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While remote observations of the linear polarization of five low-albedo near-Earth asteroids are available at large phase angles, space missions have collected materials from the surface of two of them and one of them is already back to Earth. The structure of the regolith on the surface may be different from that encountered on larger objects, because of their low gravity and thermal stress cycling. Dust particles crushed from low-albedo meteorites (i.e. Orgueil and Allende) are tentatively used as analogues to provide a better approach of such regoliths. The PROGRA2 experiment studies the light-scattering properties of dust particles of various size distributions under Earth’s gravity either deposited or with clouds lifted by an air-draught, as well as under μ-gravity conditions. Similar maximum in polarization (i.e. Pmax) values are obtained experimentally for dark particles deposited or in levitation, showing that multiple scattering is negligible. The increased sensitivity of PROGRA2 instruments, together with imaging techniques, makes it possible to study Pmax with increasing sizes of the lifted particles, up to mm-sizes and above. Our results confirm that particles constituting the regolith are mm-sized and may reach polarization values up to 50 per cent in good agreement with remote observations. Some materials are also suggested to be present on the surface of the particles. Also, huge agglomerates made by random ballistic deposition may be considered as relevant analogues e.g. for pebbles and boulders.

Dust particles in the solar system (e.g. atmospheric hazes, cometary or interplanetary dust, regolith) are likely to be irregular aggregates whose light scattering properties (phase functions of polarization) are drastically different from those of Mie spheres. However, the observation of the light they scatter may provide informations on their physical properties. If the mechanisms which lead to aggregation are invariant with time, the aggregates are likely to be fractal particles made up from individual monomers. Computations, developped in relation with the CODAG experiment, are performed using a Discrete Dipole Approximation, and each monomer is described by one or more dipoles. When the particles are formed from a few monomers made up of numerous dipoles, the polarimetric response of the aggregate is similar to the one of the constituent monomer. When the particles are formed from many monomers made up of individual dipoles, the phase curves are similar to those observed in the solar system. Our calculations suggest that dust particles have a fractal dimension of the order of 2 (Ballistic Cluster-Cluster Aggregation), and that the values of the real and imaginary part of the complex refractive index of the constituent material are high. Those results are in agreement with laboratory measurements on samples representative of astronomical organics and minerals.

International audience; Hayabusa (“peregrine falcon” in Japanese) is a scientific space mission developed by JAXA to explore a near-Earth asteroid and to return asteroidal soil, while validating new engineering technologies (propulsion, landing, and atmospheric reentry). Launched in May 2003, the Hayabusa-1 rendezvous with asteroid Itokawa was in September 2005. After 2 months of close observations, it descended and attempted to collect surface grains. It departed in 2007 and cruised back to Earth. The sample canister returned safely in July 2010. JAXA has confirmed the presence of tiny dust particles sampled on Itokawa. Hayabusa-2 is now planned, targeting the C-type asteroid 1993JU3 (launch in late 2014).

New and important data have been obtained during the 1985-1986 return of comet Halley, including in situ observations of the nucleus and the coma. Since the interpretation of the observations is not straightforward, the results are presented in a rather conservative manner. Some clues to the solar system formation are suggested, e.g. the shape of the nucleus, its low density, the estimated mass of the Oort cloud, the elemental abundances in comet Halley. Constraints related to isotopie abundances (deuterium enrichment, possible anomalies in carbon isotopes) and to cometary dust (complex organic compounds, submicron sized dust particles) are extensively discussed.

At the 1988 Baltimore General Assembly of the International Astronomical Union, members of several Commissions dealing with planetary science expressed deep concern that no work was being undertaken to identify and avoid pollution problems in interplanetary space beyond the Moon. At that time NASA had convened a conference on problems in cislunar space due to the large and growing numbers of orbiting fragments hazardous to space vehicles. In translunar space this is hardly a problem. However an alarming number of future interplanetary mission proposals were considered for other reasons to be potentially harmful to various solar system bodies and interplanetary space itself.

This report has, following the usual practice in Commission 15, been compiled primarily by the chairpersons of the two working groups. E. Tedesco prepared the entire section about Asteroids and Meteorites. H. U. Keller prepared the section about Comets using material originally prepared by E. Kuhrt (nuclei), H. Bohnhardt (dust), R. Schulz (neutral gas), and W.-H. Ip (plasma). M. F. A’Hearn did minor editing to merge the two reports and fit the report into the allotted space.

Due to the success of the March 1986 space missions to comet Halley, and to the large amount of ground, rocket or satellite observations, numerous papers have recently been published, and new and exciting problems have raised. It should nevertheless be kept in mind that the development of cometary physics is much prior to the last return to perihelion of Halley. One of the most remarkable results of the space missions has been to demonstrate that the nucleus, the coma (transient atmosphere which expands when the comet approaches the Sun), and the tails models that had been inferred from various astrophysical methods were rather in agreement with in situ observations.

The light of the night sky consists of atmospheric components (airglow, light scattered in the atmosphere) and – even in the case of spaceborne observations – of zodiacal, galactic and extragalactic light. Although all components are of similar importance, investigations on zodiacal light have profitted most by the space age since their object of research, the interplanetary dust cloud, became accessible to direct in-situ measurements. Lunar samples and measurements by micrometeoroid detectors provide individual and eventually detailed information on impact events, which however are limited in number and therefore restricted in statistical significance. Zodiacal light investigations involve scattered light of many particles in large volume elements and therefore provide global information about physical properties and spatial distribution of interplanetary dust grains, however just in terms of average values. Therefore both sources of information are complementary and a synthesis can only be achieved by synoptic interpretation of zodiacal light, micrometeoroid, and meteoroid investigations also including dynamical aspects. Measurements of zodiacal light (and emission) from rockets, manned or non manned spacecraft, and deep space probes gained drastically in importance compared to ground based observations. On the other hand investigations on airglow have become more and more a topic of geophysics Caeronomy). They remain relevant however to astronomy as far as photometric features are concerned. These general trends continued in the last triennium and have influenced the activities of our commission.

This work was carried out by an expedition organized by the S.A.F. in 1986 April to La Reunion. The aim was to observe and photograph P/Halley, within the frame-work of IHW, as part of the Island Network in the southern hemisphere. To be more precise, our work consisted of studying large-scale phenomena: the structure, dynamics and possible disconnection events in the plasma tail. We were lucky enough to observe one of the latter on the night of April 11/12, and describe it here.

Values of the temperature, of the global emissivity, and (by comparison with zodiacal light optical observations) of the albedo of interplanetary dust are retrieved from IRAS data (Hauser et al., 1984) in two localized regions of the line-of-sight, near 0.98 and 1.40 a.u. heliocentric distance. The model-dependence is reduced, compared to the values averaged on the line-of-sight. The temperature probably does not exceed 250 K at the distance of the Earth. The albedo is found to decrease from ~ 0.08 at the distance of the Earth to ~ 0.06 at the distance of Mars.

In 1985–86 when spaceprobes flew past P/Giacobini-Zinner and P/Halley, many amateurs observed these comets and submitted their results to national or international (IHW) organisations. Their visual, photographic, photometric and spectroscopic observations made a contribution to scientific knowledge. It is pertinent to ask if space programmes under consideration will allow similar cooperation to be continued.

Mass spectra of cometary dust particles measured by the PIA dust particle analyzer aboard the Giotto spacecraft show some unexpected and striking features. First, small particles below 10 to the -14th g are much more abundant than anticipated by models. Second, most of the particles are rich in light elements such as H, C, N, and O, suggesting the validity of models that describe the cometary dust as including organic material. Third, the light elements specifically seem to have a low ratio of mass to volume. Three examples of original mass spectra showing typical compositions are given; these have been measured, and are compared with a computer-simulated mass spectrum.