Belton Space Exploration Initiatives LLC

Sierra Vista, AZ, United States

Belton Space Exploration Initiatives LLC

Sierra Vista, AZ, United States
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Li J.-Y.,University of Maryland University College | A'Hearn M.F.,University of Maryland University College | Belton M.J.S.,Belton Space Exploration Initiatives LLC | Farnham T.L.,University of Maryland University College | And 4 more authors.
Icarus | Year: 2013

The photometric properties of the nucleus of Comet 9P/Tempel 1 as modeled from the Stardust-NExT images agree with those reported by Li et al. (Li, J.-Y. et al. [2007a]. Icarus 187, 41-55; Li, J.-Y., A'Hearn, M.F., McFadden, L.A., Belton, M.J.S. [2007b]. Icarus 188, 195-211) from Deep Impact images. No significant changes are detectable by comparing the two image-sets taken one comet year apart. The overall photometric variations on the ∼70% of the surface of Tempel 1 observed by Deep Impact and Stardust-NExT are small, with albedo variations of ±10% full-width-at-half-maximum and non-detectable variations in phase function and surface roughness. Some bright surface albedo features visible in the outbound images have an albedo about 25% higher than that of surrounding area. No bright albedo features similar to those ice patches reported by Sunshine et al. (Sunshine, J.M., et al. [2006]. Science 311, 1453-1455) are seen on the outbound side, which was not imaged by DI. The similar global photometric properties among cometary nuclei may indicate that these properties are dominated by cometary activity that results in constant resurfacing on comets. Tiny amounts of ice concentration on their surface can significantly change the local photometric properties. © 2012 Elsevier Inc.


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..


Farnham T.L.,University of Maryland University College | Bodewits D.,University of Maryland University College | Li J.-Y.,University of Maryland University College | Veverka J.,Cornell University | And 2 more authors.
Icarus | Year: 2013

We present results from our study of the Stardust-NExT NAVCAM images of Comet 9P/Tempel 1, in which we analyze the dust coma and derive the locations and directions of 11 isolated jets detected around closest approach. Seven of the jets form a cluster that is associated with terraced terrain near the equator. Other jets arise from the nightside of the nucleus, having been in darkness for as long as 4h, indicating that thermal lags continue to drive activity long after sunset. We compare the coma features observed here to those seen during the Deep Impact encounter on the previous apparition, and argue that much of the isolated jet activity is associated with steep slopes and the edges of smooth areas. We estimate that the cluster of jets produces 7-20% of the total dust in the coma, indicating that isolated sources play a significant role in the comet's activity. We measured an average dust production rate A(α)fρ=42±6cm at an approach phase angle of 79°, corresponding to a dust mass loss of approximately 264kgs-1. Our analysis also indicates that the Stardust-NExT spacecraft did not pass through any dust jets during the flyby. © 2012 Elsevier Inc.


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.


Belton M.J.S.,Belton Space Exploration Initiatives LLC | Thomas P.,Cornell University | Carcich B.,Cornell University | Quick A.,Cornell University | And 8 more authors.
Icarus | Year: 2013

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)×104kg 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, Δmabs, which is the visual amplitude of the outburst referenced to a standard initial brightness. We find Log10D∼0.107(±0.004)Δmabs+1.3(±0.4) and Log10E∼0.32(±0.01)Δmabs+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 7×1012-3×1015J, and that between 6×107 and 1.3×1011kg 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. © 2012 Elsevier Inc.


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.


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.


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.


Belton M.J.S.,Belton Space Exploration Initiatives LLC
Canadian Journal of Physics | Year: 2012

Recent space observations of cometary nuclei show evidence of internal (cryovolcanic) activity while retaining aspects of their primitive origins. Using discoveries made during the two most recent cometary encounters: EPOXI at 103P/Hartley 2 and Stardust-NExT at 9P/Tempel 1, we test a hypothesis for their physical evolution, which, if true, could provide a unified basis for understanding the relative ages of their surfaces and the causes of a wide range of cometary activity. We show: (i) that the categorization of 103P/Hartley 2 as hyperactive is not a reflection of the extent of activity over the surface of the nucleus for which we find a normal H 2O production rate; (ii) that the heterogeneous spatial distribution of CO 2 and H 2O in the inner comae of 9P/Tempel 1 and 103P/Hartley 2 is best explained by processes associated with cometary activity rather than the presence of primitive compositional heterogeneities in the nucleus; and (iii) that most of the quasi-circular depressions seen on the surface of 9P/Tempel are the result of outburst activity. The apparent absence of circular depressions and large scale layering on 103P/Hartley 2 present a challenge to the evolutionary hypothesis although the small size of its nucleus may ultimately provide an explanation. © 2012 Published by NRC Research Press.


Belton M.J.S.,Belton Space Exploration Initiatives LLC | Belton M.J.S.,U.S. National Optical Astronomy Observatories
Icarus | Year: 2014

We investigate the differential size-frequency distribution (SFD) of Jupiter Family Comets (JFCs) in order to determine whether they are primordial accreted objects or collisional fragments as suggested by current models of the evolution of Trans-Neptunian Objects (TNOs). We develop a list of effective radii and their uncertainties for 161 active JFCs from published sources and compute the observed differential size-frequency distribution using a Probability Index technique. The radii range from 0.2 to 15.4. km and average 1.9. km. The peak of the distribution is near 1.0. km. This is then corrected for the effects of observational selection using a model published earlier by Meech et al. (Meech, K.J., Hainaut, O.R., Marsden, B.G. [2004]. Icarus 170, 463-491). We estimate that the total number of active JFCs between 0.2 and 15.4. km is approximately 2300 indicating that our current sample of the of active JFC population is far from complete. The active JFC size-frequency distribution, over the range from 0.6 to 10. km where it is best defined, is found to be closer to an exponential distribution in character than a power-law. We then develop a statistical model, based on the assumption of a steady state, for converting the distribution of active JFCs to the SFD of the source population among the TNOs. The model includes the effects of devolatization (that produces a large sub-class of defunct nuclei) and surficial mass-loss. Comparison with available TNO observations shows that to simultaneously attain continuity with the data on objects in the hot TNO population (Fuentes et al. [2010]. Astrophys. J. 722, 1290-1304), satisfy constraints on the number of TNOs set by the occultation detections of Schlichting et al. (Schlichting, H.E. et al. [2012]. Astrophys. J. 761, 150), and to remain within upper limits set by the Taiwanese-American Occultation Survey (TAOS; Zhang et al. [2013]. Astron. J. 146, ID 14, 10pp.) the total JFC population must contain a large fraction of small defunct nuclei. The effective power-law index of the inferred TNO differential SFD between 1 and 10. km is -4.5. ±. 0.5 indicating a population in this range that is not in fully relaxed collisional equilibrium. We conclude that the cometary nuclei so far visited by spacecraft and many JFCs are primordial accreted objects relatively unaffected by collisional evolution. We find a turndown in the slope of the predicted TNO cumulative distribution near 1. km radius rather than near 10. km that is seen in many TNO evolutionary calculations. This may or may not represent the onset of a collisional cascade. © 2013 Elsevier Inc.

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