Comet 46P/Wirtanen from Madrid
An image of Comet 46P/Wirtanen taken by Wouter Van Reeven at ESA's European Space Astronomy Centre (ESAC) near Madrid, Spain, on 6 December 2018.
The comet nucleus is at the core of the brightest spot at the centre of the image, and the green diffuse cloud is its coma. The green colour is caused by molecules – mainly CN (cyanogen) and C2 (diatomic carbon) – that are ionised by sunlight as the comet approaches the Sun. A hint of the comet's tail is visible to the upper left. The diagonal stripes are star trails.
The comet reached perihelion, the closest point to the Sun along its orbit, on Wednesday 12 December, and is on the way to its closest approach to Earth this weekend, when it might become visible to the naked eye from dark locations.
A bright comet with a period of 5.5 years, 46P had been chosen in the 1990s as the target of ESA's Rosetta mission. However, a launch delay prompted the mission team to select a new target, Comet 67P/Churyumov-Gerasimenko, which was visited by Rosetta between 2014 and 2016.
This image is a composite of 120 individual images, each with a 15 second exposure, using a William Optics ZS 71 ED (71 mm refractor) telescope and a Canon EOS 700D DSLR camera (ISO: 3200). The field of view spans 2 degrees × 1.5 degrees.