Article: "The Puzzle Of The Half Comet-Half Asteroid"

From the article...

In 1996, astronomers identified an extraordinary object orbiting the Sun between Mars and Jupiter in the region best known for its asteroids. And yet this body, called 133P, defied description: it had the orbit of an asteroid and yet was emitting dust like a comet.

Clearly, this is a rare object. After centuries of observation, not a single other object in the asteroid belt has burped gas and dust in the same way.

So how could this have got there? According to Henry Hsieh at Queen's University, Belfast in Northern Ireland, there can be only two explanations. The first is that 133P is a comet that has somehow recently become trapped in an asteroid-like orbit. This would have required a hugely unlikely combination of gravitational kicks from other planets as the comet travelled into the solar system from the Kuiper Belt or Oort cloud.

"The Puzzle Of The Half Comet-Half Asteroid"
05 August 2009

Link: Technology Review Article

Abstract:

The Hawaii Trails Project: Comet-Hunting in the Main Asteroid Belt
Authors: Henry H. Hsieh (Queen's University, Belfast)
(Submitted on 31 Jul 2009)

Abstract: 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 behavior: (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, kilometer-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 devolatilized over Gyr timescales, suggesting that 133P, and possibly the other MBCs as well, could be secondary, or even multigenerational, fragments from recent breakup events.

Link: Citation: The Hawaii Trails Project: Comet-Hunting in the Main Asteroid Belt

Link: Paper: The Hawaii Trails Project: Comet-Hunting in the Main Asteroid Belt [PDF]