Comet 46P/Wirtanen, time-exposure optical image. The comet's nucleus (centre) is surrounded by its coma (diffuse green glow). The green colour is caused by molecules such as cyanogen and diatomic carbon, that are ionised by sunlight as the comet approaches the Sun. This comet reached perihelion (closest point to the Sun) on 12 December 2018. This comet, which has an orbital period of 5.5 years, was previously the original target for the Rosetta mission. This image was obtained on 6 December 2018, using a telescope at ESA's European Space Astronomy Centre (ESAC) near Madrid, Spain.