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 \aas@@fstack { \prime \prime } 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 _ { \mathrm { 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 .