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Belton M.J.S.,Belton Space Exploration Initiatives LLC
Icarus | Year: 2010

The properties of 50 jet and jet-filament outflows from 27 active areas observed on the four comet nuclei that have been visited by spacecraft (1P/Halley, 19P/Borrelly, 81P/Wild 2, and 9P/Tempel 1) are investigated and we propose a taxonomic categorization in which there are three types of active areas: Type I that is dominated by the sublimation of H2O through the porous mantle; Type II that is controlled by the localized and persistent effusion of super-volatiles from the interior; and Type III that is characterized by episodic releases of super-volatiles.The zonally averaged distribution of active area locations associated with Type II outflows over the four comets is calculated and we find that they are distributed randomly in latitude. In longitude, the distribution shows a marginal tendency for active areas to occur more frequently in the region near the ends of the long axis or, alternatively, a tendency to avoid the region close to the ends of the intermediate axis.Combining observations of filamentary structure with exploratory hydrodynamic calculations we find that Type II outflows are likely to be relatively cold laminar flows (Re<1000) of a mixture of CO2, CO and H2O that are highly collimated (6-10° full-cone angle) during the daytime as a result of being constrained by the ambient H2O atmosphere. We propose that they become visible as a result of the turbulent momentum flux at the base of the filamentary structure that causes the friable surface to release dust at a higher rate than in surrounding areas.We present evidence that indicates that geophysical flows occur on cometary nuclei other than 9P/Tempel 1 and discuss a possible scenario for the long-term evolution of cometary surfaces near the Sun. We conclude with an exposition of a cometary activity paradigm brought up-to-date with discoveries made with recent space missions, associated Earth-based investigations, and the results of this work. © 2010 Elsevier Inc. Source


Belton M.J.S.,Belton Space Exploration Initiatives LLC | Thomas P.,Cornell University | Li J.-Y.,University of Maryland University College | Williams J.,University of Maryland University College | And 12 more authors.
Icarus | Year: 2013

We derive the spin state of the nucleus of Comet 103P/Hartley 2, its orientation in space, and its short-term temporal evolution from a mixture of observations taken from the DIXI (Deep Impact Extended Investigation) spacecraft and radar observations. The nucleus is found to spin in an excited long-axis mode (LAM) with its rotational angular momentum per unit mass, M, and rotational energy per unit mass, E, slowly decreasing while the degree of excitation in the spin increases through perihelion passage. M is directed toward (RA, Dec; J2000)=8±4°, 54±1° (obliquity=48±1°). This direction is likely changing, but the change is probably <6° on the sky over the ∼81.6days of the DIXI encounter. The magnitudes of M and E at closest approach (JD 2455505.0831866 2011-11-04 13:59:47.310) are 30.0±0.2m2/s and (1.56±0.02)×10-3m2/s2 respectively. The period of rotation about the instantaneous spin vector, which points in the direction (RA, Dec; J2000)=300±3.2°, 67±1.3° at the time of closest approach, was 14.1±0.3h. The instantaneous spin vector circulates around M, inclined at an average angle of 33.2±1.3°, with an average period of 18.40±0.13h at the time of closest approach. The period of roll around the principal axis of minimum inertia (" long" axis) at that time is 26.72±0.06h. The long axis is inclined to M by ∼81.2±0.6° on average, slowly decreasing through encounter. We infer that there is a periodic nodding motion of the long axis with half the roll period, i.e., 13.36±0.03h, with amplitude of ∼1° again decreasing through encounter. The periodic variability in the circulation and roll rates during a cycle was at the 2% and 10-14% level respectively.During the encounter there was a secular lengthening of the circulation period of the long axis by 1.3±0.2min/d, in agreement with ground-based estimates, while the period of roll around the long axis changed by ∼-4.4min/d at perihelion. M decreased at a rate of -0.038 (m2/s) per day in a roughly linear fashion. Assuming a bulk density between 230-300kg/m3 and a total volume for the nucleus of 8.09×108m3, the net torque acting on the nucleus was in the range 0.8-1.1×105kgm2/s2. In order to bring the spacecraft photometric and imaging data into alignment on the direction of M, the directions of the intermediate and short principal axes of inertia had to be adjusted by 33° (on the sky) from the values indicated by the shape model with an assumed homogeneous interior. The adjusted direction of the intermediate axis is RA, Dec=302°, -16.5°. The morning and evening terminators in the images are identified, and the variation of the insolation at three regions on the nucleus associated with active areas calculated. The plume of water vapor observed in the inner coma is found to be directed close to the direction of local gravity over the sub-solar region for a range of reasonable bulk densities. The plume does not follow the projected normal to the surface at the sub-solar point. © 2012 Elsevier Inc.. Source


Belton M.J.S.,Belton Space Exploration Initiatives LLC
Icarus | Year: 2013

We show that the unusual behavior of dust jets seen embedded in the sunward coma of 103P/Hartley 2 originate in active regions migrating over the two lobes of the nucleus following the Sun. The slowly changing orientation of the jets and their rapid changes in brightness is due to the shape and local topography of the nucleus coupled with the complex spin state. The intermittent appearance of a second jet is due to periodic deviations in the direction of the ejection of dust from the small lobe of the nucleus. The release of dust into the structures is likely due to the sublimation of H2O. The jets are characterized by injection speeds from the nucleus of 50-210m/s, a radiation pressure parameter 0.08<β<1, and a particle life-time near 7h. Within the jets, the average particle size decreases and the injection speed increases with distance from the nucleus. © 2012 Elsevier Inc. Source


Chesley S.R.,Jet Propulsion Laboratory | Belton M.J.S.,Belton Space Exploration Initiatives LLC | Carcich B.,Cornell University | Thomas P.C.,Cornell University | And 7 more authors.
Icarus | Year: 2013

Observations from the second encounter of Comet 9P/Tempel 1 by the Stardust-NExT spacecraft provide an improved shape model and rotational pole for the nucleus (Thomas, P.C. et al. [2012]. Icarus 222, 453-466) that allows us to greatly improve our knowledge of its rotational evolution beyond that outlined earlier in Belton et al. (Belton, M.J.S. et al. [2011]. Icarus 213, 345-368). Model light curves are shown to fit observations at both perihelia with a single pole direction indicating that polar precession during a single perihelion passage is small. We show that the rotational phasing associated with observations taken far from perihelion in the previous work was incorrectly assessed by approximately half a cycle leading us to a significant reassessment of the evolution of the non-gravitational torques acting on the nucleus. We present an updated spin rate profile (torque model) for the 2005 perihelion passage and show that retardation of the spin rate well before perihelion is no longer a required feature. With the exception of the spin rate before the 2000 perihelion passage, the evolution of rotational rates through the three most recent perihelion passages is largely unaffected as is the prediction of the rotational phase of the comet's nucleus at the Stardust-NExT near-perihelion encounter. We find a spin rate of 209.4 ± 0.01°/d likely applies in the quiescent period before the 2000 perihelion, a 0.2% change, and that the rotational period shortened by 12.3 ± 0.2. min during the 2000 perihelion passage.We present an analysis of Stardust-NExT time-series photometry that yields a spin rate near 213.3 ± 0.8°/d at the time of encounter. An application of the 2005 torque model suggests that, while roughly similar, the torques were probably weaker during the 2011 perihelion passage. © 2012 Elsevier Inc. Source


Li J.-Y.,University of Maryland University College | Besse S.,University of Maryland University College | A'Hearn M.F.,University of Maryland University College | Belton M.J.S.,Belton Space Exploration Initiatives LLC | And 7 more authors.
Icarus | Year: 2013

We have studied the photometric properties of the nucleus of a hyperactive comet, 103P/Hartley 2, at visible wavelengths using the DIXI flyby images with both disk-integrated and disk-resolved analyses. The disk-integrated phase function of the nucleus has a linear slope of 0.046 ± 0.002. mag/deg and an absolute magnitude of 18.4 ± 0.1 at V-band. The nucleus displays an overall linear, featureless spectrum between 400. nm and 850. nm. The linear spectral slope is 7.6 ± 3.6% per 100. nm, corresponding to broadband solar-illuminated color indices B-V of 0.75 ± 0.05 and V-R of 0.43 ± 0.04. Disk-resolved photometric analysis with a Hapke model returns a best-fit single-scattering albedo of 0.036 ± 0.006, an asymmetry factor of the Henyey-Greenstein single-particle phase function of -0.46 ± 0.06, and a photometric roughness of 15 ± 10°. The model yields a geometric albedo of 0.045 ± 0.009 and a Bond albedo of 0.012 ± 0.002. The overall photometric variations of the nucleus are small, with an equivalent albedo variation of 15% FWHM, and a color variation of 12% FWHM. Some areas near the terminator visible in the inbound images show an albedo of more than twice the global average value, and a much bluer color than the average nucleus. The overall photometric properties and variations of the nucleus of Hartley 2 are similar to those of the nuclei of Comets Wild 2 and Tempel 1 as studied from previous spacecraft flyby missions at similar resolutions. © 2012 Elsevier Inc. Source

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