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Home , Earth, Europa (moon), Europa (rocket), Jupiter, Jupiter mass, Moon

Voyage to !

1. Introduction: Is there on one of 's ? The Setting The Challenge Heads Up! 2. Activities: Getting the Images Imaging Jupiter and it's Moons Scope it Out Reflecting on your Images Which is Europa? Using the Images to Investigate Jupiter and Europa Preparing for the Trip: How far is Jupiter? Go Figure A Base Camp on Jupiter? What is the scale of my ? How large is Jupiter? How large is Europa? How much does Jupiter weigh? How dense is Jupiter? How strong is Jupiter's ? Mission assessment: Jupiter as a base camp? Search for Life on Europa What is Europa Like? Does Europa have an ? What is Europa's Environment? Can We Communice to from Europa? Artificial Gravity? Try this/Tabletop Experiment: Mysterious motion experiment An on Europa? Try this/Tabletop Experiment: Cooool ! Did you know/fun fact: whales Is there Life at the extremes?

3. Wrap-Up & Reference: Data Page

Briefing Room Timeline Ideas you'll need

From the Ground Up!: Jupiter v. 031301 -1- Harvard-Smithsonian Center for VOYAGE TO EUROPA

1. INTRODUCTION:

Is there life on one of Jupiter's moons?

Circling the giant , Jupiter, are several moons that may contain liquid . One of those moons Ñ named Europa Ñ may even be covered by a vast ocean. In fact, according to recent evidence from a NASA probe to Jupiter, this salt-water ocean may be 60 deep and as warm as the of Bermuda!

If true, this would make Europa the likeliest place in our solar to search for extra- terrestrial life. The problem: Europa's ocean is hidden under a thick layer of .

THE CHALLENGE:

You and your firm, Investigation Inc., have been asked by NASA to report on the prospects for sending an expedition to search for life beneath Europa's frozen . NASA will provide the to get your crew to Jupiter, and will develop any technologies you may need.

Your challenge is to:

• Obtain images of the planet Jupiter and its four brightest moons, and identify your destination, Europa.

• Evaluate the prospects for setting up a base camp on Jupiter. Use your images Ñ and your knowledge of Ñ to find out as much as you can about Jupiter. If you tried to on Jupiter, would you hit a solid surface, or would you keep on falling and never be heard from again?

If you could land on Jupiter, how much would you weigh? Would your body be to support you? Would you be able to take off again? From the Ground Up!: Jupiter v. 031301 -2- Harvard-Smithsonian Center for Astrophysics • Evaluate the prospects for a base camp on Europa: Do you expect Europa to have an atmosphere? How much does Europa get compared to Earth? If you can only explore the surface while the is up, how long can you explore before you must get to base camp? How do you expect it to be? How much will you weigh there, compared to Earth?

• Evaluate the prospects for life under Europa's icy surface: The ocean on Europa could contain bizarre life forms Ñ or it could contain no life at all. Make a case for the likelihood of finding life on Europa. Consider these questions: Does life need an source Ñ and if so, what kinds of energy will do? Does life need to exist? Does life need ?

In discussing these questions, you may wish to research and report on one or more of the following:

-What kind of life near deep- volcanic vents on Earth?

-What are the most extreme conditions on Earth, and what kinds of living things thrive under those conditions?

This mission guide will take you step by step through the activities you'll need for your report.

HEADS UP!

Next you find yourself under the , look for the planet Jupiter. When it's above the , Jupiter is easy to spot, even in the city, because it often appears brighter than even the brightest stars.

If the other or the Moon are also visible, you'll see that they all lie along a nearly straight path across the . If extended below the horizon, this line would also pass through the Sun. That's because the Sun, planets and their moons lie in nearly the same . When seen from a point within that plane, they appear to lie along a line.

Try this dizzying feat: Look up at the sky and picture the plane that the planets lie in. You'll suddenly become aware that you're standing at an to that plane. When it comes to , which direction is "up"?!

From the Ground Up!: Jupiter v. 031301 -3- Harvard-Smithsonian Center for Astrophysics 2. ACTIVITIES

Getting the Images

IMAGING JUPITER AND ITS MOONS

Your first challenge is to use the to get good images of your destination: Jupiter and its moons.

In this challenge, you'll investigate the motion of the moons by taking images about once an for four or five . Amazingly, there will be enough information in these images for you to determine several properties of Jupiter that you'll need to know for your mission Ñ such as Jupiter's , , and gravity. This information will help you in deciding whether to establish a base camp on Jupiter.

'SCOPE IT OUT

Selecting the Target: Use the pull-down menu to select Jupiter. (The telescope's computer will automatically determine Jupiter's location in the sky for the time you selected. Jupiter does not have a permanent address Ñor RA and DEC Ñ in the sky, because it moves from to night relative to the background stars. In fact, the word "planet" means "wanderer.")

Camera: Use the MAIN camera, ZOOMED IN. (If some of the moons are out of the of view, you can use ZOOMED OUT instead.)

Filter: Try using the filter ("ND-40") to cut down on Jupiter's glare.

Exposure time: Use a 10 second exposure if you are using the grey filter.

Downloading Your Image: You should be able to see Jupiter and its moons clearly in the GIF-format image on the Web, without any image processing. Be sure to download both the image AND its Image Info file, because this contains the information about how and when you took the image.

It's a good idea to also download the FITS file for each image as well for your records. (Click and HOLD on the underlined link, then select "Save As...SOURCE" and download.

Printing the Image: The simplest way to compare your images is to print them. TIP: Use an image processing program to INVERT your image Ñ that is, to reverse and . Then when you print, Jupiter and its moons will appear black against a white background. That's much easier to measure, and you'll be saving your printer's ink as well!

Making Measurements: You can make measurements directly from your computer monitor, or from printed images. For an image printed at 100% scale, 1 inch = 72 pixels.

From the Ground Up!: Jupiter v. 031301 -4- Harvard-Smithsonian Center for Astrophysics REFLECTING ON YOUR IMAGES:

Size of Jupiter. Why does Jupiter appear so small, compared to the Moon?

Point of view. Why do we see the moons arranged on a more or less straight line?

Forces and motion. What keeps the moons in around Jupiter? Why don't they fly off into space?

Speed of the moons. Which moons appear to have moved, from image to image? Why have some moons moved more than others?

Getting the big picture. Jupiter and its moons look like a miniature "." How does the plane of the moons compare to the plane of the solar system? Why might that be?

From the Ground Up!: Jupiter v. 031301 -5- Harvard-Smithsonian Center for Astrophysics WHICH MOON IN YOUR IMAGE IS EUROPA?

You'll probably see up to four of Jupiter's moons in your image. Can you figure out which moon is Europa? The moon closest to Jupiter in is called , followed by Europa, and . But the moon that looks closest to Jupiter in your image need not be Io. That's because, from Earth, we're looking at the moons edge-on to the plane of their orbit. For example, the image at right shows how the moons might look if you were to look DOWN on the plane of their orbit. (The image shows the relative size of the , to scale.) The edge-on view below it shows how this scene would appear through the telescope. Can you label which moon is which? When you look at your images, you won't have the benefit of a top-down view to compare them to. But can you think of a way to figure out which moon is which by following the moons through several images? If you could follow the moons for long enough, you could see the furthest distance that each one gets from Jupiter. That would tell which moon is which. A second way to tell the moons apart is to see which moon moves FASTEST -- i.e., moves the furthest in several successive images taken, say, an hour apart. The moons closest to Jupiter orbit the fastest, as predicted by 's laws of motion. But even here you'll have to be careful: Take a look at the figure on the next page, where the 's mark equal time intervals. As seen from edge-on, the moon seems to up and slow down. At what part of the orbit will the moon appear to moving fastest? Where will it seem to be moving slowest? After the Image and Analysis Team has examined the images and is confident they can identify the moons, label the images with the team's results.

From the Ground Up!: Jupiter v. 031301 -6- Harvard-Smithsonian Center for Astrophysics Getting the Facts on Jupiter and Europa

PREPARING FOR THE TRIP: HOW FAR IS JUPITER?

How far is Jupiter anyway Ñ and how long will it take you to get there?

You can use your knowledge of physics, and a few basic observations, to estimate the distance to Jupiter. Knowing this distance will also help you interpret your images.

GO FIGURE

Start with this observation: Jupiter takes about 12 to orbit once around the sun and return to the same part of the sky (same background of stars). • How long does it take the Earth to orbit once around the sun? • Based on the periods of their orbits around the sun, is Jupiter further or closer to the sun than Earth is? By how much? (See the Briefing Room: Ideas You'll Need) • The Earth is about 93 million miles from the Sun. How far is Jupiter from the Sun? Record your results on the handy DATA SHEET for the mission to Jupiter. You'll need them later. Using your results, what is the closest that Jupiter gets to Earth? What is the furthest? (It helps to draw a diagram with Earth and Jupiter on the SAME side of the Sun, and then one with the planets on OPPOSITE sides of the Sun.)

Scale drawing. In the scale drawing below, how far from the sun should you draw Jupiter? In the drawing, 6 inches represents 100 million miles.

From the Ground Up!: Jupiter v. 031301 -7- Harvard-Smithsonian Center for Astrophysics REFLECTING ON YOUR RESULTS

About how long will it take you to get to Jupiter? Assume that your spacecraft can average about 100,000 miles per hour. (This is 3 faster than current spacecraft.)

How would you choose your crew members for a trip this long? What qualities should they have?

A BASE CAMP ON JUPITER?

What are the prospects for landing on Jupiter? Can you use it to set up a base camp from which to explore Europa? How much would you weigh on Jupiter? Is there a solid surface, or would you sink in? Would you be able to blast off again?

Before planning a landing, you'd better find out as much as you can about the planet. Amazingly, you'll be able to tell a lot just from the images you took Ñ and by applying the physics you've learned. Here are some measurements and estimates you'll need to make. Each one builds on one or more of the previous measurements.

• What is the scale of my image? • How large is Jupiter? • How large is Europa? • How far is Europa from Jupiter? • How long does Europa take to orbit Jupiter? • How much does Jupiter weigh? • How dense is Jupiter? • How much would you weigh on Jupiter?

From the Ground Up!: Jupiter v. 031301 -8- Harvard-Smithsonian Center for Astrophysics WHAT IS THE SCALE OF MY IMAGE?

How many miles does one pixel represent in your image? You'll need the following information:

• The distance to Jupiter is ______miles. (Use your result from activity above.)

• An object that is 57 times further than it is wide will appear 1 wide. (See the activity, "A Wrangle with .")

• In your telescope image, one degree spans 720 pixels. So 1 pixel = 1 / 720 degree.

So 1 pixel represents ______miles wide (for an object at Jupiter's distance).

HOW LARGE IS JUPITER?

Now that you have the scale of your image, you can determine how large Jupiter really is.

In your image, how many pixels wide is Jupiter? How many miles is this? How does Jupiter's size compare to Earth's?

Width of Jupiter = ______miles

Width of Earth = 8000 miles

How many would fit inside Jupiter? (How much larger is Jupiter's than Earth's?)

Jupiter has about______times the volume of Earth.

Record your results on the mission DATA PAGE.

From the Ground Up!: Jupiter v. 031301 -9- Harvard-Smithsonian Center for Astrophysics REFLECTING ON YOUR RESULTS

Does the size of Jupiter alone tell you anything about SIZE ... how strong the gravity will be at Jupiter's surface? What other information would you need? Could there be a planet ten times larger than Jupiter? Strangely enough, Jupiter may be about the largest a planet can get... and still be a planet! Although planets two or three times as massive Jupiter have been detected orbiting other stars beyond our Sun, believe that an object much larger than Jupiter would collapse under its own to form a . In a star, the and density of is so great that the matter undergoes nuclear reactions, liberating enormous amounts of energy and causing the star to shine. You can learn more about stars in the investigation, "To the Stars!"

STORM ON JUPITER URL:

Knowing the size of Jupiter, students use digital movies of Jupiter to estimate the in a violent there.

From the Ground Up!: Jupiter v. 031301 -10- Harvard-Smithsonian Center for Astrophysics HOW MUCH DOES JUPITER "WEIGH"?

You can actually "weigh" Jupiter Ñ that is, determine FINDING JUPITER'S its mass ÑÊjust from observations of the motions of its (see Briefing Room) moons. Remember that the mass of a planet influences how fast a moon will revolve around it. (See the Box The (T) of a moon's orbit at right, and the Briefing Room in the appendix.) depends on its distance (d) from the planet and the mass (M) of the planet.:

To determine Jupiter's mass, you'll need to know the 2 3 DISTANCE from Europa to Jupiter, and the TIME it T ~ d / M takes Europa to orbit once. You can find the planet's mass if you know the distance and period of a moon. STEP 1: DETERMINE EUROPA'S DISTANCE FROM JUPITER.

You could determine the distance from Europa to Jupiter (that is, the radius of Europa's orbit) by following Europa's motion and measuring the maximum separation between Europa and Jupiter. Then, knowing the scale of your image in miles per pixel, you could determine the radius of the orbit.

To save you time, the drawings at left show the maximum distance for the four inner Ñ at the same scale as the MicroObservatory zoomed-out and zoomed-in images. Can you use this guide to determine the distance from Europa to Jupiter? (Hint: Use the scale of your image in miles per pixel from the activity above.)

STEP 2: DETERMINE HOW FAST EUROPA ORBITS JUPITER.

Our own Moon takes a little more than 27 days to orbit the Earth once (one "moonth" or about a !) How long would you guess it would take Europa to orbit Jupiter, given that they are roughly the same distance apart as our Earth and Moon?

Use your images to find out. To estimate Europa's period:

• Measure the distance that Europa moved in the first and last images that you took. (A ruler that measures millimeters is best for this.)

From the Ground Up!: Jupiter v. 031301 -11- Harvard-Smithsonian Center for Astrophysics Europa moved ______(pixels? millimeters?) over ______hours

(Don't remember the time interval? Check the Image Info headers on your images to see when they were taken!)

• Compare the distance you measure with the maximum width of Europa's orbit, as shown in the drawing above. What fraction of the orbit does Europa move in the time interval you've observed.

If Europa takes, say, two hours to move some fraction of the orbit, then how many hours will it take to move all the way around Jupiter?

IMPORTANT: Discuss with your team how you will make this estimate. It's tricky: As you can see from the drawing at right, Europa will appear to move more slowly near the turning points of its orbit (furthest from Jupiter) and will APPEAR to move fastest as it crosses the face of Jupiter. How will you correct for this?

Also, don't forget that the period is the time to go all the way around Europa and back to the starting point.

The period of Europa's orbit is about ______hours.

How does the period of Europa's orbit compare with the time it takes our own Moon to go around the Earth?

If Europa is at roughly the same distance from Jupiter as the Moon is from Earth, then why does Europa orbit so much faster than the Moon?

From the Ground Up!: Jupiter v. 031301 -12- Harvard-Smithsonian Center for Astrophysics PUTTING IT ALL TOGETHER: HOW MUCH DOES JUPITER WEIGH? "WEIGHING" JUPITER Use your results for the and Use your measurements for the orbital period and distance of Europa to determine the mass distance of Europa to determine the mass of of Jupiter. See the Briefing Room for Jupiter. (See box at right.) details.

M ~ d3 / T2 The mass of Jupiter is about ______(grams) M = (2¹)2 d3 G T2

M is the mass of Jupiter How many times more massive is Jupiter compared d is the distance from Europa to the center of to the Earth? Jupiter T is the period of Europa's orbit G is Newton's (G = 6.7 x 10-8 cm3 / g sec2 ) . TIP: You can compare Jupiter's mass to the Earth's just by knowing the orbital period and distance of If d is in centimeters and T is in seconds, one for each planet. then M will be in grams. 2 3 2 3 MJupiter / MEarth = (T d )Europa / (T d )Earth's Moon

Use your results for Europa, and the results for the Moon from "The Incredible Shrinking Moon" challenge, or from a published source.

HOW DENSE IS JUPITER?

Can you land on Jupiter, or would you sink right in? Is Jupiter solid ? Is it liquid or gas? One line of evidence to use is the average density of Jupiter, which is the mass per unit volume.

Use your findings for the mass and volume of Jupiter to determine its average density. Then compare your result with for the following materials (under normal pressure): Liquefied : 1 gram/ cubic centimeter Water: 1 g/ cc. Rock: 3 g/ cc. : 5 g/ cc.

How does the density of Jupiter compare to Earth density (about 3 g/ cc.)? Could Jupiter be solid rock?

From the Ground Up!: Jupiter v. 031301 -13- Harvard-Smithsonian Center for Astrophysics Is it safe or unsafe to attempt to land on Jupiter?

HOW STRONG IS JUPITER'S GRAVITY?

How much stronger is Jupiter's gravity than Earth's (at the surface of each planet)? To answer this question, discuss with your team what factors influence the gravitational pull you feel from a planet. Then use any of your previous results that you need.

How much would you weigh at the surface of Jupiter?

MISSION ASSESSMENT: JUPITER AS A BASE CAMP?

Given your results, how does your team rate Jupiter as a potential base camp for your of the planet's moon, Europa?

From the Ground Up!: Jupiter v. 031301 -14- Harvard-Smithsonian Center for Astrophysics Searching for Life on Europa

What kind of environment will you find when you get to Europa? And what are the prospects for life? Try some or all of the activities on the following pages and include your findings in your mission report.

WHAT IS EUROPA LIKE?

A good place to start: Data and images from Europa sent by NASA's spacecraft. What can you learn from this evidence about what Europa is like?

Diameter of Europa: 1882 miles Mass (Earth = 1): .0083 Density of Europa: 3.01 (grams/ cubic centimeter)

For full-color images: http://www.jpl.nasa.gov/galileo/images/europa/eurimages.html

What do you think the bright spot and dot are in the lower right portion of this image of Europa?

How does this image compare to Earth's moon? What might account for the lack of craters?

What might the dark, reddish- material be? What are possible sources of this material? What evidence would you need to support your hypotheses?

From the Ground Up!: Jupiter v. 031301 -15- Harvard-Smithsonian Center for Astrophysics The surface of Europa is very bright. What might this indicate?

What might the cracks on the surface indicate? Where are on Earth might you see cracks like this?

From the Ground Up!: Jupiter v. 031301 -16- Harvard-Smithsonian Center for Astrophysics DOES EUROPA HAVE AN ATMOSPHERE?

Your environment team has been asked to assess the chances that Europa has an atmosphere. What determines whether a moon or planet has an atmosphere?

Consider the following:

• Our has an atmosphere, but our Moon does not. Why not?

• The planet does NOT have an atmosphere, but Ñ a moon of that's the same size as Mercury Ñ DOES have atmosphere. Why?

DISCUSSION: What factors do you think influence whether a planet or moon has an atmosphere?

If the Earth had been much smaller than it is, would it have kept its atmosphere? If the Earth had been much smaller, would you be here now?!

From your telescope images, can you say anything about the size of Europa? Do you expect it to have an atmosphere?

From the Ground Up!: Jupiter v. 031301 -17- Harvard-Smithsonian Center for Astrophysics WHAT IS EUROPA'S ENVIRONMENT?

What kind of environment can your mission team expect on Europa? Can you use light from the sun as your power source?

Solar energy on Europa. How much light does Europa get, compared to Earth? Use your findings from a previous activity about the relative distances of Earth and Jupiter from the Sun.

Temperature on Europa. Using your results for the amount of sunlight and the lack of an atmosphere on Europa, how would you expect the temperature to compare to that on Earth?

Gravity on Europa. How does gravity on Europa compare to Earth? (Use data from preceding pages.) How much would you weigh on Europa? If you can carry 70 pounds maximum on Earth, how heavy a could you wear on Europa?

From the Ground Up!: Jupiter v. 031301 -18- Harvard-Smithsonian Center for Astrophysics COMMUNICATING FROM EUROPA

From Earth, we always see the same side of the Moon. That's because our Moon rotates on its axis at the exactly the same rate (once every 27.5 days) as it revolves around the Earth. This used to be thought a coincidence Ñ but now we know it always happens when a smaller moon orbits close enough to a larger planet.

Europa also rotates exactly as fast as it revolves around Jupiter Ñ so it always shows one side to Jupiter.

Suppose you wanted to have a in orbit above Europa that always remained above a particular spot on the surface (e.g., above your base camp on Europa). How far above the surface of Europa should the orbit be? Use your finding for Europa's period of revolution (and therefore ). Apply Kepler's law to figure out how high the satellite should be to have a period equal to Europa's rotation rate.

Is this satellite feasible? How does its orbit compare to the distance from Europa to Jupiter? Will the satellite risk crashing into Jupiter at this distance?

If you leave base camp to explore Europa, what's the maximum you'll have before you are plunged into darkness? (You'll need to draw a model showing the location of the Sun, Jupiter, and Europa, and take into account the motions of Europa.)

From the Ground Up!: Jupiter v. 031301 -19- Harvard-Smithsonian Center for Astrophysics CAN YOU DESIGN ARTIFICIAL GRAVITY?

When are deprived of Earth's gravity for too long, their bodies start to feel the ill effects. For example, bones start to dissolve, and muscles lose their fitness.

Can you design vehicle that orbits Europa, and that spins on its axis just enough to provide one Earth gravity? How large a vehicle should it be, and how fast should it spin?

Some of your colleagues want a box-shaped orbiting vehicle. Trying the following experiment, report on the dangers of a box-shaped vehicle and the importance of shape.

MYSTERIOUS MOTION EXPERIMENT:

Using an empty cereal box, try the following experiment. Toss the box in the air so that it spins around an axis through its largest side (1). Can you get it to spin WITHOUT wobbling?

Try it again, this time spinning around an axis through its smallest side (2). Is it again stable? Now try spinning it about the axis through its medium side (3). What happens?

The motion you observe is an example of "chaotic motion." Newton's laws can predict that such motion will take place, but cannot describe in detail the motion itself!

From the Ground Up!: Jupiter v. 031301 -20- Harvard-Smithsonian Center for Astrophysics IS THERE AN OCEAN ON EUROPA?

Scientists have evidence that a vast ocean exists under the surface ice on Europa. But if Europa is so cold, then where does the heat come from to keep the underground ocean from freezing solid?

Simple may be at work: As Europa orbits around Jupiter, the enormous gravity from Jupiter (and the other moons) alternately stretches and squeezes Europa. The friction created by this process is enough to heat the interior and melt the ice on Europa, from the inside out. (The giant on Jupiter's nearest moon, Io, is thought to be powered in the same way. Image at right.)

To see how this continual stretching might provide an energy source to melt Europa's water, try this experiment:

EXPERIMENT: COOOOL HEAT!

Materials: A wire coat hanger.

SLOWLY bend the coat hanger back and forth at one point along its length. CAREFULLY and BRIEFLY touch the area around the bend. What do you feel?

Why is the hot? Why does the hanger get cool again? Would a larger coat hanger cool faster? Slower? The same?

DISCUSSION: Objects can generate heat through their entire volume, but they can lose heat only through their surface. What happens to the volume/ surface ratio as objects get larger? What examples in nature can you think of where size plays a role in an object's temperature?

Fun fact. When whales are about to give birth to their calves, they migrate to warmer waters. Reason: The smaller babies would lose heat too fast in the colder waters. As soon as the babies gain weight, it's back to the frigid waters again!

From the Ground Up!: Jupiter v. 031301 -21- Harvard-Smithsonian Center for Astrophysics REPORT ON: LIFE AT THE EXTREMES

An ocean on Europa could contain bizarre life forms Ñ or it could contain no life at all. Make a case for each of these possibilities, citing existing evidence, or evidence you would need to get.

What are the major requirements for life? What energy source could be present under the ice? Are the chemical elements for life present? Could life survive without light or oxygen? What kinds of creatures could live there?

In support of the view that there may be life, you may wish to research and report on the following:

On Earth, is there life near deep-sea volcanic? E.g., see the Web site: http://www.discovery.com/stories/science/ seavents/archive/entry1.html

What are the most extreme conditions on Earth, and what kinds of living things thrive under those conditions?

For a skeptic's view, visit the following site and discuss with your team the reports found there: http://www.spaceviews.com/1999/08/05a.html

In your view, would it be worth exploring Europa if only microscopic life Ñ but no larger life forms Ñ existed? Why?

From the Ground Up!: Jupiter v. 031301 -22- Harvard-Smithsonian Center for Astrophysics VOYAGE TO EUROPA DATA PAGE

Distance to Jupiter:

Jupiter is ______times further from the Sun than is Earth.

Jupiter is ______miles from the Sun.

Jupiter's closest approach to Earth is ______miles.

Size of Jupiter:

Jupiter is ______miles wide, compared to 8000 miles wide for Earth.

Jupiter's volume is______times the volume of Earth.

Jupiter's volume is ______cubic centimeters

Mass of Jupiter:

The mass of Jupiter is ______grams

Density of Jupiter:

The density of Jupiter is ______grams per cubic centimeter

Gravity on Jupiter:

A 100 person would weigh______pounds at the surface of Jupiter.

Size of Europa:

Europa is less than ______miles wide

Radius of Europa's orbit:

The distance from Europa to Jupiter is about ______miles

Period of Europa's orbit:

It takes Europa ______days (hours) to orbit once around Jupiter.

From the Ground Up!: Jupiter v. 031301 -23- Harvard-Smithsonian Center for Astrophysics BRIEFING ROOM TIMELINE

Ancient Roman times: The planet Jupiter is named after one of the Roman gods.

1610. Using one of the first , discovers four moons orbiting the planet Jupiter. This shows that Earth is not the only "center of attraction" in the , and makes it easier to accept Copernicus' conclusion that the Earth orbits the Sun.

Some townspeople refuse to look through Galileo's telescope, calling it a distortion of reality.

1979. Three scientists from the Jet Propulsion Laboratory conclude that Jupiter's strong gravity may flex and heat its nearby moons.

1979. Three days later, the Voyager spacecraft sends the first images of volcanoes on Io, the closest moon to Jupiter.

1996. NASA's Galileo spacecraft, is launched to explore the planet Jupiter and its moons.

2000. The Galileo spacecraft arrives at Jupiter sends back images and data about the planet Jupiter and several of its moons.

2000. Kivelson and her team from the University of California conclude that an ocean exists under the surface of Europa. Evidence: Telltale magnetic fields from the sloshing of salt-water inside Europa.

The of .

Ever wonder how the days of the got their names? The Romans named the days after the Sun, Moon, and 5 gods corresponding to the five visible planets, including Jupiter. The Norse god equivalent to Jupiter was . So every time you make plans for Thor's , or Thursday, you're paying homage to the planet Jupiter!

From the Ground Up!: Jupiter v. 031301 -24- Harvard-Smithsonian Center for Astrophysics BRIEFING ROOM IDEAS YOU'LL NEED

Newton's law of gravity. Newton discovered that the of gravity between any two objects depends only on the mass of the two objects and the distance between them. He showed that the force is proportional to the mass of each object, and inversely proportional to the square of the distance between their centers:

F ~ mM / d2

Does this relationship make sense?: The more massive (M) a planet, the stronger its gravitational pull on you. The more massive (m) you are, the STRONGER the pull (that's why a person with twice your mass weighs twice as much). But the more distant (d) you are from the Earth, the WEAKER the pull of gravity. (Just as the brightness of a light appears more feeble as the square of your distance from it, so does gravity become more feeble as the square of the distance.)

[The Art of : "Laws"... Or Relationships?]

Kepler's law. discovered that the time it takes a moon to orbit a planet depends only on the mass of the planet and the distance between their centers:

T2 ~ d3 / M

Does this relationship make sense? Yes: The more distant you are from a planet, the MORE TIME it takes to go around the planet. (Two reasons: You have further to travel, and you are moving more slowly, because gravity is weaker further out.) And the more massive a planet, the stronger its gravity, and so an object in orbit must whip around faster to stay in orbit.

Where does Kepler's law come from? Kepler discovered his law through careful observations of the motions of the planets around the sun. But the law can also be derived from Newton's law of gravity and his law of motion. If you're interested in seeing how, see box X.

From the Ground Up!: Jupiter v. 031301 -25- Harvard-Smithsonian Center for Astrophysics AND CIRCULAR MOTION

An old puzzle: Name three parts of a car that cause it to accelerate. Any ideas?

One is the gas pedal (causes car to speed up, called acceleration). Second is the brake (causes car to slow down: negative acceleration). But what's the third part? Give up?

Answer: The steering wheel.

According to Newton's description of motion, an object in motion will keep going at the same speed in a straight line UNLESS a force acts on it, causing it to accelerate. Accelerate means to speed up, slow down, or CHANGE DIRECTION. All of these are examples of acceleration.

For an object that changes its speed but keeps the same direction, it's easy to see how we might measure acceleration: For example, we say that a sports car goes "from 0 to 60 miles per hour in six seconds." So its acceleration is 60 miles per hour per six seconds, or 10 miles per hour per second. But how do we measure the acceleration of an object that keeps the same speed but changes its direction Ñ such as a car traveling in a circle at constant speed?

Your teacher may show you how to derive the simple relationship for the acceleration, the radius of the circle, and the speed of the object:

a = v2 / d where a is the acceleration, v is the speed of the object, and d is the distance to the center of the circle.

To see why this relationship makes sense, think about what you feel when you're a passenger in a car going around a curve. The tighter the curve, the more the acceleration (i.e., small d leads to big a). If the car tries to take the curve twice as fast (greater v, in the drawing) then the acceleration is four times as great. (Many drivers lose control of a car going into a curve, because they don't realize how strongly the acceleration Ñ and the on the car Ñ depends on speed.)

From the Ground Up!: Jupiter v. 031301 -26- Harvard-Smithsonian Center for Astrophysics WHERE KEPLER'S LAW COMES FROM.

The nice thing about physics and math is that we can use them to help us find out what we want to know.

For our mission, we want to weigh Jupiter, knowing the size and period of Europa's orbit. We can use our understanding of gravity and acceleration to help us.

From Newton's description of gravity, we know that the acceleration on Europa comes from the force of gravity that Jupiter exerts on it:

a = F / m

This force of gravity is

F = mMG / d2

So the acceleration of Europa is

a = MG/ d2

But we also know how to relate the acceleration of an object to its speed, when it is moving in a circle:

a = v2 / d

v2 / d = MG/ d2

The time that it takes Europa to make one orbit is just its speed divided by the distance around the circle. (T = 2¹ d / v, or v = 2¹ d / T)

So replacing the velocity with 2¹d / T, the equation becomes

(2¹) 2 d / T2 = MG/ d2

So the mass of Jupiter is related to the size and period of Europa's orbit:

M = (2¹)2 d3 G T2

M is the mass of Jupiter d is the distance from Europa to the center of Jupiter T is the period of Europa's orbit G is Newton's gravitational constant (G = 6.7 x 10-8 cm3 / g sec2 ) .

If d is in centimeters and T is in seconds, then M will be in grams.

From the Ground Up!: Jupiter v. 031301 -27- Harvard-Smithsonian Center for Astrophysics