DIY Comet

A comet consists of:

A nucleus of ice and dust.
A tail of gas and dust that boils off the nucleus when the comet comes close to the Sun.

The ice is made from water and frozen gases including carbon dioxide, and ammonia. The dust is made from silicates and carbon compounds. The nucleus is typically a few kilometres across, while the tail can stretch up to millions of kilometres in length. This recipe is for a comet nucleus. Each ingredient has a purpose. The water will turn to ice in contact with the dry ice. Dry ice is frozen carbon dioxide. Window cleaner contains ammonia. Dirt contains silicates. Coca-Cola and cornflour both contain various carbon compounds.

Storing Dry Ice

Dry ice can be obtained from BOC and they provide it in an insulated bag. Place this bag in a chilly bin, leave the chilly bin closed until you need it, and store it in a cool place. If you have room, put the chilly bin itself into a freezer. Dry Ice, at -80°C, can cause a lot of damage so don't store it in the fridge. Packed into a chilly bin with a bit of extra dry ice it will last for days.

Ingredients

Water – 400 ml per kg of dry ice, plus extra to pour on
Dry Ice – between 2 and 5 kg
Ammonia window cleaner (or oven cleaner) – 2 squirts
Fine dirt – a handful (no worms!)
Coca-Cola – ½ can (not sugar-free)
Cornflour – 1 teaspoon

Equipment

Large cooking pot
Buckets – at least 2
Plastic bin liner (rubbish bag)
Wooden stirrer
Rubber gloves (waterproof)
Piece of 4x2 to use as crusher for the dry ice
Clear safety goggles

Method

Crush the dry ice to a powder

Do this just before you make the comet, but note that it takes a while. Wear the safety goggles to avoid a bit of dry ice getting in your eye and burning it. Pour a few pellets at a time into a large cooking pot, and crush them with the piece of 4x2, just as you would use a mortar and pestle. Pour the crushed powder into a bucket as you go, until all of the dry ice is crushed.

Assemble the comet

Line an empty bucket with the plastic bin liner.
Pour the measured water into the bin liner.
Add the window cleaner, the dirt, the Coca-Cola, and the cornflour. Mix well.
Put on the gloves.
Gradually add the powdered dry ice, stirring all the time. There will be lots of vapour formed. The mixture should form a thickening slush. Keep stirring for a few seconds as it thickens.
Using the bin liner, lift the slush from the bucket, and using gloved hands, pack the slush into a ball. Keep packing and forming until the ball forms a big lump. The secret of getting a good round comet is to pack the ice into a ball and hold the bag tightly around it until it solidifies sufficiently. Don’t be too perfect – real comets are never spherical and have lumps and bumps.
Peel the bin liner back and scatter some more dirt over the lump.
Pour some water over it, turning as you do, so that a layer of water ice forms.

Observe the comet

Observe the behaviour of your miniature cometary nucleus. It can be handled without gloves if the water ice coating is intact. If a spot feels sticky, pour water on it. It hisses and pops as carbon dioxide ice sublimes (turns directly from solid into gas) and forces its way through the water ice crust. On a real nucleus, this results in jetting forces, which cause the nucleus to spin, slightly altering the comet’s orbit, or even causing the comet to split into pieces.

Reference

"Kitchen Comets" in New Zealand Astronomical Yearbook 1998 Stan Walker and Grant Christie, published by Auckland Observatory, page 24.

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