In my February story, ò??What Halleyò??s Comet taught us,ò?? I mentioned that astronomers characterized the makeup of the famous celestial visitor as a ò??dirty snowball.ò?? Once Halley made its most recent close approach, in 1986, the suite of probes that studied it in detail confirmed this view ò?? mostly. It turns out that the cometò??s nucleus is actually less volatile than scientists had suspected, meaning it was made up of more inert material than they thought, making it more of a ò??snowy dirtball.ò??
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