Explorer 38
First NASA satellite for radio astronomy, launched 1968.
Explorer 38, designated Radio Astronomy Explorer A (RAE-A) and RAE-1, was NASA’s initial satellite built for radio astronomy. It launched on July 4, 1968, from California’s Vandenberg Air Force Base aboard a Delta J rocket, marking the first of two RAE missions in the Explorer program.
Quick Facts
- Names list
- RAE-A
- RAE-1
- Radio Astronomy Explorer-1
- Operator
- NASA
- Satcat
- 03307
- Mission duration
- 1 year (achieved); July 4, 1968, 17:26:50 (in orbit)
- Spacecraft
- Explorer XXXVIII
- Spacecraft bus
- RAE
- Manufacturer
- Goddard Space Flight Center
- Launch mass
- 602 kg / 1,327 lb
Facts from the source article.
Spacecraft
The spacecraft measured the intensity of celestial radio sources, particularly the Sun, as a function of time, direction, and frequency from 0.2 to 20 MHz. It was gravity-gradient stabilized and carried two 230 m long V-antennas, one facing Earth and one facing away, plus a 37 m long dipole antenna oriented tangentially to Earth's surface. A 136-MHz telemetry turnstile was also aboard. Experiments included four step-frequency Ryle-Vonberg radiometers (0.45 to 9.18 MHz), two multichannel total power radiometers (0.2 to 5.4 MHz), a step-frequency V-antenna impedance probe (0.24 to 7.86 MHz), and a dipole antenna capacitance probe (0.25 to 2.2 MHz). The tape recorder began to deteriorate after two months, but good data were obtained from all three antenna systems despite instrument malfunctions and continuous radio interference from Earth.
Experiments
The impedance measurements determined reactive and resistive components of antenna impedance as a function of local electron density, electron temperature, magnetic field, and vehicle potential at ten frequencies from 0.25 to 8 MHz. Two planar electron traps were mounted on opposite sides of the spacecraft; each trap had a collector positively biased to repel ions and reduce photoemission, with a sawtooth voltage applied to a grid. Electron density was derived from the grid voltage-collector current profile, with the ambient value taken from the probe facing the direction of satellite motion. Attitude was determined from electron density or solar and magnetic sensors. Data were tape recorded and telemetered once per orbit, and the sensors operated nominally since launch, providing electron density mapping at spacecraft altitude. Thirty-two channel step frequency radiometers connected to the lower 230 m antenna and the 37 m dipole via high-impedance preamplifiers. The burst radiometer on the dipole stepped through 32 discrete frequencies between 0.2 and 5.4 MHz every 7.7 seconds to generate dynamic spectra, measuring amplitude, rate of change of frequency, and decay time of solar bursts and other noise. Operating in two sensitivity modes, these receivers could measure signals up to 50 dB above the cosmic background. After about 18 months, one preamplifier on the lower V burst radiometer failed, reducing sensitivity and altering the antenna pattern.
Results
By 1971, results included the absolute spectrum and average cosmic noise up to 0.5 MHz. Data from type III solar radio bursts in the 0.2 to 5 MHz band provided a first estimate of solar corona electron density gradient, solar wind speed, and density inhomogeneities between 10 and 30 solar radii. An upper limit on Jupiter's HF radio flux was determined from Moon occultations. Earth's natural and artificial radio emissions were widespread and often 40 dB above the cosmic background in the 0.2 to 9.2 MHz range.
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