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Figure 7.-Anticipated cometary meteoroid event rate for Pioneer F and G A/MD as a function of heliocentric distance with inferred asteroidal event rates.

TABLE IV.-Probability of Detecting a Large Asteroid
(a > 10 m) During a Mission, Wideband Mode

cy
r
1.75 2.0 2.5 3.0
0.07 . . . 0.03 0.7 10 89
0.2 . . . . .10 1.9 26 ~ 100
0.3 . . . . .15 2.8 33 ~ 100

Values are in percent.

immaterial. Should there prove to be a lack of particles within the wideband threshold capability, the option of switching to narrowband would be exercised. This option may also be exercised if representative data have already been obtained for the small particles. The result of switching to the

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Figure 8.—Computer-generated cyclic threshold variation for Pioneer F and GA/MD in the narrowband mode. Galactic longitude = 333°, galactic latitude = 0°, bandwidth = 13 kHz. x indicates threshold, o indicates noise.

TABLE V.—Probability of Detecting a Large Asteroid
(a > 10 m) During a Mission, Narrowband Mode

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narrowband mode in terms of the probability of seeing a single large asteroid is shown in table V.

ACKNOWLEDGMENTS

This effort was partially supported by NASA under contract NAS2-5609. The authors are indebted to Marie Wise for her assistance, patience, and forbearance.

REFERENCES

Anders, E. 1965, Fragmentation History of Asteroids. Icarus 4,399. Dohnanyi, J. S. 1969, Collisional Model of Asteroids and Their Debris. J. Geophys. Res. 74(10), 2531.

Gehrels, T. 1970, Photometry of Asteroids. Surfaces and Interiors of Planets and Satellites
(ed., A. Dollfus), p. 317. Academic Press, Inc. New York.
Houten, C. J. van, Houten-Groeneveld, I. van, Herget, P., and Gehrels, T. 1970,
Palomar-Leiden Survey of Faint Minor Planets. Astron. Astrophys. Suppl. Ser. 2, 339.
Kessler, D. J. 1968, Upper Limit on the Spatial Density of Asteroidal Debris. AIAA J.
6(12), 2450.
Kuiper, G. P., Fujita, Y., Gehrels, T., Groeneveld, I., Kent, J., Van Biesbroeck, G., and
Houten, C. J. van. 1958, Survey of Asteroids. Astrophys. J. Suppl. Ser. 3, 289.
NASA PT-204. 1970, Pioneer F Asteroid Analysis. NASA Ames Research Center.
NASA SP-8013. 1969, Meteoroid Environment Model—1969 (Near Earth to Lunar
Surface).
Piotrowski, S. 1953, The Collisions of Asteroids. Acta Astron. 5, 135.
Roach, F. E., and Megill, L. R. 1961, Integrated Starlight Over the Sky. Astrophys.J. 133,
228.
Soberman, R. K., and Neste, S. L. 1971, Sisyphus Threshold Program (THRESH1).
General Electric PIR 2R90-34.

DISCUSSION

GREYBER: It seems to me that a telescope using a computer-controlled imaging system, such as an image-orthicon tube, would be preferable for optical detection of asteroids from a spacecraft traversing the asteroid belt to the method chosen by Soberman, especially on a stabilized, nonspinning spacecraft. Stirling Colgate has developed such technology for the rapid detection of supernovas in external galaxies. In principle for asteroid detection one would write a computer program that would “ignore” the background of stars and search for objects moving rapidly across the field of view; i.e., planets, comets, and asteroids.

[Editorial note: The Pioneer Mission to Jupiter is described in NASA SP-268.]

OBSERVATIONS IN THE ASTEROID BELT WITH THE IMAGING PHOTOPOLARIMETER OF PIONEERS F AND G

C. E. KENKW/GHT
University of Arizona

GENERAL DESCRIPTION

Astronomical measurements of the brightness, color, and polarization of light from solar system bodies at phases not visible from Earth are planned for Pioneers F and G. An imaging photopolarimeter (IPP) in the scientific payloads will make these measurements and produce line-scan color images of Jupiter. The IPP collects light with a 2.5 cm aperture telescope that is able to turn in the plane containing the spin axis. Data are taken in scans around the spin axis along a cone whose apex angle is periodically changed by stepping the telescope. The light is analyzed for polarization, divided into blue and red components, and detected by four photomultipliers having electron channel multipliers. Appendix A gives a summary of instrument characteristics.

SCIENTIFIC EXPERIMENTS

Modes of operation of the IPP include (1) standby, (2) zodiacal light, (3) polarimetry, and (4) imaging. Standby is not a data mode but power is applied to the instrument except for the detectors. Imaging is designed for the bright surface of Jupiter at encounter so that the instantaneous field of view (0.0297) and dwell time per field of view (1 ms) are too small to be of interest in the asteroid belt. Both polarimetry and zodiacal light modes allow detection of objects brighter than 5 mag. (See app. B.) The polarimetry mode is more suited to stellar objects provided their sky position is known, whereas the zodiacal light mode is designed to measure the distribution of sky glow periodically between launch and Jupiter encounter. The sky accessible to the instrument is from 29° from the earthward spin axis (limited by the communication antenna) to 170° from that axis.

Observations in the zodiacal light mode will yield brightness and polarization maps of the sky. Subtraction of the maps made at different radial distances from the Sun will permit separation of the zodiacal light component from unresolved stars, galactic scattered light, etc. After separation, the radial distribution of zodiacal light scatterers can be inferred. Strips of the zodiacal

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