Extinction in the Coma of Comet 17P/Holmes

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