In Situ Measurements of Interplanetary Dust in the Inner Solar System

Abstract

The Helios 1 spacecraft was launched in December 1974 into a heliocentric orbit of 0.3 AU perihelion distance. It carries on board a micro-meteoroid experiment which contains two sensors with a total sensitive area of 121 cm2. The ecliptic sensor measures dust particles which have trajectories with elevations from −45° to +55° with respect to the ecliptic plane. The south sensor detects dust particles from −90° to −4°. The ecliptic sensor is covered by a thin film (3000 Å parylene coated with 750 Å aluminium) as protection against solar radiation. The other sensor is shielded by the spacecraft rim from direct sunlight and has an open aperture. Micrometeoroids are detected by the electric charge produced upon impact and the ions are mass analysed in a time-of-flight-spectrometer. During the first 6 orbits of Helios 1 around the sun the experiment registered a total of 168 meteoroids, 52 particles were detected by the ecliptic sensor and 116 particles by the south sensor. Most impacts on the ecliptic sensor were observed when it was pointing in the direction of motion of Helios (apex direction). In contrast to that the south sensor detected most impacts when it was facing in between the solar and antapex directions. Orbit analysis showed that the “apex” particles which are predominantly detected by the ecliptic sensor have eccentricities e < 0.4 or semimajor axes a < 0.5 AU. From comparison with corresponding data from the south sensor it is concluded that the average inclination of these particles is below 30°. The excess of impacts on the south sensor have orbit eccentricities e > 0.5 AU. β-meteoroids which leave the solar system on hyperbolic orbits are directly identified by the imbalance of outgoing (away from the sun) and ingoing particles. Mass analyses of the spectra showed that 40% of the observed spectra have the peak abundance above mass 35 amu which are preliminarily identified as iron meteoroids. 40% of the spectra have the peak abundance below mass 35 amu which correspond to chondritic composition. 20% of the spectra could not be identified in either class.