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In this cropped and zoomed detail the coma is about 2 arc min wide, if we force the contrast the coma grows to double this size, see Hergenrother's post introducing comet Hartley 2
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The coma profile above the background level (thin red line) is about 4 arc minutes wide.
A false colour rendition.
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Comet Hartley 2 in its current (2010) return will pass at only 0.12 AU from Earth on October the 20th.
The Deep impact spacecraft will approach 103P Hartley at just 1000 Km on November the 4th.
References :
Gary Kronk's Cometography, Wiki, BAA