Космическая пыль комет: состав, размеры и происхождение

Cosmic dust that originates from a comet
Comet dust refers to cosmic dust that originates from a comet. Comet dust can provide clues to comets' origin. When the Earth passes through a comet dust trail, it can produce a meteor shower.

The majority of dust from comet activity is sub micrometer to roughly micrometer in size. However, this fraction is short lived, as radiation pressure causes them to blow out of the Solar System or spiral inwards. The next size class is large, "fluffy" aggregates of the above grains. These are typically 20 100 micrometers, a size not arbitrary but observed as the porous aggregates tend to fracture or compact. Larger particles are micrometeoroids, not dust. In the absence of a definition from the IAU, groups devised their own definitions of dust: smaller than 100 micrometers, 50, 40, 30, and 20 microns, some overlapping or self conflicting. The IMO noted the new definition, but still displays a prior definition on their site. The Meteoritical Society site retains its prior definition, 0.001 cm. The AMS has posted no rigorous definition.

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. 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.

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" Encke actually emits most of its solid mass as meteoroids or "rocks,"