additional bodies in the
Kuiper belt. To get the current list of Kuiper belt bodies, look at the web site
Chapter 15 - Solar System Debris
5. Where Did Comets Form?
6. Collisions with Earth
The announcement in spring 1998 that a newly discovered asteroid might impact the Earth
in about 30 years created a news circus. The subsequent announcement that archival
observations of the asteroid showed that it would clearly miss the Earth did nothing for the
credibility of the argument that a collision with an Earth-crossing asteroid is inevitable and
that we needed a much fuller inventory of such asteroids in order to assess the risks. That
inventory, however, is being carried out. The list of Earth-crossing asteroids is
considerably longer than it was a decade ago.
The relationship between impacts and extinctions may not be completely settled, but the
advocates of other extinction models such as volcanic activity seem to be less vocal than
they were a decade or so ago. Certainly the public seems to have accepted the idea that
extinctions are related to impacts.
achondrite — A stony meteorite lacking chondrules.
Amor asteroid — A member of a class of asteroids having orbits that cross the orbital distance
of the Earth.
Apollo asteroid — A member of a class of asteroids having orbits that cross the orbital distance
of the Earth.
asteroid — A small, planetlike solar system body. Most asteroids are rocky in makeup and
have orbits of low eccentricity and inclination.
asteroid belt — The region of the solar system lying between 2.1 and 3.3 AU from the Sun.
The great majority of asteroids are found in the asteroid belt.
Aten asteroid — An asteroid having an orbit with semimajor axis smaller than 1 AU.
C-type asteroid — One of a class of very dark asteroids whose reflectance spectra show no
absorption features due to minerals.
carbonaceous chondrite — A stony meteorite that contains carbon–rich material. Carbonaceous
chondrites are thought to be primitive samples of material from the early solar system.
chondrite — A meteorite containing chondrules.
chondrule — A small, spherical body embedded in a meteorite. Chondrules are composed of
iron, aluminum, and magnesium silicate rock.
coma — A spherical gaseous region that surrounds the nucleus of a comet. The coma of a
comet may be 100,000 km or more in diameter.
comet — A small, icy body in orbit about the Sun. When a comet is near the Sun, it displays a
coma and tail.
cosmic ray — An extremely energetic ion or electron that travels through space almost at the
speed of light. Most cosmic rays come from great distances and may be produced in
supernovae and pulsars.
cosmic ray exposure age — The length of time that has passed since a meteorite broke off from
a larger body and became exposed to radiation damage from cosmic rays.
dust tail — A comet tail that is luminous because it contains dust that reflects sunlight. The
dust in a comet tail is expelled from the nucleus of the comet.
Chapter 15 - Solar System Debris
fireball — An especially bright streak of light in the sky produced when an interplanetary dust
particle enters the Earth’s atmosphere, vaporizing the particle and heating the atmosphere.
iron meteorite — A meteorite composed primarily of iron and nickel.
long–period comet — A comet with an orbital period of 200 years or longer.
M-type asteroid — One of a class of asteroids that have reflectance spectra like those of
metallic iron and nickel.
meteor — A streak of light produced by a meteoroid moving rapidly through the Earth’s
atmosphere. Friction vaporizes the meteoroid and heats atmospheric gases along the path
of the meteoroid.
meteor shower — A temporary increase in the normal rate at which meteors occur. Meteor
showers last for a few hours or days and occur on about the same date each year.
meteorite — The portion of a meteoroid that reaches the Earth’s surface.
meteoroid — A solid interplanetary particle passing through the Earth’s atmosphere.
micrometeorite — A meteoritic particle less than 50 millionths of a meter in diameter.
Micrometeorites are slowed by atmospheric gas before they can be vaporized, so they drift
slowly to the ground.
minor planet — Another name for asteroid.
new comet — A comet that has entered the inner solar system for the first time.
nucleus — An irregularly shaped, loosely packed lump of dirty ice several kilometers across
that is the permanent part of a comet.
Oort cloud — The region beyond the planetary system, extending to 100,000 AU or more,
within which a vast number of comets orbit the Sun. When comets from the Oort cloud
enter the inner solar system, they become new comets.
plasma tail — A narrow, ionized comet tail pointing directly away from the Sun.
radiant — The point in the sky from which the meteors in a meteor shower seem to come.
reflectance spectrum — The reflectivity of a body as a function of wavelength.
S-type asteroid — One of a class of asteroids whose reflectance spectra show an absorption
feature due to the mineral olivine.
short–period comet — A comet with an orbital period shorter than 200 years.
solidification age — The amount of time since a meteorite solidified from the molten state.
stony meteorite — A meteorite made of silicate rock.
stony–iron meteorite — A meteorite made partially of stone and partially of iron and other
terminal velocity — The speed with which a body falls through the atmosphere of a planet
when the force of gravity pulling it downward is balanced by the force of air resistance.
Trojan asteroid — One of a group of asteroids that orbit the Sun at Jupiter’s distance and lie
60° ahead of or behind Jupiter in its orbit.
V-type asteroid — The asteroid Vesta, which is unique in having a reflectance spectra
resembling those of basaltic lava flows.
zodiacal light — The faint glow extending away from the Sun caused by the scattering of
sunlight by interplanetary dust particles lying in and near the ecliptic.
Chapter 15 - Solar System Debris
Conceptual Questions
1. A meteoroid is a small body in interplanetary space. When it enters the Earth’s atmosphere
it produces a meteor. The parts of it that reach the Earth’s surface become meteorites.
2. (a) Slowed by gases high in the atmosphere and gently fall to the ground. (b) Burns up
completely in the atmosphere. (c) Burns away the outer few cm of material, the rest is
slowed so that it falls to the ground at terminal velocity. (d) Strikes the ground before it can
be slowed to terminal velocity. Makes a crater when it hits the ground.
3. They are traveling along parallel paths from the direction of the radiant.
4. They recur whenever the Earth recrosses the orbit of the meteor swarm, which occurs on
the same date each year.
5. They appear to be unaltered samples of material from the early solar system.
6. They show that the iron in the meteorite cooled very slowly from a molten state.
7. It tells how long it has been since the meteorite broke off from a larger parent body.
8. They must have been at least 100 km in diameter in order to have cooled slowly enough to
account for the sizes of crystals in iron meteorites.
9. It is too faint to be seen with the unaided eye.
10. Meteorites are fragments of asteroids.
11. When the nucleus comes within about 3 AU of the Sun it is warmed by sunlight to the
point where water and other ices evaporate and flow outward carrying dust particles with
12. From the nucleus
13. Dust is pushed away from the Sun by solar radiation. The solar wind pushes the ionized
gas in the plasma tail away from the Sun.
14. The new comets have aphelia between 10,000 and 100,000 AU, showing that the new
comets come from a swarm orbiting the Sun at enormous distances.
15. Their orbits are altered by the gravity of passing stars.
16. From a disk of comets that orbit between Neptune’s orbit and about 1000 AU
17. They strike planets, are eroded by many passages near the Sun, or are fragmented by
passing too near the Sun or a planet.
18. Meteor showers are produced by swarms of dust particles left behind when the icy material
in a comet is used up.
19. No comet has ever been found to have a hyperbolic path when it first entered the planetary
system. Thus none of them have speeds as great as escape velocity.
20. The flash and heat would ignite vegetation triggering large fires. Ejected molten and solid
material would cover the region near the impact crater. Vaporized material would solidify
in the upper atmosphere and block sunlight. There would also be strong acid rain and
temporary heating of the atmosphere.
21. An iridium-rich layer of sediment was laid down at the time that widespread extinction
took place at the end of the Cretaceous era. Iridium is abundant in meteorites but rare in the
Earth’s surface rocks.
Chapter 15 - Solar System Debris
1. 3.0 years, 6.0 years
2. 11.9 years, the same as Jupiter’s orbital period
3. Twice each 1.12 year-long orbital period
4. 29,000 AU