The image on the left was taken on the day of Rosetta’s rendezvous from about 121 kilometres out.
The image on the right was taken exactly a year later from a distance of 261 kilometres. During this time, the comet has moved from almost 540 million kilometres from the Sun last year to around 186 million kilometres this week.
As the intensity of the sunlight has increased 700% over the year, ices have sublimated, turning into gas, and pouring out into space. The gas drags the dust along with it and together they form the comet’s fuzzy atmosphere, or coma.
The comet today reaches its closest point to the Sun along its orbit, known as perihelion. The European Space Agency is celebrating with a Google+ Hangout scheduled for 13:00–15:00 GMT (15:00–17:00 CEST).