Earth and Space

How to make a comet

Practical Activity for 14-16 PRACTICAL PHYISCS


Teaching about comets? Bring them to life in class with this memorable demonstration!

Apparatus and Materials

  • Large polythene sheet to protect floor
  • Bin-liner bag, to line bowl and draw comet together
  • Mallet, to crush some dry ice to powder
  • Substantial plastic bag, in which to crush dry ice
  • Gardening gloves (heavy duty type)
  • Gardening gloves (heavy duty type)
  • Balance, to find mass of comet (and hence calculate a hypothetical kinetic energy)

For two comets...

  • Dry ice pellets, 10 kg
  • Garden sand, 1 kg
  • Water, 2 litres
  • Soil, 1 handful (organic constituent)
  • Worcestershire sauce (organic constituent)
  • Smelling salts (organic constituent)

Health & Safety and Technical Notes

Dry ice sublimes at -78°C and will cause serious skin burns on contact, but momentary contact is unlikely to be a problem.

Do not confine in a sealed container as it will explode.

10 kg of dry ice will produce 5 m3 of gas, raising the level of CO2 from 0.035% (natural) to safe-limit (USA) of 0.5% in a room 3 m high by 19 m on a side.

Make sure there is adequate ventilation, although if the dry ice is transported in a substantial expanded polystyrene box, little will sublime.

CO2 is heavier than air therefore pools at ground level.

In theory trapped gas could fracture the comet or cause it to split, but this has never been recorded.

Read our standard health & safety guidance

The essential ingredients are dry ice, sand and water. The other items represent the organic molecules thought to be present in a comet. If it feels as if the comet will not bind into a snowball, it is because you have not used enough water. There is a natural tendency not to want to use too much water for fear of evaporating all the dry ice.

Do this in a well-ventilated area. Wear safety spectacles and gardening gloves.


  1. Line mixing bowl with bin liner.
  2. Pour in half a litre of water and several handfuls of sand.
  3. Stir and add crushed dry ice.
  4. Stir and add Worcestershire sauce, soil and smelling salts.
  5. Add more water. Make sure that there is a fairly violent release of CO2, which indicates that you are cooling the mixture.
  6. Draw the mixture together with the bin liner and squeeze between your gloved hands. You will feel the comet is binding into a solid mass. If it feels loose you require more water and may require more crushed dry ice. Uncrushed pellets on their own will not cool the water fast enough to form a solid mass.

Teaching Notes

  • We gave a talk about comets before making our comet. We then found its mass and from this calculated the kinetic energy it would have if it struck the Earth at speeds of order 30 kilometres per second (the Earth orbits the Sun at 30 km s-1) and compared this with the kinetic energy of an aeroplane.
  • At the start of the class, we put a banana skin in with the dry ice, taking it out just before we made the comet. When the cold banana skin is dropped on the bench it shatters like china. This is a fun demonstration and also provides a vivid warning of the danger of dry ice.
  • Comets, long thought to be fearful omens of trouble and doom, are popularly described as "dirty snowballs".
  • Recent comet space missions reveal dry dusty or rocky surfaces, suggesting that ices are hidden beneath their crusts. It is now thought that short-period comets originate in the Kuiper Belt (beyond Neptune's orbit) whereas long-period comets originate much further from the Sun, in the Oort cloud.

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