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How big is a comet nucleus ?

Comet nuclei come in a range of sizes. Most are between a few and ten kilometres in size while the largest ones have an average size of about 20 km. The comet nucleus is nearly 50 % water ice and therefore the typical density may be around 1 gm/c.c. Thus an average comet has a mass that is billion times smaller than that of the earth. The nucleus of comet Halley was found to be peanut shaped by the spacecraft Giotto, 15 km long and 8 km cross section. Almost all the comet nuclei whose shapes have been determined, have a similar shape. Mass comaparison
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