"The new images from the OSIRIS camera onboard Rosetta from July 14th are truly amazing!!
In my opinion this is the best case yet for a cometary contact binary, and to see one up close with this particular configuration is extremely fortuitous for the mission. Exciting times ahead. We knew there was something strange in relation to the shape a while back, based on earlier modelling of remote telescope imaging of the nucleus, which we actually thought may have been due to "large concavities", like large impact craters (see http://bit.ly/1k8mfRW). A contact binary shape turned out to be responsible, and to be honest, I was quietly hoping that we would see such a 'bi-lobed' structure.
Several formation scenarios have already been discussed so far to explain its shape, which range from rapid spin-up - perhaps due to surface outgassing - that caused material to leave the surface and coalesce into a satellite which later merged with the larger primary body, to close encounter(s) with a planet that tore the comet apart by tidal forces. A nice discussion is given at the ESA blog site above. All of these are possible and there will be many avenues to explore when the more detailed images start coming in."
Image (above) shows a rotating view of comet 67P/C-G taken on 14 July 2014. Credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA