Astronomy HOMEWORK Chapter 9 – 9th Edition 1. A piece of space debris that you pick up from the ground is called a(n): (a) asteroid; (b) meteoroid; (c) meteor; (d) meteorite; (e) comet. Answer: (d), meteorite. A meteoroid is space, meteor burning up in the atmosphere, meteorite is what reaches Earth’s surface. 2. Space debris that is a roughly equal mix of rock and ice is called a(n): (a) asteroid; (b) comet; (c) meteoroid; (d) meteorite; (e) meteor. Answer: (b) comet. “Cometary nucleus” might be a little more precise. 4. What part of a comet is solid? (a) nucleus; (b) halo; (c) gas tail; (d) dust tail; (e) coma. Answer: (a) nucleus. The coma and dust tail are gas and dust particles. The coma and gas tail are gas. 5. Suppose you were standing on Pluto. Describe the motions of Charon relative to the horizon. Under what circumstances would you never see Charon? Answer: Pluto and Charon are both in synchronous rotation. That means that each always presents the same face to the other, and keeps the “‘away” face out of sight of the other. If you lived on the “toward” side of Pluto, Charon would always be in the sky. If you lived on the “away” side of Pluto, you would never see Charon. On earth, some satellites are placed in synchronous orbit, so that they are always at the same spot in the sky, as seen by your satellite dish. This is the way Charon is, from Pluto; or Pluto, from Charon. 7. What role did Charon play in enabling astronomers to determine Pluto’s mass? Answer: Kepler’s 3rd Law is rewritten so that the mass of the central object becomes a variable. In that way, by measuring both the orbit size (semimajor axis) and the period, the mass of the central object (Pluto, in this case) can be solved for. The equation is P 2 = (4π2/G)(a3/m). The episode of Charon passing in front of Pluto allowed Charon’s size to be determined, but had less to do with determining its mass. 13. In what direction do comet tails point, and why? Answer: Comet tails point away from the Sun, because it is the solar wind (subatomic particles) and radiation (light) which blows them away from the Sun. 17. Why do astronomers believe that many isolated meteors come from asteroids, whereas the debris that creates meteor showers comes from comets? Answer: Meteorites are predominantly stony or iron. This strongly indicates that they are pieces of asteroids which underwent planetary differentiation and were subsequently pulverized in collisions. The Wid- manst¨atten patterns provide especially strong evidence: these patterns are long crystals which form only when the iron has solidified over millions of years. The fact that all meteor showers are debris from burned out comets is indicated by the fact that they occur at the same point on Earth’s orbit (which is to say, the same date) every year. In many cases, the comet is known. 28. Assuming a constant rate of meteor infall, how much mass has Earth gained in the past 4.6 billion years? Answer: The gained mass is the gain rate (300 tons per day) times the number of days in 4.6 billion years. Number of days = 365 × 4.6 × 109 =1.7 × 1012 days Mass gained = (300 ton/day) × (1.7 × 1012 days) = 5 × 1014 tons. 5 × 1014 tons = 5 × 1017 kg. Now, Earth’s mass is 6 × 1024 kg, so the fractional change is Fractional change = (5 × 1017 kg)/(6 × 1024 kg) = 8 × 10−8. This is 80 parts in one billion, a very small fraction. 32. 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? H ow many NEOs are now known, and how many has this program found? Will any of these NEOs pose a threat in your lifetime? Answer: According to NASA/JPL NEO site: http://neo.jpl.nasa.gov/index.html. Ten thousand NEOs, some very small impact possibilities..