Chasing Comets: Notes for Project Leaders #1

In this activity, the most important idea is to explore and experiment with models and games to understand how a comet’s tail behaves as the comet hurtles around the sun. The key concept is that the comet’s tail is being pushed away from the sun by the ionizing radiation, solar wind and even the light itself blasting out of the sun. This means that when the comet is inbound, approaching the sun, its tail streams behind it, like a horse’s tail. But on the outbound journey, as the comet leaves the sun behind, its tail flies out in front of it. What we hope the participants will take away from these activities is a picture of what a comet looks like as it moves and the knowledge of why it looks that way.

Comet-tail behavior simply makes sense when “experienced” from the comet’s point of view.  If by any chance some of these facts are a discovery for you, too, don’t feel like you have to keep it a secret that you are learning–have fun with it. A key ingredient in the formula for growing a scientist is that finding out how the universe works is fun. Or, in the words of one physicist profiled in the film Particle Fever: The real answer to “why do we do this is . . . because it’s cool.”)

Keep in mind the constraints of your particular situation when assembling your materials and pre-planning the project. For instance, if there aren’t enough classroom scissors or if session time is tightly constrained, you can pre-cut the ribbon for the individual comet models into 3-foot lengths. Be aware of opportunities for participants with special needs—for instance, the comet-running activity does require at least one person to be standing still. In return, that one who just can’t stand still could be a pinch-runner. If the group as a whole isn’t particularly fast-moving, the “running” game can be done at whatever pace suits the team.   (One can be a “student” at any age—most of us middle-aged folks are not exactly speed-demons.)  If you’re planning this as a home-schooling project, this is one you’ll want to save for a get-together with other home-schoolers–you need at least three players and it is ever so much more fun with a group.

Stage 1: The Small-Scale Experiment

This description may look long, but that’s just to let you walk through it easily and to share some photos to help. This whole Stage 1 should take about fifteen minutes, tops.   I’ll spare your weary eyes and park the “Stage 2” and “Stage 3” activities in the next posting–but don’t worry, the entire activity fits into a single science session if you can claim an hour’s time to play with.

Before distributing materials, bring out one individual model comet, the sample to be used for the models everyone will take home. It’s simply an ordinary badminton birdie with long streamers of ribbon tied to it. For now, keep the ribbons bunched up inside the net of the birdie. Explain that the ball at the end of the birdie is the comet’s nucleus, the frilly part can be its atmosphere, or coma, which begins to form as the gas and dust which jets away from the outer layers comet as it warms up.

One Small Comet

Notes: I’d suggest that you relax and let your sample comet be imperfect—comets are messy creatures by nature and you don’t need that one super-meticulous individual slowing down the whole event by striving to exactly matching a perfect sample. If you have an older, more experienced group of comet enthusiasts to work with, you can interject the extra information about the distinction between the ion and dust tails—perhaps even represent them by different ribbon colors. On the other hand, if you’re working with anyone between the ages of 5 and 15, and you don’t want to deal with distracting snickers and giggles erupting through the group, simply refrain from using the technical term for a birdie. Oh, come on, you know why.

OK, back to it. The ribbon represents those gases and dust particles that make up the comet’s tail(s). Now, if we toss our model across the room, what happens to the streamers tied to it? Right . . . they float out behind. They don’t stretch out in front or clump in a bunch around the head of the “birdie”. You can demonstrate by trying to throw your comet backwards: hold the tail in front and toss, but the tail will just fall back to the head and—if your throw is a mighty one—end up in back again..

Now, invite answers to a key question: why does the ribbon float behind? What pushes the tail behind the cone as it flies through the room? With a little nudging, you should get general agreement that it is the air pushing on the lightweight streamers, shoving them behind the “head” of our comet.

But now we must turn to a more difficult line of questioning. Pull out playground or soccer ball (a handy model for the sun), and ask one student to stand and hold up your Sun so everyone can see the next portion. Bunch up the comet’s tail in the back of the shuttlecock again, and carry the comet in a “flight” around the “Sun”. As you move, ask the students to think hard about what happens to the comet’s tail as it whips around the sun.

Start easy. Shake out the streamers, and stretch them out with your free hand. Move the comet towards the sun. Which way should I point the streamers? Everyone will be quick to tell you to pull them backwards, away from the sun. Now, place the comet at its closest approach to the sun, just before it curves back to head into deep space again. “I’m at the Sun now,” you can say, “zooming around the back of it. And moving as fast as I’ll go in this journey. Which way should the streamers point?”

Usually this question generates some disagreement. A reasonable argument would be that you should hold the streamers behind the comet, as it moves, which would mean the comet’s tail would point along a tangent to its orbit around the Sun. (Even if the students are covering tangents in math, please don’t interrupt yourself to pause and discuss tangents right now! Use this lesson later to enliven the math session.)

Tail Behind?

Tail In Front?

Tail Sideways?

Some students may suggest—quite logically–that when you are that close, the Sun’s gravity should pull the tail towards it. If the group is large enough, you should also get someone who can argue that the tail should point away from the sun—for now, it doesn’t matter if this is a knowledge-based claim or just a contrarian viewpoint from snarkiest person in the room. Whatever hypotheses are offered, just accept them as proposed solutions and demonstrate what each would look like.

Finally, move to the “outbound” portion of your comet’s orbit. “Our comet now flies on away from the sun, perhaps to return in another century or two. Now, which way should the comet’s tail point?” Again, if you have managed to keep a poker face so far, the most popular answer is likely have the tail streaming behind the comet. As before, accept and demonstrate each of the guesses. If students have reasons for their theories, let everyone hear them. Discussing and justifying hypotheses is an integral part of the real scientific process.

If you have access to a blackboard (oh, well, it’s modern times, so, okayokayokay, you can use your smelly whiteboard or that fancy tablet-linked projector), now is the moment to leave off demonstrating with the model and sketch the competing hypotheses for everyone to see. Your picture will look kind of like this. Please remember to Keep It Messy.

Discussing Possible Tail Directions

Have you ever read one of those annoying mystery stories in which the author leaves you in the dark about a critical fact that solves the entire case? Well, here too, we have denied our puzzle-solvers an important clue. So, tell the group it’s time for a change of topic. But actually what we’re doing is rolling out the narrative twist that makes the whole thing so cool.

Here on Earth, it is air that pushes the streamers on our comet model. But how much air is there out in space? (So little that you might as well say “zero”!) But without air, why should any comet have a tail at all?

What comes out of the sun? You should hear the following answers: heat, light, maybe even radiation. But has anyone heard of the solar wind? The sun blasts out particles, too? The sun is shooting out plasma, protons and electrons flying through the solar system at thousands of miles per hour. This is the solar wind, which blows through the solar system all the time, at thousands of miles per hour. The particles are tiny, not even as big as atoms, so it is an invisible wind. And like wind, it’s not perfectly even, it gusts and changes from moment to moment as the Sun itself changes.

All of those things we named help to make our comets look the way they do. Consider your audience…

Explanation #1: You are all correct. All of that stuff blasting out of the sun–light, radiation, heat, and the solar wind–shove all that stuff leaking out of the comet into a tail. And since all that stuff is coming from the sun, the only way the tail can point is away from the sun.

Explanation #2: All of those answers are correct . . . and they all combine to make a comet’s tail. The heat of the sun warms the comet to free the gases and dust. The solar wind blasts the gases—and the particles in the solar wind also interact with those gases, stripping some of their electrons to make that part of the tail a glowing stream of ionized gas. The radiation from the sun actually can push things, and that pressure is just strong enough to shove those tiny dust particles enough to counteract their tendency to fall towards the sun. And the visible sunlight reflects from the spread-out cloud of dust, making the comet shine in our night sky.

Again, with older/experienced participants, now is the time to clue them in that radiation pressure—the totally cool idea that sunlight itself exerts pressure—exists because light is electromagnetic radiation and electromagnetic radiation is a wave and a wave [http://physics.info/em-waves/] pushes on the objects it encounters. You may not feel battered and bruised by the TV and radio waves powering through you day and night or be physically bowled over by the sunlight forming a gorgeous rainbow. But: it’s enough to push fine grains of dust. The only sad thing about radiation pressure is it’s not common knowledge yet—it’s been proven since 1873.

To represent these solar forces, we need to make a breeze. For that job, a fan does the trick. When we turn it on, it blasts a healthy “solar” wind. (Be sure to experiment in advance with your fan and sample comet–there’s a lot of variation in fan settings.)

Inbound Comet

Hold the comet in the “inbound” position, with the front of the birdie pointed at the Fan Sun.  Yes! We were all correct: the tail points behind the comet as it moves towards the sun.

If the fan is strong enough, you can also use the model to hint at how the length of the comet’s tail changes. Far from the sun, the comet has no tail; far from the fan, our streamers dangle to the floor. A little closer in, a real comet’s tail appears as a pale streak behind it; as you approach your fan, the model’s streamers lift up and begin to flutter weakly behind it. Near the sun, the tail stretches out millions of miles behind a real comet’s head; near the fan, the your streamers stretch their full length.

Now, what about when the comet is heading away from the sun? Which way will the tail be pointing, now that we know about the solar “wind”? Nearly everyone will see, now, that it must point away from the sun.

Outbound Comet

Demonstrate that this works: you point the birdie’s nose away from the fan, turn on the blast, and the streamers flow out over the front of the birdie. The shape of the birdie helps emphasize the incongruity of our expectation—that the tail goes behind—with the reality: the solar forces push the tail.

If the class has patience for one more test, add the third question: what happens when the comet is rounding the far side of the sun, and is pointed “sideways”? Hold the comet model perpendicular to the flow of the fan.

Comet At Perihelion

Let everyone see how the tail sweeps out to the side of the comet. It always points away from the sun, no matter what direction the comet is pointing.

Here’s 13 seconds of one model comet in action:

Coming Real Soon:  Stage 2