Description

Pioneer 7 was the second 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 positive ions and electrons in the solar wind, the interplanetary electron density (radio propagation experiment), solar and galactic cosmic rays, and the interplanetary magnetic field. 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 approximately 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 contained primarily scientific data and consisted of 32 seven-bit words per frame. One scientific data format was used for the two highest bit rates. Another was used for the three lowest bit rates. The third contained data from only the radio propagation experiment. The fourth data format contained mainly 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 period between which successive frames were collected and stored 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 7 was launched on 17 August 1966 into a solar orbit with a mean radius of 1.1 AU. The spacecraft was last tracked successfully in March 1995, there are no further plans to track or attempt communication with Pioneer 7.

Spacecraft and Subsytems

Instruments

Single-Axis Magnetometer

A single, boom-mounted, uniaxial fluxgate magnetometer, with a dynamic range of plus or minus 32 nT and plus or minus 0.125-nT resolution, obtained a vector magnetic field measurement by means of three scalar 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, time-averaged field data were returned from the spacecraft. The detector performed well until February 1969, after which no further data were obtained. For further details see Mariani et al, J. Geophys. Res., v. 75, p. 6037, 1970. NSSDC has all the useful data that exist from this investigation.

Solar Wind Plasma Faraday Cup

A multigrid Faraday cup with two semicircular, coplanar collectors was used to study solar wind ions and electrons. The instrument had 14 contiguous energy-per-charge channels between 75 and 9485 V for positive ions and four energy-per-charge channels between 115 and 1600 V for electrons. The instrument view axis was perpendicular to the spacecraft spin axis and parallel to the ecliptic plane. The line separating the two collectors lay in the ecliptic plane, enabling a rough determination of solar wind bulk flow perpendicular to the ecliptic plane. During every second spacecraft rotation and at one voltage level, the sum of the currents from the collectors was obtained in 28 contiguous 11.25-deg angular sectors (from -45 deg to 270 deg, with 0 deg being the spacecraft-sun line). The eight measurements about the sun-earth line (-45 deg to +45 deg) were telemetered, but only the largest measurement in each succeeding 45-deg interval (45 deg to 270 deg) was telemetered. In addition, during this rotation the current from one of the collectors was measured in all twenty-eight 11.25-deg sectors, and the largest was identified and telemetered (both magnitude and sector). A complete set of positive ion measurements and one electron measurement were completed every 32 s. The time between each 32-s group of measurements varied with the bit rate. The experiment worked well from launch until it became inoperable in November 1972. For more complete information, see J. Geophys. Res., v. 71, pp. 3787-3791, August 1966.

Electrostatic Analyzer

A quadrispherical electrostatic analyzer with eight contiguous current collectors was used to study the directional intensity of the electrons and positive ions in the solar wind. Ions were detected in 16 logarithmically equispaced energy per unit charge (E/Q) steps from 200 to 10,000 V. There was an electron mode of operation in which electrons were measured in eight logarithmically equispaced energy per charge steps ranging from 0 to 500 V. The eight collectors measured particles incident from eight different contiguous angular intervals relative to the spacecraft equatorial plane (same as the ecliptic plane). There were four 15-deg intervals, two 20-deg intervals, and two 30-deg intervals. As the spacecraft was spinning, fluxes were measured in 15 azimuthal angular sectors. Eight of the these sectors were 5-5/8 deg wide, were contiguous, and bracketed the solar direction. The remaining seven sectors were 45 deg wide. Three different modes of data collection were used. At the highest bit rate (512 bps), the full scan mode was alternated with the maximum flux mode at each E/Q step. In the full scan mode, the maximum flux observed in each of the 15 azimuthal sectors as the spacecraft rotated was recorded for a given single collector at a given E/Q step. During 24 successive operations of the full scan mode (48 spacecraft revolutions), the 16 ion E/Q steps and eight electron E/Q steps were exercised for a given collector. During eight successive such periods, each of the eight collectors was exercised. The full cycle of full scan mode data required 400 spacecraft revolutions (about 400 s). Such cycles were repeated without interruption at the high bit rate. In the maximum flux mode, for the E/Q step used in the preceding revolution of full scan mode operation, all collectors were observed for one revolution, and the maximum flux observed was reported along with the number of the collector that observed it and the angular direction (2-13/16-deg resolution) of the observation. At the next highest bit rate (256 bps), the short scan mode was alternated every spacecraft revolution with the maximum flux mode. The short scan mode was the same as the full scan, except that only the peak flux in each of the eight 5-5/8-deg-wide azimuthal sectors was recorded, thus, this cycle also took 400 spacecraft revolutions. At the low bit rates (64, 16, and 8 bps), the maximum flux mode alone was used. Thus, no azimuthal distributions were measured. At the low bit rates, it took 32 s for a complete set of ion measurements and 16 s for a complete set of electron measurements. At 64 bps, the ion and electron measurements were taken and telemetered every 84 s. At 16 bps, they were taken and telemetered every 36 s. At 8 bps, they were taken and telemetered every 672 s.

Two-Frequency Beacon Receiver

Both 423.3-MHz and its 2/17 subharmonic 49.8-MHz signals were transmitted from a 4.6-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) was subtracted to produce data describing the interplanetary electron content of the solar wind and its variations. The experiment operated nominally from launch to May 20, 1969. For similar experiments covering other time periods, see 68-100A-03, 67-123A-03, 65-105A-04, and 67-060A-02. More detailed descriptions of the experiment can be found in J. Geophys. Res., v. 71, p. 3325-3327, 1966, and in Radio Sci., v. 6, p. 55-63, 1971.

Cosmic-Ray Anisotropy

This experiment was designed primarily to measure the directional characteristics of galactic and solar cosmic ray fluxes. The particle detector was a CsI (Tl) scintillator crystal that was set into an anticoincidence plastic scintillator collimator cup. Separate photomultiplier tubes viewed the two scintillators. Pulses from the CsI crystal that were not accompanied by pulses from the plastic scintillator were sorted by a three-window pulse-height analyzer, the windows corresponding to energy depositions of 7.2 to 47.4, 47.4 to 64.5, and 64.5 to 81.2 MeV. No positive species identification was made, although most of the counts in each window were usually due to protons with the window energies. For each energy window, counts were separately accumulated in each of four angular sectors as the spacecraft spun. Each angular sector was normally 89.5 deg in width, with the sun either near a sector boundary or in the middle of a sector, depending on the operating mode. However, when large fluxes were encountered, each angular sector was reduced to 11.2 deg, with the sun either in a sector or near the midpoint between two sectors. A spin-integrated (isotropic) mode, in which all particles depositing 7.2 MeV in the CsI crystal (no anticoincidence requirement) were counted, was also used. Accumulation times for each of the 12 directional modes and for the omnidirectional mode varied between 14 and 112 s (spacecraft spin period was about 1 s) depending on the telemetry bit rate. See Bartley et al., Rev. Sci. Instrum., v. 38, p. 266, 1967, for a more detailed experiment description.

Cosmic-Ray Telescope

This experiment used a charged particle telescope composed of four silicon solid-state detectors to study the anisotropy and fluctuations of solar protons and alpha particles. The proton energy ranges sampled were 0.6 to 12.7 MeV, 12.7 to 73.0 MeV, 73.0 to 165 MeV, and E>165 MeV. The alpha particle energy ranges sampled were 2.5 to 52 MeV, 52 to 280 MeV, and E>280 MeV. The time resolution ranged from about one measurement per 0.4 s to about one measurement per 28 s depending on the telemetry bit rate. The detector was mounted so that it made a 360-deg scan in the ecliptic plane about once per second.

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 were transmitted continuously and were received at ground-based deep space network stations with 26.5-m diameter antennas and with the 64-m antenna in California.

Superior Conjunction Faraday Rotation

This experiment utilized measurements of the polarization of the spacecraft telemetry signal to obtain measurements of the relative Faraday rotation due to the interplanetary medium and the earth's ionosphere.

Launch-Orbit Information

Launch Information

Launch Date: 1966-08-17 at 15:20:00 UTC
Launch Vehicle: Delta
Launch Site: Cape Canaveral, United States
Mass: 138.0 kg

Trajectory Details

Type
Central Body: Sun
Epoch start: 1976-02-12 00:00:00 UTC

Orbital Parameters
Periapsis Apoapsis Period Inclination Eccentricity
1.01 AU 1.12 AU 402.9 days 0.098° 0.05436
Summary