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OUR PLANETARY SYSTEM

Earth, photographed from the outskirts of our solar system by the Voyager spacecraft. The “sunbeam” surrounding Earth is an artifact of light scattering in the camera.

LEARNING GOALS

1 STUDYING THE SOLAR SYSTEM

■ What does the solar system look like?■ What can we learn by comparing the planets to one

another?

2 PATTERNS IN THE SOLAR SYSTEM

■ What features of our solar system provide clues to how it formed?

3 SPACECRAFT EXPLORATION OF THE SOLAR SYSTEM

■ How do robotic spacecraft work?

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O U R P L A N E T A R Y S Y S T E M

We succeeded in taking that picture [left], and, if you look at it, you see a dot. Look again at that dot. That’s here. That’s home. That’s us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. … saint and sinner in the history of our species lived there—on a mote of dust suspended in a sunbeam.

—Carl Sagan

Our ancestors long ago recognized the motions of the planets through the sky, but it has been only a few

hundred years since we learned that Earth is also a planet that orbits the Sun. Even then, we knew little about the other planets until the advent of large telescopes. More recently, the dawn of space exploration has brought us far greater understanding of other worlds. We’ve lived in this solar system all along, but only now are we getting to know it.

In this chapter, we’ll explore our solar system like newcom-ers to the neighborhood. We’ll begin by discussing what we hope to learn by studying the solar system, and in the process take a brief tour of major features of the Sun and planets. We’ll also explore the major patterns we observe in the solar system. Finally, we’ll discuss the use of spacecraft to explore the solar system, examining how we are coming to learn so much more about our neighbors.


Galileo’s telescopic observations began a new era in astron-omy in which the Sun, Moon, and planets could be studied for the first time as worlds, rather than as mere lights in the sky. Since that time, we have studied these worlds in different ways. Sometimes we study them individually—for example, when we map the geography of Mars or the atmospheric structure of Jupiter. Other times we compare the worlds to one another, seeking to understand their similarities and differences. This latter approach is called comparative plan-etology. Note that astronomers use the term planetology broadly to include moons, asteroids, and comets as well as planets.

We will use the comparative planetology approach for most of our study of the solar system in this text. Before we can compare the planets, however, we must have a general idea of the nature of our solar system and of the characteristics of individual worlds.

Scale of the Universe Tutorial, Lesson 1

What does the solar system look like?

The first step in getting to know our solar system is to visualize what it looks like as a whole. Imagine having the

perspective of an alien spacecraft making its first scien-tific survey of our solar system. What would we see as we viewed the solar system from beyond the orbits of the planets?

Without a telescope, the answer would be “not much.” Remember that the Sun and planets are all quite small compared to the distances between them—so small that if we viewed them from the outskirts of our solar system, the planets would be only pinpoints of light, and even the Sun would be just a small bright dot in the sky. But if we magnify the sizes of the planets by about a million times compared to their distances from the Sun and show their orbital paths, we get the central picture in FIGURE 1.

Following Figure 1 is a brief tour through our solar system, beginning at the Sun, continuing to each of the planets, and concluding with dwarf planets such as Pluto and Eris. The tour highlights a few of the most important features of each world we visit. The side shows the objects to scale, using a 1-to-10-billion scale. The map along the bottom shows the locations of the Sun and each of the plan-ets in the Voyage scale model solar system, so that you can see their relative distances from the Sun. TABLE 1 follows the tour and summarizes key data.

As you study Figure 1, the tour, and Table 1, you’ll quickly see that our solar system is not a random collection of worlds, but a system that exhibits many clear patterns. For example, Figure 1 shows that all the planets orbit the Sun in the same direction and in nearly the same plane, and the tour pages show that the planets fall into two distinct groups.

T H I N K A B O U T I TAs you read the tour, identify one characteristic of each object that you find particularly interesting and would like to know more about. In addition, try to answer the following questions as you read: (1) Are all the planets made of the same materi-als? (2) Which planets are “Earth-like” with solid surfaces? (3) How would you organize the planets into groups with common characteristics?

Formation of the Solar System Tutorial, Lesson 1

What can we learn by comparing the planets to one another?

The essence of comparative planetology lies in the idea that we can learn more about an individual world, including our own Earth, by studying it in the context of other objects in our solar system. It is much like learning more about a person by getting to know his or her family, friends, and culture.

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1 Large bodies in the solar system have orderly motions. All planets have nearly circular orbits going in the same direction in nearly the same plane. Most large moons orbit their planets in this same direction, which is also the direction of the Sun’s rotation.

The solar system’s layout and composition offer four major clues to how it formed. The main illustration below shows the orbits of planets in the solar system from a perspective beyond Neptune, with the planets themselves magnified by about a million times relative to their orbits.

Seen from above, planetary orbits are nearly circular.

Neptune

Uranus

Jupiter

Saturn

Red circles indicate the orbital direction of major moons around their planets.

White arrows indicate the rotation direction of the planets and Sun.

Mercury

Earth

Mars

Venus

Each planet’s axis tilt is shown, with

direction of the planet’s rotation.

direction of orbital motion.

JupiterMars

Mercury

Sun

EarthAsteroid belt

Venus

Orbits are shown to scale, but planet

times relative to orbits. The Sun is not shown to scale.

Neptune

C O S M I C C O N T E X T F I G U R E 1 The Solar System

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2 Planets fall into two major categories: Small, rocky terrestrial planets and large, hydrogen-rich jovian planets.

3 Swarms of asteroids and comets populate the solar system. Vast numbers of rocky asteroids and icy comets are found throughout the solar system, but are concentrated in three distinct regions.

Terrestrial Planets: Jovian Planets:

Asteroids are made

and most orbit in the asteroid belt between Mars and Jupiter.

Comets are ice-rich, and many are found in the Kuiper belt beyond Neptune’s orbit.

Even more comets orbit the Sun in the distant, spherical

Oort cloud, and only a rare few

the inner solar system.

Kuiper belt

terrestrialplanet

jovianplanet

Saturn

Earth’s relatively large moonUranus’s odd tilt

4 Several notable exceptions to these trends stand out. Some planets have unusual axis tilts, unusually large moons, or moons with unusual orbits.

Uranus

Our own Moon is much

than most other moons in comparison to their planets.

Uranus rotates nearly on its side compared to its

major moons share this “sideways” orientation.

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The Sun

■ Radius: 696,000 km = 108REarth

■ Mass: 333,000MEarth

■ Composition (by mass): 98% hydrogen and helium, 2% other elements

The Sun is by far the largest and brightest object in our solar system. It contains more than 99.8% of the solar system’s total mass, making it nearly a thousand times as massive as everything else in the solar system combined.

The Sun’s surface looks solid in photographs (FIGURE 2), but it is actually a roiling sea of hot (about 5800 K, or 5500°C or 10,000°F) hydrogen and helium gas. The surface is speckled with sunspots that appear dark in photographs only because they are slightly cooler than their surroundings. Solar storms sometimes send streamers of hot gas soaring far above the surface.

The Sun is gaseous throughout, and the temperature and pressure both increase with depth. The source of the

Sun’s energy lies deep in its core, where the temperatures and pressures are so high that the Sun is a nuclear fusion power plant. Each second, fusion transforms about 600 million tons of the Sun’s hydrogen into 596 million tons of helium. The “missing” 4 million tons becomes energy in accord with Einstein’s famous formula, E = mc2. Despite converting 4 million tons of mass to energy each second, the Sun contains so much hydrogen that it has already shone steadily for almost 5 billion years and will continue to shine for another 5 billion years.

The Sun is the most influential object in our solar system. Its gravity governs the orbits of the planets. Its heat is the primary influence on the temperatures of planetary surfaces and atmospheres. It is the source of virtually all the light in our solar system—planets and moons shine by virtue of the sunlight they reflect. In addition, charged particles flow-ing outward from the Sun (the solar wind) help shape plan-etary magnetic fields and influence planetary atmospheres. Nevertheless, we can understand almost all the present char-acteristics of the planets without knowing much more about the Sun than we have just discussed.

Earth shownfor size comparison

b This ultraviolet photograph, from the SOHO spacecraft,shows a huge streamer of hot gas on the Sun. The image of Earth was added for size comparison.

a A visible-light photograph of the Sun’ssurface. The dark splotches are sunspots—each large enough to swallow several Earths.

FIGURE 2 The Sun contains more than 99.8% of the total mass in our solar system.

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The Voyage scale model solar system represents sizes and distances in our solar system at one ten-billionth oftheir actual values. The strip along the side shows the sizes of the Sun and planetson this scale, and the map above shows their locations in the Voyage model on the National Mall in Washington,D.C. The Sun is about the size of a large grapefruit on this scale.

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Venus

■ Average distance from the Sun: 0.72 AU

■ Radius: 6051 km = 0.95REarth

■ Mass: 0.82MEarth

■ Average density: 5.24 g/cm3

■ Composition: rocks, metals

■ Average surface temperature: 740 K

■ Moons: 0

Venus, the second planet from the Sun, is nearly identi-cal in size to Earth. Before the era of spacecraft visits, Venus stood out largely for its strange rotation: It rotates on its axis very slowly and in the opposite direction of Earth, so days and nights are very long and the Sun rises in the west and sets in the east instead of rising in the east and setting in the west. Its surface is completely hidden from view by dense clouds, so we knew little about it until a few decades ago, when spacecraft began to map Venus with cloud-penetrating radar, discovering

mountains, valleys, craters, and extensive evidence of past volcanic activity (FIGURE 4). Because we knew so little about it, some science fiction writers used its Earth-like size, thick atmosphere, and closer distance to the Sun to speculate that it might be a lush, tropical paradise—a “sister planet” to Earth.

The reality is far different. We now know that an extreme greenhouse effect bakes Venus’s surface to an incredible 470°C (about 880°F), trapping heat so effectively that nighttime offers no relief. Day and night, Venus is hotter than a pizza oven, and the thick atmosphere bears down on the surface with a pres-sure equivalent to that nearly a kilometer (0.6 mile) beneath the ocean’s surface on Earth. Far from being a beautiful sister planet to Earth, Venus resembles a traditional view of hell.

The fact that Venus and Earth are so similar in size and composition but so different in surface conditions suggests that Venus could teach us important lessons. In particular, Venus’s greenhouse effect is caused by carbon dioxide, the same gas that is primarily responsible for global warming on Earth. Perhaps further study of Venus may help us better understand and solve some of the problems we face here at home.

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FIGURE 4 The image above shows an artistic rendition of the surface of Venus as scientists think it would appear to our eyes. The surface topography is based on data from NASA’s Magellan spacecraft. The inset (left) shows the full disk of Venus photographed by NASA’s Pioneer Venus Orbiter with cameras sensitive to ultra-violet light. (Image above from the Voyage scale model solar system, developed by the Challenger Center for Space Science Education, the Smithsonian Institution, and NASA. Image by David P. Anderson, Southern Methodist University © 2001.)


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Mercury






■ Average surface temperature: 700 K (day), 100 K (night)

■ Moons: 0

Mercury is the innermost planet of our solar system, and the smallest of the eight official planets. It is a desolate, cratered world with no active volcanoes, no wind, no rain, and no life. Because there is virtually no air to scatter sunlight or color the sky, you could see stars even in the daytime if you stood on Mercury with your back toward the Sun.

You might expect Mercury to be very hot because of its closeness to the Sun, but in fact it is a world of both hot

and cold extremes. Tidal forces from the Sun have forced Mercury into an unusual rotation pattern: Its 58.6-day rota-tion period means it rotates exactly three times for every two of its 87.9-day orbits of the Sun. This combination of rotation and orbit gives Mercury days and nights that last about 3 Earth months each. Daytime temperatures reach 425°C, nearly as hot as hot coals. At night or in shadow, the temperature falls below -150°C, far colder than Antarctica in winter.

Mercury’s surface is heavily cratered (FIGURE 3), much like the surface of our Moon. But it also shows evidence of past geological activity, such as plains created by ancient lava flows and tall, steep cliffs that run hundreds of kilom-eters in length. These cliffs may be wrinkles from an episode of “planetary shrinking” early in Mercury’s history. Mercury’s high density (calculated from its mass and volume) indicates that it has a very large iron core, perhaps because it once suffered a huge impact that blasted its outer layers away.

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FIGURE 3 The left image shows that Mercury’s surface is heavily cratered but also has smooth volcanic plains and long, steep cliffs. The inset shows a global composite. (Images from the MESSENGER spacecraft.)


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Earth


■ Radius: 6378 km = 1REarth





■ Moons: 1

Beyond Venus, we next encounter our home planet, Earth, the only known oasis of life in our solar system. Earth is also the only planet in our solar system with oxygen to breathe, ozone to shield the surface from deadly solar radiation, and abundant

surface water to nurture life. Temperatures are pleasant because Earth’s atmosphere contains just enough carbon dioxide and water vapor to maintain a moderate green house effect.

Despite Earth’s small size, its beauty is striking (FIGURE 5a). Blue oceans cover nearly three-fourths of the surface, broken by the continental land masses and scattered islands. The polar caps are white with snow and ice, and white clouds are scattered above the surface. At night, the glow of artificial lights reveals the presence of an intelligent civilization.

Earth is the first planet on our tour with a moon. The Moon is surprisingly large compared with Earth (FIGURE 5b); although it is not the largest moon in the solar system, almost all other moons are much smaller relative to the planets they orbit. The leading hypothesis holds that the Moon formed as a result of a giant impact early in Earth’s history.

FIGURE 5 Earth, our home planet.

a This image (left), computer generated from satellitedata, shows the striking contrast between the dayand night hemispheres of Earth. The day side revealslittle evidence of human presence, but at night ourpresence is revealed by the lights of human activity.(From the Voyage scale model solar system,developed by the Challenger Center for Space ScienceEducation, the Smithsonian Institution, and NASA.Image created by ARC Science Simulations © 2001.)

b Earth and the Moon, shown to scale. The Moon isabout 1/4 as large as Earth in diameter, while its massis about 1/80 of Earth's mass. To show the distancebetween Earth and Moon on the same scale, you'dneed to hold these two photographs about 1 meter(3 feet) apart.

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Mars







■ Moons: 2 (very small)

The next planet on our tour is Mars, the last of the four inner planets of our solar system (FIGURE 6). Mars is larger than Mercury and the Moon but only about half Earth’s size in diameter; its mass is about 10% that of Earth. Mars has two tiny moons, Phobos and Deimos, which probably once were asteroids that were captured into Martian orbit early in the solar system’s history.

Mars is a world of wonders, with ancient volcanoes that dwarf the largest mountains on Earth, a great canyon that runs

nearly one-fifth of the way around the planet, and polar caps made of frozen carbon dioxide (“dry ice”) and water. Although Mars is frozen today, the presence of dried-up riverbeds, rock-strewn floodplains, and minerals that form in water offers clear evidence that Mars had at least some warm and wet periods in the past. Major flows of liquid water probably ceased at least 3 billion years ago, but some liquid water could persist under-ground, perhaps flowing to the surface on occasion.

Mars’s surface looks almost Earth-like, but you wouldn’t want to visit without a spacesuit. The air pressure is far less than that on top of Mount Everest, the temperature is usually well below freezing, the trace amounts of oxygen would not be nearly enough to breathe, and the lack of atmospheric ozone would leave you exposed to deadly ultraviolet radia-tion from the Sun.

More than a dozen spacecraft have flown past, orbited, or landed on Mars, and plans are in the works for more. We may even send humans to Mars within the next few decades. By overturning rocks in ancient riverbeds or chipping away at ice in the polar caps, explorers will help us learn whether Mars has ever been home to life.

FIGURE 6 The image below shows the walls of a Martian crater as photographed by NASA’s Opportunity rover, with a simulated image of the rover included at the appropriate scale. The inset shows a close-up of the disk of Mars photographed by the Viking orbiter; the horizontal “gash” across the center is the giant canyon Valles Marineris.

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Jupiter


■ Radius 71,492 km = 11.2REarth

■ Mass: 318MEarth


■ Composition: mostly hydrogen and helium

■ Cloud-top temperature: 125 K

■ Moons: at least 67

To reach the orbit of Jupiter from Mars, we must traverse a distance that is more than double the total distance from the Sun to Mars, passing through the asteroid belt along the way. Upon our arrival, we find a planet much larger than any we have seen so far (FIGURE 7).

Jupiter is so different from the planets of the inner solar system that we must adopt an entirely new mental image of the term planet. Its mass is more than 300 times that of

Earth, and its volume is more than 1000 times that of Earth. Its most famous feature—a long-lived storm called the Great Red Spot—is itself large enough to swallow two or three Earths. Like the Sun, Jupiter is made primarily of hydrogen and helium and has no solid surface. If we plunged deep into Jupiter, the increasing gas pressure would crush us long before we ever reached its core.

Jupiter reigns over dozens of moons and a thin set of rings (too faint to be seen in most photographs). Most of the moons are very small, but four are large enough that we’d call them planets or dwarf planets if they orbited the Sun independently. These four moons—Io, Europa, Ganymede, and Callisto—are often called the Galilean moons (because Galileo discov-ered them), and they display varied and interesting geology. Io is the most volcanically active world in the solar system. Europa has an icy crust that may hide a subsurface ocean of liquid water, making it a promising place to search for life. Ganymede and Callisto may also have subsurface oceans, and their surfaces have many features that remain mysterious.

FIGURE 7 This image shows what it would look like to be orbiting near Jupiter’s moon Io as Jupiter comes into view. Notice the Great Red Spot to the left of Jupiter’s center. The extraordinarily dark rings discovered during the Voyager missions are exaggerated to make them visible. This computer visualization was created using data from both NASA’s Voyager and Galileo missions. (From the Voyage scale model solar system, developed by the Challenger Center for Space Science Education, the Smithsonian Institution, and NASA. Image created by ARC Science Simulations © 2001.)

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Saturn


■ Radius: 60,268 km = 9.4REarth



■ Composition: mostly hydrogen and helium



The journey from Jupiter to Saturn is a long one: Saturn orbits nearly twice as far from the Sun as Jupiter. Saturn, the second-largest planet in our solar system, is only slightly smaller than Jupiter in diameter, but its lower density makes it considerably less massive (about one-third of Jupiter’s mass). Like Jupiter, Saturn is made mostly of hydrogen and helium and has no solid surface.

Saturn is famous for its spectacular rings (FIGURE 8). Although all four of the giant outer planets have rings, only

Saturn’s can be seen easily. The rings look solid from a distance, but in reality they are made of countless small particles, each of which orbits Saturn like a tiny moon. These particles of rock and ice range in size from dust grains to city blocks. We are rapidly learning more about Saturn and its rings through observations made by the Cassini spacecraft, which has orbited Saturn since 2004.

Cassini has also taught us more about Saturn’s moons, and has revealed that at least two are geologically active today: Enceladus, which has ice fountains spraying out from its southern hemisphere, and Titan, the only moon in the solar system with a thick atmosphere. Saturn and its moons are so far from the Sun that Titan’s surface temperature is a frigid −180°C, making it far too cold for liquid water to exist. However, studies by Cassini and its Huygens probe, which landed on Titan in 2005, have revealed a remarkably Earth-like landscape—including an erosion-carved surface with riverbeds and lakes—except that the features are shaped by extremely cold liquid methane or ethane rather than liquid water.

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FIGURE 8 Cassini ’s view of Saturn. We see the shadow of the rings on the upper right portion of Saturn’s sunlit face, and the rings become lost in Saturn’s shadow on the night side. The inset shows an infrared view of Saturn’s largest moon, Titan, which is shrouded in a thick, cloudy atmosphere.


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Uranus





■ Composition: hydrogen, helium, hydrogen compounds



It’s another long journey to the next stop on our tour, as Uranus lies twice as far from the Sun as Saturn. Uranus (normally pronounced YUR-uh-nus) is much smaller than either Jupiter or Saturn but much larger than Earth. It is made largely of hydrogen, helium, and hydrogen compounds such as water (H2O), ammonia (NH3), and methane (CH4). Methane gas gives Uranus its pale blue-green color (FIGURE 9). Like the other giants of the outer solar system, Uranus lacks a solid

surface. More than two dozen moons orbit Uranus, along with a set of rings somewhat similar to those of Saturn but much darker and more difficult to see.

The entire Uranus system—planet, rings, and moon orbits—is tipped on its side compared to the rest of the planets. This extreme axis tilt may be the result of a cataclysmic collision that Uranus suffered as it was forming, and it gives Uranus the most extreme seasonal variations of any planet in our solar system. If you lived on a platform floating in Uranus’s atmosphere near its north pole, you’d have continuous daylight for half of each orbit, or 42 years. Then, after a very gradual sunset, you’d enter into a 42-year-long night.

Only one spacecraft has visited Uranus: Voyager 2, which flew past all four of the giant outer planets before heading out of the solar system. Much of our current understanding of Uranus comes from that mission, though powerful new telescopes are also capable of studying it. Scientists hope it will not be too long before we can send another spacecraft to study Uranus and its rings and moons in greater detail.

FIGURE 9 This image shows a view of Uranus from high above its moon Ariel. The ring system is shown, although it would actually be too dark to see from this vantage point. This computer simulation is based on data from NASA’s Voyager 2 mission. (From the Voyage scale model solar system, developed by the Challenger Center for Space Science Education, the Smithsonian Institution, and NASA. Image created by ARC Science Simulations © 2001.)

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Neptune





■ Composition: hydrogen, helium, hydrogen compounds



The journey from the orbit of Uranus to the orbit of Neptune is the longest yet in our tour, calling attention to the vast emptiness of the outer solar system. Nevertheless, Neptune looks nearly like a twin of Uranus, although it is

more strikingly blue (FIGURE 10). It is slightly smaller than Uranus in size, but a higher density makes it slightly more massive even though the two planets share very similar compositions. Like Uranus, Neptune has been visited only by the Voyager 2 spacecraft.

Neptune has rings and numerous moons. Its largest moon, Triton, is larger than Pluto and is one of the most fascinating moons in the solar system. Triton’s icy surface has features that appear to be somewhat like geysers, although they spew nitrogen gas rather than water into the sky. Even more surprisingly, Triton is the only large moon in the solar system that orbits its planet “backward”—that is, in a direction oppo-site to the direction in which Neptune rotates. This backward orbit makes it a near certainty that Triton once orbited the Sun independently before somehow being captured into Neptune’s orbit.

FIGURE 10 This image shows what it would look like to be orbiting Neptune’s moon Triton as Neptune itself comes into view. The dark rings are exaggerated to make them visible in this computer simulation using data from NASA’s Voyager 2 mission. (From the Voyage scale model solar system, developed by the Challenger Center for Space Science Education, the Smithsonian Institution, and NASA. Image created by ARC Science Simulations © 2001.)

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Dwarf planets: Pluto, Eris, and more

Pluto Data:





■ Composition: ices, rock


■ Moons: 5

We conclude our tour at Pluto (FIGURE 11), which reigned for some 75 years as the “ninth planet” in our solar system. However, the 2005 discovery of the slightly larger Eris, and the fact that dozens of other recently discovered objects are not much smaller than Pluto and Eris, led scientists to reconsider the definition of “planet.” The result was that we now consider Pluto and Eris to be dwarf planets, too small to qualify as offi-cial planets but large enough to be round in shape. Several other solar system objects also qualify as dwarf planets, includ-ing the largest asteroid of the asteroid belt, named Ceres.

Pluto and Eris belong to a collection of thousands of icy objects that orbit the Sun beyond Neptune, making up what we call the Kuiper belt. As you can see in Figure 1, the Kuiper belt is much like the asteroid belt, except it is farther from the Sun and composed of comet-like objects rather than rocky asteroids.

Pluto’s characteristics help us to think about what it would be like to visit this distant realm. Pluto’s average distance from the Sun lies as far beyond Neptune as Neptune lies beyond Uranus, making Pluto extremely cold and quite dark even in daytime. From Pluto, the Sun would be little more than a bright light among the stars. Pluto’s largest moon, Charon, is locked together with it in synchronous rotation, so Charon would dominate the sky on one side of Pluto but never be seen from the other side.

The great distances and small sizes of Pluto and other dwarf planets make them difficult to study, but we are rapidly learning more. Scientists are particularly excited about two upcoming events: In February 2015, the Dawn spacecraft will arrive at Ceres, giving us our first close-up views of this world; then, in July 2015, after a 9-year journey, the New Horizons spacecraft will fly past Pluto.

FIGURE 11 Pluto and its five moons, as imaged by the Hubble Space Telescope. Aside from Pluto and its moons, the other blue light in the image is scattered light within the telescope.

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TABLE 1 The Planetary Dataa

Photo PlanetRelative Size

Average Distance from Sun (AU)

Average Equatorial Radius (km)

Mass (Earth = 1)

Average Density (g/cm3)

Orbital Period

Rotation Period

Axis Tilt

Average Surface (or Cloud-Top) Temperatureb Composition

Known Moons (2012) Rings?

Mercury 0.387 2440 0.055 5.43 87.9 days 58.6 days 0.0° 700 K (day) 100 K (night)

Rocks, metals 0 No

Venus 0.723 6051 0.82 5.24 225 days 243 days 177.3° 740 K Rocks, metals 0 No

Earth 1.00 6378 1.00 5.52 1.00 year 23.93 hours 23.5° 290 K Rocks, metals 1 No

Mars 1.52 3397 0.11 3.93 1.88 years 24.6 hours 25.2° 220 K Rocks, metals 2 No

Jupiter 5.20 71,492 318 1.33 11.9 years 9.93 hours 3.1° 125 K H, He, hydrogen

compoundsc 67 Yes

Saturn 9.54 60,268 95.2 0.70 29.5 years 10.6 hours 26.7° 95 K H, He, hydrogen

compoundsc 62 Yes

Uranus 19.2 25,559 14.5 1.32 83.8 years 17.2 hours 97.9° 60 K H, He, hydrogen

compoundsc 27 Yes

Neptune 30.1 24,764 17.1 1.64 165 years 16.1 hours 29.6° 60 K H, He, hydrogen

compoundsc 13 Yes

Pluto 39.5 1160 0.0022 2.0 248 years 6.39 days 112.5° 44 K Ices, rock 5 No

Eris 67.7 1200 0.0028 2.3 557 years 1.08 days 78° 43 K Ices, rock 1 No

aIncluding the dwarf planets Pluto and Eris.bSurface temperatures for all objects except Jupiter, Saturn, Uranus, and Neptune, for which cloud-top temperatures are listed.cInclude water (H2O), methane (CH4), and ammonia (NH3).

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While we still can learn much by studying planets indi-vidually, the comparative planetology approach has demon-strated its value in at least three key ways:

■ Comparative study has revealed similarities and differ-ences among the planets that have helped guide the devel-opment of our theory of solar system formation, thereby giving us a better understanding of how we came to exist here on Earth.

■ Comparative study has given us new insights into the physical processes that have shaped Earth and other worlds—insights that can help us better understand and manage our own planet.

■ Comparative study has allowed us to apply lessons from our solar system to the study of the many planetary systems now known around other stars. These lessons help us understand both the general principles that govern planetary systems and the specific circumstances under which Earth-like planets—and possibly life—might exist elsewhere.

The comparative planetology approach should also bene-fit you as a student by helping you stay focused on processes rather than on a collection of facts. We now know so many individual facts about the worlds of our solar system and others that even planetary scientists have trouble keeping track of them all. By concentrating on the processes that shape planets, you’ll gain a deeper understanding of how planets, including Earth, actually work.

Orbits and Kepler’s Laws Tutorial, Lessons 2–4

2 PATTERNS IN THE SOLAR SYSTEM

Our goal in studying the solar system as a whole is to look for clues that might help us develop a theory that could explain how it formed. In this section, we’ll explore the patterns of our solar system in more depth, and organize these patterns into a set of general features that tell us about our solar system’s formation.

What features of our solar system provide clues to how it formed?

We have already seen that our solar system is not a random collection of worlds, but rather a family of worlds exhibit-ing many traits that would be difficult to attribute to coin-cidence. We could make a long list of such traits, but it is easier to develop a scientific theory by focusing on the more general structure of our solar system. For our purposes, four major features stand out, each corresponding to one of the numbered steps in Figure 1:

1. Patterns of motion among large bodies. The Sun, planets, and large moons generally orbit and rotate in a very organized way.

2. Two major types of planets. The eight planets divide clearly into two groups: the small, rocky planets that are close together and close to the Sun, and the large, gas-rich planets that are farther apart and farther from the Sun.

3. Asteroids and comets. Between and beyond the planets, vast numbers of asteroids and comets orbit the Sun; some are large enough to qualify as dwarf planets. The locations, orbits, and compositions of these asteroids and comets follow distinct patterns.

4. Exceptions to the rules. The generally orderly solar system also has some notable exceptions. For example, among the inner planets only Earth has a large moon, and the planet Uranus is tipped on its side. A successful theory must make allowances for such exceptions even as it explains the general rules.

Because these four features are so important to our study of the solar system, let’s investigate them in a little more detail.

Feature 1: Patterns of Motion Among Large

Bodies If you look back at Figure 1, you’ll notice several clear patterns of motion among the large bodies of our solar system. (In this context, a “body” is simply an individual object such as the Sun, a planet, or a moon.) For example:

■ All planetary orbits are nearly circular and lie nearly in the same plane.

■ All planets orbit the Sun in the same direction: counter-clockwise as viewed from high above Earth’s North Pole.

■ Most planets rotate in the same direction in which they orbit, with fairly small axis tilts. The Sun also rotates in this direction.

■ Most of the solar system’s large moons exhibit simi-lar properties in their orbits around their planets, such as orbiting in their planet’s equatorial plane in the same direction as the planet rotates.

We consider these orderly patterns together as the first major feature of our solar system. Our theory of solar system forma-tion explains these patterns as consequences of processes that occurred during the early stages of the birth of our solar system.

Feature 2: Two Types of Planets Our brief plan-etary tour showed that the four inner planets are quite differ-ent from the four outer planets. We say that these two groups represent two distinct planetary classes: terrestrial and jovian.

The terrestrial planets (terrestrial means “Earth-like”) are the four planets of the inner solar system: Mercury, Venus, Earth, and Mars. These planets are relatively small and dense, with rocky surfaces and an abundance of metals in their cores. They have few moons, if any, and no rings. We count our Moon as a fifth terrestrial world, because its history has been shaped by the same processes that have shaped the terrestrial planets.

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The jovian planets (jovian means “Jupiter-like”) are the four large planets of the outer solar system: Jupiter, Saturn, Uranus, and Neptune. The jovian planets are much larger in size and lower in average density than the terrestrial planets, and they have rings and many moons. They lack solid surfaces and are made mostly of hydrogen, helium, and hydrogen compounds—compounds containing hydrogen, such as water (H2O), ammonia (NH3), and methane (CH4). Because these substances are gases under earthly conditions, the jovian planets are sometimes called “gas giants.” TABLE 2 contrasts the general traits of the terrestrial and jovian planets.

Feature 3: Asteroids and Comets The third major feature of our solar system is the existence of vast numbers of small objects orbiting the Sun. These objects fall into two major groups: asteroids and comets.

Asteroids are rocky bodies that orbit the Sun much like planets, but they are much smaller (FIGURE 12). Most known asteroids are found within the asteroid belt between the orbits of Mars and Jupiter (see Figure 1).

Comets are also small objects that orbit the Sun, but they are made largely of ices (such as water ice, ammonia ice, and methane ice) mixed with rock. You are probably familiar with the occasional appearance of comets in the inner solar system, where they may become visible to the naked eye with long, beautiful tails (FIGURE 13). These visitors, which may delight sky watchers for a few weeks or months, are actually quite rare among comets. The vast majority of comets never visit the inner solar system. Instead, they orbit the Sun in one of the two distinct regions shown as Feature 3 in Figure 1. The first is a donut-shaped region beyond the orbit of Neptune that we call the Kuiper belt (Kuiper rhymes with piper). The Kuiper belt contains at least 100,000 icy objects, of which Pluto and Eris are the largest known. Kuiper belt objects all orbit the Sun in the same direction as the planets. The second cometary region, called the Oort cloud (Oort rhymes with court), is much farther from the Sun and may contain a trillion comets; its most distant comets may sometimes reside nearly one-quarter of the distance to the nearest stars. Comets of the Oort cloud have orbits randomly inclined to the ecliptic plane, giving the Oort cloud a roughly spherical shape.

Feature 4: Exceptions to the Rules The fourth key feature of our solar system is that there are a few notable exceptions to the general rules. For example, while most of the planets rotate in the same direction as they orbit, Uranus rotates nearly on its side, and Venus rotates “backward” (clockwise as viewed from high above Earth’s North Pole). Similarly, while most large moons orbit their planets in the same direction as their planets rotate, many small moons have much more unusual orbits.

One of the most interesting exceptions concerns our own Moon. While the other terrestrial planets have either no moons (Mercury and Venus) or very tiny moons (Mars), Earth has one of the largest moons in the solar system.

Summary Now that you have read through the tour of our solar system and the description of its four major features, review them again in Figure 1. You should now see clearly that these features hold key clues to the origin of our solar system.

TABLE 2 Comparison of Terrestrial and Jovian Planets

Terrestrial Planets Jovian Planets

Smaller size and mass Larger size and massHigher average density Lower average densityMade mostly of rocks and metals

Made mostly of hydrogen, helium, and hydrogen compounds

Solid surface No solid surfaceFew (if any) moons and no rings

Rings and many moons

Closer to the Sun (and closer together), with warmer surfaces

Farther from the Sun (and farther apart), with cool temperatures at cloud tops

FIGURE 13 Comet Hale-Bopp, photographed over Boulder, Colorado, during its appearance in 1997.

FIGURE 12 The asteroid Eros (photographed from the NEAR spacecraft). Its appearance is probably typical of most asteroids. Eros is about 40 kilometers in length. Like other small objects in the solar system, it is not spherical.

5 km

VIS


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3 SPACECRAFT EXPLORATION OF THE SOLAR SYSTEM

How have we learned so much about the solar system? Much of our knowledge comes from telescopic observations, using both ground-based telescopes and telescopes in Earth orbit such as the Hubble Space Telescope. In one case—our Moon—we have learned a lot by sending astronauts to explore the terrain and bring back rocks for laboratory study. In a few other cases, we have studied samples of distant worlds that have come to us as meteorites. But most of the data fueling the recent revolution in our understanding of the solar system have come from robotic spacecraft. To date, we have sent robotic spacecraft to all the terrestrial and jovian planets, as well as to many moons, asteroids, and comets. In this section, we’ll briefly investigate how we use robotic spacecraft to explore the solar system.

How do robotic spacecraft work?

The spacecraft we send to explore the planets are robots designed for scientific study. All spacecraft have computers used to control their major components, power sources such as solar cells, propulsion systems, and scientific instru-ments to study their targets. Robotic spacecraft operate primarily with preprogrammed instructions, but also carry radios that allow them to communicate with controllers on Earth. Most robotic spacecraft make one-way trips, never physically returning to Earth but sending their data back from space in the same way we send radio and television signals.

Broadly speaking, the robotic missions to other worlds fall into four major categories:

■ Flyby. A spacecraft on a flyby goes past a world just once and then continues on its way.

S P E C I A L TO P I CHow Did We Learn the Scale of the Solar System?

This chapter presents the layout of the solar system as we know it today, when we have precise measurements of planetary sizes and distances. But how did we learn the scale of the solar system?

By the middle of the 17th century, Kepler’s laws had provided planetary distances in astronomical units (AU), or distances relative to the Earth-Sun distance, but no one yet knew the value of the AU in absolute units like miles or kilometers. A number of 17th-century astronomers proposed ideas for measuring the Earth-Sun distance, but none were practical. Then, in 1716, Edmond Halley (best known for the comet named after him) hit upon the idea that would ulti-mately solve the problem: He realized that during a planetary transit, when a planet appears to pass across the face of the Sun, observers in different locations on Earth would see the planet trace slightly different paths across the Sun. Comparison of these paths could allow calculation of the planet’s distance—which would in turn allow determination of the AU—through the simple geometry shown in FIGURE 1.

Only Mercury and Venus can produce transits visible from Earth. Halley realized that although Mercury transits occur more often, the measurements would be easier with Venus because its closer distance to Earth means greater separation between the paths in Figure 1. Unfortunately, Venus transits are rare, occurring in pairs 8 years apart about every 120 years. Halley did not live to see a Venus transit, but later astronomers followed his plan, mounting expeditions to observe transits in 1761 and 1769.

The transit observations turned out to be quite difficult in prac-tice, partly from the inherent challenge that long expeditions posed at that time, and partly because getting the geometry right required very precise timing of the beginning of the transit. Astronomers discovered that this timing was more difficult than Halley had guessed, because of optical effects that occur during a transit. Nevertheless, astronomers studied the data from the 1761 and 1769 transits for many decades, and by the middle of the 1800s the value of the AU had been pinned down to within about 5% of its modern value of 149.6 million kilometers. The next Venus transits occurred in 1874 and 1882. Photography had been invented by then, making observations more reliable, so in principle those transits could have allowed refinement of the AU. However, by that time photography and better telescopes had also made it possible to observe parallax of

planets against stars, and by 1877 such observations had given us the value of the AU to within 0.2% of its modern value.

The most recent transits occurred in 2004 and 2012. While they were amazing spectacles to observe, they weren’t important for interplanetary measurements. Nowadays, we measure the distance to Venus very precisely by bouncing radio waves off its surface with radar, a technique known as radar ranging. Because we know the speed of light, measuring the time it takes for the radio waves to make the round trip from Earth to Venus tells us the precise distance. We then use this distance and Venus’s known distance in AU to calculate the actual value of the AU. Once we know the value of the AU, we can determine the actual distances of all the planets from the Sun, and we can determine their actual sizes from their angular sizes and distances. Indeed, we now know the layout of the solar system so well that we can launch spacecraft from Earth and send them to precise places on or around distant worlds.

FIGURE 1 During a transit of Venus, observers at different places on Earth will see it trace slightly different paths across the Sun. The precise geometry of these events therefore allows computa-tion of Venus’s true distance, which in turn allows computation of the AU distance. (Adapted from Sky and Telescope.)

Venus

Sun

path seen from south

Not to scale!

path seenfrom north

So, using geometry, we cancalculate the distance to Venus.

…and careful observations duringthe transit allow us to measure this parallax angle.

Earth

S

N

We know the distancebetween two pointson Earth…

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■ Orbiter. An orbiter is a spacecraft that orbits the world it is visiting, allowing longer-term study.

■ Lander or probe. These spacecraft are designed to land on a planet’s surface or probe a planet’s atmosphere by flying through it. Some landers carry rovers to explore wider regions.

■ Sample return mission. A sample return mission makes a round trip to return a sample of the world it has studied to Earth.

The choice of spacecraft type depends on both scientific objectives and cost.

Flybys Flybys tend to be cheaper than other missions because they are generally less expensive to launch into space. Launch costs depend largely on weight, and onboard fuel is a significant part of the weight of a spacecraft heading to another planet. Once a spacecraft is on its way, the lack of friction or air drag in space means that it can maintain its orbital trajectory through the solar system without using any fuel at all. Fuel is needed only when the spacecraft must change from one trajectory (orbit) to another.

Moreover, some flybys gain more “bang for the buck” by visiting multiple planets. For example, Voyager 2 flew past Jupiter, Saturn, Uranus, and Neptune before continuing on its way out of our solar system (FIGURE 14). This trajectory allowed additional fuel savings by using the gravity of each planet along the spacecraft’s path to help boost it onward to the next planet. This technique, known as a gravitational slingshot, can not only bend the spacecraft’s path but also speed it up by essentially stealing a tiny bit of the planet’s orbital energy, though the effect on the planet is unnoticeable.

densities. Like the backlit views of the rings, these types of data cannot be gathered from Earth. Indeed, most of what we know about the masses and compositions of moons comes from data gathered by spacecraft that have flown past them.

Orbiters An orbiter can study another world for a much longer period of time than a flyby. Like the spacecraft used for flybys, orbiters often carry cameras, spectrographs, and instruments for measuring the strength of magnetic fields. Some missions also carry radar, which can be used to make precise altitude measurements of surface features. Radar has proven especially valuable for the study of Venus and Titan, because it provides our only way of “seeing” through their thick, cloudy atmospheres.

An orbiter is generally more expensive than a flyby for an equivalent weight of scientific instruments, primarily because it must carry added fuel to change from an interplanetary trajectory to a path that puts it into orbit around another world. Careful planning can minimize the added expense. For example, recent Mars orbiters have saved on fuel costs by carrying only enough fuel to enter highly elliptical orbits around Mars. The spacecraft then settled into the smaller, more circular orbits needed for scientific observations by skimming the Martian atmosphere at the low point of every elliptical orbit. Atmospheric drag slowed the spacecraft with each orbit and, over several months, circularized the space-craft orbit. (This technique is sometimes called aerobraking.) We have sent orbiters to the Moon, to the planets Venus, Mars, Jupiter, and Saturn, and to two asteroids.

Landers and Probes The most “up close and personal” study of other worlds comes from spacecraft that send probes into the atmospheres or landers to the surfaces. For example, in 1995, the Galileo spacecraft dropped a probe into Jupiter’s atmosphere. The probe collected temperature, pres-sure, composition, and radiation measurements for about an

EarthAug. 20, 1977

JupiterJuly 9, 1979 Saturn

Aug. 25, 1981

UranusJan. 24, 1986

NeptuneAug. 25, 1989

Voyager 2

FIGURE 14 The trajectory of Voyager 2, which made flybys of the four jovian planets in our solar system.

T H I N K A B O U T I TStudy the Voyager 2 trajectory in Figure 14. Given that Saturn orbits the Sun every 29 years, Uranus orbits the Sun every 84 years, and Neptune orbits the Sun every 165 years, would it be possible to send another flyby mission to all four jovian planets if we launched it now? Explain.

Although a flyby offers only a relatively short period of close-up study, it can provide valuable scientific information. Spacecraft on flybys generally carry small telescopes, cameras, and spectrographs. Because these instruments are brought relatively close (typically thousands of kilometers or less) to other worlds, they can obtain much higher-resolution images and spectra than the largest telescopes on Earth or in Earth orbit. In addition, flybys sometimes give us information that would be very difficult to obtain from Earth. For example, Voyager 2 helped us discover Jupiter’s rings and learn about the rings of Saturn, Uranus, and Neptune through views in which the rings were backlit by the Sun. Such views are possible only from beyond each planet’s orbit.

Spacecraft on flybys may also carry instruments to measure local magnetic field strength or to sample interplanetary dust. The gravitational effects of the planets and their moons on the spacecraft itself provide information about object masses and

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hour as it descended; it was then destroyed by the heat and pressure of Jupiter’s interior.

On planets with solid surfaces, a lander can offer close-up surface views, local weather monitoring, and the ability to carry out automated experiments. Landers have successfully reached the surfaces of the Moon, the planets Venus and Mars, and Saturn’s moon Titan. Several of our Mars landers have included rovers to explore wider areas of the surface, including the Spirit and Opportunity rovers that landed on Mars in 2004, and the Curiosity rover that landed in August 2012. Because of its weight, Curiosity’s landing required a particularly spectacu-lar feat of engineering (FIGURE 15). The spacecraft carrying the lander first used a parachute to slow it down in the Martian atmosphere and then fired rockets that slowed it to a halt about 7 meters above the surface. Finally, a “sky crane” lowered the rover to the surface.

Sample Return Missions Although probes and landers can carry out experiments on surface rock or atmospheric samples, the experiments must be designed in advance and must fit inside the spacecraft. One way around these limita-tions is to design missions in which samples from other worlds can be scooped up and returned to Earth for more detailed study. To date, the only sample return missions have been to the Moon (brought back by the Apollo astronauts and by robotic spacecraft sent in the 1970s by the then–Soviet Union) and to an asteroid (Japan’s Hayabusa mission). Many scien-tists are working toward a sample return mission to Mars, and they hope to launch such a mission within the next decade or so. A slight variation on the theme of a sample return mission is the Stardust mission, which collected comet dust on a flyby and returned to Earth in 2006.

Combination Spacecraft Many missions combine more than one type of spacecraft. For example, the Galileo mission to Jupiter included an orbiter that studied Jupiter and its moons as well as the probe that entered Jupiter’s atmos-phere. The Cassini spacecraft included flybys of Venus, Earth, and Jupiter during its 7-year trip to Saturn. The spacecraft itself is an orbiter that is studying Saturn and its moons, but it also carried the Huygens probe, which descended through the atmosphere and landed on the surface of Saturn’s moon Titan.

Exploration—Past, Present, and Future Over the past several decades, studies using both telescopes on Earth and robotic spacecraft have allowed us to learn the general characteristics of all the major planets and moons in our solar system as well as the general characteristics of asteroids and comets. Telescopes will continue to play an important role in future observations, but for detailed study we will probably continue to depend on spacecraft.

TABLE 3 lists some significant robotic missions of the past and present. The next few years promise many new discov-eries as missions arrive at their destinations. Over the longer term, all the world’s major space agencies have hopes of launching numerous and diverse missions to answer many specific questions about the nature of our solar system and its numerous worlds.

S E E I T F O R YO U R S E L FIt can be easy with a text full of planetary images to forget that these are real objects, many of which you can see in the night sky. Search the Web for “planets tonight” and then go out and see if you can find any of the planets in tonight’s sky. Which planets can you see? Why can’t you see the others?

FIGURE 15 An artist’s conception of the landing sequence that brought the Curiosity rover to Mars, along with a photo of its descent taken from orbit.

1 Friction slows spacecraft as it entersMars atmosphere.

2 Parachute slows spacecraft to about350 km/hr.

3 Rockets slow spacecraft to halt; “skycrane” tether lowers rover to surface.

4 Tether released, the rocket heads offto crash a safe distance away.

As it flew overhead, the Mars ReconnaissanceOrbiter took this photo of the spacecraft withits parachute deployed.

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■ Each planet has its own unique and interesting features. Becoming familiar with the planets is an important first step in understand-ing the root causes of their similarities and differences.

■ Much of what we now know about the solar system comes from spacecraft exploration. Choosing the type of mission to send to a planet involves many considerations, from the scientific to the purely political. Many missions are currently under way, offering us hope of learning much more in the near future.

TABLE 3 Selected Robotic Missions to Other Worlds

Destination MissionArrival Year Agency*

Mercury MESSENGER orbiter studies surface, atmosphere, and interior 2011 NASAVenus Magellan orbiter mapped surface with radar 1990 NASA Venus Express focuses on atmosphere studies 2006 ESAMoon The United States, China, Japan, India, and Russia all have current or planned

robotic missions to explore the Moon— —

Mars Spirit and Opportunity rovers learn about water on ancient Mars 2004 NASA Mars Express orbiter studies Mars’s climate, geology, and polar caps 2004 ESA Mars Reconnaissance Orbiter takes very high-resolution photos 2006 NASA Phoenix lander studied soil near the north polar cap 2008 NASA Curiosity rover explores Gale Crater to understand prospects for life 2012 NASA MAVEN orbiter to study how Mars has lost atmospheric gas over time 2014 NASAAsteroids Hayabusa orbited and landed on asteroid Itokawa; returned sample to Earth in 2010 2005 JAXA Dawn visited asteroid Vesta and will visit the dwarf planet Ceres 2011/2015 NASAJovian planets Voyagers 1 and 2 visited all the jovian planets and left the solar system 1979 NASA Galileo's orbiter studied Jupiter and its moons; probe entered Jupiter’s atmosphere 1995 NASA Cassini orbits Saturn; its Huygens probe (built by ESA) landed on Titan 2004 NASA Juno orbiter to study Jupiter’s deep interior 2016 NASAPluto and comets New Horizons, the first mission to Pluto, passed Jupiter in 2007 2015 NASA Stardust flew through the tail of Comet Wild 2; returned comet dust in 2006 2004 NASA Deep Impact observed its “lander” impacting Comet Tempel 1 at 10 km/s 2005 NASA Rosetta to orbit Comet Churyumov-Gerasimenko and release a lander 2014 ESA

*ESA = European Space Agency; JAXA = Japan Aerospace Exploration Agency.

The Big Picture

Putting This Chapter into Perspective

This chapter introduced the major features of our solar system and discussed some important patterns and trends that provide clues to its formation. As you continue your study of the solar system, keep in mind the following “big picture” ideas:

■ Our solar system is not a random collection of objects moving in random directions. Rather, it is highly organized, with clear patterns of motion and with most objects falling into just a few basic categories.


■ What does the solar system look like? The planets are tiny compared to the distances between them. Our solar system consists of the Sun, the plan-ets and their moons, and vast numbers of asteroids and comets. Each world has its own unique

character, but there are many clear patterns among the worlds.

■ What can we learn by comparing the planets to one another? Comparative studies reveal the similarities and

differences that give clues to solar system formation and highlight the underlying processes that give each planet its unique appearance.

S U M M A RY O F K E Y C O N C E P T S

Neptune

Uranus

Jupiter

Saturn

Mercury

Earth

Mars

Venus


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