All-round activity – CometWatch 25-26-27 February

Today’s CometWatch features not one, but four single-frame NAVCAM images taken between 25 and 27 February at distances around 80 to 100 km from the centre of Comet 67P/Churyumov-Gerasimenko. More details on the distance, size, and resolution for the individual images are provided in the captions below. While most of Rosetta’s NAVCAM images are taken for navigation purposes, these images were obtained to provide context in support of observations performed at the same time with the Alice ultraviolet (UV) imaging spectrograph on Rosetta. Alice makes its observations through a narrow, long slit (you can read about Alice’s first far-UV spectra and find an animation showing the size and shape of its field of view in an earlier blog post), and scientists use NAVCAM images taken at the same time as the spectra to identify where the slit was located on the comet, including which parts were pointing to an illuminated portion of nucleus, which to a region in shadow, or off the nucleus altogether. Earlier in the mission, these NAVCAM context images were taken as strips centred on the position of the Alice slit, but lately full-frame NAVCAM images have been taken in support of the Alice observations instead. The exposure time used for the four images in today’s CometWatch post was roughly 2 seconds. Also, the phase angle was very small at the time, meaning that the Sun was illuminating the comet from behind Rosetta. This geometrical configuration combined with the relatively long exposure time has caused some parts of the nucleus to be slightly over-exposed. The over-exposure is not a concern for Alice, as the scientists only need these images to locate the position of the slit with respect to the comet’s nucleus. However, as the images are partly saturated, we decided to process them in a way […]



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