## Charting The Planets

An Educational Publication of the National Aeronautics and Space Administration

NASA Educational Brief
EB-111 12/92

Topic: Planetary Science-Explore and Interpret Characteristics of the Planets

Learning to use a chart of data can be helpful in several ways. First, a chart can provide an orderly list of individual facts. For example, if one needs to know a single datum, like Neptune's mean distance from the sun, it is easy to find. In the chart MEAN DISTANCE OF PLANETS, you would look to the left-hand column for the "Mean Distance From Sun" row, and then look across the row to the "Neptune" column. Your answer lies where row and column cross: 4,497 (millions of kilometers).

A second way to use a chart is to look for trends and patterns in the data. This approach will give you "the big picture" about the set of objects the chart describes. Take a look at the chart SILHOUETTES OF THE PLANETS. The planets are shown in approximate relative sizes, and arranged according to their order from the Sun: Mercury, on the left, is the closest to the Sun and Pluto (normally) is the farthest away. The obvious pattern, moving away from the Sun, is that there are four small planets, then two huge ones, two large ones, and a final, tiny one. Please note that two characteristics are plotted in the silhouettes: diameter and distance. Diameter is to scale and distances are not to scale. To find out the real distances from the Sun, and then establish a scale we ll need to look at the data in the chart MEAN DISTANCE FROM THE SUN.

Looking at the chart, we can see that the columns are ranged, like the silhouettes, according to their order from the Sun. Let's look at the mean distance from the Sun of each of the planets to see if there is a pattern. A quick glance tells you that there is a big jump from Mars to Jupiter, and the jumps stay pretty big after that. A look at the mathematical progression is more revealing. (You can do these calculations with a calculator or on paper by entering the larger number, dividing by the smaller number, and rounding off the result.)

Venus is a bit less than twice as far (1.8x) from the Sun than Mercury. Earth is about 1.4 times farther out than Venus. Mars is about 1.5 times farther than Earth. So in this group of small planets, each orbits from 1.4 to 1.8 times farther out than its inner neighbor.

Then we get to Jupiter, which orbits 3.4 times farther out than Mars. The pattern predicts that the step after Mars should have been in the 1.4 to 1.8 range, but it is just about twice that. From Jupiter to Saturn is a step of about 1.8; Saturn to Uranus is 2.0; Uranus to Neptune, 1.6; and Neptune to Pluto, 1.3.

Looking at all the numbers in this row, you can see that the pattern across the nine planets is broken most notably by the distance from Mars' orbit to Jupiter's. A scientist developing a theory about the planets' orbits would need to account for this break in the pattern.

These two examples of chart-reading illustrate how charts can be read for patterns, and pattern-breakers, without spending too much time figuring out the math behind each point of data. You can look at other rows of the chart on the next page to determine patterns and pattern-breakers as well. After listing all of the pattern-breakers, you may find there is a pattern in the pattern-breakers!

### What the Categories on the Chart Describe

Refer to the file named CHARTING THE PLANETS (full page) for the chart designed for this activity.

1. Mean Distance from the Sun. A "mean" value is an average value. When there is more than one value for a measurement, and you want a single meaningful approximate value, you can take the average of the measurements. One way of averaging is by taking the mean. The mean is determined by summing all the values and dividing by the number of values summed. When you see the term "average" or "mean" on a chart, it tells you that there are multiple values for this category, and they have been averaged to produce one representative value.

Q: What does the fact that the distance is shown as an average tell you about a planet's distance from the Sun?

2. Period of Revolution. A revolution of a planet is a single trip, or orbit, of that planet around the Sun. The "period" of any event is the amount of time the event takes to occur. The term "period" is usually applied to the duration of events that take place over and over again.

Q: Note that the chart does not say "mean" or "average" period of revolution. What does this tell you about the value of any planet's period of revolution?

3. Equatorial Diameter. This measurement is the diameter of a planet from one surface through to the other, measured at the equator.

Q: Why do you think the measurement is most often used to compare the size of the planets (as opposed to volume of the sphere, or circumference, for example)?

4. Atmosphere (Main Components). An atmosphere is the layer of gases held close by a planet's gravity. The components are the individual gases in the atmosphere. Most atmospheres have one or two dominant components, in much greater abundance than others, and then many minor and "trace" components as well.

Q: Looking at Venus, Earth, and Mars, which one has a different atmosphere from the other two? What else do you know about that planet that is different from the others?

5. Moons. A moon is a natural satellite in orbit around a planet. There are two classes of moons, regular and irregular. Regular moons are spherical in shape, like small planets. Irregular moons are roughly shaped. They may be asteroids captured by a planet's gravity. Asteroids are chunks of rocky, metallic, and icy material that circle the Sun, mostly between Mars and Jupiter. They are probably pieces of planets that never formed, and/or were broken up in collisions.

Q: Do you think Earth's moon is regular or irregular?

6. Rings. Jupiter, Saturn, Uranus, and Neptune all have ring systems of variously sized particles in orbit around them. These rings are made up of countless icy and rocky particles, probably from moons that collided or never formed. Saturn's rings are bright and visible from Earth, but the rings around the other three ringed planets were not seen until NASA's Voyager spacecraft flew by in the 1980's.

Q: There are several reasons for the rings of Jupiter, Uranus, and Neptune being invisible from Earth. Can you guess one or two?

7. Inclination of Orbit to the Ecliptic. The path a planet takes around the Sun is the planet's orbit. The planets orbit the Sun in or close to a geometric plane called the ecliptic. The plane of the ecliptic corresponds to the equator of the Sun. If you were to slice through the equator of the Sun with a huge flat plane, stretching across the entire solar system, that plane would be on the ecliptic. If a planet's orbit follows a path exactly on the ecliptic plane, then its inclination of orbit is 0 degrees. If it orbits off the ecliptic plane, the planet has an inclined orbit, measured in degrees from the ecliptic plane. This measurement is the inclination of orbit.

Q: Will an inclined orbit always be above the ecliptic plane? Why or why not?

8. Eccentricity of Orbit. Orbits that are not round are called eccentric. Round orbits are said to have an eccentricity of zero. Most planetary orbits are not quite round, that is, sometimes the planet is closer to the Sun, and sometimes it is farther away. Such orbits are elliptical in shape, and have eccentricities between 0 and 1.

Q: A very elliptical orbit, with an eccentricity approaching 1, would mean a body spends much of its time very far from the Sun, but twice each orbit passes much closer in. (During part of Pluto's very eccentric orbit, the planet is actually closer to the Sun than Neptune is!) What do you think this means for the environment of the planet?

9. Rotation Period. "Rotate" means spin on an axis, like a bicycle wheel or a compact disc (CD). If you ride your bicycle around the block, you have made an "orbit," or revolution, when you arrive back in front of your house; but during the whole trip, your bicycle tires were rotating. The tires' mean period of rotation may have been 1/8th of a second during the ride, while the period of revolution of you and the bike may have been 5 minutes.

Q: What other machines or objects can you think of that have a period of rotation and a period of revolution?

10. Inclination of Axis. A planet's axis is the center line of its rotation, like the axle on a bicycle wheel or the center spindle on a CD player. If a planet rotates standing straight up in the ecliptic, then the inclination of its axis is said to be 0 degrees . A planet that rotates not quite straight up would have an inclination of axis of less than 10 degrees. A planet that rotates completely over on its side, rolling along like a bicycle wheel, would have an inclination of axis of 90 degrees. Six of the planets rotate in the same direction, counterclockwise as seen from above (north), and three do not. Rotating or revolving in a direction opposite from the norm is called retrograde motion. Venus, Uranus, and Pluto exhibit retrograde rotation.

Q: A planet that rotates in a retrograde direction has an axis of inclination of greater than 90 degrees. Why would this be so?

### For the Classroom

Studying patterns and pattern-breakers usually leaves you with a set of questions to be answered. The search for these answers can be illuminating. For example, from examining the general pattern of planetary motion, scientists have come to believe that every body in the solar system formed out of the same huge uniformly spinning cloud of dust and gas (something like a cosmic cyclone). This would account for why most things in the solar system spin in the same direction now. The bodies that don't, the pattern-breakers that exhibit retrograde motion, are believed to have been in violent collisions that flipped them over. In Venus' case the collision also slowed down its rotation quite a bit.

From the general pattern of orbital distance from the Sun, we believe that there should have been a planet where the asteroid belt now is, between Mars and Jupiter. Asteroids are probably the stuff from which this planet would have formed. There are several possible reasons why it didn't form, or formed and then broke up. The influence of the huge pull of Jupiter's gravity nearby was no doubt a key factor. And there may have been a lot of collisions in that part of space during the time the planets were forming.

Pluto shows a great variance from the pattern in size, orbital inclination and eccentricity, and probable composition (not shown in this chart). These peculiarities have led scientists to speculate that tiny, icy Pluto may have been a comet that was pulled into a planetary orbit by the Sun's gravity. If Pluto is like a comet, then it may include frozen remnants of the original materials from which the planets formed. We'll need a closer study of the distant planet before we can be sure.

Note: Before addressing these questions, you may want to skip ahead to Activity 2 and make the planet cards. The cards may prove helpful in thinking about what is in the chart.

### Activity 1

This activity utilizes the PLANETARY CARD SET files. There are two parts to this set, the fact sheet and the image sheet. Both parts of this set are necessary to conduct this activity.

1. There is a jump in the pattern for mean distance from the Sun, from Mars to Jupiter. Is there a jump in the same place in any of the other data?

2. There is a clear correspondence between period of revolution and what other characteristic in the chart on page 2? How would you account for this correspondence?

3. Which is the smallest planet? Are we sure of the size of its diameter? Why do you think we might not be?

4. As we noted previously, Earth has a decidedly different atmosphere than the other small, inner planets. The outer planets show a simple pattern of their own in atmospheric composition. What is it? Using your knowledge of chemistry (in particular, the periodic table of the elements), what can you say about the two main gases in the atmospheres of the four large, outer planets?

5. What general statement can you make about the number of moons held by each of the two main classes of planets? What other characteristics of the planets follow the same general pattern as the number of moons?

6. The rings of Saturn are believed to have a lot of icy particles in them, which is why they reflect light so brightly and appear so colorful. The rings of the other ringed planets are darker and less numerous. These rings are probably made up of dustier and rockier materials. Is there something resembling a ring system orbiting the Sun? Where does it orbit? What do you remember about this orbital slot from our previous discussion?

7. What is the pattern in inclination of orbit? What is the pattern-breaker? Keep track of this pattern-breaking planet's characteristics; it is different from the other planets in many intriguing ways.

8. There are two planets with very eccentric orbits. What else do these two planets have in common? Are there other categories in which they are both pattern-breakers? Which one(s)?

9. Is there a pattern in rotation period? Is there one planet whose rotation period is very different from the rotation periods of the other planets in its size range?

10. As noted previously, there is a correspondence between inclination of axis and retrograde motion. Planets that are "upside down" exhibit retrograde rotation. A planet spinning perfectly on its "head" has an inclination of axis of 180 degrees. Examining all nine planets, can you say what the trend is in axial inclination, and what the trend-breakers are? Is Mercury a trend-breaker? Why or why not?

### Activity 2

These activities involve the use of the PLANETARY CARD SET. This could be photocopied, preferably onto heavy paper (cover stock or card stock), or may be glued onto heavier stock. Students should then cut out each card so they have a set of nine.

1. For each planet, fill out the missing data on the back of the card.

2. Make a card for the Sun. The Sun's diameter is 1,395,161 km. The scale at which the planets are drawn on the cards is 1 mm = 2604 km. Therefore, the diameter of your drawing of the Sun should be 1,395,161 divided by 2604, or 536mm. You will need a large piece of paper, at least 536 mm on each side, for the Sun.

3. On the front of each planet's card, use a protractor and ruler to draw the axis of rotation through the planet. The axis should pass through the center of the planet. Remember that an inclination of 0 degrees is straight up, and 90 degrees means a planet is on its side.

4. Draw rings on each planet that has rings. Rings orbit a planet around its equator, which is at 90 degrees to the axis of rotation.

5. Draw moons around each planet that has moons. Like rings, moons orbit a planet around its equator, but farther out than the rings. Most of the moons can be small dots, since most moons are tiny compared to the size of the planet. Earth's Moon is about 1/4th the diameter of Earth. Pluto's moon, Charon, is about 1/3rd the diameter of Pluto.

6. For each category (row) on the chart, arrange the planets in order, from least to greatest. Discuss what you have learned about each category. Note relationships between categories. Which categories seem to stand alone, in little or no relation to the others? Which single category seems the most random to you? Why?

### Activity 3

This activity utilizes the U.S. PLANETARY MISSIONS chart.

#### Background

Much of what we know about the Moon and planets has come from robotic spaceflight missions - spacecraft sent from Earth to photograph and measure characteristics of our neighbors in the solar system. The next page of this Brief is a graphic depiction of all the robotic U.S. missions to other bodies in the solar system. All of the "visitors" to each body are listed, along with the type and year of the mission. Photo missions were spacecraft that were crashed into the body, taking pictures right up to the end. Flyby missions made a single pass by a planet. Orbiters went into orbit around the planet, in some cases dropping probes through the atmosphere or landers onto the surface. The Magellan mission of 1989 was an orbiter with a special instrument called aradar mapper, which penetrated the thick cloud cover of Venus to map the surface below.

1. Discuss the difference in the way data are shown in this graphic compared to the chart of planetary characteristics seen earlier in this Brief. What purpose can a graphic like this one serve?

2. What general statements can you make about planetary exploration based on the graphic? Consider where we have gone and not gone, and what we have done there.

3. See if you can relate any of the information on this graphic to information on the chart of planetary characteristics.

4. Write a set of discussion questions for the graphic.

5. See if you can set up a chart, like that of planetary characteristics, for the planetary missions, using some or all of the data in the graphic. Then look at item 3 again.

6. Choose one mission as a research topic. Gather information about the objectives of the mission, what was learned from the mission, the status of the spacecraft, and how the mission may have influenced later exploration of the target body.

7. After you have learned about past missions, locate newspaper and magazine articles about current and future planetary missions. Add these missions her's guide, including discussion questions and student section, is included with the video. These resources are available from NASA CORE, Teacher Resource Centers, and Regional Teacher Resource Centers.

NASA CORE
Lorain County Joint Vocational School
15181 Route 58 South
Oberlin, OH 44074
(216) 774-1051, Ext. 293 or 294

NASA Spacelink is a computer-information service that allows individuals to receive news about current NASA programs, activities, and other space-related information, including historical and astronaut data, lesson plans, classroom activities, and even entire publications. Although primarily intended as a resource for teachers, anyone with a personal computer and a modem can access the network.

The Spacelink computer access number is (205) 895-0028. Users need a computer, modem, communications software, and a long-distance telephone line to access Spacelink. The data word format is 8 bits, no parity, and 1 stop bit. Spacelink is also available through the Internet at spacelink.msfc.nasa.gov. For more information, contact:

NASA Marshall Space Flight Center, Mail Code CL01
Marshall Space Flight Center, AL 35812
Phone: (205) 961-1225

## Charting The Planets

An Educational Publication of the National Aeronautics and Space Administration

NASA Educational Brief
EB-111 12/92

Topic: Planetary Science-Explore and Interpret Characteristics of the Planets

Learning to use a chart of data can be helpful in several ways. First, a chart can provide an orderly list of individual facts. For example, if one needs to know a single datum, like Neptune's mean distance from the sun, it is easy to find. In the chart MEAN DISTANCE OF PLANETS, you would look to the left-hand column for the "Mean Distance From Sun" row, and then look across the row to the "Neptune" column. Your answer lies where row and column cross: 4,497 (millions of kilometers).

A second way to use a chart is to look for trends and patterns in the data. This approach will give you "the big picture" about the set of objects the chart describes. Take a look at the chart SILHOUETTES OF THE PLANETS. The planets are shown in approximate relative sizes, and arranged according to their order from the Sun: Mercury, on the left, is the closest to the Sun and Pluto (normally) is the farthest away. The obvious pattern, moving away from the Sun, is that there are four small planets, then two huge ones, two large ones, and a final, tiny one. Please note that two characteristics are plotted in the silhouettes: diameter and distance. Diameter is to scale and distances are not to scale. To find out the real distances from the Sun, and then establish a scale we ll need to look at the data in the chart MEAN DISTANCE FROM THE SUN.

Looking at the chart, we can see that the columns are ranged, like the silhouettes, according to their order from the Sun. Let's look at the mean distance from the Sun of each of the planets to see if there is a pattern. A quick glance tells you that there is a big jump from Mars to Jupiter, and the jumps stay pretty big after that. A look at the mathematical progression is more revealing. (You can do these calculations with a calculator or on paper by entering the larger number, dividing by the smaller number, and rounding off the result.)

Venus is a bit less than twice as far (1.8x) from the Sun than Mercury. Earth is about 1.4 times farther out than Venus. Mars is about 1.5 times farther than Earth. So in this group of small planets, each orbits from 1.4 to 1.8 times farther out than its inner neighbor.

Then we get to Jupiter, which orbits 3.4 times farther out than Mars. The pattern predicts that the step after Mars should have been in the 1.4 to 1.8 range, but it is just about twice that. From Jupiter to Saturn is a step of about 1.8; Saturn to Uranus is 2.0; Uranus to Neptune, 1.6; and Neptune to Pluto, 1.3.

Looking at all the numbers in this row, you can see that the pattern across the nine planets is broken most notably by the distance from Mars' orbit to Jupiter's. A scientist developing a theory about the planets' orbits would need to account for this break in the pattern.

These two examples of chart-reading illustrate how charts can be read for patterns, and pattern-breakers, without spending too much time figuring out the math behind each point of data. You can look at other rows of the chart on the next page to determine patterns and pattern-breakers as well. After listing all of the pattern-breakers, you may find there is a pattern in the pattern-breakers!

### What the Categories on the Chart Describe

Refer to the file named CHARTING THE PLANETS (full page) for the chart designed for this activity.

1. Mean Distance from the Sun. A "mean" value is an average value. When there is more than one value for a measurement, and you want a single meaningful approximate value, you can take the average of the measurements. One way of averaging is by taking the mean. The mean is determined by summing all the values and dividing by the number of values summed. When you see the term "average" or "mean" on a chart, it tells you that there are multiple values for this category, and they have been averaged to produce one representative value.

Q: What does the fact that the distance is shown as an average tell you about a planet's distance from the Sun?

2. Period of Revolution. A revolution of a planet is a single trip, or orbit, of that planet around the Sun. The "period" of any event is the amount of time the event takes to occur. The term "period" is usually applied to the duration of events that take place over and over again.

Q: Note that the chart does not say "mean" or "average" period of revolution. What does this tell you about the value of any planet's period of revolution?

3. Equatorial Diameter. This measurement is the diameter of a planet from one surface through to the other, measured at the equator.

Q: Why do you think the measurement is most often used to compare the size of the planets (as opposed to volume of the sphere, or circumference, for example)?

4. Atmosphere (Main Components). An atmosphere is the layer of gases held close by a planet's gravity. The components are the individual gases in the atmosphere. Most atmospheres have one or two dominant components, in much greater abundance than others, and then many minor and "trace" components as well.

Q: Looking at Venus, Earth, and Mars, which one has a different atmosphere from the other two? What else do you know about that planet that is different from the others?

5. Moons. A moon is a natural satellite in orbit around a planet. There are two classes of moons, regular and irregular. Regular moons are spherical in shape, like small planets. Irregular moons are roughly shaped. They may be asteroids captured by a planet's gravity. Asteroids are chunks of rocky, metallic, and icy material that circle the Sun, mostly between Mars and Jupiter. They are probably pieces of planets that never formed, and/or were broken up in collisions.

Q: Do you think Earth's moon is regular or irregular?

6. Rings. Jupiter, Saturn, Uranus, and Neptune all have ring systems of variously sized particles in orbit around them. These rings are made up of countless icy and rocky particles, probably from moons that collided or never formed. Saturn's rings are bright and visible from Earth, but the rings around the other three ringed planets were not seen until NASA's Voyager spacecraft flew by in the 1980's.

Q: There are several reasons for the rings of Jupiter, Uranus, and Neptune being invisible from Earth. Can you guess one or two?

7. Inclination of Orbit to the Ecliptic. The path a planet takes around the Sun is the planet's orbit. The planets orbit the Sun in or close to a geometric plane called the ecliptic. The plane of the ecliptic corresponds to the equator of the Sun. If you were to slice through the equator of the Sun with a huge flat plane, stretching across the entire solar system, that plane would be on the ecliptic. If a planet's orbit follows a path exactly on the ecliptic plane, then its inclination of orbit is 0 degrees. If it orbits off the ecliptic plane, the planet has an inclined orbit, measured in degrees from the ecliptic plane. This measurement is the inclination of orbit.

Q: Will an inclined orbit always be above the ecliptic plane? Why or why not?

8. Eccentricity of Orbit. Orbits that are not round are called eccentric. Round orbits are said to have an eccentricity of zero. Most planetary orbits are not quite round, that is, sometimes the planet is closer to the Sun, and sometimes it is farther away. Such orbits are elliptical in shape, and have eccentricities between 0 and 1.

Q: A very elliptical orbit, with an eccentricity approaching 1, would mean a body spends much of its time very far from the Sun, but twice each orbit passes much closer in. (During part of Pluto's very eccentric orbit, the planet is actually closer to the Sun than Neptune is!) What do you think this means for the environment of the planet?

9. Rotation Period. "Rotate" means spin on an axis, like a bicycle wheel or a compact disc (CD). If you ride your bicycle around the block, you have made an "orbit," or revolution, when you arrive back in front of your house; but during the whole trip, your bicycle tires were rotating. The tires' mean period of rotation may have been 1/8th of a second during the ride, while the period of revolution of you and the bike may have been 5 minutes.

Q: What other machines or objects can you think of that have a period of rotation and a period of revolution?

10. Inclination of Axis. A planet's axis is the center line of its rotation, like the axle on a bicycle wheel or the center spindle on a CD player. If a planet rotates standing straight up in the ecliptic, then the inclination of its axis is said to be 0 degrees . A planet that rotates not quite straight up would have an inclination of axis of less than 10 degrees. A planet that rotates completely over on its side, rolling along like a bicycle wheel, would have an inclination of axis of 90 degrees. Six of the planets rotate in the same direction, counterclockwise as seen from above (north), and three do not. Rotating or revolving in a direction opposite from the norm is called retrograde motion. Venus, Uranus, and Pluto exhibit retrograde rotation.

Q: A planet that rotates in a retrograde direction has an axis of inclination of greater than 90 degrees. Why would this be so?

### For the Classroom

Studying patterns and pattern-breakers usually leaves you with a set of questions to be answered. The search for these answers can be illuminating. For example, from examining the general pattern of planetary motion, scientists have come to believe that every body in the solar system formed out of the same huge uniformly spinning cloud of dust and gas (something like a cosmic cyclone). This would account for why most things in the solar system spin in the same direction now. The bodies that don't, the pattern-breakers that exhibit retrograde motion, are believed to have been in violent collisions that flipped them over. In Venus' case the collision also slowed down its rotation quite a bit.

From the general pattern of orbital distance from the Sun, we believe that there should have been a planet where the asteroid belt now is, between Mars and Jupiter. Asteroids are probably the stuff from which this planet would have formed. There are several possible reasons why it didn't form, or formed and then broke up. The influence of the huge pull of Jupiter's gravity nearby was no doubt a key factor. And there may have been a lot of collisions in that part of space during the time the planets were forming.

Pluto shows a great variance from the pattern in size, orbital inclination and eccentricity, and probable composition (not shown in this chart). These peculiarities have led scientists to speculate that tiny, icy Pluto may have been a comet that was pulled into a planetary orbit by the Sun's gravity. If Pluto is like a comet, then it may include frozen remnants of the original materials from which the planets formed. We'll need a closer study of the distant planet before we can be sure.

Note: Before addressing these questions, you may want to skip ahead to Activity 2 and make the planet cards. The cards may prove helpful in thinking about what is in the chart.

### Activity 1

This activity utilizes the PLANETARY CARD SET files. There are two parts to this set, the fact sheet and the image sheet. Both parts of this set are necessary to conduct this activity.

1. There is a jump in the pattern for mean distance from the Sun, from Mars to Jupiter. Is there a jump in the same place in any of the other data?

2. There is a clear correspondence between period of revolution and what other characteristic in the chart on page 2? How would you account for this correspondence?

3. Which is the smallest planet? Are we sure of the size of its diameter? Why do you think we might not be?

4. As we noted previously, Earth has a decidedly different atmosphere than the other small, inner planets. The outer planets show a simple pattern of their own in atmospheric composition. What is it? Using your knowledge of chemistry (in particular, the periodic table of the elements), what can you say about the two main gases in the atmospheres of the four large, outer planets?

5. What general statement can you make about the number of moons held by each of the two main classes of planets? What other characteristics of the planets follow the same general pattern as the number of moons?

6. The rings of Saturn are believed to have a lot of icy particles in them, which is why they reflect light so brightly and appear so colorful. The rings of the other ringed planets are darker and less numerous. These rings are probably made up of dustier and rockier materials. Is there something resembling a ring system orbiting the Sun? Where does it orbit? What do you remember about this orbital slot from our previous discussion?

7. What is the pattern in inclination of orbit? What is the pattern-breaker? Keep track of this pattern-breaking planet's characteristics; it is different from the other planets in many intriguing ways.

8. There are two planets with very eccentric orbits. What else do these two planets have in common? Are there other categories in which they are both pattern-breakers? Which one(s)?

9. Is there a pattern in rotation period? Is there one planet whose rotation period is very different from the rotation periods of the other planets in its size range?

10. As noted previously, there is a correspondence between inclination of axis and retrograde motion. Planets that are "upside down" exhibit retrograde rotation. A planet spinning perfectly on its "head" has an inclination of axis of 180 degrees. Examining all nine planets, can you say what the trend is in axial inclination, and what the trend-breakers are? Is Mercury a trend-breaker? Why or why not?

### Activity 2

These activities involve the use of the PLANETARY CARD SET. This could be photocopied, preferably onto heavy paper (cover stock or card stock), or may be glued onto heavier stock. Students should then cut out each card so they have a set of nine.

1. For each planet, fill out the missing data on the back of the card.

2. Make a card for the Sun. The Sun's diameter is 1,395,161 km. The scale at which the planets are drawn on the cards is 1 mm = 2604 km. Therefore, the diameter of your drawing of the Sun should be 1,395,161 divided by 2604, or 536mm. You will need a large piece of paper, at least 536 mm on each side, for the Sun.

3. On the front of each planet's card, use a protractor and ruler to draw the axis of rotation through the planet. The axis should pass through the center of the planet. Remember that an inclination of 0 degrees is straight up, and 90 degrees means a planet is on its side.

4. Draw rings on each planet that has rings. Rings orbit a planet around its equator, which is at 90 degrees to the axis of rotation.

5. Draw moons around each planet that has moons. Like rings, moons orbit a planet around its equator, but farther out than the rings. Most of the moons can be small dots, since most moons are tiny compared to the size of the planet. Earth's Moon is about 1/4th the diameter of Earth. Pluto's moon, Charon, is about 1/3rd the diameter of Pluto.

6. For each category (row) on the chart, arrange the planets in order, from least to greatest. Discuss what you have learned about each category. Note relationships between categories. Which categories seem to stand alone, in little or no relation to the others? Which single category seems the most random to you? Why?

### Activity 3

This activity utilizes the U.S. PLANETARY MISSIONS chart.

#### Background

Much of what we know about the Moon and planets has come from robotic spaceflight missions - spacecraft sent from Earth to photograph and measure characteristics of our neighbors in the solar system. The next page of this Brief is a graphic depiction of all the robotic U.S. missions to other bodies in the solar system. All of the "visitors" to each body are listed, along with the type and year of the mission. Photo missions were spacecraft that were crashed into the body, taking pictures right up to the end. Flyby missions made a single pass by a planet. Orbiters went into orbit around the planet, in some cases dropping probes through the atmosphere or landers onto the surface. The Magellan mission of 1989 was an orbiter with a special instrument called aradar mapper, which penetrated the thick cloud cover of Venus to map the surface below.

1. Discuss the difference in the way data are shown in this graphic compared to the chart of planetary characteristics seen earlier in this Brief. What purpose can a graphic like this one serve?

2. What general statements can you make about planetary exploration based on the graphic? Consider where we have gone and not gone, and what we have done there.

3. See if you can relate any of the information on this graphic to information on the chart of planetary characteristics.

4. Write a set of discussion questions for the graphic.

5. See if you can set up a chart, like that of planetary characteristics, for the planetary missions, using some or all of the data in the graphic. Then look at item 3 again.

6. Choose one mission as a research topic. Gather information about the objectives of the mission, what was learned from the mission, the status of the spacecraft, and how the mission may have influenced later exploration of the target body.

7. After you have learned about past missions, locate newspaper and magazine articles about current and future planetary missions. Add these missions her's guide, including discussion questions and student section, is included with the video. These resources are available from NASA CORE, Teacher Resource Centers, and Regional Teacher Resource Centers.

NASA CORE
Lorain County Joint Vocational School
15181 Route 58 South
Oberlin, OH 44074
(216) 774-1051, Ext. 293 or 294

NASA Spacelink is a computer-information service that allows individuals to receive news about current NASA programs, activities, and other space-related information, including historical and astronaut data, lesson plans, classroom activities, and even entire publications. Although primarily intended as a resource for teachers, anyone with a personal computer and a modem can access the network.

The Spacelink computer access number is (205) 895-0028. Users need a computer, modem, communications software, and a long-distance telephone line to access Spacelink. The data word format is 8 bits, no parity, and 1 stop bit. Spacelink is also available through the Internet at spacelink.msfc.nasa.gov. For more information, contact:

NASA Marshall Space Flight Center, Mail Code CL01
Marshall Space Flight Center, AL 35812
Phone: (205) 961-1225

## Charting The Planets

An Educational Publication of the National Aeronautics and Space Administration

NASA Educational Brief
EB-111 12/92

Topic: Planetary Science-Explore and Interpret Characteristics of the Planets

Learning to use a chart of data can be helpful in several ways. First, a chart can provide an orderly list of individual facts. For example, if one needs to know a single datum, like Neptune's mean distance from the sun, it is easy to find. In the chart MEAN DISTANCE OF PLANETS, you would look to the left-hand column for the "Mean Distance From Sun" row, and then look across the row to the "Neptune" column. Your answer lies where row and column cross: 4,497 (millions of kilometers).

A second way to use a chart is to look for trends and patterns in the data. This approach will give you "the big picture" about the set of objects the chart describes. Take a look at the chart SILHOUETTES OF THE PLANETS. The planets are shown in approximate relative sizes, and arranged according to their order from the Sun: Mercury, on the left, is the closest to the Sun and Pluto (normally) is the farthest away. The obvious pattern, moving away from the Sun, is that there are four small planets, then two huge ones, two large ones, and a final, tiny one. Please note that two characteristics are plotted in the silhouettes: diameter and distance. Diameter is to scale and distances are not to scale. To find out the real distances from the Sun, and then establish a scale we ll need to look at the data in the chart MEAN DISTANCE FROM THE SUN.

Looking at the chart, we can see that the columns are ranged, like the silhouettes, according to their order from the Sun. Let's look at the mean distance from the Sun of each of the planets to see if there is a pattern. A quick glance tells you that there is a big jump from Mars to Jupiter, and the jumps stay pretty big after that. A look at the mathematical progression is more revealing. (You can do these calculations with a calculator or on paper by entering the larger number, dividing by the smaller number, and rounding off the result.)

Venus is a bit less than twice as far (1.8x) from the Sun than Mercury. Earth is about 1.4 times farther out than Venus. Mars is about 1.5 times farther than Earth. So in this group of small planets, each orbits from 1.4 to 1.8 times farther out than its inner neighbor.

Then we get to Jupiter, which orbits 3.4 times farther out than Mars. The pattern predicts that the step after Mars should have been in the 1.4 to 1.8 range, but it is just about twice that. From Jupiter to Saturn is a step of about 1.8; Saturn to Uranus is 2.0; Uranus to Neptune, 1.6; and Neptune to Pluto, 1.3.

Looking at all the numbers in this row, you can see that the pattern across the nine planets is broken most notably by the distance from Mars' orbit to Jupiter's. A scientist developing a theory about the planets' orbits would need to account for this break in the pattern.

These two examples of chart-reading illustrate how charts can be read for patterns, and pattern-breakers, without spending too much time figuring out the math behind each point of data. You can look at other rows of the chart on the next page to determine patterns and pattern-breakers as well. After listing all of the pattern-breakers, you may find there is a pattern in the pattern-breakers!

### What the Categories on the Chart Describe

Refer to the file named CHARTING THE PLANETS (full page) for the chart designed for this activity.

1. Mean Distance from the Sun. A "mean" value is an average value. When there is more than one value for a measurement, and you want a single meaningful approximate value, you can take the average of the measurements. One way of averaging is by taking the mean. The mean is determined by summing all the values and dividing by the number of values summed. When you see the term "average" or "mean" on a chart, it tells you that there are multiple values for this category, and they have been averaged to produce one representative value.

Q: What does the fact that the distance is shown as an average tell you about a planet's distance from the Sun?

2. Period of Revolution. A revolution of a planet is a single trip, or orbit, of that planet around the Sun. The "period" of any event is the amount of time the event takes to occur. The term "period" is usually applied to the duration of events that take place over and over again.

Q: Note that the chart does not say "mean" or "average" period of revolution. What does this tell you about the value of any planet's period of revolution?

3. Equatorial Diameter. This measurement is the diameter of a planet from one surface through to the other, measured at the equator.

Q: Why do you think the measurement is most often used to compare the size of the planets (as opposed to volume of the sphere, or circumference, for example)?

4. Atmosphere (Main Components). An atmosphere is the layer of gases held close by a planet's gravity. The components are the individual gases in the atmosphere. Most atmospheres have one or two dominant components, in much greater abundance than others, and then many minor and "trace" components as well.

Q: Looking at Venus, Earth, and Mars, which one has a different atmosphere from the other two? What else do you know about that planet that is different from the others?

5. Moons. A moon is a natural satellite in orbit around a planet. There are two classes of moons, regular and irregular. Regular moons are spherical in shape, like small planets. Irregular moons are roughly shaped. They may be asteroids captured by a planet's gravity. Asteroids are chunks of rocky, metallic, and icy material that circle the Sun, mostly between Mars and Jupiter. They are probably pieces of planets that never formed, and/or were broken up in collisions.

Q: Do you think Earth's moon is regular or irregular?

6. Rings. Jupiter, Saturn, Uranus, and Neptune all have ring systems of variously sized particles in orbit around them. These rings are made up of countless icy and rocky particles, probably from moons that collided or never formed. Saturn's rings are bright and visible from Earth, but the rings around the other three ringed planets were not seen until NASA's Voyager spacecraft flew by in the 1980's.

Q: There are several reasons for the rings of Jupiter, Uranus, and Neptune being invisible from Earth. Can you guess one or two?

7. Inclination of Orbit to the Ecliptic. The path a planet takes around the Sun is the planet's orbit. The planets orbit the Sun in or close to a geometric plane called the ecliptic. The plane of the ecliptic corresponds to the equator of the Sun. If you were to slice through the equator of the Sun with a huge flat plane, stretching across the entire solar system, that plane would be on the ecliptic. If a planet's orbit follows a path exactly on the ecliptic plane, then its inclination of orbit is 0 degrees. If it orbits off the ecliptic plane, the planet has an inclined orbit, measured in degrees from the ecliptic plane. This measurement is the inclination of orbit.

Q: Will an inclined orbit always be above the ecliptic plane? Why or why not?

8. Eccentricity of Orbit. Orbits that are not round are called eccentric. Round orbits are said to have an eccentricity of zero. Most planetary orbits are not quite round, that is, sometimes the planet is closer to the Sun, and sometimes it is farther away. Such orbits are elliptical in shape, and have eccentricities between 0 and 1.

Q: A very elliptical orbit, with an eccentricity approaching 1, would mean a body spends much of its time very far from the Sun, but twice each orbit passes much closer in. (During part of Pluto's very eccentric orbit, the planet is actually closer to the Sun than Neptune is!) What do you think this means for the environment of the planet?

9. Rotation Period. "Rotate" means spin on an axis, like a bicycle wheel or a compact disc (CD). If you ride your bicycle around the block, you have made an "orbit," or revolution, when you arrive back in front of your house; but during the whole trip, your bicycle tires were rotating. The tires' mean period of rotation may have been 1/8th of a second during the ride, while the period of revolution of you and the bike may have been 5 minutes.

Q: What other machines or objects can you think of that have a period of rotation and a period of revolution?

10. Inclination of Axis. A planet's axis is the center line of its rotation, like the axle on a bicycle wheel or the center spindle on a CD player. If a planet rotates standing straight up in the ecliptic, then the inclination of its axis is said to be 0 degrees . A planet that rotates not quite straight up would have an inclination of axis of less than 10 degrees. A planet that rotates completely over on its side, rolling along like a bicycle wheel, would have an inclination of axis of 90 degrees. Six of the planets rotate in the same direction, counterclockwise as seen from above (north), and three do not. Rotating or revolving in a direction opposite from the norm is called retrograde motion. Venus, Uranus, and Pluto exhibit retrograde rotation.

Q: A planet that rotates in a retrograde direction has an axis of inclination of greater than 90 degrees. Why would this be so?

### For the Classroom

Studying patterns and pattern-breakers usually leaves you with a set of questions to be answered. The search for these answers can be illuminating. For example, from examining the general pattern of planetary motion, scientists have come to believe that every body in the solar system formed out of the same huge uniformly spinning cloud of dust and gas (something like a cosmic cyclone). This would account for why most things in the solar system spin in the same direction now. The bodies that don't, the pattern-breakers that exhibit retrograde motion, are believed to have been in violent collisions that flipped them over. In Venus' case the collision also slowed down its rotation quite a bit.

From the general pattern of orbital distance from the Sun, we believe that there should have been a planet where the asteroid belt now is, between Mars and Jupiter. Asteroids are probably the stuff from which this planet would have formed. There are several possible reasons why it didn't form, or formed and then broke up. The influence of the huge pull of Jupiter's gravity nearby was no doubt a key factor. And there may have been a lot of collisions in that part of space during the time the planets were forming.

Pluto shows a great variance from the pattern in size, orbital inclination and eccentricity, and probable composition (not shown in this chart). These peculiarities have led scientists to speculate that tiny, icy Pluto may have been a comet that was pulled into a planetary orbit by the Sun's gravity. If Pluto is like a comet, then it may include frozen remnants of the original materials from which the planets formed. We'll need a closer study of the distant planet before we can be sure.

Note: Before addressing these questions, you may want to skip ahead to Activity 2 and make the planet cards. The cards may prove helpful in thinking about what is in the chart.

### Activity 1

This activity utilizes the PLANETARY CARD SET files. There are two parts to this set, the fact sheet and the image sheet. Both parts of this set are necessary to conduct this activity.

1. There is a jump in the pattern for mean distance from the Sun, from Mars to Jupiter. Is there a jump in the same place in any of the other data?

2. There is a clear correspondence between period of revolution and what other characteristic in the chart on page 2? How would you account for this correspondence?

3. Which is the smallest planet? Are we sure of the size of its diameter? Why do you think we might not be?

4. As we noted previously, Earth has a decidedly different atmosphere than the other small, inner planets. The outer planets show a simple pattern of their own in atmospheric composition. What is it? Using your knowledge of chemistry (in particular, the periodic table of the elements), what can you say about the two main gases in the atmospheres of the four large, outer planets?

5. What general statement can you make about the number of moons held by each of the two main classes of planets? What other characteristics of the planets follow the same general pattern as the number of moons?

6. The rings of Saturn are believed to have a lot of icy particles in them, which is why they reflect light so brightly and appear so colorful. The rings of the other ringed planets are darker and less numerous. These rings are probably made up of dustier and rockier materials. Is there something resembling a ring system orbiting the Sun? Where does it orbit? What do you remember about this orbital slot from our previous discussion?

7. What is the pattern in inclination of orbit? What is the pattern-breaker? Keep track of this pattern-breaking planet's characteristics; it is different from the other planets in many intriguing ways.

8. There are two planets with very eccentric orbits. What else do these two planets have in common? Are there other categories in which they are both pattern-breakers? Which one(s)?

9. Is there a pattern in rotation period? Is there one planet whose rotation period is very different from the rotation periods of the other planets in its size range?

10. As noted previously, there is a correspondence between inclination of axis and retrograde motion. Planets that are "upside down" exhibit retrograde rotation. A planet spinning perfectly on its "head" has an inclination of axis of 180 degrees. Examining all nine planets, can you say what the trend is in axial inclination, and what the trend-breakers are? Is Mercury a trend-breaker? Why or why not?

### Activity 2

These activities involve the use of the PLANETARY CARD SET. This could be photocopied, preferably onto heavy paper (cover stock or card stock), or may be glued onto heavier stock. Students should then cut out each card so they have a set of nine.

1. For each planet, fill out the missing data on the back of the card.

2. Make a card for the Sun. The Sun's diameter is 1,395,161 km. The scale at which the planets are drawn on the cards is 1 mm = 2604 km. Therefore, the diameter of your drawing of the Sun should be 1,395,161 divided by 2604, or 536mm. You will need a large piece of paper, at least 536 mm on each side, for the Sun.

3. On the front of each planet's card, use a protractor and ruler to draw the axis of rotation through the planet. The axis should pass through the center of the planet. Remember that an inclination of 0 degrees is straight up, and 90 degrees means a planet is on its side.

4. Draw rings on each planet that has rings. Rings orbit a planet around its equator, which is at 90 degrees to the axis of rotation.

5. Draw moons around each planet that has moons. Like rings, moons orbit a planet around its equator, but farther out than the rings. Most of the moons can be small dots, since most moons are tiny compared to the size of the planet. Earth's Moon is about 1/4th the diameter of Earth. Pluto's moon, Charon, is about 1/3rd the diameter of Pluto.

6. For each category (row) on the chart, arrange the planets in order, from least to greatest. Discuss what you have learned about each category. Note relationships between categories. Which categories seem to stand alone, in little or no relation to the others? Which single category seems the most random to you? Why?

### Activity 3

This activity utilizes the U.S. PLANETARY MISSIONS chart.

#### Background

Much of what we know about the Moon and planets has come from robotic spaceflight missions - spacecraft sent from Earth to photograph and measure characteristics of our neighbors in the solar system. The next page of this Brief is a graphic depiction of all the robotic U.S. missions to other bodies in the solar system. All of the "visitors" to each body are listed, along with the type and year of the mission. Photo missions were spacecraft that were crashed into the body, taking pictures right up to the end. Flyby missions made a single pass by a planet. Orbiters went into orbit around the planet, in some cases dropping probes through the atmosphere or landers onto the surface. The Magellan mission of 1989 was an orbiter with a special instrument called aradar mapper, which penetrated the thick cloud cover of Venus to map the surface below.

1. Discuss the difference in the way data are shown in this graphic compared to the chart of planetary characteristics seen earlier in this Brief. What purpose can a graphic like this one serve?

2. What general statements can you make about planetary exploration based on the graphic? Consider where we have gone and not gone, and what we have done there.

3. See if you can relate any of the information on this graphic to information on the chart of planetary characteristics.

4. Write a set of discussion questions for the graphic.

5. See if you can set up a chart, like that of planetary characteristics, for the planetary missions, using some or all of the data in the graphic. Then look at item 3 again.

6. Choose one mission as a research topic. Gather information about the objectives of the mission, what was learned from the mission, the status of the spacecraft, and how the mission may have influenced later exploration of the target body.

7. After you have learned about past missions, locate newspaper and magazine articles about current and future planetary missions. Add these missions her's guide, including discussion questions and student section, is included with the video. These resources are available from NASA CORE, Teacher Resource Centers, and Regional Teacher Resource Centers.

NASA CORE
Lorain County Joint Vocational School
15181 Route 58 South
Oberlin, OH 44074
(216) 774-1051, Ext. 293 or 294

NASA Spacelink is a computer-information service that allows individuals to receive news about current NASA programs, activities, and other space-related information, including historical and astronaut data, lesson plans, classroom activities, and even entire publications. Although primarily intended as a resource for teachers, anyone with a personal computer and a modem can access the network.

The Spacelink computer access number is (205) 895-0028. Users need a computer, modem, communications software, and a long-distance telephone line to access Spacelink. The data word format is 8 bits, no parity, and 1 stop bit. Spacelink is also available through the Internet at spacelink.msfc.nasa.gov. For more information, contact: