On 14 February 2015, the Optical, Spectroscopic and Infrared Remote Imaging System (OSIRIS) on the Rosetta spacecraft observed the surface of comet 67P Churyumov-Gerasimenko with the Sun directly behind it, so the only shadow seen in the image is that of the photographer, the orbiter itself. "An image with this arrangement of the light source and camera really reveals the differences in brightness on the comet's surface. As there are no shadows, this difference must be due to the scattering of the light by the dust particles across the comet's surface," explains Ekkehard Kührt, a cometary researcher at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) and a scientist on the OSIRIS team. "This means, for instance, that it is possible to calculate the sizes of the dust particles spread across the surface, even though these are far below the camera's resolution threshold." The image was acquired as the orbiter passed over the comet at an altitude of just six kilometres.
Smooth plains and rugged areas
Measuring 228 by 228 metres, the terrain shows abruptly terraced steps separating flat ground from fissured areas. "The camera system is looking straight down from above, which makes estimating the actual height of the terraces quite difficult." Scientists have given this region, which is situated not far from the equator of the larger part of the comet nucleus, the name Imhotep. Unfortunately, it is on the opposite side to Philae's landing site, which means the scientists were denied the possibility of discovering the landing craft's location during this overflight. Rosetta's shadow is visible as a dark patch in the lower half of the image. The adjacent surface of the comet is brighter than the rest, as here the comet, the orbiter and the Sun are aligned precisely along the same axis.
The low-altitude overflight of 67P Churyumov-Gerasimenko did not last long; by 17 February 2015, the Rosetta orbiter had already reached a distance of 253 kilometres from the comet, before returning to within 76 kilometres on 25 February. It re-examined the surface from an altitude of 110 kilometres on 28 February.
Up close to the comet
The OSIRIS image of 14 February – the day the orbiter came closest to the comet – has a resolution of 11 centimetres per pixel. So far, only the Rosetta Lander Imaging System (ROLIS), installed on the bottom of the Philae lander, has been able to acquire higher resolution photographs of the comet's surface, taking images at a resolution of four centimetres per pixel as it descended towards 67P Churyumov-Gerasimenko. Scientists are currently analysing photographs of the comet's surface, which were taken immediately after landing using artificial light. It is hoped that these images, which have a resolution of less than one millimetre per pixel, will provide definitive information on the celestial body’s fine structure. Preliminary results are expected in April 2015.
Rosetta is an ESA mission with contributions from its member states and NASA. Rosetta's Philae lander is funded by a consortium headed by DLR, the Max Planck Institute for Solar System Research (MPS), CNES and the Italian Space Agency (ASI).
The OSIRIS camera was built by a consortium led by the Max Planck Institute for Solar System Research (Germany), in collaboration with the Center of Studies and Activities for Space (CISAS) at the University of Padua (Italy), the Laboratoire d'Astrophysique de Marseille (France), the Institute of Astrophysics of Andalusia (IAA-CSIC) (Spain), ESA's Scientific Support Office, the National Institute for Aerospace Technology (Instituto Nacional de Técnica Aeroespacial) (Spain), the Technical University of Madrid (Universidad Politécnica de Madrid) (Spain), the Department of Physics and Astronomy at Uppsala University (Sweden), and the Institute of Computer and Network Engineering at the Braunschweig University of Technology (Germany). OSIRIS was funded by national agencies in Germany (DLR), France (CNES), Italy (ASI), Spain (MEC) and Sweden (SNSB), as well as ESA's Technical Directorate.
ROLIS was developed by the DLR Institute of Planetary Research in Berlin.