Comets are the most volatile-rich minor bodies in the solar system. It has been suggested that impacts with comets and asteroids provided Earth with much of its water. Although most comets are less accessible than near-Earth

asteroids, their high water content makes them an economically attractive resource for space mining. The possibility that some near-Earth asteroids are extinct or dormant cometary nuclei means that this water-rich resource may be more accessible than was once thought.

Recent spacecraftand ground-based studies of comets have confirmed and refined Whipple's "dirty snowball" model for cometary nuclei. Cometary material is composed principally of water ice and other ices (including CO, CO₂, CH₄, C₂ H₆, and CH₃ OH) mixed with cosmic dust grains. The passages of most Oort cloud comets through the inner solar system...