Extinction in the Coma of Comet 17P/Holmes

On 2007 October 29, the outbursting comet 17P/Holmes passed within 0.79" of a background star. We recorded the event using optical, narrowband photometry and detect a 3%-4% dip in stellar brightness bracketing the time of closest approach to the comet nucleus. The detected dimming implies an optical depth τ ≈ 0.04 at 1.5" from the nucleus and an optical depth toward the nucleus center τ_n < 13.3. At the time of our observations, the coma was optically thick only within ρ <~ 0.01" from the nucleus. By combining the measured extinction and the scattered light from the coma, we estimate a dust red albedo ρ_d=0.006±0.002 at α=16° phase angle. Our measurements place the most stringent constraints on the extinction optical depth of any cometary coma.