Download Orbits Explorer

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Orbits Explorer
Brief Summary
The Orbits Explorer is a specially built table equipped with 12 motor-driven rings that rotate
independently of each other. When small model planets or satellites are placed on the rings,
they move at different rates, accurately mimicking planets as they go around the Sun (or
satellites as they orbit Earth). Mounted above the Orbits Explorer, a projector throws
computer-generated images onto it. Using this unique combination of tools, the visitor can
explore the movements of planets in the solar system, and many other phenomena.
Equipment Required
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Balls to represent the Sun, Earth and several planets
Laminated sheets showing Table of Planetary Data, Table of Satellite Data and
Resonance Pairs
Other small props such as the Space Shuttle, an astronaut in a space suit, and Skylab
Rectangular board used to show comet orbits in different planes
Laptop computer
Main Teaching Points
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Objects in orbit around the Sun (or a planet) travel faster when near the Sun (or planet)
and slower when orbiting at a distance. Kepler’s Third Law expresses the exact
mathematical relationship.
The shape of the orbits of the Earth and other planets in the solar system are ellipses
(elongated circles) that come close to being perfect circles. This relationship is called
Kepler’s First Law.
The physical driving force that causes the planets to move the way they do is the
gravitational field of the Sun, which gets weaker with increasing distance away from the
sun.
Comets are icy bodies traveling through the solar system. Many comets orbit the Sun, but
unlike planets, their orbits are very elongated ellipses, not circles, and are not always in
the plane of the ecliptic.
Educational Strategy
As an amazing piece of kinetic sculpture, this model stimulates the visitor's sense of wonder.
In addition to its aesthetic appeal, the Orbits Explorer is a highly accurate solar system
simulator with many degrees of freedom. The Orbits Explorer gives visitors a chance to test
informal hypotheses and discover the basic principles of planetary motion for themselves.
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The aesthetic appeal of this exhibit heightens positive affect, strengthening the formation of
long-term memories.
Set Up
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Turn on the main driving motor.
Unlock the drawer, take out the small props (a set of various sized balls and other
models) and place them into the container on the top of the apparatus.
Get laptop out of the drawer and boot up the Star Field program on the laptop.
Use the operator control console to select the specific show you will be presenting.
Get out wireless microphone and turn it on.
Suggested ways of presenting demo
Try this:
 Place a few balls randomly onto the moving rings, and allow visitors to just watch the
action and move the balls around. (Note that the scale of the distances is correct, but
the size of the planets is entirely exaggerated. If the Sun were the size of a basketball,
Earth would be the size of a BB (~1/8-inch in diameter) 100 feet away. Pluto would be
4000 feet away (nearly 1 mile).
 To represent the true relative speeds of Mercury, Venus, Earth and Mars, place the
balls representing these planets on rings 1, 4, 7 & 12.
 Ask visitors to find two rings whose speeds produce RESONANCE. Resonance is the
situation that occurs when objects on two different rings return to the same place, for
example the north side of the table. The objects travel with different speeds, but the
ratio of their speeds is a simple ratio such as 2:1. (The 2:1 resonance rings are 1 & 3
and 4 & 8. The 3:1 resonance pair is rings 4 & 12.) Gravitational force between the two
bodies is strongest at these points. The larger body at these points can perturb the
orbits of smaller objects such as asteroids.)
 Turn on the comet preset program and prompt the visitor to notice the changing speed
of a comet as it completes one orbit. This motion is the key to understanding planetary
motion: the closer a body is to the sun in its orbit, the faster it moves. Ask visitors if
they think all comets orbit in the same plane. To demonstrate how comets move in
other planes, place the large cardboard square at about 45 degrees over the center of
the ORBITS EXPLORER and let the computer image of the moving comet fall on it.
Or, for a game-like approach, try this:
 Put the speed controller to the slowest speed.
 Place the Earth ball in the center of the circle.
 Place the space shuttle prop on ring 3. Place an astronaut prop on ring 3 about 90
degrees behind the space shuttle, and ask the visitor to move the astronaut just twice
to get it to the space shuttle. Set up the rule that you can move the astronaut only
radially (along a line of radius) one ring in or one ring out.
 After the visitor tries various strategies, draw out by questioning the conclusion that to
catch the shuttle, the astronaut will have to go to an inner ring (which moves faster),
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not to an outer ring as most people believe. (The outer ring will work, but it will take
longer for the shuttle to catch up.)
Note that the speed of the orbit is determined only by the distance from the sun, NOT
by the size or mass of the object, assuming that we are talking about circular orbits.
The astronaut’s wrench, the astronaut, and the shuttle, though very different masses,
orbit at the same speed and seem to float together.
Operating Tips
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Laptop screen shows control buttons on panel at left. This window does NOT project onto
screen.
Ask visitors NOT to sit on the apparatus or stop the rings from moving. (A mechanism is
designed to prevent damage if this occurs, but let’s try to reserve it for accidental slips.)
Turn on the projector that shows the star field right away, but leave the comet package
and other effects till later.
Be sure to keep track of the props as the smaller pieces are choking hazards and should
not be handled by children under three.
Questions and Answers
Can the Orbits Explorer represent all nine planets in the whole solar system at once?
No, there is a problem of scale. If you put Mercury on ring 1, Jupiter would be on ring
37 and Pluto would be on ring 300. The planets are spaced such that the ORBITS
EXPLORER can show no more than four planets at a time. Demo: Show each set of planets
as follows: Jupiter, Saturn, and Uranus: rings 1, 4 & 11; Saturn, Uranus, Neptune, and Pluto:
rings 1, 5, 9, 12.
When is the best time to launch a spacecraft to Mars? Is there a set time of year that is
best?
How long does it take to get to Mars?
There is no set date of the year that is best to launch, but you need to land your spacecraft
when Mars is on the opposite side of the Sun from Earth (when the spacecraft is launched
from the Earth). (This occurs every 26 months). Just like a quarterback who throws a football
well ahead of the receiver, NASA launched the Pathfinder spacecraft before Mars reached
the rendezvous point. The spacecraft traveled on a path that followed half an ellipse with
Mars on one end of the ellipse and Earth at the other. (This energy saving path is called a
Hohmann Transfer Orbit.) To get to Mars via Hohmann Transfer requires about 8.5 months,
but spacecraft are usually sent via modified Hohmann transfer orbits that require only about 6
months. Demo #1: Show video animation of NASA’s pathfinder mission. Demo #2: To
represent the true motions of Earth and Mars, place Earth on ring 7 Mars on ring 12. It is a
difficult thing to guess the launch opportunity, but fun to try.
How was the solar system formed?
Planetary systems, like our own solar system, are a byproduct of star formation. In the denser
parts of molecular clouds, such as the giant molecular cloud behind the Orion Nebula or
those which preceeded the famous Pillars of Creation in the Eagle Nebula, gravity causes
gas and dust to collapse into objects called cloud cores.
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Star Birth
Molecular cloud cores tend to collapse "inside-out." The center collapses under the
force of its own gravity, and then the outer layers "rain-in" on top. The collapse stops
only when the object forming at the center starts to heat up, becoming a protostar.
Leftover dust and gas form a disk around the spinning protostar.
Magnetic fields erupt from the forming protostar and interact with the disk. Some of the
disk's spin energy converts into a pair of oppositely-directed jets of gas and plasma.
The luminous shockwaves of these jets, and the surrounding gases, act as signposts
of starbirth.
Planet Building
Clumps of dust in the disk around the protostar grow by colliding with other particles.
Some accumulate mantles of ice. As they settle into the plane of the disk, the clumps
can double in size every few hundred to few thousand years. Eventually their own
gravity starts to attract other particles and gas, forming protoplanets with diameters
from tens to thousands of kilometers. Collisions of ever-increasing violence mark the
last stages of planet formation, which does not end until only about a dozen Moon-toJupiter-sized objects remain, in well-separated orbits.
Protoplanets formed in the outer disk contain ices; those formed close to the parent
stars do not. Smaller rocky planets tend to form closer to the star, while giant gaseous
planets such as Jupiter form in the outer reaches of a forming planetary system. It
takes about 100 million years to grow a planetary system by this process, though there
are other models that indicate slightly different processes and shorter timescales.
Moons, Asteroids, and Comets
Most of the gas and dust that remains in the star’s disk will either be expelled by
violent stellar winds and strong magnetic fields, or dragged into the star. However,
ongoing collisions break up some of the larger protoplanets. A collision between Earth
and a Mars-sized object is thought to have formed the Earth-Moon system.
The large "maria" and giant craters on the Moon date from the end of this era of
bombardment. Some of the debris from this era survives as asteroids and meteorites.
Smaller bodies in the outer disk are ejected into the far reaches of a young planetary
system by the "gravitational slingshot" effect of the largest forming planets. The
ejected objects become comets that loop into the inner planetary system occasionally.
Demo: Show “Formation of the Solar System” animation in Cosmic Atlas.
Could there be a sister world to Earth that orbits at 180 degrees behind us in the same
orbital track so that it would always be hidden behind the sun?
No. The Earth’s orbit is an ELLIPSE, not a perfect circle. Every time we had a solar eclipse,
the counter Earth would be visible just behind the edge of the eclipsed sun. Demo: Place two
identical balls on the same ring of the OI, but 180 degrees apart. Ask a visitor to explain how
the real situation in the solar system differs from this model, and what it would take to make
the model more correct.
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Why are there gaps between the rings of Saturn?
Cassini Division in Saturn’s
rings
The gaps are maintained by the gravitational
effect
of small moons. The moons need not be within
the rings. In fact the moon that causes the
Cassini Division is outside the ring. These moons
create blank zones by exerting a series of
gravitational tugs on the smaller moons (and
orbiting debris) in the ring, lifting this material into
a more elliptical (oblong) orbit, vacating a zone. This process, resulting from a condition
called RESONANCE, can occur only when the ORBITAL PERIOD (time to circle the planet
once) of the larger moon and the smaller moon (or space debris) are “in sync”, that is, where
the orbital periods are a ratio of small whole numbers such as 1:2, 1:3, or 3:2.
Demo: Place balls on rings 1 & 3. Have a visitor mark the place where the two objects are
next to each other. See if these objects come back to that same spot after the inner ball has
gone around twice and the outer ball has gone around once. Other similar resonances occur
using rings 4 & 8 (3:1 resonance) and rings 8 & 12 (3:2 resonance).
More explanation of RESONANCE
A familiar example of RESONANCE occurs when pushing a child in a swing. To increase the
amplitude (the amount of arc of each swing) you must push the child at just the right time.
This “just-right” time occurs as the child begins his or her downward motion. If you push at
any other time, your push will work against the swinging motion and reduce the amplitude of
the child’s swing (and this is trouble for the pusher!).
Two moons in space can be in resonance only if they orbit the same planet. The gravitational
pulling effect of the moons on each other is significant only when the moons are in
conjunction, that is, when the two moons are at the closest point to each other on the same
side of their orbits. Though each moon exerts a gravitational tug on the other, the larger
moon, being more massive, doesn’t move very much. But the smaller moon is nudged ever
so slightly into a more elliptical orbit. Without resonance, the gravitational tugs do not affect
the smaller moon’s orbit very much. But when in resonance, the gravitational tugs exerted on
the smaller moon accumulate at each conjunction, just as the pushes given to the swinging
child are additive. The smaller moon’s orbit becomes more and more elliptical. The smaller
moon’s new orbit takes it across the path of other moons so it is more likely to collide with
them and be destroyed. Near misses are far more common than actual collisions. Such “long
range” interactions can alter the orbit further. Such alterations can change the orbital period,
moving the small moon out of resonance. To move a body out of resonance, the resonant
interaction must not only make the orbit elliptical, but must also change the orbital period.
One way of doing this is with near misses of other moons (other than the one responsible for
resonant interaction). Another possibility is that the smaller moon’s orbit becomes so altered
that it drifts out of the resonance alignment and settles into a stable non resonance orbit. In
either case, the smaller moon vacates the orbital zone where it was originally found, thus
leaving an empty zone such as the famous Cassini Division that separates the rings of
Saturn.
Demo: Ask the visitor to put a smaller ball on ring 1 and a larger one on ring 3. Have the
visitor pull the inner ball outward a fraction of an inch on each conjunction. After 4 or 5
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passes, the smaller moon ball will be moved into ring 2. Now the resonance effect no longer
works, so the smaller object does not continue to climb all the way out to the moon on ring 3.
Note: The Orbits Explorer cannot show that the smaller moon’s orbit becomes more and
more elliptical, which may take it into closer proximity to other debris. With an increasingly
elliptical orbit, conjunctions become closer and closer, thus accelerating the effect of the
small gravitational tugs. It is interactions with other debris that disrupts the resonance. In the
case of the asteroid belt, material is totally ejected from Jupiter’s resonant orbits.
Do the orbits of satellites going around Earth operate the same as planets around the
sun?
Yes. The scale and absolute speeds change, but the ratio of speeds does not. Demo: Place
the Earth ball in the center. Place small props representing Earth satellites as follows:
International Space Station- ring 1; NIMBUS weather satellite - ring 2; GPS satellites- ring 12.
(A geosynchronous satellite such as GOES 2 would be on the 20 th ring. On the same scale,
the Moon would be on ring 210.)
Other Cool Stuff to Try
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Explain how the solar system formed.
Run the animation: Formation of Solar System
Illustrate Saturn’s rings and Roche’s limit (see below).
Demonstrate phases of Venus. (This was an important discovery by Galileo because it
meant Venus went around the Sun not around Earth.)
Run the animation of Earth to Mars Transfer Orbit.
Show the graphic of the trajectory followed by the Voyager I and Voyager II spacecraft as
they did a grand tour of the planets. The path looks something like a growing spiral.
Fast Facts
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Johannes Kepler (1571-1630) stated in his Third Law of Planetary Motion that the square
of the orbital time equals the cube of the distance from the Sun. An example: Mars is
50% farther from the Sun than Earth, but it takes 90% longer to orbit once. Pluto is about
40 times farther from the Sun than Earth, but it takes 250 years to orbit once.
 This ratio of distance to orbital time is always true for any central object and its satellites,
that is, besides the Sun and its nine planets, Kepler’s Third Law also holds for 1) the
Earth and its artificial satellites, 2) Jupiter and its 17 moons, 3) Saturn and its rings and
moons.
 The drop-off in strength of the force of gravity felt on one body by another is rapid; for
example doubling the distance between the objects reduces the gravitational force
between them to only one-quarter of what it had been. Increasing the distance tenfold
causes to the gravitational force to drop to 1% of what it was. (The Law of Gravity is an
example of an inverse square law, which says that the strength of an effect drops off with
the square of the distance.)
 If a moon ventures too close to a planet, it will be broken up because of tidal forces
caused by the gravity of the more massive planet. The closest a moon can get without
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breaking up is its ROCHE LIMIT. For the Earth’s Moon, this distance is about 11,000
miles or about 1.4 Earth diameters. Our Moon is a very safe 30 Earth diameters away.
The location of the Roche limit is determined by the mass, density, structural strength,
and size of each of the two bodies. Io is about three Jovian diameters out, well beyond
Jupiter’s Roche limit.
Potential Problems
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Some visitors may “hog” the Orbits Explorer. Be on guard to make sure people share the
time and props.
It is very easy to slip into giving explanations that use complex language or concepts to
explain another concept. It is better to refer back to actual phenomena (e.g. the spiral path
of the Voyager 1 spacecraft as it left the solar system) and try to explain it, rather than the
whole idea of gravitational fields and the inverse square law.
Some visitors may feel obligated to stay and listen to your spiel, though you may feel they
are totally interested. If a visitor hangs around for a long conversation, make sure you are
sensitive to their need to get going.
Background materials
collections links)
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(websites,
videos,
articles,
digital
http://seds.lpl.arizona.edu/nineplanets/nineplanets/ - The Nine Planets
http://antwrp.gsfc.nasa.gov/apod/astropix.html - Astronomy Picture of the Day
http://antwrp.gsfc.nasa.gov/apod/archivepix.html - AP of Day (Archives)
http://www.hawastsoc.org/solar/homepage.htm - Views of the Solar System
http://www.theastronomycafe.net - The Astronomy Cafe
http://oposite.stsci.edu/pubinfo/ - Hubble Space Telescope Public Information
Self assessment suggestions
After doing Orbits Explorer facilitation several times, complete the checklist and rubric below
by highlighting the box that best describes your performance. Have your team leader observe
your demo then complete an identical rubric. Discuss your presentation technique with your
team leader along the lines of the rubric.
Rubric for ORBITS EXPLORER DATE________ PRESENTER_____________________
A. Checklist of pre-requisite skills
1. Understand and be able to explain the central fact of planetary motion- that the
nearer to the central body an orbiting body is, the faster it moves. (Kepler’s Third Law)
2. Explain the main features of gravity- a) all bodies in the universe attract all other
bodies at any distance, but the strength of gravity weakens very rapidly as distance
increases.
3. Explain what an elliptical orbit looks like and how it differs from a perfectly circular
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one.
4. Identify and explain the misconception that the Orbits Explorer suggests: a) scale
problems, b) that space moves c) that if an object is placed into a given point in space,
it will automatically be in orbit.
5. Use the Table of Planetary data and four balls of appropriate size to illustrate the
relative speeds of Mercury Venus Earth and Mars around the sun.
6. Use the Starry Night Pro (or Cosmic Atlas) presets to illustrate the motions of
comets through the solar system.
7. Use the cardboard square to illustrate that the orbits of comets are not always in the
plane of the Earth’s path around the Sun.
8. Using small hand props and balls, illustrate the transfer orbit for a trip to Mars.
9. Use props to illustrate the problem of a rocket catching up to the ISS already in orbit.
10. Explain the Cassini division of Saturn’s rings using balls at resonance positions to
illustrate gravitational resonance
B. Rubric for Orbits Explorer Demo
QUALITY
LEVELS  OK
TRAITS 
Clarity and
appropriateness of
explanation
Give explanation that visitors of
all ages can understand.
Response to a difficult
question
Says “I don’t know” rather than
giving visitors wrong information
Continued learning
Keeps up with all in service
training for MGG’s
Engages in conversation
with visitors
Responds to visitor questions
and opens further inquiry
Content of conversation
Shows the main features of
planetary and satellite motion
Use of specifics
Uses tables of planetary and
satellite data to illustrate specific
orbits of satellites and planets.
EXCELLENT
Finds simpler ways to explain
more complex information so
that younger and older visitors
are inspired to ask questions.
When stumped by a visitor
question, finds another MGG
who knows, or works with
visitor to use digital collection,
internet or MGG resource
page to find the answer.
Studies “Other Cool Things to
Try” and does homework to
increase knowledge
Is sensitive to the visitor’s
desire to move on, but
sustains conversation
according to visitor’s level of
interest
Adds some high interest geewhiz topics to the
conversation.
Uses illustrations from specific
space history such as “the
Viking spacecraft in 1976 took
this trajectory to Mars.”