The phrase "cometary nuclei" refers to the solid center or core of a comet. It is the condensed part of a comet that contains ice, dust, and other materials. Full definition
Over the coming month the European Space Agency's (ESA) Rosetta mission will fire its main engines no less than eight times to tweak its interplanetary intercept course with Comet Churyumov - Gerasimenko; eventually sidling up to the 4 kilometer wide cometary nucleus at about 7.9 meters per second in early August. (scientificamerican.com)
The image shows the nucleus, about five kilometers across, and unlike other cometary nuclei, it seems to be round. (scientificamerican.com)
The authors suggest that the cyclic sublimationcondensation of ice triggered by varying illumination conditions may be a general process acting on cometary nuclei. (nature.com)