More than 7000 asteroids have been cataloged. Most orbit in a broad band called the asteroid belt between the orbits of Mars and Jupiter. They are probably primal rocks that never clumped together to form a planet.
The masses of the asteroids are small, totaling less than 1/10 the mass of Earth's Moon. The largest asteroids are a few hundred kilometers across. Most are much smaller. Asteroids are classified according to the properties of their reflected light. Brighter S-type (silicate) asteroids dominate the inner asteroid belt, whereas darker C-type (carbonaceous) asteroids are more plentiful in the outer regions. They are believed to have changed little since the solar system formed. Smaller asteroids tend to be irregular in shape, and may have experienced violent collisions in the past. The few asteroid densities that have been reliably measured are generally consistent with the preceding classification. The best views of asteroids obtained by unmanned space probes during the 1990s.
A few Earth-crossing asteroids have orbits that intersect Earth's orbit and will probably collide with our planet one day. The Trojan asteroids share Jupiter's orbit, remaining 60°
ahead of or behind that planet as it moves around the Sun. The Kirkwood gaps in the main asteroid belt have been cleared by Jupiter's gravity.
Comets are fragments of icy material that normally orbit far from the Sun. Unlike the orbits of most other bodies in the solar system, comet orbits are often highly elongated and not confined to the ecliptic plane. Most comets are thought to reside in the Oort cloud, a vast "reservoir" of cometary material, tens of thousands of astronomical units across, completely surrounding the Sun. A very small fraction of comets happen to have highly elliptical orbits that bring them into the inner solar system. Comets with orbital periods less than about 200 years are thought to originate not in the Oort cloud but in the Kuiper belt, a broad band lying roughly in the ecliptic plane, beyond the orbit of Neptune.
As a comet approaches the Sun its surface ice begins to vaporize. We see the comet by the sunlight reflected from the dust and vapor released. The nucleus, or core, of a comet may be only a few kilometers in diameter. It is surrounded by a coma of dust and gas. Surrounding this is an extensive invisible hydrogen envelope. Stretching behind the comet is a long tail, formed by the interaction between the cometary material and the solar wind. The ion tail consists of ionized gas particles and always points directly away from the Sun. The dust tail is less affected by the solar wind and has a somewhat curved shape.
Spacecraft visited Halley's comet in 1986 and studied its nucleus. All other comet studies have been indirect, usually based on spectroscopic measurements. Comets are icy, dusty bodies, sometimes called "dirty snowballs," that are believed to be leftover material unchanged since the formation of the solar system. Their masses are comparable to the masses of small asteroids.
Comets and stray asteroids are responsible for most of the cratering on the various worlds in the solar system. Earth is still subject to these sorts of collisions. The most recent large impact occurred in 1908, when an asteroid apparently exploded several miles above Siberia. Comet Shoemaker—Levy 9 struck Jupiter in 1994, causing violent explosions in that planet's atmosphere.
Meteors, or "shooting stars," are bright streaks of light that flash across the sky as a meteoroid, a piece of interplanetary debris, enters Earth's atmosphere. If any of the meteoroid reaches the ground, it is called a meteorite. The major difference between meteoroids and asteroids is their size. The dividing line between them is conventionally taken to be 100 m.
Each time a comet rounds the Sun, some cometary material becomes dislodged, forming a meteoroid swarm—a group of small micrometeoroids following the comet's original orbit. If Earth happens to pass through the comet's orbit, a meteor shower occurs. Larger meteoroids are probably pieces of material chipped off asteroids following collisions in the asteroid belt. Meteorite composition is thought to mirror the composition of the asteroids, and the few orbits that have been determined are consistent with an origin in the asteroid belt. Some meteorites show evidence of heating, but the oldest ones do not. Most meteorites are between 4.4 and 4.6 billion years old.
1. Most asteroids move on almost circular orbits. (Hint)
2. The Apollo asteroids have perihelion distances of less than 1 A.U. (Hint)
3. The C-type asteroids are so named because of their heavily cratered surfaces. (Hint)
4. S-type asteroids are more common in the inner part of the asteroid belt. (Hint)
5. The least reflective asteroids are C-type. (Hint)
6. The Kirkwood gaps are two broad zones within the asteroid belt, lying around 2.5 and 3.0 A.U. from the Sun and each about 0.1 A.U. wide, in which no asteroids are found. (Hint)
7. Some comets travel up to 50,000 A.U. from the Sun.
8. Cometary orbits always lie close to the ecliptic plane. (Hint)
9. Halley's comet spends most of its time beyond the orbit of Saturn. (Hint)
10. The Oort cloud is the large cloud of gas surrounding a comet while it is near the Sun.
11. Tails of comets always lie along the path of the orbit.
12. The light known as a meteor is due to the literal burning up of a meteoroid in Earth's atmosphere. (Hint)
13. Some meteorites found on Earth originally came from the Moon or Mars. (Hint)
14. Comets are the sources of meteor showers. (Hint)
15. Astronomers have succeeded in tracing the orbits of some meteorites back into the asteroid belt. (Hint)
1. Asteroids are generally _____ in composition. (Hint)
2. The asteroid belt, where most asteroids are found, lies between the orbits of _____ and _____. (Hint)
3. The largest asteroids are _____ of kilometers in diameter; the smallest (by definition) are only _____ meters across. (Hint)
4. The Trojan asteroids share an orbit with _____. (Hint)
5. The Kirkwood gaps are caused by an orbital _____ with Jupiter. (Hint)
6. Comets are generally _____ in composition. (Hint)
7. The process of sublimation is one in which a solid turns into a _____. (Hint)
8. Comets orbits are highly _____. (Hint)
9. The nucleus of a comet is typically _____ kilometers across. (Hint)
10. Passage of a comet near the Sun may leave a _____ moving in the comet's orbit. (Hint)
11. The Kuiper belt is the source of the _____ comets. (Hint)
12. Meteoroids are mostly fragments of _____ and _____. (Hint)
13. When a meteoroid enters Earth's atmosphere, we see a _____. (Hint)
14. Meteoroids that impact a planet or moon are called _____. (Hint)
15. The oldest meteorites are about _____ years old. (Hint)
1. Describe the differences among the Trojan, Apollo, and Amor asteroids. (Hint)
2. How have the best photographs of asteroids been obtained?
3. What are the Kirkwood gaps? How did they form? (Hint)
4. Compare and contrast the C-type and S-type asteroids. (Hint)
5. What are comets like when they are far from the Sun? What happens when they enter the inner solar system? (Hint)
6. Where are most comets found? (Hint)
7. Describe the various parts of a comet while it is near the Sun. (Hint)
8. What are the typical ingredients of a comet nucleus?
9. What are some possible fates of comets? (Hint)
10. Explain the difference between a meteor, a meteoroid, and a meteorite. (Hint)
11. What causes a meteor shower? (Hint)
12. What do meteorites reveal about the age of the solar system?
13. Why can comets approach the Sun from any direction, whereas asteroids generally orbit close to the ecliptic plane? (Hint)
14. Why do meteorites contain information about the early solar system, yet Earth does not? (Hint)
15. What might be the consequences if a 10-km-diameter meteorite struck Earth today? (Hint)
1. The largest asteroid, Ceres, has a radius 0.073 that of Earth and a mass 0.00015 Earth's mass. How much would a 100-kg astronaut weigh there? (Hint)
2. You are standing on the surface of a spherical asteroid 10 km in diameter, of density 3000 kg/m3. Can you throw a small rock into circular orbit around it? Give the speed required in km/s and mph.
3. Can you find a simple orbital resonance with Jupiter that can account for the small Kirkwood gap evident in Figure 14.6(a) at semi-major axis around 2.7 A.U.? (Hint)
4. The asteroid Icarus (Figure 14.4) has perihelion of 0.2 A.U. and orbital eccentricity of 0.69. Calculate its orbital semi-major axis and aphelion distance from the Sun. Do these figures, by themselves, necessarily imply that Icarus will one day collide with Earth? (Hint)
5. (a) Using the data given in the text, calculate Dactyl's orbital period as it moves around Ida. (b) NEAR's orbit around Icarus will have periapsis (closest approach) of about 45 km from the asteroid's center. If the mass of Eros is 2 
1016 kg and NEAR's orbit has an eccentricity of 0.25, calculate the spacecraft's orbital period. (Hint)
6. Using Kepler's laws of planetary motion (see Section 2.4), calculate the orbital period of a comet with a perihelion distance of 0.5 A.U. and aphelion in the Oort cloud, at a distance of 50,000 A.U. from the Sun. (b) As comet Hale—Bopp passed by the Sun, nongravitational forces changed its orbital period from 4200 years to 2400 years. By what factor did the comet's semi-major axis change? Given that the perihelion remained unchanged at 0.914 A.U., calculate the old and new orbital eccentricities. (c) A short-period comet has a perihelion distance of 1 A.U. and an orbital period of 125 years. What is its maximum distance from the Sun?
7. Astronomers estimate that comet Hale—Bopp lost mass at an average rate of about 350,000 kg/s during the time it spent close to the Sun—a total of about 100 days. Estimate the total amount of mass lost and compare it with the comet's estimated mass of 5 1015 kg. (Hint)
8. It has been hypothesized that Earth is under continuous bombardment by house-sized "minicomets" with typical diameters of 10 m, at the rate of some 30,000 per day. Assuming spherical shapes and average densities of 100 kg/ m3, calculate the total mass of material reaching Earth each year. Compare the total mass received in the past 1 billion years (assuming all rates were the same in the past) with Earth's mass. (Hint)
9. A particular comet has a total mass of 1013 kg, 95 percent of which is ice and dust. The remaining 5 percent is in the form of rocky fragments with an average mass of 100 g. How many meteoroids would you expect to find in the swarm formed by the breakup of this comet? (Hint)
10. It is observed that the number of asteroids or meteoroids of a given diameter is roughly inversely proportional to the square of the diameter. Approximating the actual distribution of asteroids in this way—a single 1000-km body (Ceres), one hundred 100-km bodies, ten thousand 10-km asteroids, and so on—and assuming constant densities of 3000Kg/m3, calculate the total mass found in the form of 1000-km bodies, 100-km bodies, 10-km bodies, and 1-km bodies. (Hint)
1. The only way to tell an asteroid from a star is to watch it over several nights; you can detect its movement in front of the star background. The astronomy magazines Sky and Telescope and Astronomy often publish charts for especially prominent asteroids. Look for the asteroids Ceres, Pallas, or Vesta. They are the brightest asteroids. Use the chart to locate the appropriate star field. Aim binoculars at that location in the sky; you may be able to pick out the asteroid from its location in the chart. If you can't, make a rough drawing of the entire field. Come back a night or two later, and look again. The "star" that has moved is the asteroid.
2. Although a spectacular naked-eye comet comes along only about once a decade, fainter comets can be seen with binoculars and telescopes in the course of every year. Sky and Telescope often runs a "Comet Digest" column announcing the whereabouts of comets. A comprehensive list of periodic comets expected to return in a given year can be found in Guy Ottewell's Astronomical Calendar. This calendar contains a wealth of other sky information as well, including monthly star charts. At this writing, it costs $15 a year and can be purchased from:
Astronomical Workshop
Furman University
Greenville, South Carolina 29613
(803) 294-2208
3. There are a number of major meteor showers every year, but if you plan to watch one, be sure to notice the phase of the Moon. Bright moonlight or city lights can obliterate a meteor shower. A common misconception about meteor watching is that most meteors are seen in the direction of the shower's radiant point. It's true that if you trace the paths of the meteors backward in the sky, they all can be seen to come from the radiant. But most meteors don't become visible until they are 20 or 30° from the radiant. Meteors can appear in all parts of the sky! Just relax and let your eyes rove among the stars. You will generally see many more meteors in the hours before dawn than in the hours after sunset. Why do you suppose meteors have different brightnesses? Can you detect their variety of colors? Watch for meteors that appear to "explode" as they fall. Watch for vapor trails that linger after the meteor itself has disappeared.
