Analysis of the Activity on Main Belt Comet 133P/Elst-Pizarro H. Kaluna (1,2), K. Meech (1,2), H. Hsieh (1) (1) Institute for Astronomy, Hawaii, 2680 Woodlawn Drive, Honolulu HI 96822, USA (2) NASA Astrobiology Institute (Fax: +1-808-956-9850) Abstract We will present an analysis of the mechanisms re- sponsible for the sublimation activity in the main belt comet 133P/Elst-Pizarro. This will include (i) the comparison of the spin pole orientation of 133P to that predicted by models of ice longevity in the asteroid belt, (ii) sublimation modeling of the heliocentric light curve of 133P and (iii) visual monitoring of activity of 133P along its orbit. 1. Introduction Water (and other volatile species) played a key role in the planetesimal formation and alteration, and through dynamical scattering, has led to the current planetary inventory of the ingredients necessary for life. The discovery of the main belt comets (MBCs), which are believed to have water in the form of ice (rather than hydrated minerals known to be present in primitive asteroids), suggests that water ice was more preva- lent in the asteroid belt and has a greater longevity than previously thought. We will explore the activity of 133P/Elst-Pizarro, using physical and photometric properties to gain a better understanding of the ices causing sublimation in this MBC. 2. Sublimation Activity Having shown recurrent activity during the past three perihelion passages, it is now clear that in the case of 133P, the activity is the result of a thermally driven process. To constrain sublimation models of 133P, which attempt to describe the efficiency of heat trans- port through the nucleus, we require knowledge of the distances where sublimation occurs. While 133P is at perihelion and strongly active, taking short exposure images is sufficient to see 133P’s comet like activity, but as it moves further away from the Sun, the activity decreases and requires composite images to look for faint activity. Figure 1: Observations of 133P as a function of true anomaly, with respect to perihelion (P) and aphe- lion (A). Note that 133P remains active nearly 1/3 of the way around its orbit post-perihelion. Data points shown consist of our 2002-2009 observations, as well as data published by several other groups [6]. Using composite images from our 2008 runs, we find that 133P exhibits activity out to a true anomaly of 109 ◦ , shown in Figure 1, extending the baseline over which 133P is known to be active by 20 ◦ . The thermal evolution models by [4] predict that 133P is potentially active at aphelion, so we are conducting a search for faint activity using composite images from 2009-2011, as 133P nears and moves away from aphelion. 3. Pole Orientation One of the physical characteristics critical to models describing the longevity of ice on MBCs is the ori- entation of spin axes. Two models attempt to explain EPSC Abstracts Vol. 6, EPSC-DPS2011-1375, 2011 EPSC-DPS Joint Meeting 2011 c  Author(s) 2011