Observations of comet 9P/Tempel 1 and deep impact by the OSIRIS cameras onboard Rosetta

The OSIRIS cameras onboard ESA's Rosetta spacecraft observed comet 9P/Tempel 1 near-continuously from 28 June until 14 July 2005. The Narrow Angle Camera (NAC) monitored the cometary dust, while the wide angle camera (WAC) observed through filters sensitive to emissions of OH, CN, OI, Na, and the associated continuum. The brightness of the dust increased by a factor of 4.5 within 30 minutes, measured in a circular aperture of 3000 km radius centered on the nucleus. The brightening is likely caused by ejection of material from the crater, decreasing optical depth of the ejecta, and the evaporation of icy grains in the ejecta cloud. The dust moved with a typical velocity of 160 ms-1, suggesting acceleration of the dust by gas in the inner cometary coma. Images of the dust cloud taken several days after the impact show the effect of solar radiation pressure. The size distribution derived from an analysis of the radiation pressure is comparable to that commonly observed in cometary comae. The increase of the intensity of the OH emission due to the impact corresponds to a water production of (1.5-3)x10(+32) molecules. The dust production depends on the uncertain amount of large dust grains ejected by the impact, but the dust/gas ratio is likely to be larger than one. © 2009 Springer-Verlag Berlin Heidelberg.