Comets eject gas and dust into space. Primarily, this takes place on the areas of the comet's surface exposed to direct sunlight. In mid-March, from a distance of 75 kilometres, the Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) on board the Rosetta orbiter acquired images of an extraordinary phenomenon occurring on Comet 67P/Churyumov-Gerasimenko. A new dust jet – its origins currently unknown – was suddenly released from the shadowed side. "This is the first time we have been able to observe the moment at which a new dust jet is born," says comet researcher Ekkehard Kührt, who is in charge of the scientific contribution to the ESA Rosetta mission at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). "These observations will help decipher cometary activity that we have been unable to fully understand thus far."
Over the recent weeks, Comet 67P/Churyumov-Gerasimenko has become increasingly active. As it draws closer to the Sun, its surface is heating up and releasing larger quantities of gas that carry dust particles into space. Even though its closest approach to the Sun will be in approximately four months, 67P is already enveloped by a coma. The dust escaping from its surface is clearly visible across the entire daylight side.
The dust jet recently discovered on the shadowed side of the comet poses entirely new questions for the scientists. "It is exciting to think about how comets can become active on their shadowed sides," says Jörg Knollenberg, a DLR comet researcher and a scientist on the OSIRIS team. "However, it is quite possible that rays of sunlight fell on areas previously hidden behind mountain ridges. We will need to continue observing and conducting measurements before we can come up with a plausible explanation," Knollenberg continues.
The dust disperses at eight metres per second
This unique observation also provides the scientists with an opportunity to study the dust dispersal process. "We analysed the brightness fluctuations along the unexpected dust jet and were able to estimate that the particles are travelling away from the comet at a velocity of at least eight metres per second," says Knollenberg. This confirms previous measurements on the ejection of dust from the daylight side of the comet.
Rosetta is a European Space Agency mission with contributions from its Member States and NASA. The Rosetta lander, Philae, is contributed by a consortium led by DLR, the Max Planck Institute for Solar System Research (MPS), the French Space Agency (Centre National d'Etudes Spatiales; CNES) and the Italian Space Agency (Agenzia Spaziale Italiana; ASI).
OSIRIS was developed 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.