NAVCAM’s shades of grey

Ever since early August, when Rosetta rendezvoused with Comet 67P/Churyumov-Gerasimenko at a distance of roughly 100 km, the on-board navigation camera (NAVCAM) has been returning images that depict the many different facets of its nucleus. A complex surface emerges from these images, revealing valleys, cliffs, boulders, and craters all over the comet. NAVCAM takes black-and-white images and the surface of 67P/C-G shows a wide dynamic range of light and dark regions, depending on the illumination conditions and surface characteristics at any given area. But what do “light” and “dark” mean for an object like 67P/C-G? Followers of this blog have asked this and similar questions, so here are some details on how NAVCAM images are taken and displayed to make a wide range of surface features possible. Let’s start with the light available to take pictures by. At present, 67P/C-G and Rosetta are out beyond the orbit of Mars and the Sun is roughly only 10% as bright as they would see if they were in orbit around the Earth. In addition, the surface of comets can be very dark, reflecting less than 10% of the light that falls on them – something that has been known since ESA’s Giotto flyby of Comet 1P/Halley in 1986. The technical term used is that comet nuclei have a very low ‘albedo’. For 67P/C-G in particular, astronomers have combined visible light data from the Hubble Space Telescope and ground-based observatories, with infrared data from Spitzer and WISE, to determine that it has an albedo of just 4–6%, as dark as charcoal. So combining these two facts, there’s not that much light coming from 67P/C-G with which to take a picture. But just as you would do in dimly lit situations on Earth, that can be overcome by using a longer exposure time. In […]



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