---
title: "Helios Dust Instrumentation"
chunk: 2/2
source: "https://en.wikipedia.org/wiki/Helios_Dust_Instrumentation"
category: "reference"
tags: "science, encyclopedia"
date_saved: "2026-05-05T03:41:35.304459+00:00"
instance: "kb-cron"
---

The South sensor was shielded by the spacecraft rim from direct sun light, whereas the ecliptic sensor was directly exposed to the intense solar radiation (up to 13 kW/m2). Therefore, the interior of the sensor was protected by a 0.3 μm thick aluminized parylene film which was attached to the first entrance grid. In order to study the effect of micrometeoroids penetrating the film, extensive dust accelerator studies with various materials were performed. It was shown that the penetration limit of the Helios film depends strongly on the density of meteoroids. 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. The projectile materials included iron (Fe), quartz, glass, aluminium (Al), aluminium oxide (Al2O3), polystyrene, and kaolin. The mass resolution of the mass spectra of the Helios sensors was low 
  
    
      
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        =
        
          
            
              
                
              
              
                
                  M
                
              
            
            
              
                
              
              
                
                  Δ
                  M
                
              
            
          
        
        ∼
        10
      
    
    {\displaystyle R={\cfrac {M}{\Delta M}}\sim 10}
  
, i.e. only ions of atomic mass 10 Da could be separated from ions of mass 11 Da. 
These mass spectra served as reference for the spectra obtained in space. Spectra were recorded from 10 Da to 70 Da. The mean calibration spectra are presented in a three phase diagram: low masses (10 to 30 Da), medium masses (30 to 50 Da), and high masses (50 to 70 Da).

=== Micrometeoroid data ===

During ten orbits about the sun from 1974 to 1980 the Helios 1 micrometeoroid analyzer transmitted data of 235 dust impacts to Earth. Since the onboard data storage capability was limited and the data transmission rate varied strongly depending on the distance between spacecraft and Earth not all data recorded by the sensors was received on Earth. The effective measuring time ranged from ~30% at perihel to ~75% at 1 au distance. Many noise events caused by solar wind plasma and photo-electrons were recorded by the sensors as well. Only events within a coincidence time of 12 microseconds between positive and negative signals and, mainly, the measurement of a mass spectrum following the initial trigger were considered dust impacts. Quantities determined for each impact are: the time and position, the azimuth of the sensor viewing at the time of impact, the total positive charge of the impact signal, the rise-time of the charge signal (proxy for the impact speed) and a complete mass spectrum. The micrometeoroid instrument on Helios 2 was much noisier and recorded only a handful of impacts that did not provide additional information.

== Results ==
The Zodiacal light carries information on those regions of interplanetary space along the line of sight, which
contribute significantly to its observed brightness. For Helios this covers the range of 0.09 to about 2 astronomical units.

=== Spatial distribution ===

==== Radial dependencies ====

The zodiacal light instrument observed a strong increase of the zodiacal light brightness inward the Earth orbit. The brightness was more than a factor 10 higher at spacecraft position 0.3 au than at 1 au. This brightness increase corresponds to interplanetary dust density increase corresponding to 
  
    
      
        N
        (
        r
        )
        ∼
        
          r
          
            −
            1.3
          
        
      
    
    {\displaystyle N(r)\sim r^{-1.3}}
  
. This strong increase requires that there is a source of interplanetary dust inside the Earth’s orbit. It was suggested that collisional fragmentation of bigger meteoroids generates the dust observed in the zodiacal light.
The radial flux of micrometeoroids recorded by Helios increased by a factor 5 to 10 depending on the mass from 10−17 kg to 10−13 kg. This information together with the position and azimuth measurements was used in the first dynamical model of the interplanetary dust cloud; also the zodiacal light intensities observed by the Helios Zodiacal light instrument were included in this model. The Helios data defined the core, the inclined, and the eccentric populations of this model.

==== Plane of symmetry ====
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. With its ascending node of
87°±5° and inclination of 3.0°±0.3° it lies between the
invariable plane of the Solar System and the plane of the
solar equator.

=== Orbital distribution ===
Of the 235 impacts total 152 were recorded by the South sensor and 83 by the Ecliptic sensor. This excess of impacts on the South sensor had mostly small impact (charge) signals but there was also some excess of big impacts. From thee azimuth values of Ecliptic sensor impacts it was concluded that the micrometeoroids moved on low eccentric orbits, e < 0.4, whereas South sensor impacts moved mostly on higher eccentric orbits. There was even an excess of outward compared to inward trajectories like the beta-meteoroids that were observed earlier by the Pioneer 8 and 9 dust instruments.

=== Optical, physical, and chemical properties ===

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. 
On the basis of the penetration studies with the Helios film the excess of impacts on the South sensor was interpreted to be due to low density, 
  
    
      
        ρ
      
    
    {\displaystyle \rho }
  
 < 1000 kg/m3, meteoroids that were shielded by the entrance film from entering the Ecliptic sensor.
Helios mass spectra range from those with dominant low masses up to 30 Da that are compatible with silicates to those with dominant high masses between 50 and 60 Da of iron and molecular ion types. The spectra display no clustering of single minerals. The continuous transition from low to high ion masses indicates that individual grains are a mixture of various minerals and carbonaceous compounds.

=== Cometary and interstellar dust streams ===

The Helios zodiacal light measurements show excellent stability.
This allows detecting local brightness excesses if they are crossed
by the Helios field-of-view, like it happened for comet West or for
the Quadrantid meteor shower. Repetition by about 0.2% from
orbit to orbit sufficed to detect the dust ring along the orbit
of Venus.
Inspection of the Helios micrometeoroid data showed a clustering of impacts in the same region of space on different Helios orbits. A search with the Interplanetary Meteoroid Environment for eXploration (IMEX) dust streams in space model 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.
After the discovery of interstellar dust passing through the planetary system by the Ulysses spacecraft interstellar dust particles were also found in the Helios micrometeoroid data. Based on the spacecraft position, the azimuth and impact charge 27 impactors are compatible with an interstellar source. The Helios measurements comprise interstellar dust measurements closest to the Sun.

== References ==