Figure 2. Derived mid-infrared dust emission rate vs heliocentric distance for the 9 comets in our survey. There is a general trend of steeper slope and lower absolute emission rate for periodic comets versus new or long period comets. A possible selection effect that may affect the results is our ability to detect and observe fainter periodic comets due to their known positions and orbits - although the 4 comets from the COBE dataset were detected in an unbiased all-sky survey.
particles, the emitted surface area (and optical brightness) is dominated by either 0.1-10 ¡xm particles in "dusty comets" like lP/Halley or > 10 um particles in "gassy comets" like 2P/Encke. The morphology of the coma and tail for the dusty comets is typically "normal", i.e. dominated by a bright, linear tail pointing roughly in the anti-solar direction; this tail is absent or weak and in addition to a curved tail pointing along the direction of orbital motion in the "gassy" comets.
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