Comet Recipe

• 2 cups of water
• 2 cups of dry ice (solid CO2) - ideally a day old. This can be stored in a normal freezer, but remember to label it properly.
• 2 spoonfuls of sand or other dirt
• a dash of ammonia solution - this can be left out if you don't have any available.
• a dash of organic material (e.g. syrup)

1. Cut open a plastic bag and use it to line a large plastic mixing bowl.
2. Place the water in the mixing bowl, and stir in the sand or dirt using a wooden spoon.
3. Add the dash of ammonia solution and the organic material; stir again until well mixed.
4. Crush the dry ice (for example by placing inside several plastic bags and bashing with a hammer). Remember to wear safety gloves while handling the dry ice, and goggles to protect your eyes.
5. Add the dry ice to the rest of the ingredients in the bowl, stirring vigorously.
6. Continue stirring until the mixture is almost completely frozen, and then shape into a snowball (while still wearing your gloves). You may need to use a plastic mould of some kind to keep the comet in shape until it is completely frozen.

As it begins to warm up the comet will start to melt and sublime (turn directly from a solid to a gas, due to the CO2), just as comets do in reality when they come near the sun. Jets of gas may be seen, which are due to CO2 escaping through holes in the frozen water ice, and again such phenomena are seen on real comets. After several hours the comet becomes a crater-filled ice ball, as all the CO2 sublimes before much of the water ice has melted. Ultimately real comets disintegrate completely to just a few pieces of rock, or occasionally a rocky core - with the lab made comet you'll end up with a puddle of wet sand instead!

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