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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Helios Dust Instrumentation | 1/2 | https://en.wikipedia.org/wiki/Helios_Dust_Instrumentation | reference | science, encyclopedia | 2026-05-05T03:41:35.304459+00:00 | kb-cron |
The Helios 1 and 2 spacecraft each carried two dust instruments to characterize the Zodiacal dust cloud inside the Earth’s orbit down to spacecraft positions 0.3 AU from the sun. The Zodiacal light instrument measured the brightness of light scattered by interplanetary dust along the line of sight. The in situ Micrometeoroid analyzer recorded impacts of meteoroids onto the sensitive detector surface and characterized their composition. The instruments delivered radial profiles of their measured data. Comet or meteoroid streams, and even interstellar dust were identified in the data.
== Overview ==
The two Helios spacecraft were the result of a joint venture of West Germany's space agency DLR and NASA. The spacecraft were built in Germany and launched from Cape Canaveral Air Force Station, Florida. Helios 1 was launched in December 1974 onto an elliptic orbit between 1 and 0.31 au. Helios 2 followed in January 1976 and reached 0.29 au perihelion distance. The orbital periods were about 6 months. The Helios spacecraft were spinning with the spin axis perpendicular to the ecliptic plane. The Helios 1 spin axis pointed to ecliptic north whereas the Helios 2 orientation was inverted and the spin axis pointed to ecliptic south. The despun high gain antenna beam pointed always to Earth. Because of the orbit the distance between the spacecraft and Earth varied between a few and 300 million km and the data transmission rate varied accordingly. Twice per Helios orbit the spacecraft was in conjunction (in front or behind the Sun) and no data transmission was possible for a few weeks. Helios 1 delivered scientific data for ten years and Helios 2 for five years.
== Zodiacal light instrument ==
The primary goal of the Zodiacal light instrument on Helios was to determine the three-dimensional spatial distribution of interplanetary dust. To this end, from all along its orbit, Helios performed precise zodiacal light measurements covering a substantial part of the sky. These partial sky maps, because of the rotation of Helios, consisted of a band 1° wide at ecliptic latitude ß = 16° with 32 sectors 5.62°, 11.25° and 22.5° long, a similar band 2° wide at ecliptic latitude ß = 31° and a field of 3° diameter at the ecliptic pole. All fields were in the south for Helios 1, in the north for Helios 2. The width of the sectors was chosen to be smallest for the brightest regions of zodiacal light. This map has been realized by three small (36 mm aperture) photometers, P15, P30, and P90, one for each ecliptic latitude. A stepping motor changed the observing wavelength – with or without polarization – to 360±30 nm, 420±40 nm, 540±70 nm (close to the UBV system) or to dark current and calibration measurements. 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. The sensors were photomultipliers EMR 541 N operating in photon pulse counting mode. 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 photometers were fully kept in the shadow of the Helios solar cell cone, giving 3×10−3 stray light reduction. The multiple reflection in the stray light suppressing baffle added 4×10−7. The coronograph design of the photometers provided the needed additional 3×10−6 of stray light reduction. 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.
== Micrometeoroid analyzer ==
The goal of the Micrometeoroid Analyzer was 1. to determine the spatial distribution of the dust flux in the inner planetary system, and 2. to search for variations of the compositional and physical properties of micrometeoroids. 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. Each sensor (Ecliptic sensor and South sensor) was a 1 m long and 0.15 m diameter tube with two grids and a venetian blind type impact target in front, several more grids, a 0.8 m long field-free drift tube and an electron multiplier in the inside. Micrometeoroids hitting the venetian blind type impact target generate an impact plasma. Electrons are collected by the positively biased grid in front of the target while positive ions are drawn inward by a negatively biased grid behind the target. Part of the ions reach the time-lag focusing region from which they fly through the field-free drift tube at −200 V potential. Ions of different masses reach the electron multiplier at different times and generate a mass spectrum at the multiplier output. Impact signals are recorded by charge-sensitive preamplifiers attached to the electron grid in front and the ion grid behind the target. From these signals together with the mass spectrum the mass and energy of the dust particle and the composition of the impact plasma are obtained.