1.   Frequent breakup and erosion of cometary nuclei provides not only vast quantities of meteoric dust, but also occasional larger fragments.

2.   Astronomers said the echoes clearly showed the presence of a cometary nucleus surrounded by a coma consisting partly of gravel-size particles.

3.   For instance, the Oort Cloud of cometary nuclei surrounding the Solar System is thought to have been formed from matter ejected from the region of Uranus and Neptune.

4.   Other cometary nuclei may be covered with dust, temporarily or permanently.