By now, the scenario is familiar: a distant light in the spacecraft's cameras becomes a fuzzy blob, which brightens and grows until the craft is suddenly plunging through an ionized fog. Enveloped in haze, the camera spies a dark, frozen lump - the elusive nucleus of a comet, one of the strangest and least understood bodies in the Solar System. Since a battery of probes whizzed past comet Halley in 1986, the nuclei of four different comets have been successfully imaged and studied during fly-bys (see'A gallery of surprises'). But rather than building up a simple and satisfying stereotype of what comets are like, these encounters have revealed a surprisingly diverse array of features and processes. If all goes well, on 4 November, the cometary repertoire will grow by one more, when the NASA spacecraft EPOXI passes within 700 kilometres of comet Hartley 2 (see 'How to catch a comet').
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