Helios 1, 2

Helios 1, 2 [NASA]

The Helios 1 and Helios 2 spacecraft were a pair of deep space probes developed by the Federal Republic of Germany (FRG) in a cooperative program with NASA. Experiments were provided by scientists from both FRG and the U.S. NASA supplied the Titan-3E Centaur-D1T Star-37E launch vehicles.

The spacecrafts were equipped with two booms and a 32-m electric dipole. The payload consisted of a fluxgate magnetometer; electric and magnetic wave experiments, which covered various bands in the frequency range 6 Hz to 3 MHz; charged-particle experiments, which covered various energy ranges starting with solar wind thermal energies and extending to 1 GeV; a zodiacal-light experiment; and a micrometeoroid experiment. The purpose of the mission was to make pioneering measurements of the interplanetary medium from the vicinity of the earth's orbit to 0.3 AU.

Following experiments were conducted:

• Fluxgate Magnetometer for Field Fluctuations
• Fluxgate Magnetometer for Average Fields
• Search Coil Magnetometer
• Solar Wind Plasma Wave
• Fine Frequency, Coarse Time Resolution Spectrum Analysis
• 26.5-KHz to 3 MHz Radio Wave
• Cosmic-Ray Particles
• Galactic and Solar Cosmic Rays
• Plasma Detectors
• Energetic Electron and Proton Detector
• Zodiacal Light Photometer
• Micrometeoroid Detector and Analyzer
• Celestial Mechanics

The spin axis was normal to the ecliptic, and the nominal spin rate was 1 rps. The outer spacecraft surface was dielectric, effectively (because of the sheath potential) raising the low-energy threshold for the solar wind plasma experiment to as high as 100 eV. Also, sheath-related coupling caused by the spacecraft antennae produced interference with the wave experiments. The spacecraft was capable of being operated at bit rates from 4096 to 8 bps, variable by factors of 2. While the spacecraft was moving to perihelion, it was generally operated from 64 to 256 bps; and near 0.3 AU, it was operated at the highest bit rate.

Because of a deployment failure of one axis of the 32-m, tip-to-tip, dipole antenna of Helios 1, one axis was shorted, causing the antenna to function as a monopole. The major effect of this anomaly was to increase the effective instrument thresholds, and to introduce additional uncertainties in the effective antenna length.

Because of difficulty encountered with the high-gain antenna, and scheduling conflicts with Viking, relatively fewer high-bit-rate data were obtained from Helios-B than were available from Helios-A.