Rotation and color properties of the nucleus of Comet 2P/Encke

We present results from CCD observations of Comet 2P/Encke acquired at Steward Observatory's 2.3 m Bok Telescope on Kitt Peak. The observations were carried out in October 2002 when the comet was near aphelion. Rotational lightcurves in B-, V-, and R-filters were acquired over two nights of observations, and analysed to study the physical and color properties of the nucleus. The average apparent R  -filter magnitude across both nights corresponds to a mean effective radius of 3.95±0.06 km3.95±0.06 km, and this value is similar to that found for the V- and B  -filters. Taking the observed brightness range, we obtain a/b⩾1.44±0.06a/b⩾1.44±0.06 for the semi-axial ratio of Encke's nucleus. Applying the axial ratio to the R  -filter photometry gives nucleus semi-axes of [3.60±0.09]×[5.20±0.13] km[3.60±0.09]×[5.20±0.13] km, using the empirically-derived albedo and phase coefficient. No coma or tail was seen despite deep imaging of the comet, and flux limits from potential unresolved coma do not exceed a few percent of the total measured flux, for standard coma models. This is consistent with many other published data sets taken when the comet was near aphelion. Our data includes the first detailed time series multi-color measurements of a cometary nucleus, and significant color variations were seen on October 3, though not repeated on October 4. The average color indices across both nights are: (V−R)=0.39±0.06(V−R)=0.39±0.06 and (B−V)=0.73±0.06(B−V)=0.73±0.06 (R¯=19.76±0.03). We analysed the R  -filter time-series photometry using the method of Harris et al. [Harris, A.W., Young, J.W., Bowell, E., Martin, L.J., Millis, R.L., Poutanen, M., Scaltriti, F., Zappala, V., Schober, H.J., Debehogne, H., Zeigler, K.W., 1989. Icarus 77, 171–186] to constrain the rotation period of the comet's nucleus, and find that a period of ∼11.45 h∼11.45 h will satisfy the data, however the errors bars are large. We have successfully linked our data with the September 2002 data from Fernández et al. [Fernández, Y.R., Lowry, S.C., Weissman, P.R., Mueller, B.E.A., Samarasinha, N.H., Belton, M.J.S., Meech, K.J., 2005. Icarus 175, 194–214]—taken just 2–3 weeks before the current data set—and we show that a rotation period of just over 11 h works extremely well for the combined data set. The resulting best-fit period is 11.083±0.003 h11.083±0.003 h, consistent with the Fernández et al. value.