Zodiacal light and the asteroid belt: The view from Pioneer 10

Abstract

Brightnesses measured by the Pioneer 10 imaging photopolarimeter in two regio,ns of sky were compared on sky maps at sun-spacecraft distances from 2.4 to 4.8 AU to determine the spatial extent of the zodiacal light. Data in the ecliptic at elongations greater than 90 ø show negligible contribution to the zodiacal light beyond 3.3 AU, the 2:1 Jupiter resonance. The zodiacal light brightness at 2.4 AU is less than 10% of that observed at I AU.The imaging photopolarimeters (IPP) on Pioneer 10 and 11 have provided two-color observations of the brightness and polarization of the zodiacal light at varying sun-spacecraft distances. The change in brightness with sun-spacecraft distance yields information on the spatial distribution of the zodiacal dust that cannot be determined solely from observations at 1 AU [Hanner and Leinert, 1972].In this paper we analyze brightnesses measured by the Pioneer 10 IPP between 2.41 and 4.82 AU to determine (1) the heliocentric distance at which we are no longer able to detect a zodiacal light component and (2) the contribution of the asteroid belt to the zodiacal light. These answers will provide limits on the dust concentration in and beyond the asteroid belt. INSTRUMENT OPERATION AND CALIBRATION, The IPP measures orthogonal brightness components in blue (3900-5000 A) and red (5950-7200 A) light to provide sky brightness, color, and polarization information [Pellicori et al., 1973; Weinberg et al., 1974]. A complete data cycle consists of ten spacecraft rolls, one each for dark current and calibration measurements and eight for sky measurements with a 2.3 ø by 2.3 ø field of view. As the spacecraft spins, the 2.3 ø instantaneous field of view sweeps out an annulus on the sky. The IPP output signals are integrated and sampled 64 times during one spacecraft spin to give 64 sectors of simultaneous blue and red data. To build up a map, the IPP steps by 1.8 ø in look angle L (angle between the spacecraft spin axis and the telescope line of sight) after each data cycle until the desired look angle range is covered. The effective field of view is 2.3 ø wide and 2.3 ø q-(360ø/64) sin L long. Figure 1 shows the field of view pattern near look angle 90 ø (upper diagram) and from 141ø to 170 ø (lower diagram). Look angle increases toward the right. In the upper diagram the fields of view have their maximum length of 7.9 ø . The shaded areas show one sector at two successive look angles, the darker shading representing the overlap between adjacent look angles. In the lower diagram the changing size of the field of view with look angle is evident. The shaded area has a length of two sectors, the darker shading showing the sector overlap of 2.3 ø . The direction of the spacecraft spin axis is near the lower right-hand edge of the diagram, at right ascension a 322 ø and declination • -16 ø. Owing to mechanical constraints the IPP cannot observe within 10 ø of the spin axi s. Absolute calibration of the IPP is based on preflight measurements of a •4C standard source, as is described by Copyright ¸ 19...

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