Helios Dust Instrumentation

The two Helios spacecraft were the result of a joint venture of West Germany's space agency DLR and NASA.

The primary goal of the Zodiacal light instrument on Helios was to determine the three-dimensional spatial distribution of interplanetary dust.

[1] To this end, from all along its orbit, Helios performed precise zodiacal light measurements covering a substantial part of the sky.

Each of the 36 resulting different brightness maps represents an average over 512 Helios rotations, leading to a cycle of total length 5.2 hours, which is continually repeated.

Throughout their mission the Helios space probes were exposed to full sunlight, which exceed the typical zodiacal light intensity by factor of 1012 to 1013.

For accurate (1%) measurements demanding stray light suppression by a factor of 1015 was required, the main design goal to be met.

This could be achieved in three steps: The Zodiacal light instrument was developed at the Max Planck Institute for Astronomy in Heidelberg by Christoph Leinert and colleagues and built by Dornier systems.

[3] The instrument consisted of two impact ionization time-of-flight mass spectrometers and was developed by PI Eberhard Grün, Principal Engineer Peter Gammelin, and colleagues at the Max Planck Institute for Nuclear Physics in Heidelberg.

Part of the ions reach the time-lag focusing region from which they fly through the field-free drift tube at -200 V potential.

Impact experiments with a lab version of the Helios micrometeoroid sensor were performed using several materials at the accelerators at the Max Planck Institute for Nuclear Physics in Heidelberg and at the Ames Research Center, ARC, in Moffet Field.

During ten orbits about the sun from 1974 to 1980 the Helios 1 micrometeoroid analyzer transmitted data of 235 dust impacts to Earth.

Only events within a coincidence time of 12 micro seconds between positive and negative signals and, mainly, the measurement of a mass spectrum following the initial trigger were considered dust impacts.

The Zodiacal light carries information on those regions of interplanetary space along the line of sight, which contribute significantly to its observed brightness.

From the difference between the measured zodiacal light brightness during inbound and outbound parts of the orbit and between right and left of the Sun the plane of symmetry of the interplanetary dust cloud was determined.

[13] The measurements of zodiacal light color - essentially constant along the Helios orbit - and of polarization - showing a decrease closer toward the Sun - also contain information on properties on interplanetary dust particles.

[14] On the basis of the penetration studies with the Helios film[5] the excess of impacts on the South sensor was interpreted to be due to low density,

The continuous transition from low to high ion masses indicates that individual grains are a mixture of various minerals and carbonaceous compounds.

[16] This allows detecting local brightness excesses if they are crossed by the Helios field-of-view, like it happened for comet West[17] or for the Quadrantid meteor shower.

A search with the Interplanetary Meteoroid Environment for eXploration (IMEX) dust streams in space model[19] identified the trails of comets 45P/Honda-Mrkos-Pajdušáková and 72P/Denning-Fujikawa that Helios traversed multiple times during the first ten orbits around the Sun.

Schematic view of the Helios spacecraft with the three photometers (P15, P30, and P90) of the Zodiacal light instrument and the two sensors of the Micrometeoroid analyzer. The hatched areas indicate the respective field of view .
Bottom view of the Helios spacecraft. The three Zodiacal light photometers P15, P30, P90 show the black openings of their stray light baffles. The drift tube of the South (S) micrometeoroid sensor is seen next to the photometers.
P30, one of the three small photometers making up the zodiacal light instrument. In operation, the light enters from upper right through the black Carbon fibre stray light suppressing baffle.
Display of the South and Ecliptic sensors and the common electronics box of the Helios Micrometeoroid Analyzer.
Calibration of the Helios micrometeoroid analyzer with different projectile materials.
Helios 1 Orbit with meteoroid impacts and sensor viewing direction at time of impact. The length of the bars indicate the magnitude of the impact charge measured.
Brightness of the zodiacal light along the orbits of Helios 1 and 2 as observed by the P15 photometers; blue: inward, red: outward motion. The peak at 0.8 AU was caused by Comet West passing through the field of view .
Helios mass spectroscopic analysis of interplanetary and interstellar grains.
Repeatabiliy of Helios zodiacal light measurements from minimum (1975/1976) through maximum (1979/1980) of solar sunspot cycle 21. Red symbols indicate days with coronal mass ejections and enhanced plasma scattering . For comparison the brightness of Procyon was monitored.