Lesson 2 The Solar System...Lesson 2 • The Solar System 21 The solar system is vast and varied. It...

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20 Unit 1 • Space Systems

Each photo shows an object found in our solar system. With a partner, discuss the photos and try to identify the objects. Record your observations about each of the objects.

Imagine yourself outside at night, watching a planet such as Mercury or Saturn through a telescope. Would the planet stay in one place or move across the sky? Explain your ideas.

The Solar SystemLesson 2

NASA and E. Karkoschka (University of Arizona)

NASA and E. Karkoschka (University of Arizona)

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Lesson 2 • The Solar System 21

The solar system is vast and varied. It stretches outward to the farthest objects that revolve around, or orbit, the sun. It includes everything from giant planets to clouds of dust and empty space. At the center of it all is the sun, one of the billions of stars in our corner of the universe.

The picture above was taken in 1986. It shows Comet Halley. The comet is named after Edmond Halley, an early English astronomer. Halley studied the records of comets that had appeared in 1531, 1607, and 1682 and concluded that they were the same comet. The comet was swinging by Earth at regular intervals. Halley correctly predicted that the comet would reappear in 1758. He did not live to see its return.

Where do you think Comet Halley is when it is not visible from Earth? Explain your ideas.

Look AheadThe sun, planets, comets, and other bodies make up the solar system. What are the different parts of the solar system and how are they related? Where are they located? What paths do they follow? You will find answers to these and other questions in the rest of this lesson.

NASA, NSSDCA, and W. LillerNASA, NSSDCA, and W. Liller

Explore!

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A Long Way to NeptuneMaterials• Piece of poster paper • Meterstick• Masking tape • String• Drawing compass • Chalk

Steps

Tape the paper to a clear area of the floor. Draw a point in the center of the paper and label it sun. Imagine standing at that point—on the sun—looking outward. Describe what you might see.

Use the compass to draw a circle with the sun as its center and a radius of 2.5 centimeters. Draw a point on the circle and label it Earth. Describe what the model shows.

Use the compass to draw four more circles, with the following radii. Draw a point on each circle and label it with the name of its planet.

Planet Distance from Model Sun

Mercury 1.0 cm

Venus 1.8 cm

Mars 3.9 cm

Jupiter 13.2 cm

Recall that Earth takes one year to complete a revolution around the sun. Compare an Earth year with how long it might take the other planets to complete their revolutions.

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Wrap one end of the string around a piece of chalk. Mark a point on the string 24 centimeters from the chalk. Hold that point on the sun and draw a circle. Draw a point on the circle and label it Saturn. Describe what this circle represents in your model.

Use the string and chalk to draw two more circles, with the following radii. Draw a point on each circle and label it with the name of its planet.

Planet Distance from Model Sun

Uranus 49 cm

Neptune 76 cm

Describe how much farther Uranus and Neptune are from the sun than Earth is.

Think About It

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This activity models the major bodies of the solar system. Based on your model, how might you separate the planets into groups? Explain.

Talk with a partner about what you described in Step 1. Use the lines below to explain whether you would revise your description, and how.


Understand

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The solar system includes many different kinds of bodies—and they are all moving. Like Earth, they revolve around the sun.

In the Explore activity, you created a model of the sun and the eight planets. The diagram below shows another model of the solar system.

Mercury

Sun

Venus

Earth

Mars

Jupiter SaturnUranus

Neptune

NOTE: Distances not to scale

Your model of the sun and the eight planets showed distances to scale. Explain why a diagram such as the one above does not show distances to scale.

The diagram above shows diameters of the planets to scale. Explain why your model of the sun and the eight planets could not show diameters to scale.


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The table below shows the diameters of the eight planets.

Planet Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune

Diameter (1000 km) 4.9 12.1 12.8 6.8 143.0 120.5 51.1 49.5

The four planets closest to the sun are called the inner planets. Use the data in the table to find the mean diameter of the inner planets.

The four planets farthest from the sun are called the outer planets. Use the data in the table to find the mean diameter of the outer planets.

Describe how you could use your calculations to compare the two groups of planets.

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Density is the amount of mass an object contains per unit of volume. Different materials can have very different densities. Lead has a density of about 11 grams per cubic centimeter. Chalk has a density of about 3 grams per cubic centimeter.

The table below shows the densities of the eight planets.


Density (g/cm3) 5.4 5.3 5.5 3.9 1.3 0.7 1.3 1.7

Use the data in the table to find the mean density of the four inner planets.

Use the data in the table to find the mean density of the four outer planets.


Chalk

Lead

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A moon is a natural body that revolves around, or orbits, a planet. Earth is not the only planet with a moon. The photo below shows Jupiter and one of its moons, Ganymede.

NASA, ESA, and E. Karkoschka(University of Arizona)NASA, ESA, and E. Karkoschka(University of Arizona)

The table below shows the number of moons orbiting each of the eight planets.


Moons 0 0 1 2 62* 62* 27* 13*

*as of 2015

Use the data in the table to find the mean number of moons orbiting the four inner planets.

Use the data in the table to find the mean number of moons orbiting the four outer planets.



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Go Further


The solar system contains more than just the sun, the planets, and their moons. Dwarf planets are much smaller than planets. They orbit the sun like planets, but at a much greater distance. Scientists think that there are many dwarf planets far beyond Neptune.

Asteroids are small, rocky bodies. They are left over from the formation of the solar system about 4.6 billion years ago. They range in size from less than a kilometer to hundreds of kilometers across.

VestaNASA, JPL-Caltech, UCAL, MPS, DLR, IDA

ErosNEAR Project, NLR, JHUAPL,

Goddard SVS, NASA

VestaNASA, JPL-Caltech, UCAL, MPS, DLR, IDA

ErosNEAR Project, NLR, JHUAPL,

Goddard SVS, NASA

Like other objects in the solar system, asteroids revolve around the sun. Most asteroids are located in a band between Mars and Jupiter. But some follow orbital paths that bring them near the sun—and Earth. Over a period of millions of years, asteroids may collide with Earth, causing immense destruction.

With a partner, discuss the pictures of asteroids. Compare them with photos and diagrams of planets earlier in this lesson. Describe how asteroids are different from planets.

If you wanted to prove that an asteroid crashed into Earth millions of years ago, what kind of evidence would you look for? Explain your ideas.


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Comets are also leftovers from the process that formed the solar system. However, comets originate much farther away than asteroids. Comets come from the outer solar system, often far past Neptune. The orbits of some comets cut across the paths of the planets and then make a close turn around the sun. Recall the picture of Comet Halley earlier in this lesson. The diagram below shows Comet Halley’s path through the solar system.

NeptuneNeptune

HalleyHalley

UranusUranus SaturnSaturn

JupiterJupiter

MarsMars

Earth

NOTE: Mercury's orbit not shown

How is the orbit of a comet different from the orbit of a planet?

Why is Comet Halley not visible from Earth for most of its 75-year orbit?


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Scientists think that there are millions of asteroids in the belt between Mars and Jupiter. Sometimes asteroids collide, breaking apart into smaller pieces. Comets, which are more fragile than asteroids, also lose bits and pieces as they journey through the solar system. Fragments of asteroids and comets are called meteoroids.

Meteoroids orbit the sun until they run into a planet or moon. You may have witnessed such a meteoroid encountering Earth. When a small meteoroid enters Earth’s atmosphere, it burns up. At night, these meteors are called “shooting stars.” Larger meteoroids may not burn up entirely in the atmosphere. When they hit the ground, they are called meteorites.

When a large meteoroid or an asteroid hits a planet or a moon, it may form a crater, a bowl-shaped hole.

Examine the second planet on the opening page of this lesson. The planet is Mercury. Describe what you see on the planet’s surface.

Spread a three-centimeter layer of flour in a cake pan, and then sprinkle a thin, even layer of cocoa powder over the flour. From a height of about one meter, drop small objects such as raisins, marbles, and seeds into the pan. Describe the holes left by the objects. Then relate your observations to meteorites.

Try This


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Check YourUnderstanding


1 Compare the four inner planets with the four outer planets.

2 Describe how a moon moves through the solar system.

3 The perihelion of a comet is the point in its orbit at which it is closest to the sun. The table shows the last three perihelion dates for three comets. Complete the table by writing the next perihelion date for each comet.

Comet Perihelion Dates

Catalina 9 1938 1973 2008

Christensen 5 1773 1889 2005

Gibbs 4 1969 1988 2007


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4 In the box below, draw a diagram of the sun, Mars, and its two moons, Phobos and Deimos. Be sure to show orbits in your diagram.

5 Select each sentence that correctly describes the solar system.

A All objects in the solar system revolve around Earth.

B Asteroids and comets may reveal information about the formation of the solar system.

C Planets are the largest bodies in the solar system.

D The solar system includes the sun, planets, moons, other bodies, and empty space.

E Objects in the solar system revolve around the sun.

Lesson 2 The Solar System...Lesson 2 • The Solar System 21 The solar system is vast and varied. It...

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