The source of the comet's twin lobes has been a subject of debate for more than a year. Rosetta settled into orbit around the comet, known as 67P/Churyumov-Gerasimenko or 67P, in August 2014 for a long-term study.
In November, Rosetta dispatched a piggyback-riding spacecraft named Philae, which descended to the comet's surface for independent studies. Scientists are still trying to re-establish contact with Philae in hopes of a follow-on mission.
Meanwhile, one of the comet's biggest mysteries has been resolved. Writing in this week's issue of the journal Nature, scientists put to rest theories that the comet's so-called neck region had eroded away, leaving two connected lobes.
Instead, analysis of high-resolution images taken by Rosetta between August 2014 and March 2015 show that the lobes originally were two independent, though similar bodies.
The comet's shape has been described as resembling a rubber duck toy. The "neck" region is where the two bodies collided and eventually merged. "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 meters below the surface," said Matteo Massironi, lead author from the University of Padova, Italy. "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,"Massironi said.
Scientists suspect the bodies merged during the solar system's early years.