EPE (Energetic Particles Explorer) were a series of
spin-stabilized, solar-cell-powered spacecraft instrumented to measure cosmic-ray
particles, trapped particles, solar wind protons, and magnetospheric and interplanetary
magnetic fields.
Explorer 12 was instrumented to measure cosmic-ray particles, trapped
particles, solar wind protons, and magnetospheric and interplanetary magnetic fields. It
was the first of the S 3 series of spacecraft. A 16-channel PFM/PM time-division
multiplexed telemeter was used. The time required to sample the 16 channels (one frame
period) was 0.324 s. Half of the channels were used to convey eight-level digital
information, and the other channels were used for analog information. During ground
processing of the telemetered data, the analog information was digitized with an accuracy
of 1/100th of full scale. One analog channel was subcommutated in a 16-frame-long pattern
and was used to telemeter spacecraft temperatures, power system voltages, currents, etc. A
digital solar aspect sensor measured the spin period and phase, digitized to 0.041 s, and
the angle between the spin axis and sun direction to about 3-deg intervals. The spacecraft
functioned well until 6 December 1961, when it ceased transmitting data apparently as a
result of failures in the power system. Good data were recorded for approximately 90% of
the active lifetime of the spacecraft. The initial spin rate was 28.0 rpm, and the spin
axis direction was right ascension 48 deg, declination -28 deg. The direction was nearly
constant with time, and the spin rate slowly increased with time to 34.3 rpm. Apogee
direction varied from about 1200 h to 0600 h local time.
- Electrostatic Analyzer of Solar Plasma
- Fluxgate Magnetometers
- Charged Particles
- Solar Cell Damage Experiment
- Proton-Electron Scintillation Detector
- Cosmic Rays
Explorer 14 was instrumented to measure cosmic-ray particles,
trapped particles, solar wind protons, and magnetospheric and interplanetary magnetic
fields. It was the second of the S 3. A 16-channel PFM/PM time-division multiplexed
telemeter was used. The time required to sample the 16 channels (one frame period) was
0.323 s. Half of the channels were used to convey eight-level digital information, and the
others were used for analog information. During ground processing of the telemetered data,
the analog information was digitized with an accuracy of 1/100th of full scale. One analog
channel was subcommutated in a 16-frame-long pattern and was used to telemeter spacecraft
temperatures, power system voltages, currents, etc. A digital solar aspect sensor measured
the spin period and phase, digitized to 0.041 s, and the angle between the spin axis and
sun direction to about 3-deg intervals. The spacecraft functioned well except for the
period from January 10 to 24, 1963, and after 11 August 1963, when the encoder
malfunctioned terminating the transmission of usable data. Good data were recorded for
approximately 85% of the active lifetime of the spacecraft. The spacecraft was coning
(37-deg maximum half-angle) until 10 January 1963. After 24 January 1963, it was
spin-stabilized at a rate of 10 rpm. This rate slowly decreased to 1 rpm on 8 July 1963.
Initially, the local time of apogee was 0700 h.
- Proton Analyzer
- Fluxgate Magnetometers
- Trapped Particle Radiation
- Cosmic Rays
- Solar Aspect Sensor
- Electrolytic Timer Experiment
- Solar Cell Damage Experiment
- Proton-Electron Scintillation Detector
Explorer 15 was instrumented to study the artificial radiation belt
produced by the Starfish high-altitude nuclear burst of July 1962. The backup payload for
Explorer 14 was modified and used for Explorer 15. The instrumentation included three sets
of particle detectors to study both electrons and protons, and a two-axis fluxgate
magnetometer to determine magnetic aspect. A 16-channel PFM/PM time-division multiplexed
telemeter was used. The time required to sample the 16 channels (one frame period) was
0.323 s. Half of the channels were used to convey eight-level digital information, and the
others were used for analog information. During ground processing of the telemetered data,
the analog information was digitized with an accuracy of 1/100th of full scale. One analog
channel was subcommutated in a pattern 16 frames long and was used to telemeter spacecraft
temperatures, power system voltages, currents, etc. A digital solar aspect sensor measured
the spin period and phase, digitized to 0.041 s, and the angle between the spin axis and
the sun direction to about 3-deg intervals. During launch the spacecraft failed to despin.
The spin rate ranged from 72.9 to 73.2 rpm during the life of the spacecraft. The spin
axis pointed at right ascension 80.97 deg and declination 20.9 deg.
- Electron and Proton Solid-State Detectors
- Angular Distribution
- Proton-Electron Scintillation Detector
- Fluxgate Magnetometers
- Directional and Omnidirectional Energetic Protons and Electrons
Explorer 26 was instrumented to measure trapped particles and the
geomagnetic field. A 16-channel PFM/PM time-division multiplexed telemeter was used. The
time required to sample the 16 channels (one frame period) was 0.29 s. Half of the
channels were used to convey eight-level digital information. The other channels were used
for analog information. During ground processing, the analog information was digitized
with an accuracy of 1/800th of full scale. One analog channel was subcommutated in a
16-frame-long pattern and used to telemeter spacecraft temperatures, power system
voltages, currents, etc. A digital solar aspect sensor measured the spin period and phase,
digitized to 0.036 s, and the angle between the spin axis and sun direction to about 3-deg
intervals. The spacecraft systems functioned well, except for some undervoltage turnoffs,
until 26 May 1967, when the telemeter failed. The initial spin rate was 33 rpm, and the
spin axis direction was right ascension 272.8 deg and declination 21.5 deg. The spin rate
decreased with time to 2 rpm on 9 September 1965. For the balance of its life, the
spacecraft was coning or tumbling at a rate of about 1 rpm.
- Solid-State Electron Detector
- Omnidirectional and Unidirectional Electron and Proton Fluxes
- Fluxgate Magnetometers
- Proton-Electron Scintillation Detector
- Solar Cell Damage
A back-up satellite of the Explorer-12 is on display in the Space Science exhibition station at the Steven F. Udvar-Hazy Center in Chantilly, VA.
