Rosetta: The dark side of the comet

Light scattered from dust particles allows a first glance at the side of comet 67P that has been trapped in the darkness of polar night for the last months.

Rosetta’s scientific imaging system OSIRIS has caught a glimpse of the southern side of comet 67P/Churyumov-Gerasimenko. During the past months, this side has continuously faced away from the Sun making it impossible to determine shape and surface structures. Only the light scattered from dust particles in the comet’s coma very slightly illuminates this uncharted territory.

Since ESA’s space probe Rosetta arrived at comet 67P/Churyumov-Gerasimenko in August, the scientific camera system OSIRIS has mapped most of its surface revealing stunning structures such as steep ravines, sharp cliffs and numerous boulders. 67P’s southern side, however, is still a mystery. As the comet’s rotation axis is not perpendicular to its orbital plane, but is tilted, parts of its surface can at times remain in total darkness. During the past months, 67P’s southern side has seen such a polar night, comparable to the weeks of complete darkness in Earth’s polar regions.

At the same time, 67P’s dark side promises to hold the key to a better understanding of the comet’s activity. “During perihelion, when 67P comes within approximately 186 million kilometers of the Sun, the comet’s southern side is illuminated and thus subjected to especially high temperatures and radiation”, says OSIRIS Principal Investigator Holger Sierks from the Max Planck Institute for Solar System Research (MPS) in Germany. Scientists therefore believe this side to be shaped most strongly by cometary activity. “We can hardly wait until May 2015, when the polar night ends and we can finally take a good look”, says Sierks.

Until then, a recent image offers a first taste of what will come. In this image sunlight backscattered from dust particles in the comet’s coma illuminates the comet’s dark side revealing a hint of surface structures.

“For a normal camera, this tiny bit of scattered light would not help very much”, says OSIRIS team member Maurizio Pajola from the Center of Studies and Activities for Space at the University of Padua in Italy who first spotted the amazing image. Unlike standard cameras that encode information in 8 bits per pixel and can thus distinguish between 256 shades of grey, OSIRIS is a 16-bit-camera. This means that one image can comprise a range of more than 65000 shades of grey – much more than a standard computer monitor can display. “In this way, OSIRIS can see black surfaces darker than coal together with white spots as bright as snow in the same image”, says Pajola.

Scientists from the OSIRIS team exploit this high dynamic range not only to peer into the total darkness of 67P’s polar night, but also to gather information from regions that are only temporarily shaded in certain images.

Rosetta is an ESA mission with contributions from its member states and NASA. Rosetta's Philae lander is provided by a consortium led by DLR, MPS, CNES and ASI. Rosetta will be the first mission in history to rendezvous with a comet, escort it as it orbits the Sun, and deploy a lander to its surface.

The scientific imaging system OSIRIS was built by a consortium led by the Max Planck Institute for Solar System Research (Germany) in collaboration with CISAS, University of Padova (Italy), the Laboratoire d'Astrophysique de Marseille (France), the Instituto de Astrofísica de Andalucia, CSIC (Spain), the Scientific Support Office of the European Space Agency (The Netherlands), the Instituto Nacional de Técnica Aeroespacial (Spain), the Universidad Politéchnica de Madrid (Spain), the Department of Physics and Astronomy of Uppsala University (Sweden), and the Institute of Computer and Network Engineering of the TU Braunschweig (Germany). OSIRIS was financially supported by the national funding agencies of Germany (DLR), France (CNES), Italy (ASI), Spain (MEC), and Sweden (SNSB) and the ESA Technical Directorate.