Principal Investigator: Henry Hsieh, Planetary Science Institute

Program Summary:

We propose a multi-year optical observational campaign to characterize the activity and nuclei of a number of known main-belt comets (MBCs). Our objectives are (1) to monitor and analyze the active phases of seven known MBCs that are expected to exhibit activity between 2016 and 2019, (2) to use these observations to study the evolution of that activity within the same active epoch and also with respect to previous active episodes, and (3) to constrain the physical properties of two MBC nuclei during periods of inactivity. MBCs are extremely rare objects that are dynamically indistinguishable from main-belt asteroids, yet display cometary activity indicative of sublimating ice. Dynamical analyses suggest that MBCs are native to the main belt. The discovery of present-day ice in the asteroid belt is significant considering that dynamical models suggest that icy asteroids may have been a significant primordial source of terrestrial water. We will focus on MBCs whose activity has been identified as strongly likely to be sublimation-driven through observations of repeated activity or dust modeling of single active episodes indicating the action of prolonged emission events. Of the seven objects we expect to observe to be active, four have not yet been confirmed to show repeated activity, and so this program will help to ascertain the source of their activity. For objects for which repeated activity has already been identified, monthly monitoring of later active episodes will allow us to track changes in activity strength over time, helping us to better understand activity lifetimes. The characterization of inactive MBC nuclei provides indications of what characteristics other icy (but inactive) main-belt asteroids might have, as well as important inputs for thermal evolution models. This work will give us valuable insights into the nature and origin of this population, and into volatile abundance and preservation in the inner solar system. Opportunities to study active MBCs are extremely precious as MBCs only exhibit activity over limited portions of their ~6-year orbits. As such, this observational program should be considered extremely urgent, especially as these objects are also considered high-priority targets for potential upcoming NASA missions. This research is currently being funded by a NASA Solar System Observations program (NNX16AD68G; PI: H. Hsieh) for which Gemini observations constitute a central component.

Co-Investigators:

• Masateru Ishiguro, Seoul National University
• Matthew Knight, University of Maryland
• Nicholas Moskovitz, Lowell Observatory
• Scott Sheppard, Carnegie Institution of Washington
• Chad Trujillo, Gemini Observatory