We analyze a set of 760 475 observations of 333 026 unique main-belt objects obtained by the Pan-STARRS1 ( PS1 ) survey telescope between 2012 May 20 and 2013 November 9 , a period during which PS1 discovered two main-belt comets , P/2012 T1 ( PANSTARRS ) and P/2013 R3 ( Catalina-PANSTARRS ) .
PS1 comet detection procedures currently consist of the comparison of the point spread functions ( PSFs ) of moving objects to those of reference stars , and the flagging of objects that show anomalously large radial PSF widths for human evaluation and possible observational follow-up .
Based on the number of missed discovery opportunities among comets discovered by other observers , we estimate an upper limit comet discovery efficiency rate of \sim 70 % for PS1 .
Additional analyses that could improve comet discovery yields in future surveys include linear PSF analysis , modeling of trailed stellar PSFs for comparison to trailed moving object PSFs , searches for azimuthally localized activity , comparison of point-source-optimized photometry to extended-source-optimized photometry , searches for photometric excesses in objects with known absolute magnitudes , and crowd-sourcing .
Analysis of the discovery statistics of the PS1 survey indicates an expected fraction of 59 MBCs per 10 ^ { 6 } outer main-belt asteroids ( corresponding to a total expected population of \sim 140 MBCs among the outer main-belt asteroid population with absolute magnitudes of 12 < H _ { V } < 19.5 ) , and a 95 % confidence upper limit of 96 MBCs per 10 ^ { 6 } outer main-belt asteroids ( corresponding to a total of \sim 230 MBCs ) , assuming a detection efficiency of 50 % .
We note however that significantly more sensitive future surveys ( particularly those utilizing larger aperture telescopes ) could detect many more MBCs than estimated here .
Examination of the orbital element distribution of all known MBCs reveals an excess of high eccentricities ( 0.1 < e < 0.3 ) relative to the background asteroid population .
Theoretical calculations show that , given these eccentricities , the sublimation rate for a typical MBC is orders of magnitude larger at perihelion than at aphelion , providing a plausible physical explanation for the observed behavior of MBCs peaking in observed activity strength near perihelion .
These results indicate that the overall rate of mantle growth should be slow , consistent with observational evidence that MBC activity can be sustained over multiple orbit passages .
[ Accepted for publication in Icarus , 2014 Oct 19 ]