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I show that the size-distribution of small scattered-disk trans-neptunian objects when derived from the observed size-distribution of Jupiter Family comets (JFCs) and other observational constraints implies that a large percentage (94–97%) of newly arrived active comets within a range of 0.2–15.4 km effective radius must physically disrupt, i.e. , macroscopically disintegrate, within their median dynamical lifetime. Additional observational constraints include the numbers of dormant and active nuclei in the near-Earth object (NEO) population and the slope of their size distributions. I show that the cumulative power-law slope (−2.86 to −3.15) of the scattered-disk TNO hot population between 0.2 and 15.4 km effective radius is only weakly dependent on the size-dependence of the otherwise unknown disruption mechanism. Evidently, as JFC nuclei from the scattered disk evolve into the inner Solar System only a fraction achieve dormancy while the vast majority of small nuclei ( e.g. , primarily those with effective radius <2 km) break-up. The percentage disruption rate appears to be comparable with that of the dynamically distinct Oort cloud and Halley type comets (Levison, H.F., Morbidelli, A., Dones, L., Jedicke, R., Wiegert, P.A., Bottke Jr., W.F. [2002]. Science 296, 2212–2215) suggesting that all types of comet nuclei may have similar structural characteristics even though they may have different source regions and thermal histories. The typical disruption rate for a 1 km radius active nucleus is ∼5 × 10 −5 disruptions/year and the dormancy rate is typically 3 times less. We also estimate that average fragmentation rates range from 0.01 to 0.04 events/year/comet, somewhat above the lower limit of 0.01 events/year/comet observed by Chen and Jewitt (Chen, J., Jewitt, D.C. [1994]. Icarus 108, 265–271). [ABSTRACT FROM AUTHOR]

Highlights: [•] The size–frequency distribution for Jupiter family comets is derived. [•] We show that Jupiter family comet nuclei are primordial accretion objects. [•] The cumulative size–frequency distribution inferred for small TNOs. [•] The power-law index is much larger than for a collisionally evolved distribution. [•] Our results imply a large population of defunct comets. [Copyright &y& Elsevier]

Abstract: 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)×10−3 m2/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 8.09×108 m3, the net torque acting on the nucleus was in the range 0.8–1.1×105 kgm2/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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: We consider the origin of ∼380 quasi-circular depressions (pits) seen to be distributed in a broad band across the surface of 9P/Tempel 1 and show that possibly ∼96% may be due to outburst activity. Of the rest, <4%, are probably due to a mix of cryo-volcanic collapse events and collisional impacts with asteroidal material. We estimate the mass ejected during the June 14, 2005, mini-outburst on 9P to be in the range (6–30)×104 kg and find that the resulting pit should have a diameter in the range 10–30m. Published locations of mini-outbursts are revised to account for changes in the nucleus shape, rotation rate, and rotation pole that have resulted from observations made during the Stardust-NExT mission. Both of these locations are found to fall in, or on the edge of, the band of pits that encircles the nucleus. We have identified features in high-resolution images near one of these locations as the possible places of origin of the mini-outbursts. These features show close packing of multiple pits in the appropriate diameter range. We consider the distribution of pit diameters and show that the largest pits follow a power–law with exponent −2.24±0.09. Using the June 14, 2005, mini-outburst and the Deep Impact crater to provide a calibration, we establish empirical relationships between pit diameter, D, the total outburst energy, E, and the visual magnitude change, Δm abs, which is the visual amplitude of the outburst referenced to a standard initial brightness. We find Log10 D ∼0.107(±0.004)Δm abs +1.3(±0.4) and Log10 E ∼0.32(±0.01)Δm abs +10.1(±1.2) where the uncertainties represent the range of values for the coefficient rather than formal error. We apply these approximate relationships to the mega-outburst on 17P/Holmes and predict that it left a pit-like scar on the surface with a diameter in the range 160–1300m, that the total energy released was in the range 7×1012–3×1015 J, and that between 6×107 and 1.3×1011 kg of material was ejected from the surface. While these predictions are crude they encompass, particularly near the upper end of the range, the results on kinetic energy release and mass loss found by Reach et al. (Reach, W.T., Vaubaillon, J., Lisse, C.M., Holloway, M., Rho, J. [2010]. Icarus 208, 276–292) based on IR observations of 17P. [Copyright &y& Elsevier]

Abstract: The evolution of the spin rate of Comet 9P/Tempel 1 through two perihelion passages (in 2000 and 2005) is determined from 1922 Earth-based observations taken over a period of 13year as part of a World-Wide observing campaign and from 2888 observations taken over a period of 50 days from the Deep Impact spacecraft. We determine the following sidereal spin rates (periods): 209.023±0.025°/dy (41.335±0.005h) prior to the 2000 perihelion passage, 210.448±0.016°/dy (41.055±0.003h) for the interval between the 2000 and 2005 perihelion passages, 211.856±0.030°/dy (40.783±0.006h) from Deep Impact photometry just prior to the 2005 perihelion passage, and 211.625±0.012°/dy (40.827±0.002h) in the interval 2006–2010 following the 2005 perihelion passage. The period decreased by 16.8±0.3min during the 2000 passage and by 13.7±0.2min during the 2005 passage suggesting a secular decrease in the net torque. The change in spin rate is asymmetric with respect to perihelion with the maximum net torque being applied on approach to perihelion. The Deep Impact data alone show that the spin rate was increasing at a rate of 0.024±0.003°/dy/dy at JD2453530.60510 (i.e., 25.134 dy before impact), which provides independent confirmation of the change seen in the Earth-based observations. The rotational phase of the nucleus at times before and after each perihelion and at the Deep Impact encounter is estimated based on the Thomas et al. (Thomas et al. [2007]. Icarus 187, 4–15) pole and longitude system. The possibility of a 180° error in the rotational phase is assessed and found to be significant. Analytical and physical modeling of the behavior of the spin rate through of each perihelion is presented and used as a basis to predict the rotational state of the nucleus at the time of the nominal (i.e., prior to February 2010) Stardust-NExT encounter on 2011 February 14 at 20:42. We find that a net torque in the range of 0.3–2.5×107 kgm2 s−2 acts on the nucleus during perihelion passage. The spin rate initially slows down on approach to perihelion and then passes through a minimum. It then accelerates rapidly as it passes through perihelion eventually reaching a maximum post-perihelion. It then decreases to a stable value as the nucleus moves away from the Sun. We find that the pole direction is unlikely to precess by more than ∼1° per perihelion passage. The trend of the period with time and the fact that the modeled peak torque occurs before perihelion are in agreement with published accounts of trends in water production rate and suggests that widespread H2O out-gassing from the surface is largely responsible for the observed spin-up. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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±0.002mag/deg and an absolute magnitude of 18.4±0.1 at V-band. The nucleus displays an overall linear, featureless spectrum between 400nm and 850nm. The linear spectral slope is 7.6±3.6% per 100nm, 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. [Copyright &y& Elsevier]

Abstract: Data from the Extrasolar Planet Observation and Deep Impact Extended Investigation (EPOXI) mission show Comet 103P/Hartley 2 is a bi-lobed, elongated, nearly axially symmetric comet 2.33km in length. Surface features are primarily small mounds <40m across, irregularly-shaped smooth areas on the two lobes, and a smooth but variegated region forming a “waist” between the two lobes. Assuming parts of the comet body approach the shape of an equipotential surface, the mean density of Hartley 2 is modeled to be 200–400kgm−3. Such a mean density suggests mass loss per orbit of >1%. The shape may be the evolutionary product of insolation, sublimation, and temporary deposition of materials controlled by the object’s complex rotation. [Copyright &y& Elsevier]