A Further Investigation of Apparent Periodicities and the Rotational State of Comet 103P/Hartley 2 from Combined Coma Morphology and Lightcurve Datasets

TL;DR

This study combines ground-based and spacecraft observations of comet 103P/Hartley 2 to analyze its apparent periodicities and rotational state, revealing evolving activity patterns and complex rotation behavior.

Contribution

It provides new measurements of apparent periods, compares ground-based and spacecraft data, and demonstrates the evolution of the comet's rotation and activity during 2010.

Findings

Apparent period increased linearly during the apparition

Change in apparent period slowed or stopped by late November

Lightcurve shape evolved from one cycle to the next

Abstract

We present an analysis of Kitt Peak National Observatory and Lowell Observatory observations of comet 103P/Hartley 2 obtained from August through December 2010. The results are then compared with contemporaneous observations made by the EPOXI spacecraft. Each ground-based dataset has previously been investigated individually; the combined dataset has complementary coverage that reduces the time between observing runs and allows us to determine additional apparent periods at intermediate times. We compare CN coma morphology between ground-based datasets, making nine new measurements of apparent periods. The first five are consistent with the roughly linearly increasing apparent period during the apparition found by previous authors. The final four suggest that the change in apparent period slowed or stopped by late November. We also measure an inner coma lightcurve in both CN and R-band ground-based images, finding a single-peaked lightcurve which repeats in phase with the coma morphology. The apparent period from the lightcurve had significantly larger uncertainties than from the coma morphology, but varied over the apparition in a similar manner. Our ground-based lightcurve aligns with the published EPOXI lightcurve, indicating that the lightcurve represents changing activity rather than viewing geometry of structures in the coma. The EPOXI lightcurve can best be phased by a triple-peaked period near 54-55 hr that increases from October to November. This phasing reveals that the spacing between maxima is not constant, and that the overall lightcurve shape evolves from one triple-peaked cycle to the next. These behaviors suggest that much of the scatter in apparent periods derived from ground-based datasets acquired at similar epochs are likely due to limited sampling of the data.