The european Space Agency’s Rosetta spacecraft arrived at comet 67P/Churyumov–Gerasimenko on 6 August. Launched more than 10 years ago, it has now taken up an orbit around the Sun that parallels the comet’s path, to keep the comet in its cameras from a distance of only a few tens of kilometres. The next two months will see intense preparation for the final stage of the mission: in mid-November, a lander will be sent to the comet’s dark surface with instruments to measure its composition in detail. If all goes well, it will drill about 20cm into the comet, to pierce the dusty crust and reach the icy material beneath.Comets are accumulations of ice and dust. When they approach the sun and warm up, the ice turns to gas, producing the glorious comet tail and a “c