An introduction to definitions and methods used for interpretation of zodiacal light is given together with a short summary of observational results. After this, predictions for probing the zodiacal light by an out of ecliptic space probe and possibilities of explaining the scattering of interplanetary and cometary dust by irregular and fluffy particles are presented.

The zodiacal light experiments on Helios 1 and Helios 2 have been continually operating since launch in December 1974 and January 1976, respectively. The data support the view that the distribution of interplanetary dust is rather simple in space and quite constant in time. Instrument and data reduction have been described by Leinert et al. (1978). In the following we present Helios 2 data only.

In an earlier paper Sparrow et al. (1976) found the polarized brightness of zodiacal light to have solar color at five sky positions for which there were fixed-position observations from Skylab: north celestial pole, south ecliptic pole, vernal equinox, and two places near the north galactic pole. The brightness and degree of polarization of zodiacal light at these sky positions are derived using Pioneer 10 observations of background starlight from beyond the asteroid belt (Weinberg et al., 1974; Schuerman et al., 1976) and the assumption that the zodiacal light is also solar color in total light.

The International Solar Polar 1983 Mission will for the first time provide the opportunity to perform measurements out of the ecliptic and above the solar poles. The Zodiacal Light/Background Starlight Experiment of the Ruhr-University Bochum, FRG, (in collaboration with the State University of New York at Albany) which was selected for the NASA-S/C is a scanning photometer which is based on the HELIOS 1, 2 - Zodiacal Light Experiment.

The Skylab flight spare 10-color photopolarimeter (Weinberg, et al., 1975; Sparrow, et al., 1977) is being refurbished for use in Space Shuttle mission STS-4, a test flight currently scheduled for October 1980. Observations will be made of zodiacal light, background starlight, and the Shuttle-induced atmosphere (spacecraft corona), with emphasis on regions of sky closer than 90° to the sun.

Observations of the infrared emission of the interplanetary dust enable a more direct analysis to be made of interplanetary dust material than is possible from observations of the scattered sunlight. A liquid nitrogen cooled photometer with Indium Antimonide detectors is described which has been developed for balloon-borne observations of the zodiacal light. The provision of both difference and absolute flux measurement modes will allow the observation of the zodiacal light, and the presumably much more intense emission of the high atmosphere, respectively.

The Gegenschein observed at the Pic du Midi Observatory during a solar cycle has a behaviour similar to that of the brightness of the eclipsed Moon. It is assumed that in both cases the solar wind ejected through the polar coronal holes is responsible for these variations.

Preliminary results for the photometry of a photograph of the inner zodiacal light taken aboard Saliout 6 are reported. The Very Wide Field Camera to be flown aboard Spacelab 1 is described and the program of observations of the zodiacal light is presented.

Solar System dust particles reflect sunlight, producing the so-called zodiacal light (Leinert 1975). The spectrum of the zodiacal light in the far ultraviolet has been a matter of controversy in the past, and remains a subject of great interest.

Measurement of the position of the photometric axis of the zodiacal light at large elongations (90 ° < λ − λ⊙ < 270°; λ:ecliptic longitude, λ⊙: ecliptic longitude of the sun) provides information about the spatial distribution of the interplanetary dust outside the orbit of the Earth. However, modern photoelectric measurements in this part are scarce, except for the Gegenschein region, because of the observational difficulty due to faintness of this part of the zodiacal light.

Analysis of zodiacal light observations from Mt. Haleakala, Hawaii show that the symmetry plane of the zodiacal cloud near 1 A.U. is close to the invariable plane of the solar system. Since the symmetry plane of the inner zodiacal cloud is close to the orbital plane of Venus (Misconi and Weinberg, 1978; Leinert et al., 1979), we suggest that the symmetry plane changes inclination with heliocentric distance.

A method of determination of the scattered light intensity, , by a unit-volume of interplanetary space is presented. From ground base Zodiacal Light measurements and the experimental results of Pioneer X the density, ρ(r), and phase, σ(θ), functions are obtained without any previous assumptions about them.

The number density of interplanetary dust particles as a function of altitude above the symmetry plane can be determined from ground-based measurements by a new method.

The information provided by measurements of the zodiacal brightness, Z, is discussed as a function of the looking direction, with respect to the velocity vector of the observer through the solar system. In addition to a few directions which allow inversion of the brightness integral completely free of assumptions, it is shown that the orbital plane gives inverting possibilities, based on a single assumption on the heliocentric dependence of the volume scattering efficiency. Important features of the zodiacal cloud distribution, very difficult to obtain from earthbound observations, will easily be derived from the data of the Out-Of-Ecliptic Zodiacal Light Experiment.

Traditionally, earth-based observations of the zodiacal light (ZL) require two assumptions for further analysis: (A1) the dust density (n) is a power of heliocentric distance (R), n ∝ R−ν; (A2) the nature (scattering cross section, σ) of the dust is independent of location, σ(r,h,θ)=σ(θ). Observations from Pioneer 10 do not verify these assumptions.

The compatibility of the observed properties of the zodiacal light with a model made of two populations is investigated. It is shown that a population of submicron grains may play a non-negligible role.