no dust can be retained. Therefore, the thickness of dust layers covering the nuclei of a comet can indicate how closely and how often a comet's perihelion travels are to the Sun. If a comet has an accumulation of thick dust layers, it may have frequent perihelion passages that don't approach the Sun too closely. A thick accumulation of dust layers might be a good description of all of the short period comets, as dust layers with thicknesses on the order of meters are thought to have accumulated on the surfaces of short-period comet nuclei. The accumulation of dust layers over time would change the physical character of the short-period comet. A dust layer both inhibits the heating of the cometary ices by the Sun (the dust is impenetrable by sunlight and a poor conductor of heat), and slows the loss of gases from the nucleus below. A comet nucleus in an orbit typical of short period comets would quickly decrease its evaporation rate to the point that neither a coma or a tail would be detectable and might appear to astronomers as a low-albedo near-Earth asteroid. Further assemblages and bodies Dust particles, aided by ices and organics, form "aggregates" (less often, "agglomerates") of 30 to hundreds of micrometers. These are fluffy, due to the imperfect packing of cluster-type (large) dust particles, and their subsequent, imperfect packing into aggregates. The next size category is pebbles, of millimeters to centimeters scale. Pebbles were inferred at 103P/Hartley 2, and imaged directly at 67P/Churyumov-Gerasimenko. Astrophysical use of the word "pebble" differs from its geological meaning. In turn, the next-larger geological term, "cobble," has been skipped by Rosetta scientists. Even larger bodies are "boulders" (decimeter-scale and above) or "chunks." These are rarely seen in the coma, as gas pressure is often insufficient to lift them to significant altitude or escape velocity. The building blocks of comets are the putative cometesimals, analogous to planetesimal. Whether the actual cometesimals/planetesimals were pebble-scale, boulder-scale, or otherwise has been a key topic in Solar System and exoplanet research. (Mis)Use of the term "dust" At best, "dust" is a collective noun for the non-gas portion of the coma and tail(s). At worst, the term is an English usage, understood well by astronomers in the field, but not to the general public, teachers, and scientists from other fields. The larger solids are more properly called "debris" or, for all non-gases, the