Beatrice E. A. Mueller

The spin state and homogeneity of Comet Halley's nucleus

Abstract We determine a model for the spin state of Comet Halleys nucleus that simultaneously satisfies imaging data from the Vega and Giotto encounters, and a wide range of ground-based data including observations of CN-jets, CN-shells, C 2 production rates, and photometric variability. The model is that of an excited, axially symmetric, rotator whose shape is taken to be that of a prolate spheroid. The motion is assumed to be unaffected by jet-induced torques. The long-axis of the nucleus is inclined to the total angular momentum vector, M, by 66°.0, and rotates around M with a period, P φ , of 3.69 days. The component of spin around the long axis has a period, P Ψ , of 7.1 days which compounds with P φ to produce a total spin period, P T = 2.84 days. The total spin vector, S, is inclined to M by 21°.4 and freely precesses around the angular momentum vector with a period of 3.69 days. The model has a ratio of maximum to minimum moments of inertia of 2.28, which, when compared to the observed linear dimensions of the nucleus, implies an approximately constant density distribution throughout its interior. When compared to the sense of orbital motion, the spin is direct and M points toward right ascension, declination (1950) = (6°·2, -60°·7). The model is characterized by five, localized, areas on the nucleus which dominate the observed activity. One of these, located near the “waist” of the nucleus, appears to be solely responsible for the initiation of CN-shell boundaries seen propagating through the coma and may also be responsible for photometric activity seen at large (≥5 AU) heliocentric distances on approach to the sun. This behavior may indicate the presence of either a large-scale chemical inhomogeneity in the nucleus or, possibly, an extensive region on the surface with unusual physical structure.

Nucleus properties of P/Schwassmann-Wachmann 1

Time series photometric measurements are presented of Comet P/Schwassmann-Wachmann 1 at a heliocentric distance of 5.886 AU when the comet possessed an extensive coma. The light curve shows a modulation caused by the rotation of the nucleus. The rotation period is considerably shorter than the 5 day period found by Whipple (1980), and we find substantial evidence that the nucleus may be in a complex spin state characterized by two periods 14.0 and 32.3 hr. Models of the rate at which the rotational light curve range decreases as a function of the amount of coma in the aperture have determined that the projected maximum to minimum axis ratio of the comet is 2.6 and that the product of the albedo times the rotationally averaged nucleus radius size is 9.54 +/- 0.3 sq km. Assuming a minimum geometric albedo of pR = 0.04, the maximum projected average nucleus radius is 15.44 +/-0.2 km, which is only 44 percent of the size estimated by Roemer (1966). However, using the albedo determined by Cruikshank & Brown (1983) of p = 0.13, the nucleus radius is only RN = 8.6 +/-0.l km. Because of the unknown nucleus orientation, these will be upper limits to the nucleus size. It appears that the nucleus of P/Schwassmann-Wachmann 1 is not the large nucleus that it has been believed to be for nearly 40 yr.

The slow rotation of 253 Mathilde

Abstract CCD photometry of the NEAR mission fly-by target 253 Mathilde is presented. Measurements taken during 52 nights of observations, from February to June 1995, allow a rotation period of 17.406±0.010 days and a lightcurve amplitude of 0.45±0.02 mag to be determined. A B-V color index of 0.67±0.02 and a V-R of 0.35±0.02 are measured, which are compatible with C-type membership. The determination of the phase relation results in H = 10.28±0.03 and G = 0.12±0.06. Indications that the lightcurve is not strictly singly-periodic are found. A power-spectrum analysis detects a secondary frequency f 2 = 0.0322±0.0010 d −1 , which is interpreted as evidence for a complex rotation state.

Extraordinary colors of asteroidal object (5145) 1992 AD

Abstract The recently discovered outer Solar System object, (5145) 1992 AD, in a somewhat Chiron-like orbit, has colors far redder than any other known asteroids or comets, and represents a hitherto-unknown spectral class. The red color may be associated with exposure of organics that are purer or more pristine than those found on the surfaces of C, P, and D asteroids, and comets, and such materials are likely to show diagnostic spectral features in the infrared.

ROTATION OF COMET 103P/HARTLEY 2 FROM STRUCTURES IN THE COMA

The CN coma structure of the EPOXI mission target, comet 103P/Hartley 2, was observed during twenty nights from September to December 2010. These CN images probe the rotational state of the comets nucleus and provide a ground-based observational context to complement the EPOXI observations. A dynamically excited cometary nucleus with a changing rotational rate is observed, a characteristic not seen in any comet in the past. The lack of rotational damping during the four-month observing interval places constraints on the interior structure of the nucleus.

Comments on the rotational state and non-gravitational forces of comet 46P/Wirtanen

Abstract Experience of modeling the rotational state and non-gravitational forces of comet 1P/Halley and other comets is applied to comet 46P/Wirtanen. While the paucity of physical data on 46P/Wirtanen makes this process somewhat speculative, this comets place as a target for the important Rosetta mission gives significance to such a study. The arguments are based on the summary of observational data provided by Jorda and Rickman (Planet. Space Sci. 43, 575, 1995) and a comparative study of the behavior of other periodic comets. It is found that 46P/Wirtanen has a level of surface activity relative to its mass that is dynamically more akin to that found in comet 1P/Halley than in a typical periodic comet. It is shown through an illustrative numerical example that this apparent fact should likely lead to an excited spin state for this comet and that significant changes in the spin period could occur in a single pass through perihelion. It is argued that the available observations are not sufficient to substantiate the claim of Jorda and Rickman (Planet. Space Sci. 43, 575, 1995) that the nucleus is undergoing retrograde rotation and it is possible that the rotation is prograde as well as retrograde. The substantial requirements that must be placed on any future observing program necessary to determine the precise rotational state are outlined. An extended (∼2 month) southern hemisphere observing campaign is advocated to determine the nuclear rotational state in 1996 if possible before activity turns on.

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).

CYANOGEN JETS AND THE ROTATION STATE OF COMET MACHHOLZ (C/2004 Q2)

Extensive observations of Comet Machholz (C/2004 Q2) from 2005 February, March, and April were used to derive a number of the properties of the comets nucleus. Images were obtained using narrowband comet filters to isolate the CN morphology. The images revealed two jets that pointed in roughly opposite directions relative to the nucleus and changed on hourly timescales. The morphology repeated itself in a periodic manner, and this fact was used to determine a rotation period for the nucleus of 17.60 ± 0.05 hr. The morphology was also used to estimate a pole orientation of R.A. = 50°, decl. = +35°, and the jet source locations were found to be on opposite hemispheres at mid-latitudes. The longitudes are also about 180° apart, although this is not well constrained. The CN features were measured to be moving at about 0.8 km s-1, which is close to the canonical value typically quoted for gas outflow. Future modeling of the CN features will be used to improve and extend these results.

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).