Orbital and physical characteristics of micrometeoroids in the inner solar system as observed by Helios 1

The Helios 1 spacecraft was launched into a heliocentric orbit of 0.3 AU perihelion distance. Micrometeoroid experiments each of which contains two sensors are carried. The ecliptic sensor measures dust particles which have trajectories with elevations from -450 to +550 with respect to the ecliptic plane. The south sensor detects dust particles with trajectory elevations from -900 (ecliptic south-pole) to -40. The ecliptic sensor is covered by a thin film as protection against solar radiation. The other sensor is shielded by the spacecraft rim and has an open aperture. During the first 6 orbits 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). The south sensor detected most impacts when it was facing in between the solar and antapex direction. Orbit analysis showed that the 'apex' particles which are predominantly detected by the ecliptic sensor have eccentricities e <0.4 or semi-major axes a <= 0.5 AU. From a comparison with corresponding data from the south sensor it is concluded that the average inclination of 'apex' particles is <300. The excess of impacts on the south sensor, called 'eccentric' particles, have orbit eccentricities >0.4 and semimajor axes >0.5 AU. β-meteoroids leaving the solar system on hyperbolic orbits are directly identified by the observed imbalance of outgoing (away from the sun) and ingoing particles. It is shown that 'eccentric' particles, due to their orbital characteristics, should be observable also by the ecliptic sensor and it is concluded that the entrance film in front of the ecliptic sensor, prevented them from entering it. Particles which do not penetrate the film must have bulk densities below an upper limit. It is shown that approximately 30% of the 'eccentric' particles have densities below 1 g/cm3. (author)