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AU Asteroidal Cometary Total Cumulative Asteroidal Cometary Total cuo
1.00 to 1.33 0 13 13 13 0 5 5 5
1.33 to 1.66 1 10 11 24 0 4 4 9
1.66 to 2.00 7 6 13 37 2 3 5 14
2.00 to 2.33 32 6 38 75 11 3 14 28
2.33 to 2.66 37 4 41 116 15 2 17 45
2.66 to 3.00 35 2 37 153 7 2 9 54
3.00 to 3.33 8 1 9 164 7 2 9 63
3.33 to 3.66 2 1 3 167 3 2 5 68
3.66 to 4.00 0 0 0 167 0 1 1 69
4.00 to 4.33 0 1 1 168 0 1 1 70
4.33 to 4.66 0 2 2 170 0 1 1 71
4.66 to 5.00 0 1 1 171 0 1 1 72


The pressure cell type of penetration detector was chosen for the Pioneer F and G missions for a number of reasons. It is an extremely simple detector, the data from it are easy to interpret, and it is essentially unaffected by the environments encountered, other than, of course, the meteoroid environment.

Successful flight experiments on Explorers 13, 16, and 23, and all Lunar Orbiter spacecraft have proven the pressure cell penetration detector to be the most reliable meteoroid detector yet used in space. The actual penetration measurements are valuable to spacecraft technology. There remains, of course, an uncertainty in the interpretation of the penetration events in terms of the mass of the impacting meteoroid. However, there does exist a background of many years in penetration research and, based on this background, it is felt that the uncertainty in the interpretation of penetration data in terms of the mass of the impacting meteoroid is a minimum uncertainty.


NASA SP-8038. 1970, Meteoroid Environment Model—1970 (Interplanetary and Planetary).

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


General Electric Space Sciences Laboratory and Drexel University


General Electric Space Sciences Laboratory


The Pioneer asteroid/meteoroid detector (Sisyphus' or A/MD) is an optical instrument designed primarily to make measurements of small interplanetary particles that pass within about 1 km of the spacecraft. Because it is an optical system, it can also detect larger bodies at greater distances. We can approximate the amount of light incident upon the instrument resulting from an assumed spherical object by

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where Io is the solar irradiance at 1 AU, r is the reflectivity of the object (equal to 3/2 the “geometric albedo”), a is the radius of the object, s is the distance from the Sun in astronomical units, and R is the range from the object to the detector. A Sun-object-instrument angle of approximately 45° is assumed. For a single detector, one would have no way of distinguishing objects with the same as R ratio. The Sisyphus concept provides a means of determining the range and, hence, the size of the object. Consider three optically alined telescopes equipped with photomultipliers as defining three parallel cones in space. If the telescopes are identical, then the edges of the fields of view remain at a fixed distance from each other regardless of range. Any luminous object that crosses through the intersecting fields of view is then detected by each of the photomultipliers. From the entrance and exit times in each field of view, one can completely calculate the trajectory of the object in space, provided only that one has sufficiently good optics and a sufficiently long baseline between telescopes.

'Refers to man's never-ending confrontation with the environment; i.e., space rocks.

For the mathematics of the system, we define three cones as shown in figure 1; their half angles are 6. Lines joining their apexes form an arbitrary triangle in the plane perpendicular to their axes. For purposes of convention, the vector from the base of the ith cone to the particle's entrance into that cone is designated pi and the vector to the particle's exit is of Times of entrance and exit at the ith cone are designated to where j is 1 for an entrance point or 2 for an exit point. v is an arbitrary velocity vector and li is the distance between the ith and jth cone.

Using this convention, five independent vector equations result:

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By breaking these into components, we have 15 equations in 15 unknowns, therefore, a solution exists. Because the derivation is long and tedious, it is omitted here. The solution has been programed for computer use. The above vector equations remain unchanged if the cone axes are misalined (i.e., not parallel). However, the 15 component equations are more complex

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