In the course of one orbit around the Sun, the comet Churyumov-Gerasimenko goes through different phases of activity. At a distance of 3.4 astronomical units (AU) a significant increase in brightness can be observed. Shortly before crossing the orbit of Mars the comet has developed it characteristic tail. Departing from the Sun, Churyumov-Gerasimenko is still very active and shows a dust trail, a structure composed of large dust particles emitted during the previous orbits of the comet. This trail can still be discerned at a distances of 4.9 astronomical units from the Sun. Credit: MPS Enlarge

( —On its way towards the Sun comet Churyumov-Gerasimenko, next year's destination of ESA's spacecraft Rosetta, will start emitting gas and dust earlier than previously expected. The comet's activity should be measurable from Earth by March 2014. This is one of the results of a new study performed by a group of researchers under the lead of the Max Planck Institute for Solar System Research (MPS) in Germany. The scientists analyzed numerous images from the comet's past three orbits around the Sun, obtained with ground based telescopes. For the first time, they were able to reconstruct the comet's activity in all phases of its orbit.

 spends the main part of its existence far from the Sun as an unchanged lump of ice and rock. When it approaches the Sun, however, a metamorphosis takes place: highly  vaporize from the nucleus carrying fountains of  with them. These accumulate to form the comet's atmosphere, the coma, and are the origin of its tail, a comet's most characteristic feature. However, the principles governing these processes are still only poorly understood. What instances spark the ejection of gas and dust? How does this activity evolve? And which processes on the surface and within the comet's nucleus are decisive?

Next year, ESA's  Rosetta will try to answer these questions. The space probe is scheduled to rendezvous with comet Churyumov-Gerasimenko in spring, deposit a lander on its surface in the autumn of 2014, and accompany the comet on its way toward the Sun. The mission therefore offers the unique chance to study all phases of the onset of cometary activity from close-up. The new results presented by researchers from the MPS now suggest that Churyumov-Gerasimenko could allow for exciting insights very early in the course of the mission.

"Churyumov-Gerasimenko could be active by March of next year", Dr. Colin Snodgrass from the MPS summarizes the new results. Two months prior to this, in January 2014, the space probe will be awakened from its hibernation phase.

The scientists base their predictions on 31 data sets recorded by them and other professional groups in the years between 1995 and 2010 with telescopes like the Very Large Telescope (VLT) at the European Southern Observatory (ESO). The images show the comet at different points during its orbit and thus in different phases of activity.

"We were able to analyze data from the entire activity-cycle of Churyumov-Gerasimenko with the same method. For the first time, this allows for a meaningful comparison of all data sets", says Snodgrass. "In this way we compiled a comprehensive picture of how the comet's activity develops during its journey around the Sun", his colleague Dr. Cecilia Tubiana from the MPS adds.

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