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Context. Knowledge of the size distribution of cometary nuclei and, more generally, of their physical properties is important for constraining models of the formation and evolution of the Solar System.

Context. Near-Earth asteroid-comet transition object 107P/ (4015) Wilson-Harrington is a possible target of the joint European Space Agency (ESA) and Japanese Aerospace Exploration Agency (JAXA) Marco Polo sample return mission. Physical studies of this object are relevant to this mission, and also to understanding its asteroidal or cometary nature.

Context.In 2004 asteroid (2867) Steins has been selected as a flyby target for the Rosetta mission. Determination of its spin period and the orientation of its rotation axis are essential for optimization of the flyby planning.

The Optical, Spectroscopic, and Infrared Remote Imaging System (OSIRIS) experiment onboard the Rosetta spacecraft currently orbiting comet 67P/Churyumov‐Gerasimenko has yielded unprecedented views of a comet's nucleus. We present here the first ever observations of meter‐scale fractures on the surface of a comet. Some of these fractures form polygonal networks. We present an initial assessment of their morphology, topology, and regional distribution. Fractures are ubiquitous on the surface of the comet's nucleus. Furthermore, they occur in various settings and show different topologies suggesting numerous formation mechanisms, which include thermal insulation weathering, orbital‐induced stresses, and possibly seasonal thermal contraction. However, we conclude that thermal insolation weathering is responsible for creating most of the observed fractures based on their morphology and setting in addition to thermal models that indicate diurnal temperature ranges exceeding 200 K and thermal gradients of ~15 K/min at perihelion are possible. Finally, we suggest that fractures could be a facilitator in surface evolution and long‐term erosion. Fractures are observed on the surface of a comet for the first timeDifferent fracture settings and topologies suggest various formation mechanismsFractures may be a key driver in the evolution of the surface

We have used the Spitzer Space TelescopeInfraRed Spectrograph (IRS) 22-?m peakup array to observe thermal emission from the nucleus and trail of comet 103P/Hartley 2, the target of NASA's Deep Impact Extended Investigation (DIXI). The comet was observed on UT 2008 August 12 and 13, while 5.5 AU from the Sun. We obtained two 200 frame sets of photometric imaging over a 2.7 hr period. To within the errors of the measurement, we find no detection of any temporal variation between the two images. The comet showed extended emission beyond a point source in the form of a faint trail directed along the comet's antivelocity vector. After modeling and removing the trail emission, a NEATM model for the nuclear emission with beaming parameter of 0.95 ± 0.20 indicates a small effective radius for the nucleus of 0.57 ± 0.08 km and low geometric albedo 0.028 ± 0.009 (1?). With this nucleus size and a water production rate of 3 × 1028molecules s-1at perihelion, we estimate that ?100% of the surface area is actively emitting volatile material at perihelion. Reports of emission activity out to ?5 AU support our finding of a highly active nuclear surface. Compared to Deep Impact's first target, comet 9P/Tempel 1, Hartley 2's nucleus is one-fifth as wide (and about one-hundredth the mass) while producing a similar amount of outgassing at perihelion with about 13 times the active surface fraction. Unlike Tempel 1, comet Hartley 2 should be highly susceptible to jet driven spin-up torques, and so could be rotating at a much higher frequency. Since the amplitude of nongravitational forces are surprisingly similar for both comets, close to the ensemble average for ecliptic comets, we conclude that comet Hartley 2 must have a much more isotropic pattern of time-averaged outgassing from its nuclear surface. Barring a catastrophic breakup or major fragmentation event, the comet should be able to survive up to another 100 apparitions (?700 yr) at its current rate of mass loss.

In 2005 July, the NASA Deep Impactmission successfully completed a flyby of comet 9P/Tempel 1. A flaw in the prelaunch calibration system resulted in an inability to accurately focus the telescope's High Resolution Instrument, resulting in a significant loss of resolution. However, because of the nature of the blurring process, much of the high-frequency information is retained and can be recovered using deconvolution. We discuss the data-processing requirements and noise modeling needed prior to deconvolution and compare a few popular deconvolution algorithms. All of the algorithms give very similar results. More importantly, we find that none of the algorithms correctly handle the errors resulting from the data compression used on board the spacecraft to compress the 16 bit CCD data to 8 bits prior to transmission of the image to the ground. We present a modification to the Richardson-Lucy deconvolution algorithm that we have successfully used to account for these compression errors.

Every three years the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements revises tables giving the directions of the north poles of rotation and the prime meridians of the planets, satellites, and asteroids. This report introduces a system of cartographic coordinates for asteroids and comets. A topographic reference surface for Mars is recommended. Tables for the rotational elements of the planets and satellites and size and shape of the planets and satellites are not included, since there were no changes to the values. They are available in the previous report (Celest. Mech. Dyn. Astron., 82, 83–110, 2002), a version of which is also available on a web site.

The nucleus of Comet 9P/Tempel 1, the target of the Deep Impact mission, was detected during Hubble Space Telescope observations taken with the wide-field planetary camera 2 (WFPC2) on 31 December 1997 when the comet's solar phase angle was 3.°8, its heliocentric distance was 4.48 AU, and its geocentric distance was 3.53 AU. Sixteen images were taken through the F675W filter, and all of them revealed a point-like source without any detectable coma. From these images, we derived the R magnitude of the nucleus in the Johnson–Kron–Cousins photometric system for the entire 11.5-h time span of the observations. Assuming a prolate spheroid whose spin axis lies close to the plane of the sky, the partial lightcurve indicates semi-axes a=3.9 km and b=2.8 km (assuming a geometric albedo of 0.04 for the R band and a phase coefficient of 0.04 mag deg−1) and a rotational period in the range of ∼25–33 h. The upper limit of the parameter Afρ, which characterizes the dust production rate, is 1 cm. We also derive a fractional active area at 1.78 AU of ∼4%.

The solution of the statistical equilibrium equations for the SO molecule is used to estimate the expected intensities of microwave lines. The observed fluxes of the microwave lines of SO in Comet Hale–Bopp require approximately 1.8 1028mol/sec which give fluxes in the A3Π - X3Σ−bands below the observable limit, consistent with the observations.

Comet Bradfield (1979l), discovered shortly after perihelion, on Christmas day 1979, is characterized by a retrograde orbit of period ≈250yr. This orbit was very favourable for observation by the IUE satellite1, and the first observations, the preliminary results of which are reported here, were made on 10 and 11 January 1980. Previously, comprehensive vacuum UV cometary spectra were available only for Comet West (1976VI) from rocket observations2,3and Comet Seargent (1978m) obtained with IUE4, hence three comets have now been observed in the wavelength region where most of the major constituents are detectable spectroscopically. The UV spectra of these comets are remarkably similar, despite large differences between them in the gas-to-dust ratio and the different heliocentric distances at which the observations were made, and although the statistical basis is still very small, the implication for cosmogony is that nearly all comets have the same primary composition and origin.

We present infrared imaging and photometry of the bright, giant comet C/1995 O1 (Hale-Bopp). The comet was observed in an extended infrared and optical observing campaign in 1996–1997. The infrared morphology of the comet was observed to change from the 6 to 8 jet “porcupine” structure in 1996 to the “pinwheel” structure seen in 1997; this has implications for the position of the rotational angular momentum vector. Long term light curves taken at 11.3 μm indicate a dust production rate that varies with heliocentric distance as ∶ r−1.4. Short term light curves taken at perihelion indicate a rotational periodicity of 11.3 hours and a projected dust outflow speed of ∶ 0.4 km s−1. The spectral energy distribution of the dust on October 31, 1996 is well modeled by a mixture of 70% silicaceous and 30% carbonaceous non-porous grains, with a small particle dominated size distribution like that seen for comet P/Halley (McDonnell et al., 1991), an overall dust production rate of 2 × 105kg s−1, a dust-to-gas ratio of ∶5, and an albedo of 39%.

Outbursts are thought to originate from the nucleus of comet Halley; cameras on the Vega and Giotto spacecraft have recorded the images of jets1,2, and rapid fluctuations are seen in the visible light curve, detected, for example, by the International Ultraviolet Explorer satellite during the time the spacecraft were making their observations3. This activity does not appear to be limited to when the comet is near perihelion, but has been reported for heliocentric distances >4.6AU pre-perihelion4. The outbursts are probably triggered by the heating of relatively volatile material within the ice. We report here spectroscopic data from the IUE showing a strong correlation between a visible outburst in the near-nucleus region on 18.8 March 1986 and the sudden appearance of a large column abundance of CO+2ions in the tail. These data suggest that CO2may play a significant role in the outburst process. This outburst may have been the source of the detached clumps of CO+emission recorded by ground-based observers on 19 and 20 March 19865,6

Comets C/Okazaki-Levy-Rudenko 1989 XX (C/OLR), C/Austin 1990 V, (C/Austin) P/Schwassmann-Wachmann 3 1990 VIII (P/SW3), and C/Levy 1990 XX (C/Levy) were detected by the COBE/Diffuse Infrared Background Experiment (DIRBE) with broadband photometry at 1-240 ?m between 1989 November and 1990 September. Extended dust tails were found at 12 and 25 ?m, with detailed structure due to variations in particle properties and mass-loss rate. Spectrophotometry of the central 42' × 42' was found to agree with that of a graybody of temperature 1.1 times the local blackbody temperature for C/OLR and C/Austin, while a nongraybody distribution with a spectral index of emissivity 0.26 ± 0.15 and temperature 1.25 times the local temperature was found for C/Levy. A model using modified Mie dust particles composed of fractal mixtures of vacuum, silicates, and carbonaceous material was found to fit the observations. Comparison with IUEand ground-based observations indicates that large dark particles of radius greater than 20 ?m predominate by surface area for C/OLR and C/Austin, but 1-10 ?m particles predominate for C/Levy. The detection of P/SW3, an optically faint comet, was surprising, especially since four optically brighter comets were not detected by DIRBE. This may be related to the nuclear breakup observed during its next apparition. The total estimated mass loss from these comets in a perihelion passage is ~10 times larger than that expected from optical observations, and the loss rate is similar to that needed to supply the interplanetary dust cloud. No comet trails were detected to a limiting surface brightness of 1 MJy sr-1, although large, ? < 5 × 104particles, which could evolve into a dust trail, were detected in C/Austin.

Arguments are presented for a substantial, unexplored population of comets with radii less than 1 km. Known examples confirm this population and extrapolation of any plausible size-distribution function indicates large numbers. However, their accurate numbers, orbital characteristics, and physical properties are unknown. Thus, even though the small comets may be the most frequent cometary bodies impacting the planets, a quantitative evaluation is not currently possible. We advocate an optimized, dedicated search program to characterize this population.

The recent availability of bright comets has given us an excellent opportunity to study cometary chemistry. Comet Hale-Bopp (1995 O1)gave us the particularly rare opportunity to study a bright and active comet for almost two years.

The impacts of Comet Shoemaker-Levy 9 left spots on Jupiter with diameters on the order of tens of thousands of kilometers, which have the appearance of debris fields strewn upon the Jovian cloud tops. In this note we employ a measurement of the optical depth of the debris at the impact site of fragment G to estimate mass in the debris field and lower limits to the G fragment mass of 4×1012– 4×1013g and diameter of 0.1 – 0.3 km.

Context. The asteroid (21) Lutetia is the target of a planned close encounter by the Rosettaspacecraft in July 2010. To prepare for that flyby, Lutetia has been extensively observed by a variety of astronomical facilities.Aims. We used the Hubble Space Telescope (HST)to determine the albedo of Lutetia over a wide wavelength range, extending from ~1500 Å to ~7000 Å.Methods. Using data from a variety of HSTfilters and a ground-based visible light spectrum, we employed synthetic photometry techniques to derive absolute fluxes for Lutetia. New results from ground-based measurements of Lutetia's size and shape were used to convert the absolute fluxes into albedos.Results. We present our best model for the spectral energy distribution of Lutetia over the wavelength range 1200-8000 Å. There appears to be a steep drop in the albedo (by a factor of ~2) for wavelengths shorter than ~3000 Å. Nevertheless, the far ultraviolet albedo of Lutetia (~10%) is considerably larger than that of typical C-chondrite material (~4%). The geometric albedo at 5500 Å is 16.5 ±1%. Conclusions. Lutetia's reflectivity is not consistent with a metal-dominated surface at infrared or radar wavelengths, and its albedo at all wavelengths (UV-visibile-IR-radar) is larger than observed for typical primitive, chondritic material. We derive a relatively high FUV albedo of ~10%, a result that will be tested by observations with the Alicespectrograph during the Rosettaflyby of Lutetia in July 2010.

Context. The asteroid (21) Lutetia is the target of a planned close encounter by the Rosettaspacecraft in July 2010. To prepare for that flyby, Lutetia has been extensively observed by a variety of astronomical facilities.