![](\"http:\/\/spaceweekly.com\/wp-content\/uploads\/2019\/08\/An_unexpected_companion_small.jpg\")
<\/p>\n<\/p>\n
\nLast week marked five years since\u00a0ESA\u2019s Rosetta<\/a> probe\u00a0arrived at its target, a comet named 67P\/Churyumov-Gerasimenko (or 67P\/C-G).\u00a0Tomorrow, 13 August, it will be four years since the comet, escorted by Rosetta, reached its perihelion<\/a> \u2013 the closest point to the Sun along its orbit. This image, gathered by Rosetta a couple of months after perihelion, when the comet activity was still very intense, depicts the nucleus of the comet with an unusual companion: a chunk of orbiting debris (circled).\n<\/p>\n \nComet 67P\/C-G is a dusty object. As it neared its closest approach to the Sun<\/a> in late July and August 2015, instruments on Rosetta recorded a huge amount of dust enshrouding the comet. This is tied to the comet\u2019s proximity to our parent star, its heat causing the comet\u2019s nucleus to release gases into space, lifting the dust along. Spectacular jets<\/a> were also observed, blasting more dust away from the comet. This disturbed, ejected material forms the \u2018coma\u2019<\/a>, the gaseous envelope encasing the comet\u2019s nucleus, and can create a beautiful and distinctive tail.\n<\/p>\n \nA single image from Rosetta\u2019s OSIRIS<\/a> instrument can contain hundreds of dust particles and grains<\/a> surrounding the 4 km-wide comet nucleus. Sometimes, even larger chunks of material left the surface of 67P\/C-G \u2013 as shown here.\n<\/p>\n \nThe sizeable chunk in this view was spotted a few months ago by astrophotographer Jacint Roger<\/a>\u00a0from Spain, who mined the Rosetta archive, processed some of the data, and posted the finished images on Twitter<\/a> as an animated GIF. He spotted the orbiting object in a sequence of images taken by Rosetta\u2019s OSIRIS narrow-angle camera on 21 October 2015. At that time, the spacecraft was at over 400 km away from 67P\/C-G\u2019s centre. The animated sequence is available for download here<\/a>.\n<\/p>\n \nScientists at ESA and in the OSIRIS instrument team are now looking into this large piece of cometary debris in greater detail. Dubbed a \u2018Churymoon\u2019 by researcher Julia Mar\u00edn-Yaseli de la Parra, the chunk appears to span just under 4 m in diameter.\n<\/p>\n \nModelling of the Rosetta images indicates that this object spent the first 12 hours after its ejection in an orbital path around 67P\/C-G at a distance of between 2.4 and 3.9 km from the comet\u2019s centre. Afterwards, the chunk crossed a portion of the coma, which appears very bright in the images, making it difficult to follow its path precisely; however, later observations on the opposite side of the coma confirm a detection consistent with the orbit of the chunk, providing an indication of its motion around the comet until 23 October 2015.\n<\/p>\n \nScientists have been studying and tracking debris around 67P\/C-G<\/a> since Rosetta\u2019s arrival in 2014. The object pictured in this view is likely the largest chunk detected around the comet, and will be subject to further investigations.\n<\/p>\n