In Table 3 we present the nongravitational parameters A1, A2, A3 determined as constant values within sets of three (or four) consecutive apparitions for each of six erratic comets. Temporal variations of most sensitive nongravitational parameter A2 are plotted in Fig. 3 in open circles and thin lines. For three investigated comets (21P/Giacobini Zinner, 32P/Comas Solá and 37P/Forbes) the deceleration detected in the motion changed into acceleration; remaining three comets have negative and variable A2. In our investigations the estimates of A1, A2 and A3can be treated only as a qualitative indication of changes of nongravitational effects within a whole observational time interval, especially at the moments of unexpected changes of trends in behaviour are important for more detailed and more accurate calculations presented in Sects. 4-5. In other words, we show that long-term behaviour of nongravitational parameters resulting from forced precession models may differ from the nongravitational parameters obtained in Marsden's formalism.
Copyright ESO 2001