As hinted in the previous post, for our universe-building project we’re doing two construction activities related to elementary particles. So, we’ll have two “Lists of Requirements” this time around. The model atoms use marshmallows, miniature candy chips, and gelatin mix. You’ll need just one packet of mixed-flavor candies for even a fairly large group–in advance, you can separate out flavors into the amounts needed. For sub-atomic particles, we’ll use multi-flavor candies, such as “Life-Savers”…we need six flavors, so you get to buy both peppermint and five-flavor mixtures. Depending on your workspace, you may choose to have participants work in table groups of of 3-4 people or to set up supplies assembly-line style in a relatively mess-friendly zone. The assembly-line method reduces the need for extra supplies, though these are quite inexpensive materials. For pre-preparation, it helps to count out supplies for each participant–small paper cups are ideal and stack neatly once your supplies are set up. Another helpful side item is a roll of waxed paper or a stack of paper plates for setting out the end-products while they dry or for taking them home.
One extra item, for your wrap-up, is highly recommended if your budget permits: pick up one humongous balloon–the 36-inch diameter size, in any color or design that delights you.
The recommended quantities are generous, to allow for after-project treats. Ice-cream sundaes, anyone?
The Atomic Marshmallow Project
For a group of 10
For a group of 30
Standard size (not miniature) marshmallows
Miniature candies, dark color*: try candy “decors” or extra-tiny chocolate chip ice-cream topping mixture
1 package of mixed candies: count out at least 20 dark-colored pieces
1 package of mixed candies: count out at least 20 dark-colored pieces
Miniature candies: light color*: try candy “decors” or extra-tiny white candy chip ice-cream topping mixture
From the same packet of mixed-flavor candies: count out at least 20 light-colored pieces
From the same packet of mixed-flavor candies: count out at least 60 light-colored pieces.
(choose a variety of fun, colorful flavors)
(one per group of 3-4 people)
For an assembly line:
(one per group of 3-4 people)
For each assembly line:
Wooden skewers (alternative: toothpicks)
10-16 ounce containers
(mugs, plastic cups, reused food containers)
For groups: 16
For each assembly line: 2
Small cups for sorting supplies
* IMPORTANT NOTE: If you’re tempted to use peanut-flavor candies, remember to be SURE to check in advance that none of the participants suffers from peanut allergy. In its worst form, this allergy can trigger anaphylaxis merely through physical contact with peanut oils or proteins, but at the very least, peanut-sensitive people should not eat anything tagged “packed in same location as peanut-handling equipment” or “may contain nuts”. There are lots of different candy chips to choose from; just be sure you end up with two different colors of “chips” for the protons and neutrons.
Sufficient Supplies For Construction of Approximately 40 Model Atoms
The second project’s list is even easier, and doesn’t require a “mess zone”:
One Side Makes You Smaller
A Top-Down Search for the Strange Charm of Putting Up With Those Quarks at Bottom of the Universe
The counts of candies in a mixed bag of five-flavor candies is a bit random, so if buying for a group you may need to grab an extra bag, just in case you need it. The package of sorting cups you purchased for the Atomic Marshmallow Project will have enough for you to sort supplies for this project as well.
Per 10 people
For 30-person group
Five-flavor Life-Savers candies
1 of each color,
a total of 5
each gets 5 total, 1 of each color
(2 bags of individually-wrapped Life-Savers)
each gets 5 total, 1 of each color
(6 bags of individually-wrapped Life-Savers)
1 extra piece of one of the five flavors
(There should be enough left over from the 2 bags you’ve purchased.)
(There should enough left over from the 6 bags you’ve purchased.)
20: each gets 2
(1 bag of individually-wrapped peppermints
60: each gets 2
(2 bags of individually-wrapped peppermints)
A Pile of Quarks, Ready for Construction of a Small Universe
I have a pair of projects to present this time–together, they are a sugar-based approach to understanding the building blocks of our universe. The goal is to build up a sense of the scale and dynamic relationships among the smallest particles identified to date, and how they combine to form the stuff we call “matter”. By the end of these activities, everyone participating should have a clearer picture of the following:
1. All of the matter in our universe is composed of just a few extremely basic and very tiny building blocks. They’re called quarks and leptons.
2.These building blocks, in the right combinations, make the next-level construction materials. The most common ones are electrons, protons, and neutrons. But there are others, too.
3. Once you have electrons, protons, and neutrons, you can build elements. Each element has particular physical and chemical properties–which arise from its unique physical composition of protons, electrons, and neutrons.
To make this activity fun (besides incorporating sweet treats), it helps to build into the presentation an element of discovery. First, we come to terms with the fact that the familiar atom is not the smallest particle. Second, we wrap our minds around the knowledge that even the tiny particles inside the atomic nucleus are made of even tinier ones. Third, at the conclusion, it’s truly mind-expanding to try to envision each of these in true relative scale.
The atom is still a meaningful idea, so long as we adjust its definition to suit modern understanding. The concept of the atom dates back over 2500 years, to Leucippus of Miletus and his more-famous student, Democritus. They reasoned out that if you keep cutting a material, you’ll eventually reach a particle that cannot be divided further. In Greek, the word “a” means “not” and “tomos” means cut, so when you call something an “atom,” you’re saying you can’t subdivide it. However, even now that we know that the structures we call “atoms” can be broken open, we still use the term. For instance, we’ll talk about “an atom of iron” or “the carbon atom”. But instead of defining the atom as “indivisible”, we now describe it as the smallest unit of a material that still retains those unique physical and chemical properties defined by its combination of electrons, protons, and neutrons.
In this project, we will build atoms from electrons, protons, and neutrons. Energized by our constructions, we will discard our preconceptions about the structure of the universe and descend to the sub-sub atomic scale, where we will capture quarks and leptons, then build ourselves some protons and neutrons and electrons. And then we will eat the lot: atoms, quarks, protons and all. It’s elemental.
We’ll proceed in two parts: “The Atomic Marshmallow Project” introduces the idea of atoms and their components, and “One Side Will Make You Smaller” takes us down into the realm of quarks. As in our other science projects, we’ll include information to share with the participants as you go along. For those who would like to delve into more detail, you’ll find links to good sources with plenty of depth.
New Horizons has flown past Pluto successfully, and is now on the way to check out other Kuiper Belt objects. Here’s Corwin Wray’s simulation (made with Pixel Gravity, his software for doing multi-body models on your laptop), which concludes with a wistful look back at our Solar System:
Like New Horizons, you can explore further too.
It’s worth your while to start by tracking down Guy Ottewell. Yes, he’s on the web, folks, and you can connect with him! Start with his Home Page, which includes all of his books, including the latest version of the book form of his Thousand-Yard Model as well as innovative ideas in several fields, from voting systems to landscape design: He has a Facebook Page on which he’s been more active as of 2014, sharing art and world news: And he joined Twitter in 2013 and tweets regularly, especially on human-rights topics, which should interest anyone who’s become aware of just how small our human community is in this huge universe: find him as simply @GuyOttewell on the tweet machine. A few of his books are available at Amazon, but take care—the latest updates are best obtained by purchasing directly from the author.
Of course, you might want to follow some of informational links given in the workbook pdf’s for this project:
The National Optical Astronomy Observatory presents Guy Ottewell’s original project description from 1989 online:
A wonderful collection of poems and quotes related to astronomy, gathered by Michele Stark, an astronomer with a wonderful page she created while lecturing in physics at the University of Michigan, Flint. l You’ll also find astronomy labs she’s created for non-majors interested in the field, under “Outreach and Education”
A relatively exhaustive listing of scale models in place around the world—most are designed for point-to-point driving or cycling tours, so scroll to the bottom portion of the list for walkable models, several of which are roughly on the same scale as that presented here. Check before you set out—some of these installations were only temporary, as part of larger events and some are virtual (i.e., online). I would like to imagine astronomy fans travelling to all of them, as baseball fans travel to all the major-league parks.
The Project Astro Notebook used to be sold as a huge expensive bulky (and still wonderful) binder. Soon, you’ll be able download at least some portions in pdf format from the free government-sponsored education resources site eric.gov. However, for now your best bet is to buy the DVD’s at http://astrosociety.org/astroshop/index.php?p=product&id=577&parent=1
Why use a FIFA 4 or 5 ball? Well, the dimensions are good for it. But any similar-sized ball will do for this project…like the tennis-ball-patterned playground ball I have. Guy Ottewell likes to use a bowling ball—but notes that it’s kind of heavy to lug around. http://www.achallenge.com/t-faq.aspx
If you need more reassurance that science and math are not only fun but also funny, visit http://www.xkcd.com (but do prescreen before sharing with students—this webcomic does sometimes use “PG-13” language.
For the jacks ball, you can pick up a jacks set anywhere. Online (e.g., www.orientaltrading.com , they’re often sold in party packs of a dozen sets. But any bouncy ball bigger than ¾” and no bigger than 1” in diameter will do the trick.
If you decide to buy a playground ball or soccer ball online, locate an air pump before your shipment arrives—they’re often shipped uninflated.
And if you buy on Amazon, be sure to sign up for smile.amazon.com first, so your purchases can support your favorite charity.
If you skipped Part 1, then you need to know know that in this activity, you will build a scale model of the Solar System as far as Pluto. You will use familiar objects and easy, approximate measurements—mostly simply pacing off distances. This is not a project about being extremely precise; the goal is to develop a strong perception of just how big the solar system is and how small the planets are within that system.
For preparation, you need only to assemble the collection of properly-sized objects listed in the requirements table (See Step 2) and print out the “cheat sheet” you’ll carry on the Walk. A glance at a map of your local area will help you decide which way to take your expedition and to identify some landmarks to stand in for more-distant things like the far edge of the Oort Cloud. To build your own interest and enjoy some discoveries of your own, check out some of the links I’ll include in the references section (Step 4).
You can feel free to substitute alternate model planets, using the scaled sizes as a guide; however, most of the items called for can be found in an average family home, borrowed from classroom parents, or purchased at a very modest outlay. While modern kids may not find the contents of kitchen spice jars terribly fascinating, using an allspice or peppercorn seed as your “Earth” model will give them a lifelong reference point–they’ll be smelling pumpkin pie or watching a chef grind pepper and that spark of memory will remind them of this project.
Because the scaled planets range from the size of a pin point to the size of a jacks ball, it also makes sense to attach each object to something larger, such as an inverted cup or a 4 by 6 index card. If you have access to sports equipment, the bright-colored cones often used for laying out a temporary playing field are helpful. You can position the planet-holder and also tape a “Please Leave Our Experiment Here” sign to the top of the cone. And the bright colors and signs help the explorers to look back and spot the distant planets. Again, be creative! There is no need to run out and buy sports equipment—any handy rock or a brick will do to keep your objects and notes in place.
Here’s my Walk kit, ready to go.
When reviewing the Cheat Sheet, you’ll see that this model describes our solar system as far as the outer edge of the Oort Cloud. However, to go all the way to the Oort Cloud in this model is a journey of 75 miles (100 km), so don’t expect to travel that far. Instead, as part of your preparation, identify a few local landmarks 1 or 2 miles from your start point and also pick some regional and further-off destinations to match the scaled distances for such key locales as the Oort Cloud, the heliopause, the estimated positions of the Pioneer and Voyager spacecraft, the far edge of the Kuiper belt, and our further neighbors in the Universe. If you’re too short on time, the Cheat Sheet includes some general destinations, but your own localized ones will be much more meaningful to the group. If your group won’t have time to walk all the way to Pluto, find out where Pluto would be in that locale and point ahead to that location before you do turn back.
Once in the classroom, before launching your exploratory mission, start with a quick review of the concept of scale. Regardless of your target age group, toys which are also scale models of cars or airplanes or trains are helpful examples. Quickly walk through a sample of numerical proportions to give a sense of how it goes when you are creating your own scale model: for instance, sketch on the board or a sheet of poster paper a rough scale drawing of the classroom room at 1 inch per foot (5 cm per m). Rather than slowing down the project with extra work, prepare for this session by making your own rough measurements of the classroom dimensions in advance—simply pace off the length and width and note any additional features to the room. Remember, the idea is to illustrate your point, not to create an architectural drawing.
Moving on to the Solar System, start with the Sun…an 8-inch-diameter playground ball or an ordinary soccer ball fits our scale. Ask if anyone can guess what size the Earth should be to go with this “Sun”. The guesses are very likely to be way off, because most “models” used in classrooms and the pictures in the textbooks are not at all to scale. In those, Earth is shown as a recognizable ball appearing as much as a tenth the size of the Sun.
Once you have a few guesses on record, share the key data. Write on the board or a flip chart as you go, to keep the presentation lively. (Nothing kills attention like a PowerPoint!) The Sun’s diameter is about 800,000 miles (1400 thousand km), and we’re using an 8-inch (18 cm) ball, so each inch stands for 100,000 miles (or, a cm stands for 75,000 km). The Earth’s diameter is only 8,000 miles (12,700 km). So how big will the model Earth be? It turns out we need something less than 1/10th of an inch across, only 0.08 inches (0.17 cm). So now you can pass around your “Earth”…a peppercorn will work, so will an allspice seed. (And, yes, you can get away with crumbling up a bit of paper and claiming it’s a spitwad you found.) If you have a spice-jar worth of seeds, everyone can have their own Earth to keep. Let the students take a moment to actually compare the sizes of Earth and Sun. It’s a dramatic difference, nothing like what their textbooks show.
Now it’s time to figure out where the Earth and Sun should be to fit in with this scale. Start by inviting students to guess…they will likely assume you can fit the Earth-Sun model easily inside the room. So now, add the distance data they need and we can “step” through the necessary calculation:
The Earth is roughly 93 million miles (150 million km) from the sun.
In our scale model, that’s 930 inches (2000 cm)
or 78 feet (20 m),
or 39 steps of about 2 feet (40 steps of 0.5 m)
In our model we’re using a pace distance reasonably close to the average woman’s step length and not too far off the step length of a child who is supposed to be walking but can’t resist running. If your group is adult men or tall women, you can use the worksheet to adjust the number of steps accordingly.
Our scale in SI (Système international, or metric) is slightly different than in English units, so that those using the SI version can also use simple round figures.
At this point, try to keep a straight face while pretending to start building the model inside the classroom. Dramatically place the “Sun” at one end of the room and try to pace off 39 or 40 steps. Unless you’re doing this activity in a large lecture hall or a cafeteria, you will quickly run out of space (pun intended). By now, it should be clear to the students that this is to be an outdoor activity.
If the group is not too insanely anxious to get outdoors, you can take one more minute to assemble a part of the model which will fit in the room—the Earth-Moon system. Our Moon is nearly ¼ the diameter of Earth, so it’s actually an important body in its own right. And it’s close by. In our scale model, the Moon—which can be represented by a single nonpareil or cake “décor” candy—is 2 3/8” or 5 cm from Earth—so Earth & Moon can be stuck to a card or piece of paper. Keep in mind that if your group is too anxious to get outside, you can choose to save this step for your arrival at the Earth’s position in the model outside.
Earth and Moon are stuck together
Set the very few ground rules for the mission plan. The model is built by counting steps—the students will be the ones to do the counting and you (the project leader) will expect them to try hard and in return will not be too fussy about precision or how the measurement accuracy may be affected when leadership shifts from short to tall students. The group will remain cohesive, so no-one misses out on any important discoveries—and no one will charge ahead lest they get “lost in space”. And everyone should understand the time constraints.
When the group is large, I’ve had success assigning small subgroups to accompany one adult leader as the “vanguard” to each planet, leaving the rest behind until they have “landed,” then allowing the followers to run full-speed to catch up. If you do this, it’s important to ensure everyone has a turn to be in the vanguard at least once. If the students have been studying the planets, the vanguard students can also be asked to provide just a few key bits of information to the other explorers as features they have “discovered” about the planet they just reached. However, resist the urge to turn each stop into a seminar—the goal is to travel as far as possible across the system quickly enough to return before class time ends.
Remind the group that it’s a long walk across the solar system and then get started for real. Carry your Sun to a central location outside. If you can park Sol near a tall landmark (such as a flagpole), you’ll find it easier to point back to the “center of the Solar System” as you move further away. Take your Cheat Sheet in hand (the page from the resource kit listing your step-off distances) and read out the number of steps from the sun to Mercury. Send the Mercury explorer team ahead to place Mercury in its position, and quickly join them with the rest of the group. If the vanguard has some cool facts to share about Mercury, give them time to speak. And move on to Venus and the rest of the inner planets.
The asteroid belt portion is the first region containing many objects. If you pause at Ceres, the biggest dwarf planet in the inner Solar System, it helps reduce the stigma of Pluto being “only” a dwarf planet. The fun part in these “belt” regions is to pretend to dodge the small asteroids or other objects—while you may mention that there really isn’t any significant risk of running into an asteroid, that is no reason to turn down the chance to pretend you’re in a crowded mess of obstacles just like in the movies. Even Neil deGrasse Tyson, in his reboot of Cosmos, includes a sequence in which his Ship of the Imagination zigs and zags through, first, a crowded Asteroid Belt and later a densely-packed Oort Cloud.
If time is short or you are working with younger children, it is reasonable to make it to Jupiter (don’t forget to dodge the asteroids on the way out) point out roughly where the outer planets, Pluto, and the further objects would be found and then head back to Earth.
In any case, carry some ordinary first-aid supplies and be sure to have extra adults on hand to slow down those who want to jump to lightspeed. Don’t worry if you don’t have a straight route to use…twisting and turning your way around the streets of a neighborhood is equally impressive. If time will permit, participants can bring lunches and picnic in the Kuiper Belt before returning. And remember, as you return to collect the planet models, it is just as fun to rediscover the distances on the way back.
Don’t worry. This is one of the least expensive major science projects you’ll put together.
I found a sunny yellow ball for my Sun.
1) Any ball roughly 8” (19mm) in diameter—a basic playground ball is likely to work, as will a standard soccer ball. FIFA size 5 works for the English-units model; the SI model is slightly smaller, so a youth-sized FIFA size 4 is appropriate—but don’t get bogged down in the details. Visually, when compared with the planet models, all of these ball sizes look the same. It’s most likely that you already own or can borrow a ball for this project; if you simply must buy a ball, you should be able to find one for under $10.
2) A set of eleven objects to represent each of the eight planets, our Moon, and two of the dwarf planets:
Mars or Venus
Pluto or Ceres
a) four pins (two pin heads represent Mars and Venus, two pin points represent Ceres and Pluto),
The Moon Is Made Of Green Candy
b) one tiny candy nonpareil (cake décor or “sprinkle”) for the Moon
Earth Gets Spicy
c) two peppercorns or allspice seeds for Earth and Venus
Having a Ball with Jupiter
d) one jacks-size ball (Jupiter)
This jellybean could be Uranus or Neptune
e) two jelly beans (or coffee beans) for Neptune and Uranus
Saturn represented by a large swirly peppermint
f) and a ¾” (19mm) “shooter” marble or a big round piece of candy (also 3/4″ or 19mm) for Saturn. (It’s just so nice to have something extra-cool and colorful for our most spectacular planet.)
Total cost: less than a dollar US; ideally, rummaging about an average home or allowing participants to bring contributions should turn up most of these objects for free. To splurge, pick up a whole jar of fresh peppercorns for around $5 and share them out among the students.
2) Eleven inexpensive holders for your objects, with the object names written on them. Empty clear yogurt containers or plastic drink cups work very well (see photos), as the pins can be pushed through the cups and others attached with glue to the cup bottoms…such that the cups then serve as mini-pedestals for the model objects. However, don’t feel bound by guidelines here—a set of index cards will do the job if that’s what you have handy. It does help to secure each object to its support. However, be sure that students can see the actual object clearly so that everyone has a feel for the scale. Cost: as much as 10 cents
3) A few signs printed on regular-sized paper to leave with objects that will be waiting for your return, such as: “Please Leave This Experiment Undisturbed — (Teacher’s Name).” Cost: 10 cents
4) Weights to keep each sign from blowing away in a breeze—anything from a handy rock to a water bottle to an actual sports-field marker from your supply closet. Cost: negligible
5) Your basic first-aid kit and/or other equipment required by local protocols for a field trip.
6) Water as needed (Up to $10 if you need to buy each student some bottled water; negligible if students can bring refillable water bottles.) You may choose to make the walk as short as a half-mile (kilometer) or as long as twice that. For a short walk, you should only need modest supplies; for a long walk, snacks and water will be welcome.
7) A printout of your “Cheat Sheet” for either the English-units or SI-units version of the project Walk to Pluto, Miles or Walk to Pluto, km (Just click to download the desired document) Whichever measurement system you’re using, it’s just one sheet, front & back, and includes short comments you can make as you take your trek. Cost: 15 cents, if your printer ink is expensive, because it does have colors.
Total cost of essential supplies: normally about a dollar, assuming most items can be gathered at home or borrowed. For bottled water, if needed, budget an additional 50 cents per student
If you purchase all new supplies, you could spend as much as $40 for a brand-new soccer ball, a jar of nonpareils, a jar of peppercorns, a packet of pins, a jacks game, a bag of marbles with a shooter, and a package of jellybeans.
(For workbook copies in Excel format, ready for editing, I can send you a copy via Facebook messaging. Just connect to one of my pages, Pixel Gravity or Cometary Tales. Say, while you’re there, “like” the page. Either way, you’ll receive the file in a return message. The beauty of this approach is that you don’t even need a copy of Excel to use the workbook—Facebook will prompt you to choose whether to open it in Office Online or to download it. The alternative is to email me via firstname.lastname@example.org.)
Compare the sizes of Earth and Pluto & Charon (Pluto’s shadow isn’t that big on Earth!) Image Credit: NASA
It’s been a super-fantastic #PlutoFlyby day (see the video for a Pixel Gravity simulation of New Horizons’ close approach path on 7/15/2015), and I can’t resist going to one of my favorite astronomy projects: building a scale model of the Solar System that takes you out of the house, out of the classroom, and under the sky. (Where maybe Pluto’s shadow, cast by a distant star, will pass over you.)
As a reminder, you can look for the following in any Messy Monday project:
A set of notes for project leaders, sketching the key elements of the project and the science topic it is meant to address
A detailed supply list, structured to make it simple to purchase supplies for either a one-shot demonstration or for a classroom-sized group activity.
A set of instructions for working through the project with students, including commentary to help cope with common classroom-management issues, questions that are likely to arise, and issues to keep in mind from safety to fairness.
A rough estimate of the cost to run the project.
As before, I’ll break down the presentation into four postings, to spare readers trying to scroll through a 5000-word document, but I’ll post them quickly, so you can jump ahead if you are raring to go or want to access the reference materials first. In other projects, we built our own comets. In this project, we travel out into the solar system, hoping to reach the source of that comet.
Step 1: Space is Big
It’s a long way to Pluto. But as far as the Universe is concerned, Pluto’s in our condo’s tiny back yard. What would it be like, though, to take a hike to Pluto? Like the New Horizons Spacecraft spacecraft buzzing past Pluto and its cluster of moons, but, well, maybe taking a bit less time about it. Nine years (the explorer was launched in early 2006) is longer than even the above-average student’s attention span. What if we could shrink the Solar System down to a reasonable size for nice walking field trip?
Paths of the nine planetary objects orbiting the Sun for many years (A Pixel Gravity simulation result.)
No surprise here: it’s been done. Six ways to Sunday, in fact. While no one person claims to own the idea of building a scale model of the solar system, my favorite advocate of such models is Guy Ottewell, who likes a scaling factor that makes the model a reasonable size for the average person to walk. You can buy his book on the subject (now with cartons!) at the books page on his website. As a bonus, you’ll also find the most current editions of all of his other books on astronomy and much more. (He self-effacingly describes his annual Astronomical Calendar as “widely used”; a more-accurate description would be “fanatically used by serious amateur astronomers”.) No disclaimer necessary; we’re not friends, I’m just one of his (many) Twitter followers.
The goal of this project is for everyone involved to obtain a personal sense of the feature of Outer Space that is hardest to conceptualize by reading books and trolling the internet: Space is BIG. (Yes, you may pause to reread the opening to The Hitchhiker’s Guide to the Galaxy, by Douglas Adams.) Indeed. Really Really Big.
On top of that, the places you can stop—the non-empty bits—are few and very tiny compared with the distances between them. And it takes a long time to get from one stop to another.
So, when assembling materials and presenting this project, keep these two key goals in mind. It’s not important whether you model Earth as a peppercorn (Ottewell’s model) or an allspice seed (easier to find in my own kitchen) or a spitwad from the ceiling that happens to be about a tenth of an inch across. What’s important is that the Earth is not only extremely teensy compared to the Sun, but you can’t even fit the Sun and Earth into an ordinary classroom. And you have to hike at least a half a mile (a kilometer) if you want to make it to Pluto. With any luck, you can make practical use of the excess energy in a classroom-full of kids and also amaze them. If you’re doing this as a classroom helper and the teacher is used to taking advantage of the time to catch up on infinite paperwork, this is a time to persuade that teacher to shove the paperwork aside and join the expedition. There will be no regrets!
The objects used to represent planets and other bodies should be chosen for familiarity, because you want the participants to absorb the scale comparisons effortlessly. “Everyone knows” how big a jellybean is, a pin is familiar—both the pushing end and the painful poking end—a soccer ball is a known object, and so on. It doesn’t matter if the object you use is not exactly the design diameter—and no one is going to care that jellybeans or coffee beans are bumpy ovoids, not spheres. The next time you’re eating a jellybean (or slurping a Starbucks), at the back of your mind will be “I had to hike a half-mile just to get to this little Neptune here”. Plus, “Yum, astronomy is delicious.”
If you’re interested in the underlying concepts, I encourage you to stop by the National Optical Astronomy Observatory’s website and read Guy Ottewell’s original 1989 description of his Thousand Yard Model; however, if you consider yourself a mathphobe, don’t let the arithmetical computations worry you. I’ve made you an Excel worksheet to do that task. Running a mind-expanding science project should help relieve that condition, not make it worse.
If you have visited a museum’s scale model, read Ottewell’s book, or done a similar project in the past, there are a few differences you may encounter in this project. In particular, I suggest you avoid having planets represented by peanuts. Including nuts in school projects, can be problematical if any student (or parent helper) with nut hyper-allergy could possibly be affected. (I have relatives with this allergy, and there is nothing quite like coping with anaphylactic shock to ruin a day’s outing.)
I’ve included a few more “destinations”—such as the ever-popular asteroid “belt” and my personal favorite of Pluto’s fellow dwarf planets. The number of steps taken between planets (and other destinations) is greater, because kids take shorter steps than grown-ups. (Also, other models I’ve seen assume a stride length more typical of men—and the majority of teachers and parent volunteers are still women, with shorter strides than men.) And I’ve included the current (for now, at least) locations for a few more distant “destinations” that we can look out towards from our turnaround point at Pluto.
The tables I’ve provided are in both English and SI units. The scales are slightly different between the two, in order to yield intuitively-scaled results in either set of units. And I’ve provided a “cheat sheet” of the key data for a teacher or other presenter to carry as a reference source on the walk. If anyone would like to get completely precise and build their own model matching their pace length exactly, or adjusting to a different scale, you can request a copy of my Excel workbook for this project to create your individualized pace-off. Or if you know a Senior Girl Scout or Boy Scout in need of a Gold Star or Eagle project, a community solar system model would be a very cool service project. (C’mon, Scouts, do you really want to build another park bench?)
Speaking of space, and coolness, and peanuts, and bigness, by the time your group finishes this project—everyone who participates should wholeheartedly agree: Space is Big
Below, you’ll find a handy supply document you can download, with shopping lists for small and large groups and a range of cost estimates, depending on how much of the supplies you can acquire from available supplies or donations by participants. With a minimal outlay, you and your group can experience being comet chasers–observers of comets.
Basically, you need a bunch of badminton birdies for your comet heads—keep in mind you don’t need performance-grade shuttlecocks or even new ones. If your high school has a badminton team, they will have worn-out birdies you can take off their hands. A grungy, beat-up birdie makes a more realistic comet head.
Birdies for Comets
And you need a bunch of ribbon—curling ribbon for the comet tails. The supply sheet estimates ribbon packages at around $8, but if you look at this photo, you’ll see the last time I bought supplies, it was out of the clearance bin at $2. And if you can get one in five of your participants to bring in a roll to share, it won’t cost you a dime.
Zoom Out–Yes! Here’s All You Need To Make Comets
The one oddball item is that tulle fabric ribbon for the big comet. This you might have a hard time finding in your junk drawer unless you’ve been helping a bride make wedding tchochkes. But for $10 you can buy enough to make three huge comets. Cut five-yard lengths and tie one end of each to a vane of a single birdie, allowing a few inches of extra length to fan out as the comet’s “coma”. Tulle scrunches up easily, so even a six-inch-wide ribbon will feed through the holes between the birdie’s vanes.
Detail–How To Tie Fabric Tails
You should be able to borrow a portable fan and a playground or soccer ball. If you can’t, it will take a roughly $25 expenditure to get those items in stock—a cost you can recoup in part by either donating it to the group you’re working with or simply deducting the expense as part of your cost of volunteering.
And it is presumed you can find a pencil, which makes holding the small model a little easier when you’re doing the demo with the fan; here’s the trick for hooking the pencil to the comet head:
Holder For Fan Experiment
Depending on how good you are at scrounging supplies and locating soccer balls, your costs will range from $10 to $85 for typical group sizes. The spreadsheet I use has a calculation column to adjust the requirements list for other class sizes So, if you want a copy of this fully-functional workbook, “like” the Facebook page & I’ll send you one via a Facebook “message”. (You can also try emailing me through the “contacts” page here, but you’ll get a faster response on FB.) Your FB contact will be used for nothing other than sending you a file and boosting the “likes”-count on my page. [Insert maniacal laughter, if desired.]
Meanwhile, you can get the static workbook as a pdf right away:
Now that you are all excited about comets, here are some fun places to go where you can find more cometary material:
A lovely one-page summary from the Spaceguard Program (sponsored by the European Space Agency) gives a clear description of comet tail structureand dynamics, including a neat animation of what both tails look like as the comet proceeds around the sun. The ion tail streams straight back, while the dust tail is curved a bit as the particles within the dust tail blend movement due to their individual orbits about the sun and the forces of the radiation pressure. Net, both tails roughly point away from the sun, as in our demonstration.
Our guy Euler was the first one to suggest that light exerts pressure, but we had to wait over 100 years to get to Maxwell, who proved it, and then another quarter-century went by before some Russians managed to measure radiation pressure. (Also, gotta love Google Books.)
OK, we’re back for part 2. Remember that our goal is to impart an intuitive, long-term understanding of how comet tails work. I’ll give you an observation worksheet that students can use during the Comet Running game, but if time or attention-spans are too short for a worksheet, dispense with that element in favor of learning through movement and Socratic dialogue. (What? You think an engineer wouldn’t have read the Greek philosophers?)
If you have time and enough outdoor space for the “Game” version of this simulation, move right along to “Stage 2” now. The promise of a chance to make their own models is what will entice the students back to the classroom. Otherwise, save the great outdoor model for another time or place and move directly to “Stage 3,” building the individual models.
Stage 2: The Game
That’s One Big Comet
This is an outdoor game, and it works to best advantage with a nice BIG comet model. Four five-yard lengths of white fabric streamers attached to a single badminton shuttlecock (“birdie”) make our Comet Chase model. A playground ball or a soccer ball (around 8” in diameter) stands for the sun. Sort the participants into groups of no more than five and no fewer than three, and move to the great outdoors. A grassy area is safest, because this game involves some complicated running; if you’re stuck with pavement, tone down the running to “jogging” and allow a little extra time.
Start by laying out the ground rules for the game. First, each group will get to play every role. There are three parts: being the sun, being the comet, and being observers back on Earth. Remind everyone of your local rules for behavior outside. It’s harder to listen to instructions out in the sunshine and fresh air!
Take a moment to review the lesson so far. Place the model Sun on the ground, at least ten yards away. Ask an adult helper or one of the students to stand about halfway between the class and the Sun and to hold the head of the comet
Large Comet Head With Coma
while you extend the tail’s long white streamers. This model is much more evocative of the scale of a real comet, which has a tail tremendously longer than the diameter of its coma, or head—but it’s still not a scale model. Allow for some oohs and aahs, but move on to your query: which direction should the comet’s tail point? Don’t move yet; both you and your helper just stand in place.
Large Comet: Incoming or Outbound?
Don’t be concerned if it takes more than one answer to get the right one! Some may still want to know which way your comet is moving. But in a few moments, you should achieve the consensus that the tail should point towards the class and away from the sun.
Now, add the movement and ask everyone to call out which way for you to move. Ask your helper to start walking (slowly, please!) towards the sun and then to loop around the sun. You will need to move quickly to keep the comet’s tail pointing away from the sun. In fact, even if your helper cooperates by walking slowly, you will need to break into a run! As you run, if the students aren’t already hollering directions to you, tel them to keep reminding you which way to point the tail: away from the Sun!
Pause partway and while you catch your breath you can demo a technique for helping to align the tail while in motion. With your outside hand, hold the streamers. With your inside hand, point at the Sun. The tail-runners should always find that pointing at the Sun also means pointing at the comet’s head.
Now, it is finally the students’ turn. Run as many iterations as necessary to ensure that each group does each job at least once. For instance, for a class of 20, allow time to run the game at least four times.
The Comet Group: The comet group needs one Head and up to four Tail-Runners. Name the comet after the person who’s serving as the Head. Comets are always named according to the last name of the comet’s discoverer. So if you have Robin Williams as the comet’s head, then this will be Comet Williams. Getting the comet named after him/her may compensate for the fact that the “head” only gets to walk slowly around the sun.
Meanwhile, the tail-runners get to hold the ends of the tail streamers and run to keep the comet’s head between themselves and the Sun. In the normal course, the “tail” group will tend to lag a little and spread out, but that actually serves to more-accurately represent the shape of the dust tail. If you’re working with a two-tails group, designate one especially determined runner to represent the ion tail by taking one ribbon and maintaining a straight line from the ribbon end through the comet head to the sun.
The Sun Group: The sun group stands in the middle of your running space. One or two group members hold the model sun overhead. This makes it easier for the Comet group to see if they have successfully aligned the comet head and the sun. If the tail-runners stray out of line, members of the sun group need to to shout out “Got you! Got you!” or “Solar Wind Coming!” to warn them that the solar forces are blasting the tail.
The Astronomer Group: The people who are not part of the sun-comet demonstration still have a critical role. They are not just watching other people play the game, but they are tracking the shape of the comet’s tail as it passes around the sun, as observers on Earth. Depending on their perspective at each point in the comet’s orbit, the tail will appear longer or shorter. For example, if the comet is roughly between Earth and the Sun, the tail may look short, because it is stretched towards us. If you have time for writing, ask the Observers to sketch the comet as they see it. (See the handout.) In an average class, each student will get to observe the comet at least twice, which is very helpful for catching the unexpected views.
When every group has had a chance to play every role, take a few minutes to review one more time. As a comet is orbiting around the sun, which way does its tail point? By now, everyone should be willing to state that the tail always points away from the sun.
Still, you may still have a few hold-outs who are not quite sure this can be true. If you are lucky and it’s a sunny day, you have a hole card to play. Invite the students to each imagine that they are comets. “Guess what? You can see exactly where your tail would be. Who can point at it? Where’s your tail, Comet Human?”
If you are not saved by the insight of a student who’s totally absorbed the lesson, it is OK to resort to hints. “Everyone has one. It’s easy to see. Yes, you can see your comet tail! Where is it? Which way does a comet’s tail point? Right: away from the sun. Where’s the sun right now? What do you have that’s pointing away from the sun? It’s not bright and shiny like a comet’s tail. It’s dark, because there are no sunbeams there.
“Yes! Your shadow is your comet tail. It points away from the sun, always, no matter what direction you run.”
Stage 3: The Reward
Finally, everyone needs a model comet of their own to take home and show off and share with family members everything about how comet tails work. This is not an art project; it’s an opportunity to review and experiment individually. If some students are fussy about carefully arranging their streamers to make a colorful pattern, that is all right, but the point is to assemble a working model.
Each participant needs 24 feet of curling ribbon and a birdie (remember what I told you earlier about calling it by its proper name—be prepared for lots of giggling and teasing if you insist on that) . Cut the ribbon into eight lengths of roughly 3 feet. It is perfectly all right—and in fact more realistic—if the streamers come out various lengths. And depending on the students’ social skills, it is also all right for them to exchange colors once the cutting is done. (There are always some who prefer to discover a multi-color comet and others who prefer monotone.)
Once each student has six streamers, have them tie one end of each streamer to the head of the birdie.
Detail–Attaching Ribbon For Comet Tail
Your meticulous planners will distribute them evenly around the netting; others will be clumped randomly. Either is fine. Every comet is unique and most are quite non-uniform.
Be real. This project is not done when it the comets have been only built. Everyone needs a chance to try them out. They will, of course, want to toss them around the classroom; if this is not acceptable, make some provision for them to try out that technique outdoors. More scientific, of course, as time permits, is to allow the participants to take turns trying out their comets in the pretend “solar wind” of the classroom fan. As long as they willing and able to mind safety rules about working around a fan, by all means have everyone try out the tail position approaching, passing, and retreating from the Fan Sun. But don’t get all hot under the collar if other comets are flying through the room while you monitor the fan users. Just imagine you’re in the Oort Cloud and you’ll be OK.
Up next: Supplies You Need and Resources You Can Use
In this activity, the most importantidea 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 In Front?
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.)
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.
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.
As a re-entry activity, let’s fall right into the project which inspired the overarching theme for this so-called blog: cometary tails. That is, in this instance, we’ll be “studying” the behavior of the tails of actual comets falling along their orbits about a star. But of course, this is a “Messy Monday” project, so it involves running, arguing, and playing with scissors (not all at the same time).
So far, the only star whose comets we’ve observed have been those of our own Sun, but as our star is not particularly unusual, it’s likely that comets ply their trade throughout the cosmos. We’ll not be delving too deeply into astrophysics, instead we’ll be building fun models of comets and playing games which illustrate the apparent motion of a typical comet’s tail. If you’re running this project as part of a school science program, you can double-count the activity as a P.E. session, as the central game involves more than a bit of running, though not likely moving as fast as a comet.
Just as a reminder, what I want to give you in these “Messy Monday” project descriptions is 1) enough background on the science that you’ll be prepared for questions and have resources to draw on if your own curiosity is triggered, 2) a play-by-play description of running the project with a group, recognizing that your time and resources are limited and your participants will vary in both interest and prior knowledge, and 3) a shopping list detailed enough to help you minimize your costs as well the time you have to spend assembling supplies.
Fragments of Comet Shoemaker-Levy heading for Jupiter (courtesy NASA-NSSDC)
So, What Do You Want to Know?
For thousands of years, humans have wondered at the strange visitations of comets.
Natural philosophers of the middle ages studying comets.
In our time, people now understand that comets are not harbingers of doom or annunciations of the births of kings but fellow travelers in our solar system, icy bodies wheeling in towards the sun and shedding a fraction of their substance as they approach the sun. However, a key aspect of the comet’s tail remains counterintuitive to us earthbound air-dwelling creatures. The tail of a running horse flows behind her as she gallops, so we naturally expect that the tail of comet simply flies behind it as it plunges along its course. But a comet’s behavior plays tricks with such expectations.
Where do comets come from? The Solar System is a big place, but for most of us, the territory ends with Pluto, the Object Formerly Known as The Ninth Planet.
The Great Comet of 1577
However, if you’re a fan of Cosmos (either Carl Sagan’s or Neil DeGrasse Tyson’s version) or if your school is lucky enough to have new textbooks, then you’ll know about the Oort Cloud , that sphere of orbiting material from which most comets emerge. Do you realize how much farther out this region is? On a scale of one inch per 100,000 miles, in which the orbit of Pluto would be one mile across, the distance from the Sun to the Oort Cloud would be the length of the state of California. It’s even been hypothesized that the Oort clouds of neighboring stars may physically interact, exchanging comets.
The Oort cloud is a long way out, but it’s still a part of the Solar System, because the objects there are still subject to the Sun’s gravity. Occasionally, a piece of this clutter is jostled from its orbit and begins the long fall towards the sun. Depending on the path it takes as it zooms around the sun, the comet may slingshot out of the solar system entirely or it may settle into a new orbit, returning to loop around the sun on a regular schedule. For instance, Comet Halley returns every 86 years. The last time round, it actually came in ’86–1986 that is. I was lucky enough to visit New Zealand that year, so I can confirm that Comet Halley was extremely unspectacular that year–only just barely visible. Fortunately, New Zealand itself is spectacular every single day of any given year. NASA was more successful, having a noticeable advantage in telescope access.
Babylonian Astronomers Wrote Down Their Observations of Halley in BCE 164
Comet Halley’s Appearance Dooms King Harold in 1066
Comet Halley in 1910
Comet Halley in 1986 (Courtesy of NASA)
But why do comets even have tails? We don’t see shiny tails glowing in the wakes of our planets. Well, it all has to do with the change in environmental conditions as the comet moves towards the Sun. Comets are composed of water ice, frozen gases, rocky matter, and even traces of organic compounds. As this frozen jumble approaches the sun, it warms up enough that the various ices in the outer layers of the comet become gaseous—water vapor, ammonia, carbon dioxide. These gases bubble and boil into a misty cloud, so the comet will have an atmosphere of sorts, called the coma, for the duration of its passage through the inner Solar System. The gas expulsions may even shoot out of the comet’s rocky layers like jets, causing the comet itself to tumble as it falls along its inward path. At the same time, very small-scale “dust” particles are swept from the cometary nucleus. This is not the heavily-organic dust we find under our furniture here on Earth (if you really want to know what’s in household dust don’t use “Google images”; stick to text searches or just ask your friendly neighborhood allergist). What we mean is that the particle size—a few microns—is extremely fine, about the same size as the particles in cigarette smoke.
We get our fabulous cometary tail once these newly-ejected gases and dust of the coma approach the sun just a bit closer, enough that the various solar emissions can have their ways with the comet’s atmosphere. First, there is sunlight itself, which acts in several ways to provide us with the visual spectacle of the comet’s tail.
The simplest role of sunlight is to shine on the cloud of dust ejected from the nucleus. That’s the main tail we see. But that still doesn’t explain why the dust forms a tail at all: the secret is that light, as electromagnetic radiation, actually exerts pressure on objects, and with tiny objects like cometary dust this radiation pressure force is enough to fan that material out from the core. Plus, there is a cool bonus “secret”: that most comets actually have two tails—one formed by the gases and one formed by the dust. The ultraviolet radiation in sunlight blasts the gas particles, stripping away electrons, and so creating a mass of ionized gas, which fluoresces (mostly blue) in sunlight. Then those glowing blue ions are blasted in a straight line away from the sun by the solar wind, a stream of high-energy particles hurtling at supersonic speeds through the solar system. The solar wind is a wonderfully intricate system in its own right, but for our purposes here it is most important to convey that, like earthly winds, it consists of particles moving at high speeds and that its direction is away from the Sun.
The result of all these combined forces is that a complex, continuously shifting cloud of gases and dust streams out from a comet during its time in the inner solar system and that tail—or, rather, pair of tails—points away from the sun, even when the comet is on its way back out to its origin. (If you’re a die-hard comet enthusiast, you’ll know that the dust tail does curve inward a bit, as the small particles of dust battle with the solar forces, striving to curl into their own individual orbits about the sun, but from our earthly perspective, the outward forces have the upper hand.)
In the next installment, we’ll get down to the nitty-gritty of building our own comet models and playing a game of As the Comet Tail Flies.