The standard word for 'comet' is
彗星 —
彗 means 'broom' (the comet's tail looks like a broom sweeping the sky).
慧 means 'wisdom' and is a frequent misspelling because the two characters look almost identical and share the pinyin huì. CC-CEDICT lists
慧星 only as a variant of
彗星. In careful writing, always use
彗星.
