Like any parent confronting the prospect of twins, the surprising discovery presents complications.
Rosetta includes a small lander named Philae that is designed to harpoon itself to the comet's body several months after the mother ship reaches orbit in August. The comet's odd shape, however, could make finding a suitable landing spot more difficult.
ANALYSIS: Comet Heating Up as Rosetta Spacecraft Closes In
"This form restricts potential landing zones," Philae navigator Eric Jurado is quoted as saying in a CNES press release.
At the time of landing, slated for November 2014, the comet will be about three times as far from the sun as Earth and heading inward. After about three months, heating from the sun most likely will end Philae's mission.
The Rosetta mothership, however, which will be conducting both independent and collaborative studies with the lander, is expected to remain operational at least until the end of 2015.