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Some details of the systems comprising the penetration experiment are discussed in the following paragraphs. The weight of the experiment hardware
is approximately 13.3 N (3 lbf) and the power required by the experiment is 1 W.
Pressurized Cell Penetration Detector
The experiment has 0.47 m2 (5 ft2) of detector area composed of 216 individual pressurized cells. The 0.47 m2 is made up of 12 panels, each with approximate overall dimensions of 20 by 30 cm (8 by 12 in.) and composed of 18 individual pressurized cells. Figure 2 is a photograph of one detector panel.
The panels are made of 21-6-9 stainless steel. Each of the panels is made by resistance welding a 25 pm (1 mil) thick and a 50 pm thick sheet of stainless steel together in an “air mattress” configuration to form the individual cells. The cells will be pressurized with a nitrogen and argon gas mixture. The mixture will be 75 percent argon and 25 percent nitrogen. Each cell will be pressurized to a pressure of 115 kN/m3 (16.7 psia).
The pressure switch that is used to indicate the loss of pressure in each cell is a cold-cathode device. The switch, as can be seen in figure 2, consists of two electrodes in a pressure cavity that is connected by a copper tube to the pressure cell. Approximately 525 V is impressed continuously across the two
electrodes, which are insulated from each other and from the panel. In the event of a cell puncture, the device will act as a glow tube because of ionization of the internal gas and it will conduct an electric current during a limited pressure range as the cell leaks down. The device will start conducting when the pressure in the cell drops below about 14 kN/m3 (2 psia) and it will stop conducting when the pressure drops below about 0.14 kN/m”. The tips of the two electrodes in the pressure cavity are electroplated with a small amount of **Ni to enhance ionization of the gas.
A functional block diagram of the experiment electronics is shown in figure 3. The experiment is divided into two essentially independent parts for maximum experiment reliability. One electronic system is used for half of the penetration detectors and another electronic system is used for the other half. A common dc/dc power converter takes power from the spacecraft power system and supplies power for the pressure switches, for each of the two signal conditioning circuits, and for each of the two recycling event counters. The pulse resulting from the discharge of any pressure switch will be shaped and stored by a counter. With a discharge, an event counter will advance one count and will be locked out so it cannot advance again for a period of 86 min. This is to insure that any multiple pulsing on the initiation of a switch discharge will not be interpreted by the system as multiple penetrations. The probability of another legitimate impact and switch discharge during this lockout period is small.
Each of the event counters has a capacity of 32 counts before recycling. The time of a sensor puncture will be assumed to be the time at which an
Figure 4.-Electronic module.
An artist’s sketch of the Pioneer F and G spacecraft is shown in figure 5. The 12 penetration detector panels will be mounted on the back side of the spacecraft high-gain antenna dish as is shown in the inserted sketch. A wiring
harness will connect the panels to the experiment electronics, which will be located inside the spacecraft scientific instrument compartment.
TRAJECTORY AND ORIENTATION
The trajectory for the Pioneer F and G missions to Jupiter is shown in figure 6. The spacecraft, flying near the plane of the ecliptic, will enter the region of the asteroid belt approximately 150 days after launch, and they will remain in the belt for approximately 200 days. The launch of Pioneer G is planned to take place approximately 390 days after the launch of Pioneer For essentially just after Pioneer F traverses the asteroid belt. The spacecraft are scheduled to reach Jupiter approximately 600 days after launch.
The spacecraft will spin about an axis through the center of the high-gain antenna dish. With the exception of the first few hours of the mission, the spin axis will essentially be oriented such that it intersects the Earth throughout the mission. This spacecraft orientation places the detector panels in a reasonably good viewing position to intercept asteroidal particles.
It is assumed that the asteroidal particles are in near-circular orbits and thus the relative impact velocity vector between the particles and the spacecraft will remain near the spin axis. As is illustrated in figure 7, the relative velocity vector will be only 28° off the spin axis at 1.6 AU, and it will diminish to only 8.5° off the spin axis at 2.5 AU. As the spacecraft leaves the asteroid belt at
Figure 7.—Spacecraft orientation. V. = velocity of asteroidal particles; v. = velocity of spacecraft; v = relative impact velocity
about 3.8 AU, the relative velocity vector will increase to about 45° off the spin axis.
It is assumed that the cometary particles in interplanetary space are near omnidirectional and thus that the detector panel orientation is not critical for their detection.
ESTIMATE OF PENETRATIONS
Table I presents the number of detected penetration events that are indicated by Kessler's model of the interplanetary meteoroid environment (ref. 1). As already discussed, this model, and thus the predicted events, can be grossly in error and one would be presumptuous to place much confidence in such predictions.