Churyumov Gerasimenko is a comet named after two Russian observers. It sparked into life in April and early May but the comet has now gone back to sleep – an extended dust cloud around the nucleus has vanished. It is the first time scientists have actually observed changes in cloud production around comets. It is assumed that as it progressively comes closer to the Sun there will be increasing warmth that will cause surface ices to sublimate and gas to escape from the body of the comet. As the gas flares away it carries with it tiny dust particles which expand into a coma. It is then perceived that as the warming continues to increase there will be more activity on the comet. The solar wind causes material to stream out into a long tail (see http://phys.org/print322463816.html). The Rosetta spacecraft is due to rendezvous with the comet in August. Various tests are underway- including measurement of the chemical make-up of both the surface and interior of the comet. In addition, the plasma environment of the comet will also be assessed as the coma develops and interacts with the solar wind. It will even collect gas and dust from the coma and analyse them in a miniature onboard laboratory.
Meanwhile, at http://phys.org/print322421396.html … Comet Siding Spring is getting closer to Mars. It is producing a lot of dust but in October it will pass within 86,000 miles of Mars – so close that gas and dust being emitted by the comet's coma will interact with the atmosphere of Mars (it is hoped). We may get to see what might have happened if a comet had come close to Earth in the past.