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Comet 2P/Encke (P=3.30a) will be visible in amateur equipment during the first weeks of 2017, passing perihelion on Mar. 10. It will loop near the borders of the constellations Pisces/Aquarius/Pegasus, being situated in the evening sky until the first week of March (then expected to be magnitude 7.0). The brightness development should follow – similar to previous apparitions - the unusual formula m = 5.5m + 5×log D + 4.8×r (with the values changing a bit for any apparition). The coma diameter should reach about 4-5' at the end of February. Whereas the coma should be extremely diffuse in December/January, it will condense thereafter, expected to be strongly condensed during the days the comet disappears.
Based on 100 observations from 38 observers, the brightness development is well documented. It may be best described by the non-standard formula
m = 6.5 mag + 5×log D + 4.3×rwhich, however, is typical for this comet. This yields a perihelion brightness of magnitude 7.0. However, at this apparition the standard parameters m0=10.7 mag / n=3 (yielding a maximum brightness of 6.2 mag at perihelion) work only slightly worse. The maximum brightnesses actually observed were 7.8 mag pre-perihelion (on Mar. 1) and 7.5 mag post-perihelion (on Mar. 23).
Total Brightness and Coma DiameterThe apparent coma diameter increased slowly from almost 2' during the first days of 2017 to the maximum value of slightly above 3' at the end of March. At the end of April it was estimated to be 1.5'. The absolute coma diameter decreased constantly throughout the apparition from 180.000 km to 80.000 km. The coma condensed significantly, with the degree of condensation increasing from DC 1-2 at the start of 2017 to DC 7 during the last days of February. Thereafter it became more and more diffuse, with the degree of condensation decreasing from DC 6-7 to DC 3. As usual a tail was not reported visually.
Andreas Kammerer