While 2014 was the year the Rosetta spacecraft celebrated making it into orbit around a comet, 2015 was the year it got down to some serious hard work. Its comet, with the tongue-twisting name 67P/Churyumov–Gerasimenko, made its closest approach (186 million kilometres) to the Sun, a period known as perihelion when the comet would be expected to be at its most active. Rosetta was there to witness this. It saw great streams of dust and water vapour spewing from the comet’s surface and watched the diurnal (day/night) cycle of ice as it sublimated (evaporated) in the sunshine and refroze in the night-time darkness. Rosetta detected a high-proportion of oxygen so ancient that it dates back to before the Solar System was born, and determined that the comet is actually two comets that have gently come together at the join of the comet’s neck. Rosetta saw the collapse of deep pits and sinkholes hundreds of metres across, from which jets spewed out, causing the occasional cometary outburst.
What of Philae, Rosetta’s intrepid little landing craft that bounced across the surface in November 2014 and was lost in the shadow somewhere on the comet? As the changing illumination lit up Philae’s resting place, giving energy to its starved solar panels, the little lander woke up, communicating with mission control on and off, starting on 13 June. Sadly, what little illumination was present proved insufficient to fully awaken Philae and contact was lost again on 9 July.
Inside the magazine
Our top ten greatest stories of 2015 first appeared in the December edition of Astronomy Now.
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