On 4 July, the world's TV screens were filled with high-fiving NASA astronomers celebrating the Deep Impact mission's direct hit on comet Tempel 1. It was an extraordinary achievement, and fully merited the celebrations. A few weeks later, though, when the cameras had gone, the astronomers were left scratching their heads in confusion. The Deep Impact team had hoped that, when the impactor spacecraft hit Tempel 1, it would kick up a relatively small cloud of dust, expose an area of pristine icy material underneath, and instigate some spectacular jet activity. This is exactly what didn't happen. The dust cloud was more than 10 times bigger than expected, and the effect on Tempel 1's activity was almost nil.
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