  An improved forced-precession model of Comet 46P/Wirtanen is presented. The
nongravitational motion of the comet has been investigated based on the forced
precession model of the rotating cometary nucleus. The least squares method
applied to observational equations allows us to determine six basic
nongravitational parameters: A, eta, I, phi, f_p, and s together with six
orbital elements. The solutions were obtained with additional assumptions: 1.
The cometary activity with respect to the perihelion is asymmetric and, 2a. The
time shift parameter which describes the displacement of maximum activity with
respect to the perihelion could change its value during the investigated
apparitions or 2b. The real activity of the comet changed during the time
interval considered. All eight recorded apparitions of the comet were linked
using all astrometric observations covering the period 1948-1997 with a mean
RMS residual of 1.6". According to the best solution, the nucleus of Comet
Wirtanen is oblate along the spin-axis with a ratio of equatorial to polar
radius of about R_a/R_b=1.1. This precession model yields a value of 4.9 hrs/km
for the ratio of P_rot/R_a. Assuming that the nucleus radius of 46P/Wirtanen
most likely is in the range of 0.5 - 2.0 km we obtain a range 2.5 - 10.0 hours
for the rotational period.

Astrophysics