Free Astronomy Magazine September-October 2021

SEPTEMBER-OCTOBER 2021 T his image features a comet located in the outer reaches of the Solar System: comet C/2016 R2 (PANSTARRS). As its name suggests, the comet was discov- ered in 2016 by the Pan-STARRS telescopes in Hawai’i. The new image seen here was captured by a project based at ESO’s Paranal Observatory in Chile named the Search for habitable Planets EClipsing ULtra-cOOl Stars − or SPECULOOS for short. This comet is particularly exciting because of the rare compounds and molecules that scientists have detected in its coma: carbon monoxide and nitrogen ions. These compounds give the comet distinctive blue emission lines — so much so that it is nicknamed “the blue comet”. This shy comet only orbits the Sun once every 20,000 years, its most recent approach being in May 2018. This image was taken over a period of time as the telescope tracked the comet’s motion; the bright streaks of light in the background are faraway stars, but the comet and its gaseous coma are all in focus, a testament to the tracking power of SPECULOOS. [ESO/SPECULOOS Team/E. Jehin] in the spectrum of the light from the objects. The Belgian team had spotted weak, unidentified spectral lines in their UVES data and on closer inspection noticed that they were signalling the presence of neutral atoms of iron and nickel. A reason why the heavy elements were difficult to identify is that they exist in very small amounts: the team estimates that for each 100 kg of water in the comets’ atmospheres there is only 1 g of iron, and about