Nalin H. Samarasinha

Observational and dynamical constraints on the rotation of Comet P/Halley

Abstract We have carried out an extensive search to determine all rotational states of the nucleus of Comet P/Halley which are capable of explaining a wide variety of observational data. We find that the orientation of the large end of the long axis during the Vega-1 encounter must be reversed from that suggested by Sagdeev et al . (1989) in order to satisfy some of the constraints. Neither pure rotations nor short axis modes (SAMs) are capable of satisfying the constraints. The most likely solutions are long axis modes (LAMs). In the most likely modes for the rotational state, the long axis executes a precessional motion around the space-fixed total rotational angular momentum vector with a period near 3.7 days while performing a rotational motion around itself with a period of about 7.3 days. The direction of the total rotational angular momentum is indicative of prograde rotation, but the exact coordinates are not well determined.

Delivery of Dust Grains from Comet C/2013 A1 (Siding Spring) to Mars

Comet C/2013 A1 (Siding Spring) will have a close encounter with Mars on 2014 October 19. We model the dynamical evolution of dust grains from the time of their ejection from the comet nucleus to the close encounter with Mars, 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 the 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 submillimeter to several millimeters can also reach Mars, 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.

Sulfur Chemistry at Millimeter Wavelengths in C/Hale-Bopp

The recent availability of bright comets has given us an excellent opportunity to study cometary chemistry. Comet Hale-Bopp (1995 O1)gave us the particularly rare opportunity to study a bright and active comet for almost two years.Our program concentrated on millimeter-wave observations of sulfur-bearing molecules in an effort to understand the total sulfur budget of the comet. Using the National Radio Astronomy Observatory 12-m telescope on Kitt Peak we monitored both the long and short-term variations in H2S, CS, and OCS, as well as observing H2CS and SO. This was the first observation of H2CS in any comet (Figure 1). Additionally, we mapped CS with the BIMA interferometer. Variations in the line profiles and changes in line intensity as large as a factor of two were seen in day to day observations of both H2S and CS. An example for H2S is shown in Figure 2.This is the first time we can attempt to study the entire group of sulfur-bearing molecules. Models of the sulfur coma have thus far largely been based on observations of the daughter products CS and atomic sulfur made over the last 18 years using the International Ultraviolet Explorer (IUE) satellite, coupled with radio observations of CS and H2S in several recent comets. Four new sulfur-bearing species have been observed in comets Hale-Bopp and Hyaku take, three of them parent species. The high resolution maps in CS will also allow spatial information to be included in the sulfur model for the first time.C/Hale-Bopp is the first comet in which so many sulfur species have been observed. Analysis of the abundances of these species in comparison to the total atomic sulfur observed should reveal whether or not we can now account for all of the primary sulfur sources in comets. Perhaps the most interesting question that these observations raised was why C/Hale-Bopp appeared to contain so much more SO and SO2 (as observed by others) than any other comet. This spurred the discovery that the UV fluorescence models of these species were incorrect (S. J. Kim, this issue). Analysis of the data and modeling of the sulfur budget are still underway.

Relating Changes in Cometary Rotation to Activity: Current Status and Applications to Comet C/2012 S1 (ISON)

We introduce a parameter, X, to predict the changes in the rotational period of a comet in terms of the rotational period itself, the nuclear radius, and the orbital characteristics. We show that X should be a constant if the bulk densities and shapes of nuclei are nearly identical and the activity patterns are similar for all comets. For four nuclei for which rotational changes are well documented, despite the nearly factor 30 variation observed among the effective active fractions of these comets, X is constant to within a factor two. We present an analysis for the sungrazing comet C/2012 S1 (ISON) to explore what rotational changes it could undergo during the upcoming perihelion passage where its perihelion distance will be ~2.7 solar radii. When close to the Sun, barring a catastrophic disruption of the nucleus, the activity of ISON will be sufficiently strong to put the nucleus into a non-principal-axis rotational state and observable changes to the rotational period should also occur. Additional causes for rotational state changes near perihelion for ISON are tidal torques caused by the Sun and the significant mass loss due to a number of mechanisms resulting in alterations to the moments of inertia of the nucleus.

Coma Morphology And Constraints On The Rotation Of Comet Hale–Bopp (C/1995 O1)

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).

Image enhancement techniques for quantitative investigations of morphological features in cometary comae: A comparative study

Abstract Many cometary coma features are only a few percent above the ambient coma (i.e., the background) and therefore coma enhancement techniques are needed to discern the morphological structures present in cometary comae. A range of image enhancement techniques widely used by cometary scientists is discussed by categorizing them and carrying out a comparative analysis. The enhancement techniques and the corresponding characteristics are described in detail and the respective mathematical representations are provided. As the comparative analyses presented in this paper make use of simulated images with known coma features, the feature identifications as well as the artifacts caused by enhancement provide an objective and definitive assessment of the various techniques. Examples are provided which highlight contrasting capabilities of different techniques to pick out qualitatively distinct features of widely different strengths and spatial scales. On account of this as well as serious image artifacts and spurious features associated with certain enhancement techniques, confirmation of the presence of coma features using qualitatively different techniques is strongly recommended.

Visible Lightcurve Observations of Comet 19P/Borrelly

The final Deep Space 1 (DS1) mission target, comet 19P/Borrelly, was observed from July 28—August 1, 2000 at the CTIO-1.5 m telescope in the R filter. The observed lightcurve has a large peak to peak variation between 0.84 mag and 1.0 mag. A period of 26.0 ± 1 hr (assuming a double-peaked lightcurve) was found using all five nights. This is in good agreement with the period of 25.02 ± 0.5 hr quoted by Lamy et al. (1998) using only 6 points of HST data and is also consistent with HST data taken around the DS1 encounter time by Weaver et al. (2002). Using the mean magnitude R = 20.8 mag and assuming a 4% albedo, we derive an effective nuclear radius of 2.6 km. The large lightcurve amplitude translates to a long to intermediate axial ratio a/b ≥ 2.2, in excellent agreement with the HST result of a/b ≥ 2.4 (Lamy et al., 1998) and with DS1 images (Soderblom et al., 2002).

CONSTRAINING THE DUST COMA PROPERTIES OF COMET C/SIDING SPRING (2013 A1) AT LARGE HELIOCENTRIC DISTANCES

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 comets 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

Spin Axis Direction of Comet 19p/Borrelly Based on Observations from 2000 and 2001

\rho

Imaging of the Structure and Evolution of the Coma Morphology of Comet Hale–Bopp (C/1995 O1)

, was 2500, 2100, and 1700 cm (5000-km radius aperture) for the three epochs, respectively. The color of the dust coma is 5.0