Tag Archives: comet

Chasing Comets

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Great_Comet_of_1577 by Georgium Jacobum von Datschitz public domain

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.

Shoemaker-Levy panoramic (courtesy NASA-NSSDC)

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.

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.

Great_Comet_of_1577 by Georgium Jacobum von Datschitz public domain

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

Babylonian Astronomers Wrote Down Their Observations of Halley in BCE 164

Comet Halley's Appearance Dooms King Harold in 1066

Comet Halley’s Appearance Dooms King Harold in 1066

Comet Halley in 1910

Comet Halley in 1910

Comet Halley in 1986 (Courtesy of NASA)

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.

Oh, yeah, and I’m not making things up about radiation pressure.  Consider the prospects for spaceflight under the power of light!

The Japanese IKAROS spaceprobe in flight (artist's depiction by Andrzej Mirecki).

The Japanese IKAROS spaceprobe in flight (artist’s depiction by Andrzej Mirecki).

Cooking With Kuiper: The Instruction Set

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(update:  2/18/2015)

Time to build a comet!

If you have adult or older-student assistants, ask them to take charge of crowd control; that is, keeping the audience from crowding around the demonstration. Everyone will get to see the comet! Spare a minute for a brief lecture on the hazards of dry ice. You may have participants who know that dry ice can “burn”, but not all will understand that idea at first. However, no one wants to get hurt. Mention that you will be protecting your hands with gloves and your eyes with safety goggles (or safety-rated eyeglasses).

Supplies for Comet Making (Just Add a Cooler-Full of Dry Ice)

Supplies for Comet Making (Keep your cooler-full of dry ice in a safe spot.)

Participation opportunities include: helping move the materials and equipment to a mess-tolerant location, measuring ingredients, and smashing dry ice. The trauma of allotting slots to help out is one important reason to try the exercise at least twice. (Crowd-control tip: sometimes it helps to announce “I’ll only choose helpers from those who do not raise hands and call out to volunteer.”)  As a first step, take one of your plastic bags and cut it open along one side, then use it to line your mixing bowl.  Take 2 other bags and put one inside the other to make a double-thickness bag.

In the first stage,  your chosen helpers will take turns measuring all the “safe” ingredients into the bag-lined mixing bowl.  Working with the dry ice needs closer control, so keep your supply of CO2 off to one side for now.  As you introduce each ingredient, explain why it’s being included.  You can use the short explanations provided here as a starting point, adding your own facts or curriculum tie-ins, but remember to keep it brief or you’ll lose your audience’s attention.

Let’s start with water: most comets are composed primarily of water ice. During the early formation of the solar system, the planets were bombarded by comets—so some of the water you will use in this experiment may have actually originated in the Kuiper Belt!  (For a popular-science overview, check out this article from Time Magazine.)  Your helper will add 2 cups of water.

Next, add sand or gravel: most comets incorporate at least some rocky material.  Have your helpers measure out about 2 Tablespoon (TB) of grit.

Next, you’ll add ammonia: real comets typically contain NH3, the active ingredient in this cleaning solution.   (Regrettably, few, if any, comets show up to help when it’s time to clean house.)  If you’re using a squirt bottle to store the solution, your helper just needs to add one “squirt” of ammonia solution.  Otherwise, your helper should measure in 1 Tablespoon.

A Dirty Soup of Rocks, Water, and Organics

A Dirty Soup of Rocks, Water, Ammonia, and Organics

And, for our last step before major excitement sets in, stir in a touch of ice-cream topping: these contain organic molecules, which are a normal component of comets. The organic molecules in real comets are not this delicious–they include hydrogen cyanide and formaldehyde–but comets often contain complex and interesting compounds such as amino acids.   Researchers at NASA’s Ames Research Center have shown that amino acids from comets striking Earth long ago during the Solar System’s early eons would not only survive impact but would form even more important compounds for life under the heat of impact.   So it may be that we are here to enjoy ice cream (and sugary toppings) thanks to ancient comets.   Let your helper squirt in one squeeze-worth (it will be about a tablespoon).

Now, finally, it is time to add the dry ice.  Comets contain significant quantities of frozen gases, especially carbon dioxide, which just happens to be the gas that we call “dry ice” when frozen.  This stage of your demonstration is a two-step process. First, you will put on safety goggles and work gloves and use the hammer to tap off about 2 pounds of dry ice (1/4 to 1/3 of your supply).  Place the chunks into the doubled plastic bag and twist the opening closed.  Then, and only then, one lucky volunteer will be asked to don a set of goggles and, once protected, may proceed to smash the contained dry ice with the hammer.

Crushing Dry Ice with Flat Side of Hammer

Crushing Dry Ice with Flat Side of Hammer

Have your crusher use a two-handed grip (this helps deflect the temptation to also handle the bag of dry ice and also limits the range of motion, protecting bystanders from the crusher’s swing) and turn the hammer sideways, to smash with a broader surface area.

Once that stage is completed, ask the crusher to rejoin the group.  Make sure that the wooden stirring spoon is at hand and that you are still wearing your work gloves and goggles. Then open the bag and quickly scoop out roughly two cups of crumbled dry ice.

2 Cups of Ice-Cold CO2

2 Cups of Ice-Cold CO2

Give the mixture a stir and then swiftly add the dry ice, stirring vigorously. There will be some dramatic vaporization of CO2 and in moments the dry ice will freeze the water solution to a slushy slurry. Quickly wrap the plastic bag around your slushy mass and—keeping those gloves on—form the contents into a snowball, using firm pressure to shape the contents.

Comet's In the Bag

Comet’s In the Bag

You will feel the mass harden as you form your iceball. At that point, it is time to unwrap the comet and reveal it to your onlookers. You will have something that looks surprisingly like the common description of a comet—“a dirty snowball”.  You may even want to use your snowball-making skills to firm up the comet a bit once you remove it from the bag–remember to keep your gloves on!

Forming Up the Proto-Comet

Firming Up the Comet

Your finished comet

Your finished comet

Set the comet aside on a cold-safe surface, in a location where the eventual water-ice-melt will not damage anything. The comet will continue to outgas CO2 vapor. If you are working outdoors, any breeze will push this plume into a fair imitation of a comet’s tail.

Gases (CO2) immediately begin to sublime from the comet's surface

Gases (CO2) immediately begin to sublime from the comet’s surface

Your experiment team will undoubtedly want to repeat this process. A typical group of students will demand about four comets. After 2 or 3 builds, it will be time to set up fresh plastic bags for mixing and crushing.  If the group is larger, find ways for students to share participation tasks. For instance, two students can take turns as dry-ice crusher, two can each measure one cup of water into the mix, and so on.  As you proceed, instead of repeating the descriptive information yourself, invite the students to call out more of what they remember about the components represent.

Comet, Starting in "Dirty Snowball"

Here’s one small starter comet, let’s call this one “Dirty Little Snowball”


These model comets will last a long time, up to a few hours depending on their size and the conditions.  You can explain that the comets which get our attention are much larger–Comet Halley is estimated to be about the size of Manhattan Island–and between visits to the inner Solar System, they orbit back to where it is too cold for water, ammonia, or CO2 to be anything other than solids.  By no means do you need to make any effort to create spherical, smooth comets.  In fact, as you create successive comets, allow them to be different, irregular, and, well, messy.  Here are a few samples from a few of my comet-making sessions:

That's one frosty, rocky, comet:  "Before"

That’s one rocky comet, frosted with ice crystals of H2O and CO2



That's one slimy, partly-dissociated comet

Here’s a comet with conspicuous dark patches

That's one tall, cone-shaped comet

That’s one tall, frosty, cone-shaped comet








If your schedule permits, allow some time to pass and return to look at the comets after they have lost more material, as if you are checking in on a comet as it approaches the sun and some of its ice has been drawn off under the combined forces of the sun’s radiation and the solar wind…forming the comet’s tail.

Holey Comet, Batman!

Holey Comet, Batman!

Cooking with Kuiper: Project Supply Chart

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All Lined Up for Comet Building

All Lined Up for Comet Building

(Update:  2/18/2015)

As mentioned in the notes for project leaders, it’s best to repeat the procedure at least twice–three times if the class is large, to ensure that everyone has a chance to participate in the “safe” portions of the activity and to produce a variety of comets to observe.

Purchase dry ice in advance by as much as a day (purchase at the higher end of the quantity range if you need to store it overnight) and  store wrapped in insulating material, but not  tightly sealed.  (Frozen CO2 will sublime to gas and can even explode a container that is sealed too tightly.)  A small non-airtight cooler tucked into another lightly-closed, non-airtight cooler works fine, especially if wrapped in a blanket and stored in a cool location.

For the ice-cream topping, choose a small bottle with a squirt-style top full of caramel- or chocolate-flavor syrup for ice-cream sundaes.  Do NOT purchase hard-shell toppings;  stick to sticky sugar syrups.  Be prepared to fend off requests to sample the syrup.

For ammonia, do not use pure ammonia;  simply choose a basic non-sudsy ammonia-based cleanser.  A “sport-top” (squirting-style) water bottle about half-full of ammonia works well and keeps the ammonia away from hands, eyes, and clothing.  However, be sure to clearly label the bottle with the contents.

For trash bags, choose a good, sturdy brand.  They’ll take significant abuse!

Please note carefully that most equipment is required to be either plastic or wood.

  Per comet

For about 3-4 comets, allowing for waste and failures


Estimated cost

(2015 prices)

Good sturdy “tall kitchen” garbage bag, cut down one long edge to make a liner for the bowl 1


(have a second on hand in case the original tears)


($12 for box of 45)

Additional “tall kitchen” garbage bags


Open the bags and layer them one inside the other, to create a triple-thick bag


Have a second layered set of 3 bags on hand in case of tears


($12 for box of 45)

Large plastic mixing bowl, 2-cup plastic measuring cup, tablespoon measure, large wooden spoon

1 of each

Reminder:  for safety, use plastic containers and a wooden spoon

1 of each Bring from home or borrow from volunteers
Water 2 cups

2 quarts on hand

(store in a pitcher for measuring out in 2-cup quantities)

Sand or fine gravel 2 tablespoons ½ cup zero

One squirt (about 1 tablespoon)


½ cup


($10 for 28-ounce bottle)

Ice-cream topping

One squirt (about 1 tablespoon)


About ½ cup (Bring at least a 4-ounce container of syrup.) $6.50
Dry ice 2 cups of dry ice, after crushing. About 7-10 pounds of dry ice. $15($1.50 per pound)
Safety goggles

1 pair, adult size

1-2 pair, adult or child size (depending on student age)

1 pair, adult size

1-2 pair, adult or child size (depending on student age)

If not available in classroom—one-time purchase for reuse in many projects. $5 each


Heavy work gloves 1 pair, to fit Project Leader 1 pair, to fit Project Leader Use own gloves or borrow from volunteer (a new pair would cost about $10-12)
Total Cost: $39.50

For an easy-to-print version:  Just Supplies Cooking with Kuiper

Cooking With Kuiper: Notes for Project Leaders

Published by:

(update:  2/18/2015)

Last week on the tvweb, this happened: astronomer Derrick Pitts turned up once more on “The Late Late Show”.  And even though science-loving Craig Ferguson has moved on to new horizons, Director Pitts stayed and showed Guest Host Wayne Brady how to make a comet.  So I looked back at my entries for this project and realized they need some updates, and particularly some visuals. Have patience–it’s a multi-entry blog feature, so look for two more entries for the complete Updated Edition of “Cooking With Kuiper.”

The Kuiper Belt–that donut-shaped aggregation of hundreds-of-thousands of rocky objects orbiting beyond Neptune–is one of the most interesting regions of the Solar System just now.  Just last year, NASA’s Deep Impact explorer hurled a probe into the surface of Comet Tempel 1, flinging up a curtain of debris to reveal more about the comet’s composition.

Deep Impact's probe sent back this image just before striking Comet Tempel 1 (Image: NASA/JPL-Caltech/UMD)

Deep Impact’s probe sent back this image just before striking Comet Tempel 1 (Image: NASA/JPL-Caltech/UMD)

NASA’s New Horizons mission is due to arrive in July 2015 at Pluto–the most famous Kuiper Belt object–to observe the newly-redesignated dwarf planet and its five moons and then head out to explore.  You can check in on the progress of the mission at NASA’s home for New Horizons.  There is a general agreement among astronomers that the comets which return again and again (periodic comets)  began in the Kuiper belt.

In this project, we’ll be building a model of a comet using household supplies to represent most of the comet’s components and dry ice to capture the icy-cold environment of the Kuiper Belt.   While most Messy-Monday projects are entirely hands-on this particular activity is meant as a demonstration with controlled audience participation.  Some students may be careful enough to work with dry ice…but too many are not, and the step at which the dry ice is added can be dynamic and unpredictable.

A study of comets draws in much of what students should know about their planetary system and extends that knowledge into new and intriguing areas.  Students in intermediate grades probably know the basics of comets…that they come from the far reaches of the solar system, that they have tails, and that a comet crashing into the earth makes a cool disaster movie.  They might be surprised to know that scientists still want to find out more about comets, because all we know about comets so far is from watching them on their travels through the solar system.  Just a few months ago, the Rosetta spacecraft launched in 2004 by the European Space Agency actually landed a robotic explorer named Philae on Comet Churyumov-Gerasimenko, so why not launch an investigation into the nature and structure of comets by building our own lumpy, irregular, gas-spewing comets?

This activity is best paired with at least one hands-on activity centering on comets.   The second activity in this series combines a crafting-style model construction project and a cometary motion simulation game.  Other resources can provide other activities.  For instance, students can make a flip-book illustrating a short-period comet’s behavior as it travels from the orbit of Neptune to the sun and back.  And users of Pixel Gravity can run a simulation of the comet impact which led to the demise of the dinosaurs.

In the next installment, we’ll assemble a supply list for this project.  I recommend you  plan to build at least two comets, to let more kids participate and also to illustrate just how different two comets can be.


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