Comet 67P/Churyumov-Gerasimenko is half the density of water. This density implies a high porosity in the range of 70 to 80 percent, depending on the assumed dust-to-ice ratio. (You can use 0.9 gram per centimeter for ice and 1.5 to 2 grams per centimeter density for the dust.) Other comets show densities similar to 67P, but those have been less accurately determined during high-velocity spacecraft flybys and via the plume ejected by the impactor of NASA’s Deep Impact mission. Thus, we can infer that cometary material is made of weakly bonded icy dust aggregates with high porosity and low strength.
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