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Rosetta's 'rubber duck' comet was once two separate comets

Scientists come to a fascinating conclusion about how Comet 67P got its unusual two-lobed shape.

Rubber ducky, you're the one that makes space time lots of fun. ESA/Rosetta/Navcam - CC BY-SA IGO 3.0

Even by comet standards, Comet 67P/Churyumov-Gerasimenko is a bit goofy-looking. It's most often compared to a rubber duck when people try to describe its shape.

The comet is the focus of the European Space Agency's Rosetta mission and scientists have been puzzling over its distinctive look since it first came into the spacecraft's view in July 2014.

Two main theories have competed with each other: either one big comet eroded in an odd way to form 67P's "neck," or two comets merged in the long-distant past. The mystery has now been solved.

"Two comets collided at low speed in the early Solar System to give rise to the distinctive 'rubber duck' shape," the ESA announced Monday. Scientists published a paper on the findings online in the journal Nature under the evocative title "Two independent and primitive envelopes of the bilobate nucleus of comet 67P."

"It is clear from the images that both lobes have an outer envelope of material organised in distinct layers, and we think these extend for several hundred metres below the surface," Matteo Massironi, lead author and Rosetta mission team member, said in a statement. "You can imagine the layering a bit like an onion, except in this case we are considering two separate onions of differing size that have grown independently before fusing together."

It took a considerable amount of research using detailed images sent back by Rosetta along with 3D models to identify the comet's split personality.

Rosetta first launched in 2004 on a decade-long journey to reach Comet 67P/Churyumov-Gerasimenko and study the icy space object up close. The mission is expected to wrap up in 2016, when the ESA may land Rosetta on the comet's surface to spend the rest of its days riding the rock through the cosmos.