Description

Pioneer 8 was the third in a series of solar-orbiting, spin-stabilized, solar-cell and battery-powered satellites designed to obtain measurements of interplanetary phenomena from widely separated points in space on a continuing basis. The spacecraft carried experiments to study the positive ions and electrons in the solar wind, the interplanetary electron density (radio propagation experiment), solar and galactic cosmic rays, the interplanetary magnetic field, cosmic dust, and electric fields. Its main antenna was a high-gain directional antenna. The spacecraft was spin-stabilized at about 60 rpm, and the spin axis was perpendicular to the ecliptic plane and pointed toward the south ecliptic pole. By ground command, one of five bit rates, one of four data formats, and one of four operating modes could be selected. The five bit rates were 512, 256, 64, 16, and 8 bps. Three of the four data formats were used primarily for scientific data and consisted of 32 seven-bit words per frame. One scientific data format was used at the two highest bit rates. Another was used at the three lowest bit rates. The third was used for data from only the radio propagation experiment. The fourth data format was used mainly for engineering data. The four operating modes were (1) real time, (2) telemetry store, (3) duty cycle store, and (4) memory readout. In the real-time mode, data were sampled and transmitted directly (without storage) as specified by the data format and bit rate selected. In the telemetry store mode, data were stored and transmitted simultaneously in the format and at the bit rate selected. In the duty cycle store mode, a single frame of scientific data was collected and stored at a rate of 512 bps. The time interval between the collection and storage of successive frames could be varied by ground command between 2 and 17 min to provide partial data coverage for periods up to 19 h, as limited by the bit storage capacity. In the memory readout mode, data were read out at whatever bit rate was appropriate to the satellite distance from the Earth. Pioneer 8 was launched on 13 December 1967 into a heliocentric orbit with a mean radius of 1.1 AU. The spacecraft was last tracked successfully on 22 August 1996, after being commanded to the backup transmitter tube (TWT). There are no further plans to track or attempt communications with Pioneer 8.

Spacecraft and Subsytems

Instruments

Single-Axis Magnetomete

Electrostatic Analyzer
A single, boom-mounted uniaxial fluxgate magnetometer, with mode-dependent ranges of plus or minus 32 nT and plus or minus 96 nT and corresponding resolutions of plus or minus 0.125 nT and plus or minus 0.375 nT, obtained a vector magnetic field measurement by means of three measurements taken at equal time intervals during each spacecraft spin period (approximately 1 s). At telemetry bit rates less than or equal to 16 bps, averages were computed on board for transmission to earth. For further details, see Mariani and Ness, J. Geophys. Res., v. 74, p. 5633, 1969. NSSDC has all the useful data that exist from this investigation.

Two-Frequency Beacon Receiver
A truncated hemispherical electrostatic analyzer (120-deg total parallel plate curvature) with three contiguous current collectors was used to study the directional intensity of the electrons and positive ions in the solar wind. Ions were detected in 30 logarithmically equispaced energy per unit charge (E/Q) steps from 150 to 15,000 V. There was an electron mode of operation in which electrons were measured in 14 logarithmically equispaced E/Q steps ranging from 12 to 1000 V. There was also a zero E/Q, or background, step. The three collectors measured particles incident from three different contiguous angular intervals relative to the spacecraft equatorial plane (same as the ecliptic plane). Two collectors measured flux from 10 to 85 deg on either side of the spacecraft equatorial plane, and the third measured flux in a 20-deg interval centered on the spacecraft equatorial plane. As the spacecraft was spinning, fluxes were measured in 23 possible 2-13/16-deg wide azimuthal angular sectors. Seventeen of these sectors were contiguous and bracketed the solar direction. The remaining six sectors were widely spaced. The instrument had three modes of data collection; polar scan, azimuthal scan, and maximum flux. At the two highest bit rates (512 and 256 bps) the polar scan mode was alternated with the azimuthal scan mode at each E/Q step. In the polar scan mode, all three collectors were observed, and the peak flux obtained and the azimuthal direction (to 2-13/16 deg) of the observation were reported for each collector. In the azimuthal scan mode, the peak flux observed in the 23 azimuthal sectors was recorded for the central collector at each E/Q step. At the low bit rates (64, 16, and 8 bps), the maximum flux mode was used at each E/Q step followed by either (1) for ions, a polar scan and an azimuthal scan at that E/Q step where the peak flux measurement during the maximum flux mode was obtained, or (2) for electrons, a polar scan and an azimuthal scan at E/Q = 100 V. In the maximum flux mode, only the central collector was observed, and the peak flux obtained and the azimuthal direction (to 2-13/16 deg) of the observation were reported. A complete set of measurements consisted of seven sets of ion measurements (at each E/Q step) and one set of electron measurements (at each E/Q step). At the high bit rates (512 and 256 bps) one set of ion measurements took 62 s and one set of electron measurements 38 s. At the low bit rates (64, 16, and 8 bps), one set of ion measurements took 37 s and one set of electron measurements 28 s. At 64 bps, a complete set of measurements (seven ions plus one electron) was taken and telemetered every 402.5 s. At 16 bps, it took 1610 s, and, at 8 bps, it took 3220 s.

Cosmic Dust Detector
Both 423.3-MHz and its 2/17 subharmonic 49.8-MHz signals were transmitted from a 46-m steerable parabolic antenna at Stanford University to the two-frequency radio receiver on the spacecraft. The high-frequency signal served as a reference signal since its propagation time was not appreciably delayed. The low-frequency signal was delayed in proportion to the total electron content in the propagation path. On the spacecraft, a phase-locked receiver counted the beat frequency zero crossings of the received signals to obtain measurements of phase-path differences. Differential delay of the group velocity was also observed, and these values were telemetered to the ground station. From calculated total electron content values, the ionospheric effect (up to a selected altitude obtained from other experimental techniques) could be subtracted to produce data describing the interplanetary electron content of the solar wind and its variations. For similar experiments covering other time periods, see 68-100A-03, 66-075A-04, 65-105A-04, and 67-060A-02. A more detailed description of the experiment can be found in J. Geophys. Res., v. 17, p. 3325-3327, and in Radio Sci., v. 6, p. 55-63.

Cosmic Ray Anisotropy
This experiment was designed to (1) measure the cosmic dust flux density in the solar system, (2) determine the distribution of cosmic dust concentrations in the earth's orbit, (3) determine the gradient, flux density, and speed of particles in meteor streams, and (4) perform an in-flight control experiment on the reliability of the microphone as a cosmic dust sensor. The experiment instrumentation, which was mounted in the equator of the satellite with its axis radial to the satellite spin axis facing in the ecliptic plane, consisted of a front film-grid sensor array and a rear film-grid sensor array, spaced 5 cm apart, and an acoustical impact plate upon which the rear film was mounted. The sensor arrays consisted of four vertical film strips crossed by four horizontal grid strips to form 16 front and 16 rear film-grid arrays (each 2.5 cm sq), creating 256 possible combinations. Each grid strip and film strip was connected to a separate output amplifier whose signals were used to determine the segment in which an impact occurred. The front film sensor, which was recessed 3 cm into the experiment housing, consisted of an eight-layer composite -- 700-A parylene encapsulation, 500-A copper, 300-A aluminum, 3000-A parylene substrate, 300-A aluminum, 500-A copper, support mesh, and 500-A parylene encapsulation. Each of the rear sensor-array film strips consisted of a 60-micrometer molybdenum sheet cemented to a quartz acoustical sensor plate. The operation of the sensors was based on two basic measurable phenomena that occur when a hypervelocity particle impacts on a surface -- (1) formation of plasma and (2) transfer of momentum. When the front film was penetrated by a particle, a time-of-flight 4-MHz electronic clock was activated. The clock was shut off when the particle impacted on the rear film thus measuring particle speed and direction. Three general cosmic dust particle types were detectable -- (1) high-energy, hypervelocity particles (greater than 1 erg), which produced responses at both front and rear film sensors, (2) low-energy, hypervelocity particles (less than 1 erg), which produced responses only at the front film sensor, and (3) relatively large high-velocity particles (greater than 0.1 nanograms), which could pass through the front and rear film sensor arrays without generating a detectable plasma but could still impart a measurable impulse to the acoustical sensor. The acoustical sensors were designed to perform an in-flight study on the reliability of the microphone as a cosmic dust sensor in addition to performing as an impact sensor for this experiment. In-flight calibration was provided and initiated by ground command and monitored the experiment electronics in addition to providing a check on the physical condition of the plasma sensors. The sensors were calibrated prior to the flight by impacts with iron spheres ranging in mass from 1 nanogram to 0.1 picogram, accelerated by a 2-mv electrostatic accelerator to 2 to 10 km/s.

Cosmic Ray Gradient
This experiment utilized a telescope comprised of five solid-state sensors, a Cerenkov detector, and an anticoincidence shield. The telescope axis was perpendicular to the spacecraft spin axis. As determined by two coincidence modes and electronic discrimination of sensor output pulses, particles measured were electrons in three contiguous energy intervals between 0.34 and 8.4 MeV, protons in six contiguous energy intervals between 3.49 and 64.3 MeV (one of five count rates was due to the sum of counts in two noncontiguous energy intervals), and alpha particles in four contiguous energy intervals between 6.64 and 64.1 MeV/nucleon (one of three count rates was due to the sum of counts in two noncontiguous energy intervals). A third coincidence mode measured the sum of counts due to electrons above 0.6 MeV and nuclei above 14 MeV/nucleon. A fourth coincidence mode measured the sum of nuclei above 42 MeV/nucleon and electrons above 5.1 MeV. Spacecraft spin-integrated directional fluxes were measured in the various modes. Accumulation times and readout intervals were dependent on the telemetry bit rate and were typically in tens of seconds. In all cases, they were longer than the spacecraft spin period. At low telemetry bit rates accumulator saturation rendered some counting modes to be of no value. For further details, see J. Geophys. Res., v. 76, p. 1605, 1971.

Plasma Wav
Electrostatic and electromagnetic plasma waves were measured in the solar wind near 1 AU using an unbalanced dipole antenna. The 423-MHz Stanford University antenna, which served as the sensor, was capacitively coupled to three channels. Channel 1 was a 15 % bandpass filter centered at 400 Hz, a typical interplanetary electron cyclotron frequency. Channel 2 was a 15 % bandpass filter centered at 22 kHz, a typical interplanetary electron plasma frequency. The broadband channel from 100 Hz to 100 kHz was fed into a count rate meter that measured the number of positive going pulses per unit time having amplitudes large enough to cross the present trigger level. The trigger level was varied in 16 steps per telemetry sequence. The trigger levels together with the count rate at each level gave a measure of the broadband power spectrum. Almost all of the time this measurement amounts to the power spectrum at near 100 Hz. At the highest telemetry rate of Pioneer 8, this sequence was repeated every 7.47 min.

Celestial Mechanics
The objectives of this investigation were to: (1) obtain primary determinations of the masses of the earth and moon and the distance between the earth and sun (AU), (2) use the tracking data from the whole series of Pioneer probes in a program designed to improve the ephemeris of the earth, and (3) investigate the possibility of a test of general relativistic mechanics using the pioneer orbits and data. The instrumentation was a two-way s-band doppler tracking mechanism using high-gain antennas with disk-like patterns in a plane perpendicular to the spin-axis of the spacecraft. When the spin-axis was perpendicular to the ecliptic, radio signals from the antenna continuously illuminated the earth. Data was transmitted continuously and was received at ground-based Deep Space Network stations with 26.5-m diameter antennas and with the 64-m antenna in California.

Launch-Orbit Information

Launch Information

Launch Date: 1967-12-13 at 14:08:00 UTC
Launch Vehicle: Delta
Launch Site: Cape Canaveral, United States
Mass: 146.0 kg

Type: Orbiter Central Body: Sun

Epoch start: 1975-09-17 00:00:00 UTC

Periapsis Apoapsis Period Inclination Eccentricity
0.99 AU 1.09 AU 387.5 days 0.057° 0.04615

Summary