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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2010.07.007 Byline: Michael J.S. Belton Keywords: Comets, Nucleus; Comets, Coma; Comet Tempel-1; Comet Borrelly 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 H.sub.2O 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 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. Author Affiliation: Belton Space Exploration Initiatives, LLC, 430 S Randolph Way, Tucson, AZ 85716, USA Article History: Received 14 September 2009; Revised 23 June 2010; Accepted 6 July 2010

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.11.012 Byline: Michael J.S. Belton (a), Jay Melosh (b) Keywords: Comets; nucleus; Geological processes Abstract: The Deep Impact mission discovered repetitive outbursts on Comet 9P/Tempel 1 and the presence of several smooth terrains on its surface. We present new measurements of the extent of the smooth terrains, the slopes along their centerlines, and the areas of their likely source regions. Our analysis of these features indicates that they are 0.3 m in size should become fully relaxed during the emplacement time explaining the smooth texture seen in the images. In contrast, the repetitive outbursts require a gas-laden reservoir to have formed in the vicinity of the phase change boundary well below their preferred location. We visualize the outbursts to be the result of either spouting or bubble transport to the surface where the release of gas is diurnally modulated by either thermal stresses or H.sub.2O sublimation back pressure. The source region for the i2 smooth terrain is found to coincide with an H.sub.2O-ice rich area and we propose that the process of elutriation, i.e., the separation of different classes of particulates depending on their drag properties, occurs in the fluidized material as it flows up to and through the surface. In this way the material becomes enhanced in H.sub.2O crystals relative to siliceous and carbonaceous particulates. Author Affiliation: (a) Belton Space Exploration Initiatives, LLC, 430 S. Randolph Way, Tucson, AZ 85716, USA (b) Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721-0092, USA Article History: Received 6 August 2008; Revised 15 November 2008; Accepted 20 November 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.07.009 Byline: Michael J.S. Belton (a), Paul D. Feldman (b), Michael F. A'Hearn (c), Brian Carcich (d) Keywords: Comets; nucleus; Comet Tempel-1 Abstract: Data on the UT 2005 June 14 mini-outburst of Comet 9P/Tempel 1 taken from different viewpoints have been examined for morphological differences and parallax. The data were taken with the Hubble Space Telescope (HST), from the Deep Impact (DI) spacecraft, and from the Calar Alto Observatory, Spain. The mini-outburst source region was found to be located near 218[+ or -]6E, 6[+ or -]5N on the Deep Impact nucleus shape model. The mini-outburst occurred at [approximately equal to]12 pm local solar time. The distribution of light in the mini-outburst is similar to that expected for an ejecta curtain. The method and software used to determine the surface location was checked using position angles of the impact ejecta plume as seen from DI and HST. The general region of impact was recovered and a downrange tilt of the ejecta curtain axis of 10.2 deg from the surface normal was found. We computed tracks of possible source regions for nine other mini-outbursts seen from DI. Five of these tracks converge on the 2005 June 14 event location. Three of the tracks converge at a second location near (60E, 20S), well separated from the first. Multiple mini-outbursts arise at each location either from a single source or from a few sources in close proximity. The mini-outbursts occur both at night and during the day indicating at most weak, if any, control by direct sunlight. The times of outburst are non-random with a preference for early afternoon, dusk and midnight. None of the mini-outbursts occurred near dawn. They occur at low latitudes (between [+ or -]40 deg) near the points where the principal axis of minimum moment of inertia cuts the surface. These regions are furthest from the center of figure and have the lowest effective surface gravity. We use these results to develop a conceptual model of the mini-outburst process and make comparisons with the theoretical calculations. We find that the tensile strength of the sub-surface material must be very low (e.g., [approximately equal to]10.sup.2-10.sup.3 dynescm.sup.-2) and, on the basis of features imaged on the western facet of the nucleus, suggest that inflation of the sub-surface may be occurring. Our model makes specific predictions about the kind of surface morphology that should result from mini-outburst activity. We show that one of the isolated rimless depressions and the close-packed depressions found in the Deep Impact images have the properties needed and identify them as possible sites of past and current mini-outburst activity. Author Affiliation: (a) Belton Space Exploration Initiatives, LLC, 430 S. Randolph Way, Tucson, AZ 85716, USA (b) Department of Physics and Astronomy, Johns Hopkins University, Charles and 34th St, Baltimore, MD 21218, USA (c) Department of Astronomy, University of Maryland, College Park, MD 20742, USA (d) Center for Radiophysics and Space Research, Cornell University, Ithaca, NY 14853, USA Article History: Received 1 March 2008; Revised 3 July 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.11.015 Byline: Jian-Yang Li (a), Michael F. A'Hearn (a), Lucy A. McFadden (a), Michael J.S. Belton (b) Keywords: Comet Borrelly; Photometry Abstract: Images returned from the Deep Space 1 (DS-1) spacecraft during its encounter with Comet 19P/Borrelly are used to study its disk-integrated and disk-resolved photometry and its thermal properties. A disk-integrated phase function was constructed from a combination of DS-1 images and ground-based observations, giving a geometric albedo of 0.072[+ or -]0.020 and a phase slope of 0.043 magadeg.sup.-1. The shape model of Borrelly [Kirk, R.L., Howington-Kraus, E., Soderblom, L.A., Giese, B., Oberst, J., 2004a. Icarus 167, 54-69] and the ephemerides of DS-1 were used to analyze the disk-resolved photometric data with Hapke's theoretical model. It was found that the surface of Borrelly displays large photometric heterogeneities in its photometric parameters. The single-scattering albedo, w, varies by a factor of 2.5 with an average of 0.057[+ or -]0.009; the asymmetry factor, g, ranges from almost isotropic (-0.1) to strongly backscattering (-0.7) with an average of -0.43[+ or -]0.07; the roughness parameter, I[cedilla], is less than 35[degrees] for most parts of surface but ranges up to 55[degrees] in some areas. Its average is 22[degrees][+ or -]5[degrees]. The observed 1-D temperature profile is modeled well by the standard thermal model (STM) for inactive regions and is found to be consistent with a very low thermal inertia. Water sublimation in the source region of the fan jet is observed to decrease the surface temperature from the STM predictions by 20-40 K. The source areas of two collimated jets could not be determined from either photometric model or thermal model. It is evident that the fan jet activity occurring on Borrelly's surface can be correlated to areas of relatively high albedo, weak backscattering, and high roughness. Author Affiliation: (a) Department of Astronomy, University of Maryland, College Park, MD 20742, USA (b) Belton Space Exploration Initiatives, LLC, 430 S. Randolph Way, Tucson, AZ 85716, USA Article History: Received 14 February 2006; Revised 15 September 2006

Ways to rationalize the different periods (e.g., 15.08 h, Luu and Jewitt, 1990, Icarus 86, 69-81; 11.01 h, Fernandez et al., 2004, Icarus, in this issue; Lowry et al., 2003, Lunar Planet. Sci. XXXIV, Abstract 2056) seen in near aphelion R-band light curves of Comet 2P/Encke are explored. We show that the comet is usually active at aphelion and it's observed light curves contain signal from both the nucleus and an unresolved coma. The coma contribution to the observed brightness is generally found to dominate with the nucleus providing from 28 to 87% of the total brightness. The amplitude of the observed variations cannot be explained by the nucleus alone and are due to coma activity We show that some seven periodicities exist in the observed light curves at various times and that this is likely the result of an active nucleus spinning in an excited spin state The changing periodicities are probably due to changes in the relative strengths of the active areas. We work out possible excited states based on experience with model light curves and by using an analogy to light curve observations of Comet 1P/Halley for which the spin state has been separately determined from spacecraft observations. There is a possibility of a fully relaxed principal axis spin state (0.538 [d.sup.-1]; P = 44.6 h) but, because it provides a poorer fit to the observed periodicities than the best fit excited state together with the absence of a peak near 1.08 [d.sup.-1] (2[f.sub.[phi]] in the frequency spectrum of the Fernandez et al. (2000, Icarus 147, 145-160) thermal IR lightcurve, we consider it unlikely Both SAM and LAM excited states are allowed by the underlying periodicities and additional information is needed to choose between these. Our choice of a low excitation SAM state, i.e., one in which the instantaneous spin axis nutates around the total angular momentum vector in a motion that is characterized by limited angular oscillations around the long axis, is based on Sekanina's (1988, Astron J. 95, 911-924, 1988, Astron. J. 96, 1455-1475) interpretation of the fan coma that this comet often displays. We argue that possible LAM states are excluded either because they are too difficult to excite or because they would be inconsistent with the formation of the observed fan morphology. Two possible SAM states emerge that provide good fits to the observed periodicities, one with a precessional frequency for the long axis about the total angular momentum vector of 1.614 [d.sup.-1] ([P.sub.[phi] = 14.9 h) and an oscillation frequency around the long axis of 0.539 [d.sup.-1] ([P.sub.[psi]] = 44.5 b) and a second with a precessional frequency of 2.162 [d.sup.-1] ([P.sub.[phi]] = 11.1 h) combined with an oscillation around the long axis of 0.502 [d.sup.-1] ([P.sub.[psi]] = 47.8 h). While either solution is possible, the latter is, in a least squares sense, more likely to be the actual spin state In both cases the direction of the total angular momentum vector ([[alpha].sub.M], [[delta].sub.M] [J2000] = 198.6, -0.3 deg) is assumed to be defined by the evolving geometry and morphology of the coma (Sekanina, 1988, Astron J. 95, 911-924, 1988, Astron. J. 96, 1455-1475; Festou and Barale, 2000, Astron J. 119, 3119-3132). We discuss the possible locations of the primary active areas found by Sekanina (1988, Astron J. 95, 911-924, 1988, Astron. J. 96, 1455-1475) and, while they are at high cometographic latitudes, they do not have to be physically located close the region were the axis of maximum moment of inertia pierces the surface (i.e., at high cometocentric latitude). We offer a new interpretation of the 10.7 [micro]m data by Fernandez et al. (2000, Icarus 147, 145-160) which yields an axial ratio a/b = 2.04. This, with the two SAM states that we have found, requires that b/c > 1.18 or > 1.09 implying a significant asymmetry in the shape of the elongated nucleus. For the observed fan morphology to be maintained, the true axial ratio b/c cannot be much larger than these limiting values otherwise the amplitude of the oscillation about the long axis becomes too large and the fan morphology would be destroyed. The precise phasing of the spin modes, i.e., the value of the Euler angles at a particular time, is not determinable from the current data set, but a set of well sampled thermal infrared observations of the nucleus covering many periods and a wide range of observing geometries could provide this information in the future as well as clearly distinguishing between the two excited spin states. Keywords: Comets; Encke; Rotation

The analysis of radar observations of the asteroid 4179 Toutatis by Hudson and Ostro (1995, Science 270, 84-86) yielded a complex spin state. We revisit the visible lightcurve data on Toutatis (Spencer et al. 1995, Icarus 117, 71-89) to explore the feasibility of using a rotational lightcurve to recover the signature of an excited spin state. For this, we apply Fourier transform and CLEAN algorithm (WindowCLEAN). WindowCLEAN yields clear and precise frequency signatures associated with the precession of the long axis about the total angular momentum vector and a combination of this precession and rotation about the long axis. For a long-axis mode state, our periodicities for Toutatis yield a mean long-axis precession period, [P.sub.[phi]], of 7.38 days and a rotation period around the long axis, [P.sub.[psi]], of 5.38 days, which compare well with the respective periods of 7.42 and 5.37 days derived by Ostro et al. (1999, Icarus 137, 122-139) and represent an independent confirmation of these values. We explain why the dramatic change in the Earth-Toutatis-Sun geometry during the time that the lightcurve was obtained has little effect on the final results obtained. Using the Toutatis example as a guide, we discuss the capabilities as well as the limitations on deriving information about complex spin states from asteroidal lightcurves. Key Words: asteroids; rotational dynamics; 4179 Toutatis.

During its first two encounters with Ganymede, the Galileo spacecraft obtained images of a 16,500 [km.sup.2] portion of Galileo Regio, a large expanse of dark terrain, at high resolution (76-86 m/pixel). Through mapping of the G1 and G2 target sites within Galileo Regio, we are able to characterize geological units based on their morphology and relative albedo. We find three generally low albedo units: an intermediate albedo plains unit, a lower albedo plains unit, and the lowest albedo unit which is found on furrow and crater floors. We also find high albedo units which include crater rims, furrow rims, and isolated knobs and massifs. Other features include an intermediate albedo lobate feature interpreted to be a palimpsest and a hummocky unit interpreted to be impact ejecta. Several processes are interpreted to have occurred within Galileo Regio. These include tectonic deformation, mass wasting, sublimation, resurfacing by impact ejecta, and possibly cryovolcanism and isostatic adjustment. We observe that the NW-SE trending furrows (Lakhmu Fossae) in Galileo Regio are degraded and are crosscut by the younger N-S trending furrows (Zu Fossae). We also find several other tectonic features which may be minor faults or fractures related to one or other of these systems. Through mapping and crater size-frequency distributions, we are able to propose a stratigraphy for the Galileo Regio target site. The oldest features in the area are high albedo knobs and massifs, which are interpreted to be remnants of early impact-related features and furrow rims. These may have formed at approximately the same time as the intermediate and low albedo plains units and the furrow systems. The lowest albedo unit of furrow floors probably subsequently evolved through sublimation and mass wasting. Much of the northeast portion of the target area was subsequently obscured by one of the youngest units, ejecta from an impact just to the north. We use our mapping of the high-resolution images of Galileo Regio to evaluate three endmember models for the formation of dark terrain: (1) the crust is dark throughout, (2) material on the surface is the result of a low albedo cryovolcanic layer over a higher albedo crust, and (3) dark material is distributed in small quantities throughout the crust, and geological processes have acted to concentrate low albedo material on the surface. Although it is possible that elements of more than one of these models are present within the dark terrain, we find that the third model, that of a thin veneer of low albedo material, best fits observations of Galileo Regio. Key Words: Ganymede; geological processes; satellites, general; satellites of Jupiter; surfaces, satellite.

The Galileo Orbiter examined several impact features on Europa at considerably better resolution than was possible from Voyager. The new data allow us to describe the morphology and infer the geology of the largest impact features on Europa, which are probes into the crust. We observe two basic types of large impact features: (1) 'classic' impact craters that grossly resemble well-preserved lunar craters of similar size but are more topographically subdued (e.g., Pwyll) and (2) very flat circular features that lack the basic topographic structures of impact craters such as raised rims, a central depression, or central peaks, and which largely owe their identification as impact features to the field of secondary craters radially sprayed about them (e.g., Callanish). Our interpretation is that the classic craters (all

Active volcanism on Io has been monitored during the nominal Galileo satellite tour from mid 1996 through late 1997. The Solid State Imaging (SSI) experiment was able to observe many manifestations of this active volcanism, including (1) changes in the color and albedo of the surface, (2) active airborne plumes, and (3) glowing vents seen in eclipse. About 30 large-scale (tens of kilometers) surface changes are obvious from comparison of the SSI images to those acquired by Voyager in 1979. These include new pyroclastic deposits of several colors, bright and dark flows, and caldera-floor materials. There have also been significant surface changes on Io during the Galileo mission itself, such as a new 400-km-diameter dark pyroclastic deposit around Pillan Patera. While these surface changes are impressive, the number of large-scale changes observed in the four months between the Voyager 1 and Voyager 2 flybys in 1979 suggested that over 17 years the cumulative changes would have been much more impressive. There are two reasons why this was not actually the case. First, it appears that the most widespread plume deposits are ephemeral and seem to disappear within a few years. Second, it appears that a large fraction of the volcanic activity is confined to repeated resurfacing of dark calderas and flow fields that cover only a few percent of Io's surface. The plume monitoring has revealed 10 active plumes, comparable to the 9 plumes observed by Voyager. One of these plumes was visible only in the first orbit and three became active in the later orbits. Only the Prometheus plume has been consistently active and easy to detect. Observations of the Pele plume have been particularly intriguing since it was detected only once by SSI, despite repeated attempts, but has been detected several times by the Hubble Space Telescope at 255 nm. Pele's plume is much taller (460 km) than during Voyager 1 (300 km) and much fainter at visible wavelengths. Prometheus-type plumes (50-150 km high, long-lived, associated with high-temperature hot spots) may result from silicate lava flows or shallow intrusions interacting with near-surface S[O.sub.2]. A major and surprising result is that [approximately]30 of Io's volcanic vents glow in the dark at the short wavelengths of SSI. These are probably due to thermal emission from surfaces hotter than 700 K (with most hotter than 1000 K), well above the temperature of pure sulfur volcanism. Active silicate volcanism appears ubiquitous. There are also widespread diffuse glows seen in eclipse, related to the interaction of energetic particles with the atmosphere. These diffuse glows are closely associated with the most active volcanic vents, supporting suggestions that Io's atmosphere is dominated by volcanic outgassing. Globally, volcanic centers are rather evenly distributed. However, 14 of the 15 active plumes seen by Voyager and/or Galileo are within 30 of the equator, and there are concentrations of glows seen in eclipse at both the sub- and antijovian points. These patterns might be related to asthenospheric tidal heating or tidal stresses. Io will continue to be observed during the Galileo Europa Mission, which will climax with two close flybys of Io in late 1999. Key Words: Io; volcanism; infrared observations; satellites of Jupiter; spacecraft.

Galileo Solid State Imaging (SSI) data taken during the E6 orbit on February 19, 1997 offer a brief, high-resolution glimpse of Jupiter's White Ovals. These data can be used to infer relative cloud heights and systematic rotations over a small area of latitude and longitude (approximately 20 [degrees] by 30 [degrees]) around the White Ovals. To fully understand the behavior of these cloud systems, however, a longer time span of global data is necessary. In this paper, we utilize ground-based Voyager and Hubble Space Telescope data to interpret the Galileo images. We find that the White Ovals' translation rates have slowed from 0.39 deg [day.sup.-1] during the Voyager era to 0.13 deg [day.sup.-1] in 1996 and they, along with three smaller anticyclonic systems, have coalesced into a system of alternating cyclonic and anticyclonic systems that has not been observed before. The largest of the ovals, BC, is now impeding the eastward motion of the system, causing more rapidly translating anticyclonic cells to catch the system and increase the westward extension of the equally spaced features. Rotational velocities for BC and a small anticyclonic system at 42 [degrees] S have also been measured and peak at 120 m [sec.sup.-1] for both systems, comparable to the Mitchell et al. (J. Geophys. Res. 86, 8751-8757) measurements of BC and the Great Red Spot (GRS) from Voyager data. The motion of individual features and of the whole system in the ambient wind flow is discussed. Key Words: Jupiter; Galileo; atmospheres.

New, high-quality Galileo images of Io's dayside and Jupiterlit nightside, examined for diurnal changes in violet albedo patterns and diurnal changes in hemispheric color, do not provide any evidence for significant nighttime condensation of the satellite's thin S[O.sub.2] atmosphere. In particular, the Galileo images do not show the major change in Io's average hemispheric color between day and night that was reported by Buratti et al. (1995, Icarus 118, 418-422) in a study of Voyager data. The lack of detectable nighttime frost deposition in the Galileo images is not surprising, given the tenuous, 'patchy' nature of Io's S[O.sub.2] atmosphere and the associated difficulty in producing optically thick nighttime frost layers. Key Words: atmospheres, dynamics; Galileo spacecraft; Io; surfaces, satellite.

We present an analysis of disk-resolved images of Io at 232.5, 260, and 285 nm taken with the FOC (Faint Object Camera) of the Hubble Space Telescope. The images at 232.5 and 260 nm were acquired in 1993 in an effort to separate the surface and S[O.sub.2] atmosphere contributions to the observed UV albedo. We combine these images to make UV color maps of the regions centered on the leading and trailing hemispheres. Between latitudes +60 [degrees] and -60 [degrees] we find that the UV colors are dominated by three distinctive components, [B.sub.0], [B.sub.1], and [B.sub.2], one more than was found to be required to fit visible wavelength Voyager data at these latitudes. The Voyager component 'B' (McEwen, A. S., T. V. Johnson, D. L. Matson, and L. A. Sonderblom 1988. Icarus 75, 450-478) appears to be a combination of two distinct spectral components: [B.sub.1] and [B.sub.2]. We find that [B.sub.1], the darker component, represents either a new compositional unit or patches of S[O.sub.2] vapor overlying compositional unit B (= [B.sub.2]). To distinguish between these two possibilities, we propose two simple models of surface reflectivities and S[O.sub.2] vapor curve of growth designed to allow a crude separation between the effects of the absorptions by surface materials and S[O.sub.2] vapor. In Model 1, we recognize that all absorption bandmodels share a linear regime at the limit of small absorption pathlengths and assume that the S[O.sub.2] vapor absorption spectrum on Io is linearly dependent on absorption pathlength at all wavelengths without temperature dependencies. In this case Io's albedo must be dominated by the surface reflectance and the spectral differences between [B.sub.1] and [B.sub.2] are the signature of different surface units. In this model the two-way S[O.sub.2] vapor column density is either below our detectability limit of [approximately][4.10.sup.17] mol [center dot] [cm.sup.-2] or is confined to denser patches below our spatial resolution limit of [approximately]250 km. In Model 2, we recognize that S[O.sub.2] absorptions on Io may be non-linear at 285 nm (a local maximum in S[O.sub.2] absorption cross sections) even in the presence of significant transmission through the gas. We retain the assumption of linearity at 232.5 nm where the S[O.sub.2] absorption cross sections are smallest and consider two variants of the model in which different assumptions are made about the underlying albedo of the surface materials. In variant A, which is characterized by a relatively high assumed UV reflectivity for S[O.sub.2] frost, the amounts of gas are inconsistent with mm-wave and UV spectroscopic observations, and with the cold temperatures found for the pervasive thermal reservoir unit in Veeder et al.'s (Veeder, G. J., D. L. Matson, T. V. Johnson, D. L. Blaney, and J. D. Goguen 1994. J. Geophys. Res. (Planets) 99, 17095-17162) thermophysical model of the surface. In variant B, S[O.sub.2] frost is characterized by the lowest UV reflectivity consistent with the data. In this case there is no detectable S[O.sub.2] vapor over S[O.sub.2] frost rich regions and the FOC UV images are consistent with the presence of S[O.sub.2] vapor in patches of column density N [approximately] [10.sup.18] [cm.sup.-2] covering [approximately]11-15% of Io's projected surface. This variant of Model 2 is found to be in agreement with both the disk integrated UV spectroscopic and mm-wave observations and Veeder et al.'s thermophysical model. In particular the longitude distribution of the S[O.sub.2] patches is similar to the longitude distribution of thermal anomalies in Veeder et al.'s model. The identification of the composition of the B unit remains problematic. Polysulfur oxides (PSO) give a reasonable accounting of the UV reflectivities but may be too bright near 700 nm; Sulfur does not satisfy the UV albedos but cannot be ruled out because of uncertainties in its behavior under Io conditions. In any of the above models, regardless of the assumptions made concerning the curve of growth of S[O.sub.2] vapor absorption, we find that the percentage coverage of S[O.sub.2] frost in the regions of Io's surface that we observed is in the range of 50-60%. This is a similar result to those found in earlier spectroscopic studies of S[O.sub.2] frost features near 2 and 4 [[micro]meter].

Models for the shape and global photometric behavior of 243 Ida derived from Galileo images have been used to model the asteroid's telescopic lightcurves. We find excellent agreement between the calculated and observed times of lightcurve extrema, confirming the sidereal rotation period of 4.633632 [+ or -] 0.000007 hr determined by Binzel et al. (1993. Icarus 105, 310-325.). The shapes and amplitudes of the synthetic and observed lightcurves agree with each other to within [approximately equal to]0.05 mag, and the average offset in absolute photometry between model and observation is only 0.01 mag. This consistency confirms the accuracy of the shape model derived for Ida by Thomas et al. (1996. Icarus 120, 20-32.) and supports the model of the asteroid's photometric behavior developed by Helfenstein et al. (1996. Icarus 120, 48-65.).

Galileo flyby images of 951 Gaspra show a crater population dominated by fresh craters several hundred meters in diameter and smaller. They must represent a production population because their spatial density is low (few overlaps) and because degraded craters are underabundant; equilibrium may be attained at diameters near to or below the resolution limit of the best image. We have counted, measured, and classified craters from the highest resolution, 'high phase' image, which shows >600 craters in 90 [km.sup.2]. The differential population index (0.2-0.6 km) for the fresh, obvious craters is very 'steep' (-4.3 [+ or -] 0.3). It probably reflects the index of asteroidal projectiles; it is much steeper than the theoretical value of -3.5 for collisional equilibrium. Gaspra's crater population differs from that observed on Phobos but resembles those observed on the Moon and Mars at these sizes (consistent also with the near-Earth asteroid population). Gaspra's fresh craters are superposed on a landscape that appears 'smoothed' at a vertical scale of hundreds of meters. Some 'soft,' subdued crater-like features, commonly [approximately]>500 m across, are visible. Some of these are associated with the linear grooves on Gaspra and may be endogenic features. Many others are probably pre-existing impact craters deeply blanketed or otherwise much degraded. Gaspra's largest-scale shape is highly irregular, perhaps faceted. The biggest facet exceeds the largest crater (relative to body radius) ever observed on a satellite or expected from collisional fragmentation models. Facets cannot be successive crater-forming impacts; later scars would have destroyed earlier ones. Far-encounter images show a more lumpy than faceted visage of Gaspra; two craters are [approximately]3 km in diameter, but not even half the radius of Gaspra. We expect that Gaspra was created by collisional fragmentation of a larger parent body. Its gross configuration may reflect collisional spallation of the parent. Certainly, megaregolithic processes of reaccumulation and blanketing and/or shaking are evident, due to subsequent sub-catastrophic collisions. Gaspra's subdued craters peek through the effects of the last such collision. That smoothed surface has been cratered ever since by the steep production function, which, however must become shallower again below 10 m. Since the overall density of fresh craters is low, Gaspra must be relatively youthful. Scaled to a calculated 0.5 Gyr age for bodies of its size, based on asteroid collision models and assuming that Gaspra does not have metallic strength, its cratering lifetime is [approximately]0.2 Gyr, with large modeling uncertainties. The cumulative volume of all visible craters could create a regolith only

Dactyl was discovered in solid state imaging (SSI) data on February 17,1994, during the long playback of approach images from the Galileo spacecraft's encounter with the asteroid 243 Ida. Forty-seven images of the Ida-Dactyl pair were obtained. A detailed search for other satellites was made. No confirmed detections were made, all other candidate features being consistent with radiation hits. We deduce a manifold of osculating two-body orbits that approximate Dactyl's motion over the observed orbital arc depending on the assumed mass of Ida. At the time of Galileo's encounter, Dactyl was found to be 85 km from the center of Ida, moving at [approximately]6 m [center dot] [sec.sup.-1] in the same direction as Ida's retrograde spin. The inclination of its orbit is [approximately]172 [degrees] in Ida's equatorial system (IAU definition). It was not possible to obtain a definitive orbit or measure of Ida's mass from the observed motion even though supplemental techniques (search for Dactyl's shadow on Ida, changes in angular diameter and brightness, and attempts to determine the spin of Dactyl) were explored. The influence of Ida's irregular gravitational field and solar perturbations on two-body motion are evaluated and found to be undetectable in the observed orbital arc. These effects may, however, strongly influence the motion over orbital time scales. Limits to the value of Ida's gravitation parameter, GM, are derived. A robust lower limit, GM > 0.0023 [km.sup.3] [center dot] [sec.sup.-2], is obtained by requiring Dactyl's orbit to be bound. Hubble Space Telescope observations, which show no evidence of Dactyl on a hyperbolic orbit, excludes values of GM in the range 0.00216 < GM < 0.0023 [km.sup.3] [center dot] [sec.sup.-2]. An upper limit, GM < 0.0031 [km.sup.3] [center dot] [sec.sup.-2], deduced by requiring that the orbital motion has a long lifetime in a realistic approximation to Ida's gravitational field, is illustrated with numerical calculations. Ida's mass is therefore constrained to the range 4.2 [+ or -] 0.6 x [10.sup.19] g, which, together with a volume of 16,100 [+ or -] 1900 [km.sup.3] (Thomas P. C., M. J. S. Belton, B. Carcich, C. R. Chapman, M. E. Davies, R. Sullivan, and J. Veverka 1996. Icarus 120, 20-32.) yields a bulk density of 2.6 [+ or -] 0.5 g [center dot] [cm.sup.-3] (Belton, M. J. S., C. R. Chapman, P. C. Thomas, M. E. Davies, R. Greenberg, K. Klaasen, D. Byrnes, L. D'Amario, S. Synnott, T. V. Johnson, A. McEwen, W. Merline, D. R. Davis, J-M. Petit, A. Storrs, J. Veverka, and B. Zellner 1995. Nature 374, 785-788.).

We provide an overview of the execution, data, and results of the solid state imaging (SSI) experiment at the encounter of the Galileo spacecraft with the asteroid 243 Ida. Ninety-six images of the asteroid, representing 18 time samples during a rotation period (4.633 h), were transmitted to Earth as a result of the UT 1993 August 28.70284 encounter. This provided coverage of [approximately]95% of the surface and achieved ground resolutions as high as 25 m/pixel. Coverage of most of Ida's surface is available in four colors, with limited regions in five colors, at resolutions up to 105 m/pixel. A natural satellite of Ida, called Dactyl, was discovered in a prograde (with respect to Ida's spin), near-equatorial, orbit moving slowly ([approximately] 6 m/sec) with a separation of 85 km from Ida. Ida's shape is highly irregular; by comparison, Dactyl's global topography is quite smooth. The best fit ellipsoid to Ida's shape has principal dimensions 59.8 x 25.4 x 18.6 km, mean radius 15.7 km, and volume 16,100 [+ or -] 1900 [km.sup.3]. Dactyl's mean radius is only 0.7 km. Ida's spin axis (right ascension: 348.76 [degrees] [+ or -] 7.5 [degrees]; declination: 87.10 [degrees] [+ or -] 0.4 [degrees]; J2000) was found to align with the principal axis of inertia to within the error of measurement. This is consistent with a homogeneous density distribution. Dactyl's rotation rate is unknown, but its long axis was pointed in the direction of Ida at the time of observation, suggesting synchronism of its orbital motion and spin. Constraints on Dactyl's orbit yield 4.2 [+ or -] 0.6 x [10.sup.19] g for Ida's mass and 2.6 [+ or -] 0.5 g/[cm.sup.3] for its bulk density. Unless Ida's bulk porosity is exceptionally high, Ida has moderate to low NiFe content. Subtle color variations across the surface of Ida are associated with fresh craters, but, unlike the case for Asteroid 951 Gaspra, are not correlated with topographic features such as ridges. This difference may be a reflection of a deeper and/or more mobile regolith on Ida. Dactyl's spectral reflectance is similar to, but quantitatively distinct from the surface of Ida itself. This difference may reflect compositional differences between Dactyl and Ida, which in turn may have originated in an only partially differentiated Koronis parent body. Results on the origin, collisional history, and geology of Ida and Dactyl are the subject of many of the papers in this special issue. There is general agreement that these asteroids originated in the catastrophic breakup of the Koronis parent body and that the formation of asteroid-satellite systems may be relatively common in such events. The age and collisional history of the pair present a dilemma: using standard interpretations of the cratering record on Ida's surface, an age > 1 byr. is indicated. However, the lifetime of Dactyl against collisional disruption is many times less than this. Novel ideas are presented concerning the collisional history of these two small objects that may resolve this dilemma. These ideas result from analysis of the geological record on the surface of Ida, Dactyl, and, by comparison, Gaspra - all of which are examined in this special issue. The execution of the Galileo flybys of Gaspra, Ida, and Dactyl provide important lessons for future flybys of small bodies. We present our views on the limitations faced by the Galileo imaging experimenters and indicate how future missions can be made more quantitative and productive through the application of innovative electronic control systems and detector technology.

A control network containing 64 points identified on Galileo images obtained during the August 1993 flyby was used to determine the direction of Ida's north pole. We find the right ascension and declination (in J2000 coordinates) to be [[Alpha].sub.0] = 348.76 [degrees] [+ or -] 7.5 [degrees], [[Delta].sub.0] = 87.10 [degrees] [+ or -] 0.4 [degrees]. This result agrees reasonably well with earlier determinations from groundbased observations of Ida's lightcurve. We confirm that the rotation is retrograde, but we lack the time base to improve the rotation period of 0.1930680 days determined from lightcurve analysis (Binzel, R. P., S. M. Sivan, P. Magnusson, W. Z. Wisniewski, J. Drummond, K. Lumme, M. A. Barucci, E. Dotto, C. Angeli, D. Lazzaro, S. Mottola, M. Gonano-Beurer, T. Michalowski, G. De Angelis, D. J. Tholen, M. Di Martino, M. Hoffmann, E. H. Geyer, and F. Velichko 1993. Icarus 105, 310-325).

Seventeen positive relief features [approximately]45-150 m across are identified as probable blocks in Galileo high-resolution images of Ida. Their presence provides direct evidence for regolith retention on asteroids. The spatial distribution, maximum size, and integrated volume of the blocks are consistent with those of blocks associated with craters on the Earth, the Moon, Phobos, and Deimos. The concordance suggests that the features are impact ejecta fragments and that cratering mechanics on Ida, an object of average diameter [approximately]32 km, are similar to those applying on previously studied rocky bodies. The blocks that lie within or near the rims of craters Lascaux and Mammoth were likely mobilized in the low-velocity tail portion of the excavation flow that formed those craters. A few blocks located near smaller craters may have migrated some distance away from their source, possibly by impact-induced spallation, hopping, rolling, and/or sliding. Some blocks on Ida could be surviving fragments from the Koronis parent body, accreted after its breakup. The lifetime of [10.sup.2]-m sized boulders against collisional disruption is estimated to be in the [10.sup.8]-[10.sup.9] year range, consistent with ages considered for the largest and oldest craters on Ida. Extrapolation of successful ejecta scaling laws to other asteroids suggests that blocks [approximately]15 and 70 m across could be present on Dactyl and Gaspra, respectively (in both cases too small to be identified in available Galileo images). Blocks 100 m in size could be present on 433 Eros, and km-sized megablocks on 4 Vesta.

A study on plausible evolutionary paths for Halley-like, near-prolate cometary nuclei rotational states shows that they are dependent on the location of multiple active areas on the nucleus. Precessional angles and nongravitational forces of a rotational state of nucleus remain the same over many orbits, for acceptable locations of active areas on the nucleus. Numerical simulations are carried out to study Halley-like cometary nuclei.

Information on shape and photometric behavior of Gaspra, an asteroid, is sent by Galileo. The information helps in modeling Earth-based lightcurves of the asteroid. Correlation of data gives an accurate value of period of rotation of 7.042024 + or _ 0.000020 hr, for Gaspra. The use of lightcurves gives a better picture of Gaspra in areas that are not properly photographed by the spacecraft.

An extended jet source from vent 1 of Halley's comet causes CO distribution in the coma of the comet. The CO distribution is obtained from measurements made by the neutral gas mass spectrometer on the Giotto spacecraft. The extended jet source causes only two-thirds of the CO distribution and the remaining is caused by direct CO sublimation at the cometary nucleus.

The north pole of asteroid 951 Gaspra was located by using a 19-point control network on the images obtained from the Galileo spacecraft during its October 1991 flyby. The right ascension is 9.5 degrees and declination is 26.7 degrees in the J2000 coordinates. The asteroid has a prograde rotation with a rotation period of 0. 2934197 days. The prime meridian passes through the crater, Charax.