Question 9.1

A piece of space debris that you pick up from the ground is called a(n):

1. asteroid
2. meteoroid
3. meteor
4. meteorite
5. comet

Question 9.2

Space debris that is a roughly equal mix of rock and ice is called a(n):

1. asteroid
2. comet
3. meteoroid
4. meteorite
5. meteor

Question 9.3

Which is the rarest type of meteorite found on Earth?

1. iron
2. stony-iron
3. stony

Question 9.4

Which part of a comet is solid?

1. nucleus
2. halo
3. gas tail
4. dust tail
5. coma

Question 9.5

Suppose you were standing on Pluto. Describe the motions of Charon relative to the horizon. Under what circumstances would you never see Charon?

Question 9.6

Describe the circumstantial evidence that supports the idea that Pluto is one of a thousand similar icy worlds that once occupied the outer regions of the solar system.

Question 9.7

What role did Charon play in enabling astronomers to determine Pluto’s mass?

Question 9.8

Why are asteroids, meteoroids, and comets of special interest to astronomers who want to understand the early history of the solar system?

Question 9.9

Describe the objects in the asteroid belt, including their sizes, orbits, and separation.

Question 9.10

To test your understanding of the asteroid belt, do Interactive Exercise 9.1 on the assigned Web site. You can print out your results, if required.

Question 9.11

Why are there many small asteroids but only a few very large ones?

Question 9.12

Describe the different chemistries of the two tails of a comet.

Question 9.13

In what directions do comet tails point, and why?

Question 9.14

What are the Kirkwood gaps, and what causes them?

Question 9.15

What are the Trojan asteroids, and where are they located?

Question 9.16

Describe the three main classifications of meteorites. How do astronomers believe that these different types of meteorites originated?

Question 9.17

Why do astronomers believe that the debris that creates many isolated meteors comes from asteroids, whereas the debris that creates meteor showers is related to comets?

Question 9.18

To test your understanding of comets, do Interactive Exercise 9.2 on the assigned Web site. You can print out your results, if required.

Question 9.19

Why is the phrase “dirty snowball” an appropriate characterization of a comet’s nucleus?

Question 9.20

What and where is the Kuiper belt, and how is it related to debris left over from the formation of the solar system?

Question 9.21

Why do scientists think the Tunguska event was caused by a large meteoroid and not a comet?

Question 9.22

What evidence in Figure 9-34 supports the labeling of the gas and dust tails?

Question 9.23

Would you expect the surfaces of Pluto and Charon to be heavily cratered? Explain.

Question 9.24

How did the regolith on asteroid Eros form?

Question 9.25

Why are comets generally brighter after passing perihelion (closest approach to the Sun) than before reaching perihelion?

Question 9.26

Can you think of another place in the solar system where a phenomenon similar to the Kirkwood gaps in the asteroid belt is likely to exist? Explain your answer.

Question 9.27

Where on Earth might you find large numbers of stony meteorites that have not been significantly changed by weathering?

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Question 9.28

* Assuming a constant rate of meteor infall, how much mass has Earth gained in the past 4.6 billion years?

Question 9.29

Suppose it was discovered that the asteroid Hermes had been perturbed in such a way as to put it on a collision course with Earth. Describe what you would do to counter such a catastrophe using present technology.

Question 9.30

From the abundance of craters on the Moon and Mercury, we know that numerous asteroids and meteoroids struck the inner planets during the very early history of the solar system. Is it reasonable to suppose that numerous comets also pelted the planets 4 to 4½ billion years ago? What effects would such a cometary bombardment have had, especially with regard to the evolution of the primordial atmospheres of the terrestrial planets and oceans on Earth?

Question 9.31

An ocean on Earth were struck by a comet nucleus a kilometer across? What physical effects would occur to Earth?

Question 9.32

We passed through the tail of a comet? What would happen to Earth and life on it?

Question 9.33

As some astronomers have recently argued, passage of the solar system through an interstellar cloud of gas could perturb the Oort cloud, causing many comets to deviate slightly from their original orbits? What might be the consequences for Earth?

Question 9.34

All of the space debris (asteroids, meteoroids, and comets) had been cleared out of the solar system 3.8 billion years ago? What would have been different in the history of Earth and in the history of life on Earth?

Question 9.35

The discovery of Charon (see Figure 9-4) was made by an astronomer at the U.S. Naval Observatory. Search the Web to find out why the U.S. Navy carries out work in astronomy.

Question 9.36

Search the Web to find out why some scientists disagree with the idea that a tremendous impact led to the demise of the dinosaurs. (They do not dispute that the impact occurred, only what its consequences were.) What are their arguments? From what you have learned, what is your opinion?

Question 9.37

Several scientific research programs are dedicated to the search for near-Earth objects (NEOs), especially those that might someday strike our planet. Search the Web for information about at least one of these programs. How does the program search for NEOs? How many NEOs are now known, and how many has this program found? Will any of these NEOs pose a threat in your lifetime?

Question 9.38

Search the Web to learn whether there are any comets visible at present. List them. What constellations are they presently in? Are any visible to the naked eye? (Recall that the unaided human eye sees objects brighter than about sixth magnitude.)

Question 9.39

What are the differences between meteors, meteorites, and meteoroids?

Question 9.40

Where are asteroids found in the solar system?

Question 9.41

Why is Pluto no longer considered a planet?

Question 9.42

Are planets the only solar system bodies with moons? If not, what other types of solar system bodies have them?

Question 9.43

What is the typical separation between asteroids in the Asteroid Belt?

Question 9.44

Of the following objects, which contains the largest bodies?

1. asteroids
2. meteoroids
3. moons
4. planetary rings
5. comets

Question 9.45

Use the Starry Night™ program to observe Pluto and Charon. Select Favourites > Explorations > Pluto. The view is centered upon Pluto from about 42,000 km above its surface. One of Pluto’s moons, Charon, is also shown, along with its orbit. Note that the south pole stick on Charon lines up with the star Atria and that pole sticks on Pluto are aligned with the red meridian line. Make a note of the time shown in the toolbar. With the Time Flow Rate at 30,000×, click the Play button and observe both the motion of Charon around Pluto and the rotation of Pluto. Use the time controls to Stop time once Charon has completed a full orbit with its south pole stick once again aligned with the star Atria. (a) What is the period of Charon’s orbit? (b) What is Pluto’s rotation period? (c) How do these two periods compare? Explain this result. (d) If Earth and the Moon were to have the same orbital and rotational relationship between them that Pluto and Charon have, which of the following statements would NOT be true?

• (i) The Moon would always appear above one particular location on Earth.
• (ii) Everyone on Earth would see the Moon pass through their sky every month.
• (iii) Earth would rotate with a period of 27.3 days.

Question 9.46

Use Starry Night™ to locate the largest asteroid Ceres, now designated a dwarf planet, and to examine the asteroid belt. Select Favourites > Explorations > Asteroids. The view shows the orbits of the major planets and of the dwarf planet Ceres, centered upon the Sun from a distance just beyond the orbit of Jupiter, at about 5.3 AU. Numerous asteroids are represented by the bright green dots. Use the location scroller to examine the extent of the asteroid belt and observe the orbits of some of the asteroids from different perspectives, particularly along the plane of the ecliptic and perpendicular to the plane of the ecliptic. (a) In which region of the solar system is the major concentration of asteroids? (b) How would you describe the plane of Ceres’s orbit relative to the plane of the ecliptic?

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Question 9.47

Use Starry Night™ to explore some of the dwarf planets of the solar system. Select Favourites > Explorations > Dwarf Planets from the menu. This view, from a position in space about 97 AU from the Sun, shows Neptune’s orbit as well as the orbits of several dwarf planets. Right-click (Ctrl-click on a Mac) on the Sun and select Centre from the contextual menu. (a) Do the dwarf planets revolve about the Sun in the same direction as the planets or in the opposite direction? (b) Use the location scroller to adjust the view so that the plane of Neptune’s orbit appears edge-on. How do the orbital planes of the dwarf planets compare to those of the planets?

Question 9.48

Make arrangements to view an asteroid. At opposition, some of the largest asteroids are bright enough to be seen through a modest telescope. You can use Starry Night™ to see which bright asteroids are visible. Click the Home button in the toolbar followed by the Stop button to stop time flow. Set the time in the toolbar to your expected observing time. Open the Find pane and clear any data in the search box to bring up a list of solar system objects. Click the + button to the left of the Asteroids entry to expand the list of asteroids. Asteroids that are visible from your location at the time of your planned observation will appear in black type, while those that are not visible will appear in light gray type. Double-click on one of the asteroids that will be visible at the time of your planned observation. Zoom in on the asteroid progressively and print a chart of its position in the sky at various zoom factors. To print a chart, move the mouse over the asteroid in the view and open its contextual menu (right-click on a PC or Ctrl-click on a Mac), and select Print Chart. In the Print Settings dialog, select the 3 Pane layout, choose 3 different fields of view, such as 90°, 50°, and 5°, and select Use current settings. As an alternative to using Starry Night™, you can check the “Minor Planets” section of the current issue of the Astronomical Almanac to see if any bright asteroids are close to opposition, or check the current issue, as well as the most recent January issue, of Sky & Telescope or Astronomy magazines, for star charts showing the paths of bright asteroids among the constellations. You will need an appropriate sky chart to distinguish the asteroid from background stars. Observe the asteroid on at least two occasions separated by a few days. On each night, draw a star chart of the objects in your telescope’s field of view. Has the position of one starlike object shifted between observing sessions? Does the position of the moving object agree with the path plotted on your star charts? Do you feel confident that you have in fact observed an asteroid? Explain.

Question 9.49

Make arrangements to view a meteor shower. The Starry Night™ database contains the positions of the radiant points from which major showers of meteors appear to come in the sky. Select an appropriate meteor shower from the list. Click the Home button and set the date to the listed date for this shower and the time to midnight. Select View > Solar System > Meteor Showers and Labels > Meteor Showers from the menu. Find the selected meteor shower in the view, move the cursor over its radiant point, right-click (Ctrl-click on a Mac) to open the contextual menu, and select Show Info. Expand the top layer of the Info pane to show the Dates of maximum intensity and the approximate Zenith Hourly Rate (ZHR) for this shower. Often, a good display can be seen a day or two before or after the maximum date. Ideally, you need a clear, moonless sky and a relatively clear horizon. The Moon’s glow in the sky will hide many of the faint meteor trails. Attempt to count the number of meteors that you see in 10-minute intervals and compare these rates with those in the Starry Night™ database. You should remove these radiant points from your view before leaving this exercise.

Question 9.50

Make arrangements to view a comet through a telescope. Since astronomers discover about a dozen comets each year, a comet is usually visible somewhere in the sky. Unfortunately, most comets are quite dim and you will need access to a moderately large telescope. You can use Starry Night™ to see which comets are visible. First, update comets online by opening the LiveSky menu at the top of the program and selecting Update Data Files. If the data files were updated, close and restart the program. Set the time in the toolbar to your expected observing time, stop time flow, and then find a comet that is visible by opening the Find pane, clearing the search box, and expanding the Comets list. The names of those comets visible at your selected observing time will be in black type. To find the expected apparent magnitude of a comet, click the menu button at the extreme left of the listing for the comet and select Show Info. The comet’s apparent magnitude will be listed under the Other Data layer of the Info pane. A comet brighter than about 5th magnitude will be visible with binoculars, while a comet fainter than about 12th magnitude will not be easily visible, even through a telescope. If the brightness of the comet you have chosen is too low, try other comets. (Hint: Remember that the larger the apparent magnitude value, the lower the brightness.) Double-click and Zoom in on a selected comet to see the direction of its tail. Of course, Starry Night™ can only depict the comet with a symbolic image and will not show the real appearance of any comet. You can print finder charts as described in the previous Observing Project on asteroids. Alternatively, consult the Web or recent issues of the IAU Circular, published by the International Astronomical Union’s Central Bureau for Astronomical Telegrams, which contains predicted positions and the anticipated brightness of comets in the sky. Also, if there is an especially bright comet in the sky, the latest issue of Sky & Telescope or Astronomy magazines might contain useful information. Observe the comet through a telescope. Can you distinguish it from background stars? Can you see its coma? How many tails do you see? (Note that a telescope with a low magnification, or a pair of binoculars, is more suitable for viewing a bright, nearby comet with an extended tail.)

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Question 9.51

Use Starry Night™ to study the motion of two comets. (a) Open Favourites > Explorations > Hyukatake to observe Comet Hyakutake from space as it approaches the inner solar system. Click the Play button and observe the comet as time progresses. Allow time to continue and observe the approach of a second comet, Hale-Bopp. To repeat the animation, select File > Revert from the menu and, if you wish, decrease the Time Flow Rate to observe the motion of the comet more closely. Use the location scroller to gain different perspectives on the motion of both comets. Describe what you see. Is each comet’s orbit in about the same plane as the orbits of the inner planets, or is it steeply inclined to that plane? (Hint: You can stop Time Flow and right-click on each comet in turn and click on Orbit in the dropdown menu to display each comet’s orbit.) How does the comet’s speed vary as it moves along its orbit? During which part of the orbit is the tail visible? In what direction does the tail point? (b) Select Favourites > Explorations > Atlas from the menu. Set the date in the toolbar to January 1, 1995. Open the Find pane and enter Hyakutake in the search box to center the view on the comet. Change the Time Flow Rate to 1 sidereal day and click the Play button. Observe the comet’s motion for at least two years of simulated time. Describe the motion, and explain why it is more complicated than the motion you observed in part (a). (c) Click the Now button in the toolbar. Set the time step to 1 lunar m, and click the Play button. Comet Hyakutake is currently moving almost directly away from the Sun and so, as seen from the Sun, its position on the celestial sphere should not change. Is this what you see in Starry Night™? Explain your observation. (Hint: You are observing from Earth, not the Sun.)