Studies of the parameters of micron and submicron ejecta particles from laboratory hypervelocity impact experiments have been accomplished using a light-gas-gun to accelerate milligram particles to velocities of 4 km/s onto a basalt like rock target.

The force acting on an interplanetary dust particle due to solar radiation pressure at a distance R from the sun is given by

The optical properties of interplanetary grains depend not only on chemical composition, size, and shape but also on lattice defects. It is argued that practically all sources for the zodiacal dust cloud yield grains with highly disturbed crystal structure. Healing of these defects can occur when the grains are heated in the vicinity of the sun (≤0.1 AU) and various orbits are considered, on which annealed grains with nearly perfect crystal structure can return to larger heliocentric distances to reveal the sharp optical features of cold well-ordered crystals. However, we do not find any processes, which produce healed particles of sufficient number to affect the properties of the general zodiacal cloud. Therefore, the optical properties of interplanetary grains are determined by a high degree of lattice perturbation.

That there is an interstellar (IS) component in cosmic dust has been demonstrated by Pioneers 8 and 9 (Wolf, Rhee, and Berg, 1975). The Pioneer spacecrafts distinguished the IS from interplanetary (IP) dust by measuring particle velocity and direction. Unfortunately, detectors that are capable of measuring a particle’s velocity and direction are so restricted in their sensitive area and/or solid angle that their event rate is very low. Pioneers 8 and 9, for example, detected 1-5 IS particles out of 20 events in 7 spacecraft-years of operation (Wolf, Rhee, and Berg, 1976). More events can be obtained, however, if one uses a detector that only measures the direction of travel. The direction alone can be sufficient to distinguish between IS and IP dust—at least on a statistical basis. For example, if most IP dust travels in directions near the ecliptic plane, then an IS flux from out of the plane should be detectable. This paper will examine the use of direction alone in detecting IS particles.

As one of the most important results of what we heard in these days I consider the density law of interplanetary dust derived from zodiacal light observations by the deep space probes going out to Jupiter and going in to 0.3 AU. The dependence on the distance to the sun R seems to be nearly as R-1. This finding is in agreement with a new discussion of ground based observations which was reported by Dumont. The density law was one of the open questions for a long time; for me this represents a break-through.

In this colloquium, my colleagues and I have reported on data from both the lunar surface and from interplanetary space. I am going to comment briefly amd individually on each of these disciplines insofar as they pertain to cosmic dust, but first I would preface those comments with a personal philosophy for each of the two stated disciplines. The two philosophies, you will find, are directly opposite. Concerning the lunar surface, one must recognize that it is a relatively unexplored area for in situ measurements. Accordingly, one must be sufficiently flexible and open-minded to include and consider all available experimental evidence and data until a somewhat consistent model of the lunar surface is derived. Further, and in a similar vein, one’s thinking must not be limited to what little we already know about the lunar surface nor to what is known about the Earth’s surface and terrestrial particles -- assuming that lunar particles are similar to Earth particles. Our lunar experimen shows strong evidence for the electrostatic transport of fines, and yet it is difficult to accommodate that phenomenon on the lunar surface in view of associated parameters that have been derived for the lunar surface, and in terms of what we know about the electrostatic surface properties of terrestrial particulates. But the electrostatic surface properties have been determined in the laboratory on terrestrial particles which are known to have vastly different properties from lunar fines. Thus, it becomes importhat to compile and consider all available data during this early exploration.

The steady-state of the meteoritic complex can be achieved by the present input rate of cometary dust, if the long-period and “new” comets release a sizeable fraction of their dust, in grains larger than 100 microns.

After hearing so many technical descriptions of different high velocity particle experiments, one easily recognizes the paramount importance of sensors for gathering the experimental data on meteoric dust. In contrast the “classical” meteor astronomy and physics is using a less expensive “sensor”, the Earth’s atmosphere. The interaction of the meteor body with the air has to be recorded and thus our experimental data are just records of the natural phenomenon itself: integral light photographs, spectral photographs, recently also image intensifier photographs and videorecords. The only “active” method is the radar observation of the ionized trail. The close distance of the phenomenon to the observational sites enables one to determine the complete geometrical and dynamical data (heights, distances, velocities); the light recording gives the intensity of the emitted light at individual points of the trajectory, the radar recording gives the ionization intensity. A suitable physical theory is necessary to convert the basic observational data into meteor mass and other parameters. The drag equation yields the “dynamic” mass, the luminosity equation yields the “photometric” mass, and the ablation equation yields the “ablation” mass. Por a single meteor, these masses are dependent on parameters defined by the drag coefficient, shape, bulk density, ablation rate and luminous efficiency. Differences in the resulting masses of a meteoroid computed from the different methods at the same trajectory point are good relative measures of the structural and compositional differences. The absolute calibration may be a problem, but laboratory and rocket measurements of the luminous efficiency, calibrations by the Lost City and Pribram fireballs and several other direct and indirect methods can be used.

My summary will certainly leave out much fine work but will stress a few of the points that impressed me.

First, I note remarkable progress since 1967 when I last worried about the interplanetary complex, particularly the new observations from space and of very small particles. I am delighted to see the ground based Zodiacal Light observers, like Weinberg and Dumont, converging on their observations both of polarization and of intensity. The larger particles are clearly the major contributors particularly away from the Sun. This forces the theoreticians and the laboratory people to find out how these larger particles produce the observed polarization including the negative polarization. Theoreticians always can prove the answer when it is known.