1 SUMMARY Our Solar System contains numerous small bodies: dwarf planets, asteroids, comets, and meteoroids. They are important astronomically because they give us information about the time of formation, composition, and physical conditions in the solar nebula. Asteroids are rocky, metallic, or carbon rich objects found mainly in the asteroid belt between the orbits of Jupiter and Mars. Comets are icy bodies found mainly in the Oort cloud, far beyond the orbit of Pluto. A comet becomes visible if a passing star or some other event disturbs the comet's orbit so that it drops in toward the inner Solar System. There, solar heating thaws the frozen nucleus and evaporates gases. Radiation pressure and the solar wind then sweep the liberated gas and dust into a tail. Some comets are caught in short period orbits where they may be melted away to a collection of dust and grit. If the Earth crosses or nears the path of such a skeletal comet, the debris falling into the atmosphere causes a meteor shower. Some asteroids and comets are large enough that their gravity has pulled them into a spherical shape. These bodies, which include Pluto, are now called dwarf planets. These are not considered planets because a large relative mass of other objects orbit at the same distance. The Earth is hit frequently by small pieces of asteroidal material which may reach the ground and are called meteorites. Occasionally a large asteroidal or cometary body strikes the Earth, producing craters or, in very rare instances, mass extinctions. Page 301 QUESTIONS FOR REVIEW 1. (11.1) What makes a shooting star? 2. (11.1) What is the difference between a meteor, a meteoroid, and a meteorite? 3. (11.1) How is a meteor heated? 4. (11.1) What kinds of meteorites are there? 5. (11.2) Where are most asteroids found? 6. (11.2) What shape are typical asteroids and how do we know? Why does Ceres not have this shape? 7. (11.2) How do we know that asteroids have a composition similar to that of some meteorites? 8. (11.2) What do asteroids tell us about the formation of the Solar System?
2 9. (11.2) What are near Earth objects? 10. (11.3) What evidence makes astronomers believe that Pluto is strongly influenced by Neptune? 11. (11.3) How did the discovery of a moon orbiting Pluto help astronomers better understand this object? 12. (11.3) Where did Pluto and other TNOs form? How did they get to where they are today? 13. (11.4) What parts make up a comet? What are they made of? How do we know? 14. (11.4) Why are there two tails to some comets? What are they made of? 15. (11.4) What is the Oort cloud? What is the Kuiper belt? 16. (11.4) What is the life history of a comet from the Oort cloud that has become an object we see? 17. (11.1/11.4) What creates meteor showers? When do some occur? 18. (11.5) What evidence is there that the Earth has been hit by asteroids or comets? 19. (11.5) What was the Tunguska event? 20. (11.5) Why do some scientists believe that asteroids and comets play a role in mass extinctions? THOUGHT QUESTIONS 1. (11.1//11.4) Explain the difference between the tail of a meteor and the tail of a comet. Are both of them hot gas and debris? 2. (10.1) You see a meteor about 45 degrees above the horizon. If the meteor is 100 km above the ground when you see it (fig 11.2), what is the farthest distance that away you think another person could be and also see it? You may find making a sketch helpful. 3. (11.2) The total mass of the asteroid belt is much less than the mass of any of the planets. If there were many, many more asteroids, do you think they could form a planet?
3 4. (11.2) The coast guard is monitoring satellite pictures of the ocean, looking for vessels traveling at night in faint moonlight. The images do not have enough detail to resolve the boats they appear as a single dot in the image. If they see a dot, how can they estimate the size of the boat? How would this depend on the color of the boat? How would infrared images help determine the size of the boat? Compare this situation to the visual and infrared study of asteroids described in this chapter. 5. (11.2) Compare the compositions of asteroids with the compositions of Mars and of Jupiter. How does the composition of bodies across the asteroid belt support the solar nebular theory? 6. (11.3) Why do some astronomers think that Pluto should be considered a planet? Do you agree? What happens if you apply this logic to Sedna and Eris? 7. (11.4) Examining the images of the nuclei of comets shown in section 11.4, do you think any of the crater like depressions were caused by impacts like the ones that cause craters on asteroids? Explain your reasoning 8. (11.5) If an asteroid were heading toward a possible future impact with Earth, what might be some advantages and disadvantages of setting off a nuclear explosion on it to deflect it to a different path? PROBLEMS 1. (11.1) The speed with which a meteoroid hits the atmosphere is roughly the speed of the Earth in its orbit (you might say the Earth hits the meteoroid). Show that the speed of the Earth in its orbit is about 30 kilometers/second 2. (11.2) Calculate the surface gravity and escape velocity for Ceres, assuming it has a radius of 487 kilometers and a mass of kilograms. 3. (11.2) The asteroid Icarus has an elliptical orbit that carries it between 0.19 and 1.97 AU from the Sun. What is its semimajor axis? How often does it cross the Earth's orbital radius? 4. (11.3) Show that Pluto's orbital period is very close to one and one half times Neptune's. Use the data in the appendix 5. (11.3) Use the modified form of Kepler's third law to calculate the mass of Pluto and Charon from the orbital data for Charon given in the text (unlike in problem 2, the satellite's mass is about 12% of the planetoid's mass, so we will not neglect it here). Compare your result to the sum of the masses given for Pluto and Charon in the appendix. Be sure to convert the orbital period to seconds and the orbital radius to meters before putting those numbers into the formula
4 6. (11.3) Calculate the density of Charon, given that its radius is approximately 600 kilometers and its mass is about grams. (Be sure to convert kilometers to centimeters or meters.) Is it likely that Charon has a large iron core? Why? 7. (11.4) Comet Swift Tuttle has a period of about 133 years and leaves the debris that causes the Perseid meteor shower. Calculate the semimajor axis of the comet's orbit. If the orbit is highly elliptical, approximately how far from the Sun is the comet at aphelion? What is found at this distance? 8. Page 302 (11.4) Use Kepler's third law to determine the period of a comet whose orbit extends to 50,000 AU, within the inner Oort cloud 9. (11.4) Given that the temperature of a body decreases as the square root of its distance from the Sun increases, estimate the temperature of a comet nucleus in the Oort cloud. Take the temperature at 1 AU to be 300 kelvin. 10. (11.1/11.5) Use the formula for kinetic energy of a moving body to estimate the energy of an SUV (mass about 2700 kilograms) traveling at 65 miles per hour (you'll need to do a few conversions on units). Compare this to the kinetic energy of a kilogram meteoroid (about as heavy as two quarters) that collides with the Earth at 30 kilometers/second. 11. (11.5) Calculate the kinetic energy of impact of a 1000 kilogram (roughly 1 ton) object hitting the Earth at 30 kilometers per second. Express your in kilotons of TNT, using the conversion that 1 kiloton is about joules. Be sure to convert kilometers/second to meters/second. TEST YOURSELF 1. (11.1) The bright streak of light we see, as a meteoroid enters our atmosphere, is caused by (a) sunlight reflected from the solid body of the meteoroid (b) radioactive decay of material in the meteoroid. (c) a process similar to the aurora that is triggered by the meteoroid's disturbing the Earth's magnetic field. (d) frictional heating as the meteoroid speeds through the gases of our atmosphere (e) the meteoroid's disturbing the atmosphere so that sunlight is refracted in unusual directions.
5 2. (11.1/11.4) Meteor showers such as the Perseids in August are caused by (a) the breakup of asteroids that hit our atmosphere at predictable times (b) the Earth passing through the debris left behind by a comet as it moved through the inner Solar System. (c) passing asteroids triggering auroral displays (d) nuclear reactions in the upper atmosphere triggered by an abnormally large meteoritic particle entering the upper atmosphere (e) none of the above 3. (11.1) What is(are) the source(s) of most meteorites? (More than one may apply.) (a) comets (b) the Moon (c) asteroids (d) Mars (e) material from the Solar Wind 4. (11.2) The asteroid belt lies between the orbits of
6 (a) Earth and Mars (b) Saturn and Jupiter (c) Venus and Earth. (d) Mars and Jupiter (e) Pluto and the Oort cloud 5. (11.2) How do astronomers estimate the sizes of most asteroids? (a) By observing angular diameters from space telescopes (b) By timing how long they take to pass in front of stars (c) By determining their gravitational influence on other asteroids (d) By measuring the amount of light they reflect 6. (11.2) Asteroids in the asteroid belt are made up of (a) iron (b) silicates (rock) (c) organic compounds (d) a and b only, in amounts that vary depending on where they orbit (and were formed).
7 (e) a, b and c, in amounts that vary depending on where they orbit (and were formed) 7. (11.3) How was the mass of Pluto determined? (a) By measuring the effect of its gravity on the terrestrial planets (b) By observing its effect on the motion of an unmanned flyby of Pluto (c) By determining the orbit of its satellite, Charon (d) By measuring how it bends light rays passing near it 8. (11.4) The tail of a comet (a) is gas and dust pulled off the comet by the Sun's gravity (b) always points away from the Sun (c) trails behind the comet, pointing away from the Sun as the comet approaches it and toward the Sun as the comet moves out of the inner Solar System. (d) is gas and dust expelled from the comet's nucleus by the Sun's heat and radiation pressure. (e) Both (b) and (d) 9. (11.4) Short period comets have a period of around and mostly come from the (a) decades to a few hundred years; Kuiper belt
8 (b) a few hundred to a thousand years; Kuiper belt (c) decades to a few hundred years; Oort cloud (d) a few hundred to a thousand years; Oort cloud (e) a thousand to a million years; Oort cloud 10. (11.5) A moving object's kinetic energy depends on its and. (a) size, density (b) density, velocity (c) velocity, mass (d) magnetic field, velocity (e) It depends only on the velocity 11. (11.5) Strong evidence that the dinosaurs were killed by a meteor impact is provided by (there may be more than one correct, select all that apply): (a) mass extinctions 65 million years ago (b) a large crater in Arizona. (c) pieces of the asteroid that have been recovered
9 (d) an unusually rich layer of a rare element in the rock record at 65 million years ago (e) a layer of 65 million year old soot in the rock record Page 303 KEY TERMS achondrite, 281 asteroid, 282 asteroid belt, 282 carbonaceous chondrite, 281 chondrite, 281 chondrule, 281 coma, 289 dust tail, 291 fluorescence, 292 ion tail, 290 Kirkwood gaps, 284 Kuiper belt, 294 meteor, 280 meteorite, 280 meteoroid, 280 meteor shower, 296 near Earth object, 285 nucleus, 289 Oort cloud, 294 radiant, 296 radiation pressure, 290
10 short period comet, 295 solar wind, 290 tail, 289 trans Neptunian object (TNO), 286 Q FIGURE QUESTION ANSWERS WHAT IS THIS? (chapter opening): This image was made by combining a number of separate images of the Leonid meteor shower taken just before dawn on November 19, You can see that the meteors appear to radiate from a small area on the sky. That area, it turns out, lies in the constellation Leo, hence the name of the shower. FIGURE 11.1: The star trails show that this was a fairly long time exposure, during which the Earth rotated, smearing the stars' images into long arcs. The meteor passed by very quickly; the Earth didn't rotate very far during its passage, and as a result the meteor's track is straight. FIGURE 11.4: The Trojan asteroids are about equidistant from the Sun and Jupiter, forming a nearly equilateral triangle. This position is known as a Lagrange point, and is a stable location for orbits. There are similar Lagrange points relative to the Moon orbiting the Earth or the Earth orbiting the Sun. FIGURE 11.8: The gaps in the distribution of the asteroids are very like the gaps in Saturn's rings, such as Cassini's division. FIGURE 11.10: Pluto's orbit is relatively steeply tilted with respect to Neptune's. As a result, when Pluto crosses Neptune's orbit, Pluto is actually well above or below Neptune's path. FIGURE 11.24: Manicouagan Crater is far enough north that it was buried by glaciers during the last ice age. Moreover, being in a wet climate, it erodes faster and more extensively than the Arizona Meteor Crater, which is in a dry climate. PROJECTS 1. Meteor Hunt: Pick a clear night and watch the night sky with a friend for half an hour. Count the number of meteors you see. Note the direction they come from. From your knowledge of these bodies, were the ones you saw (if any!) cometary or asteroidal? Note that meteors are generally more frequent before dawn, so you should try to observe then if possible. 2. Meteor Shower: Watch a meteor shower and count the number you see. Estimate the paths of the meteors on a star map and see if you can find the position they seem to radiate from and mark it on a star map. If it is summer, you might try for the Perseids in mid August (August 10 14); in autumn, the Orionids (October 18 23) or the Geminids (December 10 13); in the spring, the Lyrids (April 20 22). Other showers are listed in appendix table 6. Most are best when viewed before dawn in a dark sky without bright moonlight. What was the average time interval between meteors? Does your estimate of the radiant match the predicted position?