The solar system consists of the Sun and everything that orbits it, including the nine major planets, the moons that orbit them, and the many small bodies found in interplanetary space. The science of comparative planetology compares and contrasts the properties of the diverse bodies found in the solar system, to understand better the conditions under which planets form and develop.
The asteroids, or "minor planets," are small bodies, none of them larger than Earth's Moon, most of which orbit in a broad band called the asteroid belt between the orbits of Mars and Jupiter. Comets are chunks of ice found mostly in the outer solar system. Their importance to planetary astronomy lies in the fact that they are thought to be "leftover" material from the formation of the solar system and therefore contain clues to the very earliest stages of its development.
The major planets orbit the Sun in the same sense—counterclockwise as viewed from above Earth's North Pole—on roughly circular orbits that lie close to the ecliptic plane. The orbits of the innermost planet, Mercury, and the outermost, Pluto, are the most eccentric and have the greatest orbital inclination. The spacing between planetary orbits increases as we move outward from the Sun.
Density is a convenient measure of the compactness of any object. The average density of a planet is obtained by dividing the planet's total mass by its volume. The innermost four planets in the solar system have average densities comparable to Earth's and are generally rocky in composition. The outermost planets have much lower densities and, with the exception of Pluto, are made up mostly of gaseous or liquid hydrogen and helium.
Planetary scientists divide the eight large planets (excluding Pluto) in the solar system on the basis of their densities and composition, into the rocky terrestrial planets—Mercury, Venus, Earth, and Mars—which lie closest to the Sun, and the gaseous jovian planets—Jupiter, Saturn, Uranus, and Neptune—which lie at greater distances. Compared with the terrestrial worlds, the jovian planets are larger and more massive, rotate more rapidly, and have stronger magnetic fields. In addition, the jovian planets all have ring systems and many moons orbiting them.
All the major planets, with the exception of Pluto, have been visited by unmanned space probes. Spacecraft have landed on Venus and Mars. In many cases, the spacecrafts' trajectories have included "gravitational assists" from one or more planets to reach their destinations.
1. Moons are small bodies that orbit the Sun.
2. Most planets orbit the Sun in nearly the same plane as Earth. (Hint)
3. The Titius—Bode law, giving the spacing of planetary orbits, is not a true law at all and is not very accurate. (Hint)
4. The largest planets also have the largest densities. (Hint)
5. The total mass of all the planets is about half the mass of the Sun. (Hint)
6. A planet with a density of 5000 kg/m3 most likely has a gaseous composition. (Hint)
7. Most terrestrial planets rotate more slowly than Earth. (Hint)
8. Most jovian planets rotate more rapidly than Earth. (Hint)
9. The solar wind occurs on Earth during days of exceptional solar heating. (Hint)
10. Mars Observer was successful in mapping the surface of Mars to a resolution of 100 m. (Hint)
11. All planets have moons. (Hint)
12. The landing of the Soviet Venera 7 on the planet Venus was notable because it was the first time any spacecraft had ever landed on a planet. (Hint)
13. Mars Pathfinder is the last U.S. spacecraft scheduled to visit Mars. (Hint)
14. Both Voyager missions visited all four jovian planets, but Voyager 2 then went on to visit Pluto. (Hint)
15. The Galileo mission was designed to explore the atmosphere and moon system of Jupiter. (Hint)
1. The major bodies orbiting the Sun are known as _____. (Hint)
2. The _____ are bodies that orbit the Sun between the orbits of Mars and Jupiter
3. The two planets with the highest eccentricities and orbital tilts are _____ and _____. (Hint)
4. The inner planets Mercury, Venus, Earth, and Mars are known as the _____ planets. (Hint)
5. The outer planets Jupiter, Saturn, Uranus, and Neptune are known as the _____ planets. (Hint)
6. The largest and most massive planet in the solar system is _____. (Hint)
7. Asteroids and meteoroids have a _____ composition, in contrast with comets, which have an _____ composition. (Hint)
8. Asteroids are similar in composition to the _____ planets. (Hint)
9. Comets have compositions similar to the _____ moons of the _____ planets. (Hint)
10. The Mariner 10 spacecraft was sent by _____ (country) to photograph the planet _____. (Hint)
11. The U.S Magellan probe mapped the entire surface of Venus using _____. (Hint)
12. The Viking 1 and Viking 2 missions sent orbiters and landers to the planet _____. (Hint)
13. Jupiter was first visited by the U.S. spacecraft _____. (Hint)
14. The U.S. spacecraft _____ is the only probe to have visited each of the giant outer planets. (Hint)
15. Spacecraft visiting one planet often use a _____ assist to visit another planet. (Hint)
1. Name and describe all the different types of objects found in the solar system. Give one distinguishing characteristic of each. Include a mention of interplanetary space! (Hint)
2. What is the order of the planets, from the closest to the farthest from the Sun? (Hint)
3. What is comparative planetology? Why is it useful? What is its ultimate goal? (Hint)
4. Why has our knowledge of the solar system increased greatly in recent years? (Hint)
5. Compare and contrast Kepler's laws with the Titius—Bode "law" (Interlude 6-1). Why are Kepler's laws considered to be true natural laws, whereas the Titius—Bode "law" is not? (Hint)
6. In what sense is the solar system "flat?" (Hint)
7. Which are the terrestrial planets? Why are they given this name? Repeat the question for the jovian planets. What property actually defines these two different types of planets? (Hint)
8. Name three important differences between the terrestrial planets and the jovian planets. (Hint)
9. Compare the properties of Pluto given in Table 6.1 with the properties of the terrestrial and jovian planets in Table 6.2. What do you conclude regarding the classification of Pluto as either a terrestrial or jovian planet? (Hint)
10. Why are asteroids and meteoroids important to planetary scientists? (Hint)
11. Comets generally vaporize upon striking Earth's atmosphere. How then do we know their composition? (Hint)
12. How and why do scientists use gravity assists to propel spacecraft through the solar system? (Hint)
13. Which planets have been visited by spacecraft from Earth? On which ones have spacecraft actually landed? (Hint)
14. Why do you think Galileo and Cassini took such circuitous routes to Jupiter and Saturn, while Pioneer and Voyager did not? (Hint)
15. How do you think NASA's new policy of building less complex, smaller and cheaper spacecraft, but with shorter times between design and launch, will affect future exploration of the outer planets? Will missions like Galileo and Cassini be possible?
1. Choose two of the objects listed in Table 6.1 and calculate their densities. Compare your results with those given in the table. Assume that all objects are spherical. (Hint)
2. Use Newton's law of gravity (see More Precisely 2-2) to compute your weight (a) on Earth, (b) on Mars, (c) on the asteroid Ceres, and (d) on Jupiter (neglecting temporarily the absence of a solid surface on this planet!). (Hint)
3. Only Mercury, Mars, and Pluto have orbits that deviate significantly from circles. Calculate the perihelion and aphelion distances from the Sun (see More Precisely 2-1) of these planets. (Hint)
4. According to the Titius—Bode "law," where should the ninth, tenth, and eleventh planets in the solar system lie? (Hint)
5. Suppose the average mass of each of the 7000 asteroids in the solar system is about 1017 kg. Compare the total mass of all asteroids with the mass of Earth. Assuming a roughly spherical shape and a density of 3000 kg/m3, estimate the diameter of an asteroid having this average mass. (Hint)
6. A short-period comet is conventionally defined as a comet having an orbital period of less than 200 years. What is the maximum possible aphelion distance for a short-period comet with a perihelion of 0.5 A.U.? Where does this place the comet relative to the outer planets? (Hint)
7. How many times has Mariner 10 now orbited the Sun? (Hint)
8. A spacecraft has an orbit that just grazes Earth's orbit at perihelion and that of Mars at aphelion. What is its orbital eccentricity and semi-major axis? What is its orbital period? (This is the so-called minimum energy orbit for a craft leaving Earth and reaching Mars. Assume circular planetary orbits for simplicity.) (Hint)
9. Earth and Mars were at closest approach in March 1997. Mars Pathfinder was launched on December 4, 1996 and arrived at Mars in late June, 1997. Sketch the orbits of the two planets and the trajectory of the spacecraft. Be sure to indicate on your diagram the location of Mars at launch and of Earth when Pathfinder reached Mars. (For simplicity, neglect the eccentricity of Mars's orbit in your sketch.) (Hint)
10. How long would it take for a radio signal to complete the round trip between Earth and Saturn? Assume that Saturn is at its closest point to Earth. How far would a spacecraft orbiting the planet in a circular orbit of radius 100,000 km travel in that time? Do you think mission control could maneuver the spacecraft in real time—that is, control all its functions directly from Earth? (Hint)
1. You can begin to visualize the ecliptic—the plane of the planets' orbits—just by noticing the path of the Sun throughout the day and of the full Moon in the course of a single night. It helps if you watch from one spot, such as your backyard or a rooftop. It's also good to have a general notion of direction (west is where the Sun sets). You will see that the movements of the Sun and Moon are confined to a narrow pathway across our sky. The planets also travel along this path. The motion of the Sun, Moon, and planets is a two-dimensional reflection of the three-dimensional plane of our solar system.
2. Once you get a feeling for the whereabouts of the ecliptic, try locating the North Star. Knowing the direction to celestial north makes it easier to imagine the motion of the planets in the plane of the solar system. Don't worry about being too precise. Just get a sense of the ecliptic as a kind of merry-go-round of planets—that we on Earth also ride!
3. Go to your library and find what planetary missions are in progress or planned, other than those described in the text. Apart from the United States, what other countries have space agencies actively engaged in planetary exploration? What nonplanetary missions are in progress or planned?
