Comet Encke — Because of its proximity to the Sun and its stable orbit, comet Encke is probably one of the most evolved comets that still remains active. It may represent a transition object between an active comet and the defunct comets that are thought to make up a significant percentage of the near-Earth asteroid population.
The orbit of comet Encke is very stable both in its size and orientation because it can pass no closer to Jupiter than about 0.9 AU. This orbital stability together with the associated Taurid meteor showers that have been observed for a few thousand years suggest that comet Encke has remained in its present orbit for several millennia.
With its short period (the shortest at 3.3 years), and one of the smallest perihelion distances (0.3 AU), comet Encke has likely evolved more than other active periodic comets.
Beginning with its discovery in 1786, comet Encke has had a long observational history. Spectral observations have been made from ground-based telescopes as well as from instruments on board the OGO-5, IUE, and PVO spacecraft.
This comet has been extensively observed in wavelength regions as diverse as radio, radar, infrared, visible, and ultraviolet. This set of comprehensive observations has allowed modeling efforts to estimate an OH production rate approaching 1029 molecules per second near perihelion and about 6 x 1027 molecules/sec at the suggested encounter time.
A’Hearn et al. (1983) noted that the production rate of OH drops by a factor of 3 at 0.75 AU pre-perihelion and that all measured production rates are lower post-perihelion than at similar pre-perihelion heliocentric distances.
Although a lack of a continuous spectrum indicates a very low density of micron-sized dust, the meteor showers associated with comet Encke as well as the dust trails detected in Encke’s orbital path by the IRAS satellite suggest that larger dust particles are being emitted.
Fernandez et al. (2000) suggest that the nucleus is elongated (at least one aspect ratio is 2.6), its radius is 2.4 km, and the nucleus rotation period is likely to be 15.2 hours, a result that agrees with the earlier determination by Luu and Jewitt (1990).
Using observations of Encke’s coma morphology, Z. Sekanina (1991) suggests that the nucleus of comet Encke currently has its spin axis nearly in its orbit plane and that an active area is present at both polar regions.
He attributes the persistent pre-perihelion fan-like coma to a collimated stream of material arising from a deep vent that sinks below the surface by 500 to 1000 meters. The suggested pre-perihelion spacecraft encounter time may offer a view of a very evolved (possibly quite bizarre) cometary landscape.