Context. Optical characterization of dust particles in cometary comae is based on a comparison between observations and simulations of solar radiation scattered by cometary dust. Aims. Our recent studies suggest that all the observed features can be reproduced by dust aggregates consisting of optically dark submicrometersize grains when the overall sizes of the aggregates are much larger than visible wavelengths. Methods. To put constraints on the properties of cometary dust aggregates, we thoroughly investigate the angular and spectral dependencies of intensity and polarization for aggregate particles consisting of identical homogeneous spheres based on a rigorous light-scattering theory. Results. The optimum parameters found through our comprehensive survey are a V 0.6 µm for the radius of aggregates, a m ≈ 0.1 µm for the radius of constituent spheres, n ≈ 1.8-2.0 for the real part of the refractive index, k ≈ 0.4-0.6 for the imaginary part and dm/dλ > ∼ 0 for the spectral gradient of the refractive index. In particular, the best results are obtained for the refractive indices derived from a synthetic mixture of carbonaceous material, magnesium-rich silicate, and iron-bearing sulfide with their abundances simulating the composition of dust in the comet 1P/Halley. Conclusions. Despite the simplicity of our model, these parameters are entirely consistent with our best knowledge of the properties of cometary dust. The common angular and spectral dependencies of intensity and polarization observed for cometary dust are explained with similarities in the average size and element composition of the constituent grains forming dust aggregates. Slight differences in the observed optical data may result from variations in the processing of cometary matter.

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Aims. We consider polarimetric and thermal-emission properties of comet dust and show how and why they can be used for classification of comets. Methods. We provide a statistical analysis of comet polarimetric, thermal, and orbital characteristics. We perform computer simulations of polarization and infrared spectra considering comet particles as ballistic particle-cluster and cluster-cluster aggregates (BPCA and BCCA) consisting of submicron spherical grains. Results. Comets can be divided into two groups: type I, characterized by high gas/dust ratio, low polarization, and a weak or absent 10 µm silicate feature, and type II, for which a low gas/dust ratio, high polarization, and strong silicate feature are typical. We show that the low polarization is the apparent result of depolarization by gas contamination at low dust concentration, which, in turn, results from the dust in type I comets being concentrated near the nucleus. The simulations of thermal emission show that for more porous particles (BCCA), the silicate feature is more pronounced than more compact ones (BPCA), for which it even vanishes as the particles become larger. We also show that in both types of comets the main contribution to light scattering and emission comes from particles larger than 10 micron. Conclusions. The strength of the silicate feature in the cometary infrared spectra suggests that the dust in type II comets consists of high-porosity aggregates, whereas the dust of type I comets contains low-porosity ones. This is consistent with the polarimetric features of these comets, which indicate that the dust in type I comets tends to concentrate near the nucleus. This may result from the predominance of highly processed particles in type I comets, whereas in type II comets we see pristine or slightly-processed dust. This conclusion is in accordance with the orbital characteristics of the comets. We have found that the strength of the silicate feature correlates with the semi-major axis of periodic comets and, for short-period comets, with the perihelion distance. Thus, the silicate feature weakens due to compaction of aggregate particles if a comet spends more time in the vicinity of the Sun, which allows the comet to accumulate a mantle on the surface of its nucleus.

Some active asteroids have been proposed to be formed as a result of impact events1. Because active asteroids are generally discovered by chance only after their tails have fully formed, the process of how impact ejecta evolve into a tail has, to our knowledge, not been directly observed. The Double Asteroid Redirection Test (DART) mission of NASA2, in addition to having successfully changed the orbital period of Dimorphos3, demonstrated the activation process of an asteroid resulting from an impact under precisely known conditions. Here we report the observations of the DART impact ejecta with the Hubble Space Telescope from impact time T + 15 min to T + 18.5 days at spatial resolutions of around 2.1 km per pixel. Our observations reveal the complex evolution of the ejecta, which are first dominated by the gravitational interaction between the Didymos binary system and the ejected dust and subsequently by solar radiation pressure. The lowest-speed ejecta dispersed through a sustained tail that had a consistent morphology with previously observed asteroid tails thought to be produced by an impact4,5. The evolution of the ejecta after the controlled impact experiment of DART thus provides a framework for understanding the fundamental mechanisms that act on asteroids disrupted by a natural impact1,6.

The coma of Comet 103P/Hartley 2 has a significant population of large particles observed as point sources in images taken by the Deep Impact spacecraft. We measure their spatial and flux distributions, and attempt to constrain their composition. The flux distribution of these particles implies a very steep size distribution with power-law slopes ranging from −6.6 to −4.7. The radii of the particles extend up to 20 cm, and perhaps up to 2 m, but their exact sizes depend on their unknown light scattering properties. We consider two cases: bright icy material, and dark dusty material. The icy case better describes the particles if water sublimation from the particles causes a significant rocket force, which we propose as the best method to account for the observed spatial distribution. Solar radiation is a plausible alternative, but only if the particles are very low density aggregates. If we treat the particles as mini-nuclei, we estimate they account for < 16 − 80% of the comet's total water production rate (within 20.6 km). Dark dusty particles, however, are not favored based on mass arguments. The water production rate from bright icy particles is constrained with an upper limit of 0.1 to 0.5% of the total water production rate of the comet. If indeed icy with a high albedo, these particles do not appear to account for the comet's large water production rate.Erratum: We have corrected the radii and masses of the large particles of comet 103P/Hartley 2 and present revised conclusions in the attached erratum.