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133P/(7968) Elst-Pizarro

Title: The Hawaii Trails Project: Comet-Hunting in the Main Asteroid Belt
Authors: Henry H. Hsieh (Queen's University, Belfast)

The mysterious solar system object 133P/(7968) Elst-Pizarro is dynamically asteroidal, yet displays recurrent comet-like dust emission. Two scenarios were hypothesized to explain this unusual behaviour: (1) 133P is a classical comet from the outer solar system that has evolved onto a main-belt orbit, or (2) 133P is a dynamically ordinary main-belt asteroid on which subsurface ice has recently been exposed. If (1) is correct, the expected rarity of a dynamical transition onto an asteroidal orbit implies that 133P could be alone in the main belt. In contrast, if (2) is correct, other icy main-belt objects should exist and could also exhibit cometary activity. Believing 133P to be a dynamically ordinary, yet icy main-belt asteroid, I set out to test the primary prediction of the hypothesis: that 133P-like objects should be common and could be found by an appropriately designed observational survey. I conducted just such a survey -- the Hawaii Trails Project -- of selected main-belt asteroids in a search for objects displaying cometary activity. I made 657 observations of 599 asteroids, discovering one active object now known as 176P/LINEAR, leading to the identification of the new cometary class of main-belt comets. These results suggest that there could be ~100 currently active main-belt comets among low-inclination, kilometre-scale outer belt asteroids. Physically and statistically, main-belt comet activity is consistent with initiation by meter-sized impactors. The estimated rate of impacts and sizes of resulting active sites, however, imply that 133P-sized bodies should become significantly devolatilised over Gyr timescales, suggesting that 133P, and possibly the other MBCs as well, could be secondary, or even multigenerational, fragments from recent breakup events.

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