- Suggested Age Range: 7 – 8 years
- UK Primary Curriculum: Key Stage Two (lower)
- Suggested UK Year Group: Year 3
- UK Primary Curriculum Link: Rocks
- Science Subject: Space rocks; Meteors, asteroids and comets
- Science Question: What are comets? What are meteors? What are asteroids?
- Activity type: small groups
- Suggested linked stories: Dancing with the Stars
Brief summary: Our Solar System contains the Sun, the eight planets, as well as numerous dwarf planets, many asteroids, and an unknown number of comets. Comets have often featured in historical accounts as portents of doom or disaster (one features in the Bayeux Tapestry, for example), but the reality is that they are giant dirty snowballs. As they get close to the Sun they begin to warm, and some of the ice begins to turn to gas, creating the spectacular tails visible from Earth. This activity helps students to understand what comets are made of, and why they look the way they do in the sky.
Key concept: comets are made of ice, dust, rock, and traces of simple organic materials.
Left: a photograph of Comet Lovejoy, as seen from the International Space Station, orbiting roughly 200 kilometres above the surface of the Earth. The comet is seen against a background of stars. The dark area at the bottom of the photograph is the surface of the Earth. The fuzzy green line is a phenomenon known as air glow. Between the ground and the green air glow lies our atmosphere. The comet is a long way from the Earth and so, although it looks as though it is heading into the atmosphere, it is actually in the background.
The Science Story: Have you ever noticed that it gets cold at night? Why do you think that is? It gets cold because the Sun goes down, the Sun is what keeps the Earth warm. What do you think, would it be warmer or colder if we lived further away from the Sun? It would be colder. Just like when you sit close to a radiator or a bonfire, it is very hot close to the Sun, but if you go far away it gets much colder. Imagine if we travel in a rocket and go four thousand times further from the Sun, out where it gets very, very cold indeed. Out here, in the edge of the Solar System where it is cold and dark, this is where comets live most of the time. Has anyone ever seen a comet? What does a comet look like? Comets are fuzzy blobs in the sky, a comet looks like a tiny cloud when you look at it through a telescope. Sometimes they have tails (illustrate with a photograph, or a model “Comet on a Stick”, see linked activities, below) that can stretch across the sky, some comets look spectacular! They only look like this when they get close to the Sun though. What do you think comets are made of? Comets are made of ice, dust, and smalls bits of rock, all mashed together to make a giant dirty-snowball. Imagine a snowball as big as a city! Far away from the Sun they stay frozen, just like ice cream does in the freezer. But if a comet is unlucky and comes close to the Sun, what happens? If a comet gets too close to the Sun it begins to melt, just like a snowman in the spring. As it melts some of the ice and dust gets left behind as it travels around the Sun, and those bits that are left behind are what makes the spectacular tails. When the Earth passes through these tails, some of the rocks fall though the atmosphere and hit the ground – these are known as meteorites.
The Science: Comets are made of the material left over from the formation of our Solar System, roughly 4.5 billion years ago. They mostly consist of ice, dust, rocks of various sizes, and traces of organic compounds. There is a lot of scientific interest in studying comets because they formed so long ago, the hope is that by studying their chemistry we can better understand the formation of the Solar System and test models of how the Solar System has evolved over the last 4.5 billion years. Comets spend the majority of their time in the outer Solar System where it is very cold, many thousands of times further from the Sun than the Earth.
Comets are made of the material left over from the formation of our Solar System, roughly 4.5 billion years ago. They mostly consist of ice, dust, rocks of various sizes, and traces of organic compounds. There is a lot of scientific interest in studying comets because they formed so long ago, the hope is that by studying their chemistry we can better understand the formation of the Solar System and test models of how the Solar System has evolved over the last 4.5 billion years. Comets spend the majority of their time in the outer Solar System where it is very cold, many thousands of times further from the Sun than the Earth.
Unlike planets, comets do not move in circular orbits around the Sun, instead they travel in very elongated elliptical orbits. These elliptical orbits mean that, from time to time, one of them comes close to the Sun. As a comet moves closer to the Sun, the temperature rises and the ice on the surface starts to sublimate – it passes from a solid to a gas without passing through the liquid state (this can happen at low pressures such as those in the near-vacuum of space). As the water turns to vapour, it can take some of the dust with it. This forms a cloud around the comet nucleus, known as a coma – this is what makes comets appear fuzzy through binoculars or a telescope. The comet keeps moving in its orbit while this process happens, and as it does so it leaves behind a debris trail of gas and dust behind it. This debris that is left behind is what causes meteors – when the Earth passes through this left-behind debris, the bits of rock fall into the Earth’s atmosphere and become meteors.
The Sun is continuously shining, but it is also producing a constant “wind” of particles and gas moving out through the Solar System at hundreds of kilometres per second. This wind pushes the material in the debris trail of the comet, just like a hairdryer would blow away the steam from a pan of boiling water. The consequence of this is that the tail of a comet always points directly away from the Sun.
The activity: This is best done in small groups. Supervision is recommended as it can get messy, and it is best to use ice straight from the freezer if you can (warmer ice does not work as well). If you have access to liquid nitrogen, this works really well for this activity, freezes the comets much faster, and is a bit of a “wow” factor, but MUST be done by an adult with appropriate safety equipment.
Safety note: It may be necessary to substitute some items in this activity in case of allergies, intolerances or dietary requirements of anyone participating. Dairy-and gluten-free alternatives are available. Check the ingredients carefully.
- Large resealable food bags
- Small resealable food bags
- Lots of ice
- Whole milk (low-fat versions do not work as well)
- Cookies, crushed (for the dust)
- Chocolate chips (for the rocks)
- Vanilla essence (for the organic material)
- Gloves for handling the ice
- Small paper cups and spoons for eating the comets!
- You are going to make a model of a comet – unlike real comets, this one will be good enough to eat! First you need to collect your ingredients (see above).
- Place about 100ml of milk in the small sandwich bag – this is the ice that makes up the main part of a comet body.
- Break up your cookie and add it to the milk. The cookie pieces represent the dust and loose rocks in the comet.
- Add the chocolate chips to represent the rocks.
- Add a couple of drops of vanilla essence to represent the organic materials.
- Ensure the small bag is sealed shut, and give it a bit of a shake to mix the ingredients.
- One person in your group should put on the gloves and put lots of ice in the large bag, the more the better but leave some space for the comet.
- Place the comet bag inside the bag with the ice and shake it a bit. Keep it moving so that all the milk starts to turn to ice.
- When the milk has started to freeze, remove the smaller bag and discard the ice somewhere safe. Spoon the comet into the cups and enjoy!
Extension: This activity can be extended by making edible asteroids such as in this activity from the Lunar and Planetary Institute and doing a “compare and contrast” exercise, looking at the similarities and differences between comets and asteroids.
- Comets are often confused with shooting stars (also known as meteors). Comets are long-lived objects that can be visible in the night sky for many weeks when near to the Sun, meteors are small bits of rocks that are only visible very briefly (typically for less than a second) as they burn up in our own atmosphere.
- “Organic compounds” means there is life – organic compounds do not automatically mean that there is life, an organic compound is just a compound that contains the element carbon.
- Comet tails always show you which direction the comet came from – the tails are made up of material that is so light that the solar wind is strong enough to push it around. As a result of this, comet tails always point directly away from the Sun.
- Rocks KS2 (lower) – can be used in conjunction with an activity comparing the characteristics of different rocks and/or looking at different meteorites.
- Light KS2 (lower) – notice that light is reflected from surfaces, we see comet tails because light from the Sun reflects from the particles of ice and rock.
- States of matter KS2 (lower) – observe that some materials change state when heated or cooled.
- Earth and space KS2 (upper) – describe the movements of objects in the Solar System.
- Forces KS2 (upper) – gravity keeps the planets (and comets) in orbit around the Sun, and the solar wind produces a force that creates comet tails.
Craft activity: “comet on a stick” from NASA – can be used to make a classroom display.
A more complex version of this activity from NASA.
STFC’s Borrow the Moon – borrow some space rocks to use in the classroom!
Copyright: Megan Argo 2019