Explorer 35
NASA lunar orbiter studied plasma, fields, and particles.
Explorer 35, also known as IMP-E, AIMP-2, or Anchored IMP-2, was a spin-stabilized spacecraft from NASA's Explorer program. Its mission was to investigate interplanetary plasma, magnetic fields, energetic particles, and solar X-rays while operating from lunar orbit.
The spacecraft closely resembled the earlier Explorer 33. It weighed 104.3 kilograms and had an octagonal prism body measuring 71 centimeters across and 20.3 centimeters high. Four solar arrays, holding 6,144 n/p cells and generating an average of 70 watts, extended from the main bus. Two magnetometer booms, each 183 centimeters long, also extended outward, and four whip antennas were mounted on top. A retrorocket sat atop the bus, and power was stored in silver-cadmium batteries. Communication used a 7-watt transmitter with PFM telemetry and a digital data processor. The science payload, weighing 23.1 kilograms, included two 3-axis magnetometers, analyzers for low-energy protons and alpha particles, detectors for low-energy protons and electrons, an energetic particle detector, a plasma probe, a micrometeorite detector, a solar cell damage experiment, and instruments for gravity field and bistatic radar studies.
Part of the Interplanetary Monitoring Platform program, Explorer 35 was launched on 19 July 1967 from Cape Kennedy's Eastern Test Range aboard a Thor-Delta E1. It followed a direct ascent trajectory, reaching the Moon on 22 July 1967. After a 23-second retrorocket burn, it entered an elliptical lunar orbit with an initial altitude of 800 by 7,692 kilometers and an inclination of 147 degrees. The main engine separated two hours later. The spacecraft operated normally in lunar orbit for six years, until it was shut down on 24 June 1973. Its spin axis was nearly perpendicular to the ecliptic plane, with a spin rate of 25.6 rpm.
The Ames magnetic fields experiment used a boom-mounted triaxial fluxgate magnetometer. One sensor was aligned with the spin axis, and a motor swapped it with a spin-plane sensor every 24 hours for calibration. The noise threshold was about 0.2 nT, with three ranges covering ±20, 60, and 200 nT full scale per component. Digitization accuracy was 1% of each range. Measurements were taken every 2.05 seconds, with the same range repeated every 6.14 seconds.
- Launch date
- 19 July 1967
- End of mission
- 24 June 1973
- Mass
- 104.3 kg (230 lb)
- Orbit
- Lunar orbit, initial 800 × 7,692 km altitude at 147° inclination
- Program
- Explorer program, Interplanetary Monitoring Platform
- Power
- 70 watts average from solar cells
Lore & Background
The spacecraft was spin-stabilized with a spin rate of 25.6 rpm, its spin axis nearly perpendicular to the ecliptic plane. After a direct ascent trajectory, it reached the Moon on 22 July 1967 and entered lunar orbit following a 23-second retrorocket burn. The main engine separated two hours later. Explorer 35 provided important reference data for magnetic field measurements taken on the Moon during the Apollo program.
Reader's Guide
Explorer 35's significance lies in its long-term, six-year operation in lunar orbit, providing continuous measurements of interplanetary plasma, magnetic fields, energetic particles, and solar X-rays. Its magnetometer data served as a reference for Apollo-era lunar magnetic field studies. The bistatic radar experiment used the spacecraft's telemetry transmissions to study the electromagnetic reflective properties of the lunar surface, determining dielectric constants and mean slopes near the lunar equator. The spacecraft also carried experiments to measure micrometeorite flux, low-energy plasma, and energetic particles, contributing to understanding the Moon's environment and the solar wind's interaction with the Moon. Its design was similar to Explorer 33, and it was part of the Interplanetary Monitoring Platform series, with Explorer 41 (IMP-G) following in 1969.
Did You Know?
- Explorer 35 was launched on 19 July 1967 and reached the Moon on 22 July 1967.
- The spacecraft mass was 104.3 kg and its science payload mass was 23.1 kg.
- It carried two boom-mounted triaxial fluxgate magnetometers, one from Ames and one from GSFC.
- The bistatic radar experiment used the 136.10-Hz telemetry transmissions scattered from the lunar surface and recorded by the 46 m Stanford dish antenna.
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