2.1.2 Preliminary Results of the Micrometeoroid Experiment on Board Helios A

Abstract For the first time in situ measurements of interplanetary dust have been performed between 0.3 AU and 1 AU from the sun by the micrometeoroid experiment on board Helios A. The measured particle masses are between 10−15 g and 10−8 g and their measured speeds are between 2 km/sec and 20 km/sec. Particle impacts are identified by the time-of-flight spectra of the ions released upon impact. 15 large particles (m ≥ 10−12 g) were detected from Dec. 15, 1974 to Sept. 5, 1975. They show a strong increase of the impact rate (appr. a factor of 10) between 1 AU and 0.3 AU. The directions from which they impacted the sensor are concentrated between the solar direction and the apex direction of the Helios spacecraft.