CHANGE IN THE ROTATIONAL PERIOD OF COMET-P TEMPEL-2 BETWEEN THE 1988 AND 1994 APPARITIONS/

Comets often exhibit activity on their surface in the form of jets. Th ese jets in turn should influence the rotational state of the nucleus, producing torques that may increase or decrease the period. We have a ttempted to detect such changes in the rotational state of comet P/Tem pel 2 since the preperihelion observations of its last apparition in 1 988. This object's sidereal period had been determined by Sekanina (19 91) to be 8(h)55(m)55.2(s) + 0.2(s) for the 1988 data. CCD observation s of P/Tempel 2 made with the 0.9 m telescope on Kitt Peak within a ti me span of 2 months (October and December 1994), postperihelion, show that the rotational period of this object has changed by a small but s ignificant fraction. Fourier transform methods as well as numerical li ghtcurve models for the nucleus of P/Tempel 2 are used in the analysis of our data. We find five possible periods for this apparition (8(h)5 2(m)37(s), 8(h)54(m)29(s), 8(h)56(m)21(s), 8(h)58(m)14(s), 9(h)00(m)07 (s)) which are distinctively different from the period found by Sekani na (1991) for the previous apparition. Due to aliasing, we cannot choo se among these five periods. These periods represent both spin-up and spin-down solutions. The minimal period change is 26 sec +/- 10 sec. F uture observations during the next apparition preperihelion, using bet ter sampling, should easily confirm and constrain our result. This is the first direct evidence for a period change based on nuclear lightcu rves of a comet, We also discuss Comet Levy (1990c) and the Great Come t of 1744, for which changes in periodicity have been reported in the literature. (C) 1996 Academic Press, Inc.