Update on the Orbital Motion of Encke's Periodic Comet: Nongravitational Deceleration Gradually Gaining

Abstract

The end of an era in cometary astronomy --- as the transition from a nongravitational acceleration to a deceleration in the motion of Encke's comet was advertised in a paper of mine five years ago --- has now turned into a new era, as recent computations of the comet's nongravitational orbits performed independently by S. Nakano (CBAT) and by D. Farnocchia (JPL) appear to suggest. The new values of parameter A_2, compatible both with each other and with the earlier values in my previous paper, show that the magnitude of the deceleration has continued to grow. Critically assessed are the controversial computations at the Minor Planet Center, which contradict the above results and imply that Encke's comet was reverting to the acceleration. A plot of the number of observations used in the orbital solutions as a function of time shows an exponential rise following the introduction of CCD arrays, but not after use of photography became common five or six decades earlier. Dips in observer activity appear during the two World Wars. Because the comet has recently been observed along the entire orbit, the effective times that computed values of the nongravitational parameters apply to had to be redefined.