On the non-gravitational forces in the 1986 return of P/Halley

In this paper we study the time evolution of the non-gravitational forces (NGF) along with the observational bias, during the 1986 return of comet Halley. First, evidence is presented which shows that a NGF model with constant coefficients along the radial and transverse directions (A1 andA2), and a constant coefficient radial-only observation bias model fails to represent the observations within the conservative optical measurement noise of one second of arc. Second, we present a stochastic approach to the problem, where coefficients for the radial, transverse and normal components of both the NGF and the observation bias, are allowed to vary in time as three statistically independent Gauss-Markov processes. Finally, an orbit is estimated with this model, to fit observations made during the last apparition, employing a stochastic optimal smoother.Results are given in terms of time histories of the coefficient estimates along with their smoother computed uncertainties. A plot of the observation residuals is also included, showing a uniform and unbiased behavior. The analysis of the results confirms some of the assumptions often made when modelling cometary motion, but questions others. In particular, the normal bias coefficient shows an unexpected pre-perihelion peak (–1200±250 km) which proves that a radial-only observation bias model may lead to biased orbital estimates and unrealistic computed uncertainties.