Comet 67P was formed by collision of two smaller comets

BBC : New data from the European Space Agency’s Rosetta spacecraft indicate that comet 67P/Churyumov–Gerasimenko was probably formed by the fusion of two smaller cometesimals. The comet’s two-lobed shape, which has been variously described as resembling a rubber duck or a barbell, has perplexed astronomers since it was first imaged in July 2014. They had assumed that the odd shape was formed either by erosion or by the merging of two smaller comets. New high-resolution images now support the latter theory. According to a study published Monday in Nature, each of the two lobes is enveloped by material organized into stratified layers, such that the lobes resemble two onions stuck together. That conclusion was reinforced by comparisons of their orientations with measurements of the local gravity field. Furthermore, the lack of any significant damage indicates that the two cometesimals must have collided at a fairly low velocity. The finding is significant in that it runs counter to the earlier theory that comets only form from centimeter- or meter-sized objects.