I should have clarified that the 41.8 MWm/2 is the power received at the comet (and not absorbed) and that the Sun is assumed to be a point source. In fact at a perihelion distance of 0.0057 AU (855,000 km), the comet will only be 155,000 km above the photosphere of the Sun (radius 700,000 km) and so the Sun will subtend an angle of about 70 deg – some sight.
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