228 Minor Planet Bulletin 39 (2012) ROTATION PERIOD DETERMINATION FOR 482 PETRINA Frederick Pilcher Organ Mesa Observatory 4438 Organ Mesa Loop Las Cruces, NM 88011 USA Andrea Ferrero Bigmuskie Observatory (B88) Via Italo Aresca 12, 14047 Mombercelli-Asti ITALY Julian Oey Kingsgrove Observatory 23 Monaro Ave. Kingsgrove, NSW 2208 AUSTRALIA (Received: 1 July Revised: 21 August) A consortium of observers from Australia, Europe, and North America have obtained full phase coverage of 482 Petrina, and from these observations find a rotation period of 11.794 – 0.001 hours, amplitude 0.10 – 0.02 mag, with a highly unsymmetrical but bimodal lightcurve. Previous rotation period and amplitude determinations for 482 Petrina all obtained different results: Behrend (2002), 18 hours and a single four hour lightcurve showing a minimum followed by a steep rise; Buchheim (2007), 15.73 hours, 0.48 magnitude, based on 2 nights 2006 March 22, 24 and showing two maxima and the intervening minimum but only half of the lightcurve; and Stephens (2009), 9.434 hours, 0.06 magnitude, with a nearly symmetric lightcurve based on 10 nights 2007 July 23 - Aug. 3. One should note that the suggested 15.73 hour period is slightly less than 2/3, and 9.434 hours is slightly less than 2/5, respectively, that of Earth. In both cases the phase observable on alternate nights from a single location circulates slowly to the right. It could be interpreted as alternate sides of a symmetric lightcurve seen on successive nights. Observations by first author Pilcher on the first six nights 2012 May 26 - June 3 covered only the phases 0.00 to 0.65 in the lightcurve. These include only the maxima and the wide minimum. They appeared to be consistent with a 15.73 hour period. Starting June 8 the shallow and narrow minimum was observed. This observation ruled out the 15.73 hour period and suggested a period near 11.79 hours. Andrea Ferrero from Italy and Julian Oey from Australia were invited and kindly contributed additional observations which completed phase coverage within a short time frame. A total of 19 sessions were obtained by the three observers 2012 May 26 - June 26. When phased to a period of 11.794 hours, they show a good fit except for several dips of 0.03 - 0.04 magnitudes, typical duration less than one hour. These might suggest satellite events. However no periodicity could be found among them. Several occur near the start or end of a long session at lower altitude where photometric accuracy is reduced. Hence we consider them very likely to be spurious. Combining these new results with those of Buchheim (2007) and Stephens (2009) provides much information about the rotational properties of 482 Petrina. It can be shown that the data presented in the 15.73 hour period lightcurve by Buchheim (2007) and in the 9.434 hour period lightcurve by Stephens (2009) are both consistent with a period of 11.794 hours. Buchheim’s