Zodiacal light and local interstellar dust: predictions for an out-of-ecliptic spacecraft.

Predictions for the brightness of zodiacal light observable by the zodiacal light photometer of the International Solar Polar Mission are presented for eight positions on the out-of-ecliptic trajectory of the NASA spacecraft. During a period of 2 yr, while the space probe is between 1 AU and 2.2 AU above the ecliptic plane, the brightness scattered by interplanetary dust will be only a few S(10) over most of the antisolar hemisphere. This phase of the mission is ideally suited for star background studies. Order of magnitude changes in brightness can be expected during the period of 100 d when the spacecraft cuts through the dense layers of interplanetary dust near the ecliptic plane at perihelion. This phase is well suited to studies of spatial variations in the physical properties and density distribution of the zodiacal cloud. The possibility of detecting light scattered by local interstellar dust in the solar system is discussed for a constant density of 10 to the -13th power grains per cu cm and for models where the interstellar grains are depleted in the solar system. The expected brightness varies from a few S(10) to more than 50 S(10), depending on the position of the probe and on the solar elongation of the viewing direction.