Orbits Explorer 8 of 8
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
· 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
· 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. The aesthetic appeal of this exhibit heightens positive affect, strengthening the formation of long-term memories.
Set Up
· 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), 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
· 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.
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-to-Jupiter-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.
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.