Lightcurve observations of asteroids and bare cometary nuclei are the most widely used observational tool to derive the rotational parameters. Therefore, an in-depth understanding of how component ...periods of dynamically excited non-principal axis (NPA) rotators manifest in lightcurves is a crucial step in this process. We investigated this with the help of numerically generated lightcurves of NPA rotators with component periods known a priori. The component periods of NPA rotation were defined in terms of two widely used yet complementary conventions. We derive the relationships correlating the component rotation periods in the two conventions. These relationships were then used to interpret the periodicity signatures present in the simulated lightcurves and rationalize them in either convention.
We imaged Comet 252P/2000 G1 (LINEAR) (hereafter 252P) with the Hubble Space Telescope and both 252P and P/2016 BA\(_{14}\) (PanSTARRS) (hereafter BA\(_{14}\)) with the Discovery Channel Telescope in ...March and April 2016, surrounding its close encounter to Earth. The r'-band \(Af\rho\) of 252P in a 0.2"-radius aperture were \(16.8\pm0.3\) and \(57\pm1\) cm on March 14 and April 4, respectively, and its gas production rates were: \(Q\)(OH) = \((5.8\pm0.1)\times10^{27}\) s\(^{-1}\), and \(Q\)(CN) = \((1.25\pm0.01)\times10^{25}\) s\(^{-1}\) on April 17. The r'-band upper limit \(Af\rho\) of BA1\(_{14}\) was \(0.19\pm0.01\) cm in a 19.2"-radius aperture, and \(Q\)(CN) = \((1.4\pm0.1)10^{22}\) s\(^{-1}\) on April 17, 2017. 252P shows a bright and narrow jet of a few hundred kilometers long in the sunward direction, changing its projected position angle in the sky with a periodicity consistent with 7.24 hours. However, its photometric lightcurve is consistent with a periodicity of 5.41 hours. We suggest that the nucleus of 252P is likely in a non-principal axis rotation. The nucleus radius of 252P is estimated to be about \(0.3\pm0.03\) km, indicating an active fraction of 40% to >100% in its 2016 apparition. Evidence implies a possible cloud of slow-moving grains surrounding the nucleus. The activity level of 252P in the 2016 apparition increased by two orders of magnitude from its previous apparitions, making this apparition unusual. On the other hand, the activity level of BA14 appears to be at least three orders of magnitude lower than that of 252P, despite its ten times or larger surface area.
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.
We observed Comet C/Siding Spring using the Hubble Space Telescope (HST) during its close approach to Mars. The high spatial resolution images obtained through the F689M, F775W, and F845M filters ...reveal the characteristics of the dust coma. The dust production rate of C/Siding Spring, quantified by \(Af\rho\), is 590\(\pm\)30, 640\(\pm\)30, and 670\(\pm\)30 cm in a 420 km-radius aperture at 38\(^\circ\) solar phase angle through the three filters, respectively, consistent with other observations at similar time and geometry, and with model predictions based on earlier measurements. The dust expansion velocity is ~150-250 m s\(^{-1}\) for micron-sized dust grains, similar to the speeds found for other comets. The coma has a color slope of (5.5\(\pm\)1.5)%/100 nm between 689 and 845 nm, similar to previous HST measurements at comparable aperture sizes, consistent with the lack of color dependence on heliocentric distance for almost all previously observed active comets. The rotational period of the nucleus of C/Siding Spring is determined from the periodic brightness variation in the coma to be 8.00\(\pm\)0.08 hours, with no excited rotational state detected. The dust coma shows a broad and diffuse fan-shaped feature in the sunward direction, with no temporal morphological variation observed. The projected orientation of the dust feature, combined with the previous analysis of the coma morphology and other characteristics, suggests secular activity evolution of the comet in its inner solar system passage as one previously observed active region turns off whereas new regions exposed to sunlight due to seasonal illumination change.
We present constraints on the spin state of comet Hale-Bopp based on coma morphology. Three cases of rotational states are compatible with near perihelion observations: (1) principal-axis rotation, ...(2) complex rotational state with a small precessional angle, or (3) complex rotational state with a large ratio between the component periods. For principal axis rotators, images from 1996 (pre-perihelion) are consistent with a rotational angular momentum vector, M, directed at ecliptic longitude and latitude (250°, -5°) while images from late 1997 (post-perihelion) indicate (310°, -40°). This may suggest a change in M. A complex rotational state with small precessional angle requires only a small or no change in M over the active orbital arc. In this case, M is directed near ecliptic longitude and latitude (270°, -20°). A rotationally excited nucleus with a large ratio between component periods requires the nucleus to be nearly spherical. The transformation of dust coma morphology from near-radial jets to bright arcs and then again to near-radial jets is interpreted as a heliocentric and geocentric distance dependent evolutionary sequence. The spiral structures seen in CN filters near perihelion (in contrast to sunward side arcs seen in continuum) can be explained if the precursor of CN molecules (likely sub-micron grains) are emitted from the nucleus at low levels (≈ 10% of the peak daytime emission) during the nighttime. This may be indicative of a nucleus with a CO-rich active area(s).
The close encounter of Comet C/2013 A1 (Siding Spring) with Mars on October 19, 2014 presented an extremely rare opportunity to obtain the first flyby quality data of the nucleus and inner coma of a ...dynamically new comet. However, the comet's dust tail potentially posed an impact hazard to those spacecraft. To characterize the comet at large heliocentric distances, study its long-term evolution, and provide critical inputs to hazard modeling, we imaged C/Siding Spring with the Hubble Space Telescope when the comet was at 4.58, 3.77, and 3.28 AU from the Sun. The dust production rate, parameterized by the quantity Af\(\rho\), was 2500, 2100, and 1700 cm (5000-km radius aperture) for the three epochs, respectively. The color of the dust coma is 5.0\(\pm\)0.3\(\%\)/100 nm for the first two epochs, and 9.0\(\pm\)0.3\(\%\)/100 nm for the last epoch, and reddens with increasing cometocentric distance out to ~3000 km from the nucleus. The spatial distribution and the temporal evolution of the dust color are most consistent with the existence of icy grains in the coma. Two jet-like dust features appear in the north-northwest and southeast directions projected in the sky plane. Within each epoch of 1-2 hour duration, no temporal variations were observed for either feature, but the PA of the southeastern feature varied between the three epochs by ~30\(^\circ\). The dust feature morphology suggests two possible orientations for the rotational pole of the nucleus, (RA, Dec) = (295\(^\circ\pm\)5\(^\circ\), +43\(^\circ\pm\)2\(^\circ\)) and (190\(^\circ\pm\)10\(^\circ\), 50\(^\circ\pm\)5\(^\circ\)), or their diametrically opposite orientations.
We will present preliminary results from our observational campaign of Comet 2P/Encke during its 2013 perihelion passage. At optical wavelengths Encke is an extremely dust poor comet that has in past ...perihelion passages emitted a gas jet in the form a sunward fan. We expect to characterize both the morphology and lightcurve of the comet. The low optical dust means that even near perihelion the nuclear signature can be obtained in lightcurve data taken with narrowband continuum filters which cut out the gas emission. The campaign will consist of both narrowband and broadband imaging as well as infrared spectroscopy. Imaging will be obtained from 8 nights on the KPNO 2.1m between Sept. 7 and 14 UT. Additionally, the Murillo Family Observatory, a 0.5m telescope on the CSUSB campus which is equipped with both broadband filters and a narrowband Hale-Bopp set of filters will be used to observe the comet every clear night the moon allows between late August and early October to obtain extensive lightcurve data. These data will overlap both the Kitt Peak observations and the infrared spectroscopy which will be obtained with the SpeX instrument at the IRTF on four nights between September 26 UT and October 2 UT.
Comet C/2013 A1 (Siding Spring) will have a close encounter with Mars on October 19, 2014. We model the dynamical evolution of dust grains from the time of their ejection from the comet nucleus to ...the Mars close encounter, and determine the flux at Mars. Constraints on the ejection velocity from Hubble Space Telescope observations indicate that the bulk of the grains will likely miss Mars, although it is possible that a few-percent of grains with higher velocities will reach Mars, peaking approximately 90--100 minutes after the close approach of the nucleus, and consisting mostly of millimeter-radius grains ejected from the comet nucleus at a heliocentric distance of approximately 9~AU or larger. At higher velocities, younger grains from sub-millimeter to several millimeter can reach Mars too, although an even smaller fraction of grains is expected have these velocities, with negligible effect on the peak timing. Using NEOWISE observations of the comet, we can estimate that the maximum fluence will be of the order of \(10^{-7}\) grains/m\(^2\). We include a detailed analysis of how the expected fluence depends on the grain density, ejection velocity, and size-frequency distribution, to account for current model uncertainties and in preparation of possible refined model values in the near future.
Imaging of coma morphology of Comet Hale-Bopp from pre-perihelion through perihelion to post-perihelion is presented. Broad band images from 1996 and late 1997 show nearly radial jets streaking out ...from the nucleus. During both 1996 and late 1997, the brightest jets are approximately in a northern/northeastern direction. The slight curvature present in these radial jets is consistent with radiation pressure effects. Narrow band images around perihelion show two distinctive pictures of the CN and the continuum coma morphology. Spirals are clearly seen in the CN images but not in the continuum where structure is confined to the sunward side. The CN structure is consistent with continuous outgassing of the source of CN from the nucleus during both day and night.
