The meeting was opened by Ted Bowell, president, at 11 am. The 2006 Division III meetings were reviewed by Guy Consolmagno, secretary; as the minutes of those meetings have already been published, they were assumed to be approved.

Division III's activities focus on a broad range of astronomical research on bodies in the solar system (excluding the Sun), on extrasolar planets, and on the search for life in the Universe.

Commission 21 consists of IAU members and consultants with expertise and interest in the study of the light of the night sky and its various diffuse components, at all accessible electromagnetic frequencies. In cosmic distance scales, the subjects of Commission 21 range from airglow and tropospheric scattering in Earth's atmosphere, through zodiacal light in the solar system, including thermal emission from interplanetary dust, integrated starlight in the Milky Way galaxy, diffuse galactic light due to dust scattering in the galactic diffuse interstellar medium, thermal emissions from interstellar dust and free free emission from ionized interstellar gas, to various diffuse extragalactic background sources, including the cosmologically important cosmic microwave background (CMB). Observations of the diffuse night sky brightness at any frequency typically include signals from several of these sources, and it has been the historic mandate of Commission 21 to foster the necessary collaboration of experts from the different astronomical sub-disciplines involved.

Applications have been received from Dr. Peter Wheatley (proposed by UK), Prof. Harald Schuh (proposed by Austria), and Dr. Busaba Kramer (proposed by Thailand). All applications were endorsed, with the caveat that Dr. Kramer's application needs to be endorsed by at least one of the other commissions since she has not a publication record in our field.

Commission 21, one of IAU's smallest commissions, consists of some hundred members and consultants working to understand and describe the light of the night sky with emphasis on the diffuse components. Many more work on these topics without being members of the commission. Light is here defined in its broader sense of electromagnetic radiation of any frequency. The diffuse components of the light of the night sky encompass a variety of physical phenomena over the full range of cosmic distance scales and include scattered light, thermal emission, line emission, and any other emission phenomena producing a diffuse light source. These attract interest not only as scientific topics of study in their own right but also as an unwanted foreground or background against which all other sky phenomena are observed. Commission 21 has for mandate to promote research and availability of results on issues related to the diffuse light of the night sky. This document is a report on activities in this field and is not confined to the activities of its members, no distinction is made between work carried out by commission members and non commission members. The report is organized starting with a summary of the state of broad surveys that provide most of the observations. The report on developments in the various disciplines start with the sources closest to the observer known as airglow and progresses by way of the interplanetary and interstellar mediums to the increasingly distant integrated starlight, diffuse galactic light and diffuse emission in other galaxies ending with the extragalactic background radiation.

The conceptual design for an optical instrument capable of characterizing dust particles by in situ measurements is described. This instrument, the Integrated Dust Analyzer (IDA), measures the light scattered at several different angles as a particle passes through a pair of polarized light curtains. It is capable of defining or providing an indication of such characteristics as size, index of refraction, absorptivity, and aggregate structure.

We have studied several optical systems for dust velocity measurements on a cometary orbiter using light scattering to detect particles. Two main concepts are considered. The first relies on trajectory imaging, while the second uses time-of-flight measurements between two or more light sheets. Their merits and disadvantages are discussed. The favored system, time-of-flight between two planes with particle localization, is described.

For a long time, the dominant scattering theory used in radiative transfer and scattering calculations has been Mie theory, which is the complete solution to the problems of light scattering by single, isotropic, and homogeneous spheres. However, cosmic dust collections show that most of the largest sized interplanetary dust particles may be porous, inhomogeneous, and aggregated and may have quite different scattering properties. Arbitrary configurations of aggregated spheres may provide a reasonable first approximation to realistic light-scattering models of interplanetary dust particles. In the last few decades, progress has been made in developing light scattering theory for interacting spheres, The development of the addition theorems for scalar and vector spherical wave functions (Friedman & Russek, 1954; Stein, 1961; Cruzan, 1962) opened up a new area in the theoretical study of multisphere scattering problems.

The local cloud in which the Sun is embedded participates in a general flow from the direction of the Sco-Cen association (Weaver, 1979) while the Sun is moving in the direction of Hercules. The relative motion is well determined by the neutral gas flow (Cox and Reynolds, 1987). A prominent cone of gravitationally focused neutral helium indicates the downwind direction to within ±3°. The average flow speed outside the solar system, 26 km/s, is determined to within ±1 km/s. Measurements from the Ulysses spacecraft (Witte et al., 1993) confirmed this flow. Since interstellar matter contains dust as well as gas, efforts have been made to detect the dust flow through the solar system. However, until recently these efforts have been unsuccessful and modeling of the flow has been controversial.

We estimate the probability that specific meteoroids were produced by a specific parent body and obtain the ejection velocity and several other conditions of the ejection as a by-product. The duration of cometary activity leading to a meteor stream, or the epoch of a collision can be estimated from the ejection times of meteoroids in the same stream. The level of activity of a comet or probability of asteroid collisions can be estimated from the part of the orbit where ejection took place. We show that Phaethon ejected Geminid meteoroids over at least 3000 years lasting to as recently as ≈ AD 1600. Ejections are predominantly in the direction of the sun, i.e., from Phaethon's day side, and give insights into the ejection process which is suggestive of cometary activity. Our calculations account for the uncertainty in the observational data and include the effects of planetary perturbations, radiation pressure, Poynting-Robertson light drag and solar wind corpuscular drag.

This is a description of the beginning of a systematic investigation into the optical properties of dust structures that are likely to be representative of interplanetary dust. I delineate the development of a physical dust model to parameterize the optically important characteristics of the dust. The result is a system with two refractive indexes in an aggregate structure of varying porosity - a challenging model for most current light scattering theories. Experimental data is needed to investigate the scattering by these structures and to test new theoretical solutions (e.g., Xu 1995) as they develop. I give a brief description of the new microwave analog scattering laboratory that has been developed for this purpose at the Laboratory for Astrophysics of the University of Florida's Astronomy Department. Finally, laboratory data is shown in support of dense aggregate models for interplanetary dust.

Synthetic infrared pictures are used to illustrate changes in the shape of portions of an interplanetary dust-cloud. The dust particles, in circular heliocentric orbits, are perturbed by radiation and corpuscular forces combined with gravitational disturbances by the major planets. Dust in the inner solar system and close to the ecliptic or the orbital plane of Venus is brought to a more narrow range of ecliptic latitudes. A dust-band evolves near and inside the orbit of Venus. The cloud’s shape is less affected at high ecliptic latitudes.

Dynamical perturbations on ensembles of particles in heliocentric orbits of low eccentricity are integrated over time. The dust is perturbed by radiation pressure, Poynting-Robertson drag, their corpuscular counterparts, and by gravitation due to any number of planets. A dust cloud is represented by a set of centroids and orbital dispersions (about the centroids). Gravitational perturbations on the centroid are derived from a single matrix, valid for any planet, in the appropriate frame of reference. After transformation of the time derivatives to a common coordinate system, the perturbation rates are summed up and integrated. The time dependence of the planets’ orbital elements are evaluated inside the time integral.