Images sent back by ESA's Rosetta comet probe show that it wasn't only thing orbiting comet 67P/Churyumov-Gerasimenko (67P/C-G). A comparison of images taken four years ago when the comet was closest to the Sun shows a piece of debris about 4 m (13 ft) in diameter circling 67P/C-G like a mini-moon.

Though the Rosetta mission ended on September 30, 2016, the massive treasure trove of data sent back by the unmanned deep-space explorer is still paying dividends – some of them quite unexpected. A case in point is a series of images taken by the spacecraft as it neared 67P/C-G in late July and August 2015.

At this time, the comet was at perihelion, or the point in its orbit when it comes closest to the Sun. It's also when a comet is most active as the sunlight heats up the interior, causing the subsurface ice to flash into gas that jets out, carrying with it clouds of dust and debris. It's a time when the comet is literally boiling away, so it's not surprising when larger chunks get mixed up in the dust motes.

It was one of these chunks that was spied earlier this year by Spanish astrophotographer Jacint Roger when he was going through the Rosetta image archives – specifically, images taken by the spacecraft's OSIRIS narrow-angle camera on October 21, 2015 when it was 400 km (250 mi) from 67P/C-G. When he converted some of these into an animated GIF, he saw an object orbiting the comet.

According to ESA, the mini-moon, unofficially named "Churymoon" by researcher Julia Marín-Yaseli de la Parra, is now being studied in more detail. Computer models suggest that the object was ejected from 67P/C-G and spent its first 12 hours of independence orbiting at a distance of between 2.4 and 3.9 km (1.5 and 2.4 mi) from the comet's center. It then passed through the coma, which made it hard to see, but was later seen on the opposite side, indicating that it was, indeed, orbiting the comet until around October 23, 2015.

Though other bits of debris have been seen in the vicinity of 67P/C-G, ESA says that this is the largest one yet found and will be the subject of further study.

The Rosetta mission lifted off atop an Ariane 5 rocket on March 2, 2004 at the Guiana Space Centre in French Guiana. It arrived at 67P/C-G on August 6, 2014 and was the first mission to escort and orbit a comet as it approached the Sun on a highly elliptical orbit. After two years of studying 67P/C-G, which included the releasing of a small lander, the mission was terminated when the orbiter ran out of propellant and it was ordered to set down on the comet in a controlled landing before deactivating itself.

Source: ESA

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