There are a total of 18 scientific instruments on board Cassini-Huygens. One of these is the Cosmic Dust Analyzer (CDA), which analyses ice and dust particles in the Saturnian system. The special thing about this instrument, which is still the only one in the world, is that it can simultaneously determine the electrical charge, speed, direction of flight and mass of individual particles. The CDA, somewhat disrespectfully referred to as a 'cosmic vacuum cleaner', was developed and built as a joint project by the Institute of Planetary Research at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) in Berlin Adlershof and the Max Planck Institute for Nuclear Physics (MPI-K).
Investigating dust in the Solar System does not sound very exciting at first. But dust that can be found at apparently endless distances in the Solar System tells us something about its origins, as well as our galactic environment. The majority of cosmic dust particles are a few molecules to a micrometre – one thousandth of a millimetre – in size. This includes what is sometimes popularly known as 'stardust'. From a scientific perspective, this stardust is mineral dust that originates when stars are born and is part of the interstellar medium, so does not originate from the Solar System. Such stardust is sometimes found in small spheres, carbonaceous chondrites, which formed planetesimals (the building blocks of the bodies in the Solar System) from the disc of gas and dust circling the Sun four and a half billion years ago. 'Real' stardust is therefore older than Earth and comes from stars that existed before the Sun.
Dust is not just dust
There are different types of dust: intergalactic dust is dust that does not originate from the Milky Way but from another galaxy; interstellar dust originates from other stars in our own galaxy; and interplanetary dust is from the Solar System. The latter is responsible for the luminous phenomenon known as zodiacal light. This is a faint apparition in the ecliptic – in which the constellations of the zodiac can also be seen. Sunlight is radiated onto the particles of interplanetary dust in the ecliptic. It is thought that some 40,000 tons of cosmic dust will enter Earth's atmosphere this year and burn up there.
Following the Galileo mission to Jupiter, researchers decided to include a dust analyser as an experiment on board Cassini. The collaboration between the MPI-K andDLR for its development and construction was agreed upon in 1991. The MPI-K held the scientific lead under Eberhard Grün; when Grün retired, he passed the leadership on to Ralf Srama, who today is a researcher and lecturer at the Institute of Space Systems at the University of Stuttgart. Under the leadership of Franz Lura, who is also a member of the CDA team, DLR took responsibility for the development of the thermal design, the construction of all the mechanics including the rotating platform and the development of data compression algorithms. It also conducted contamination studies to guarantee that the results for the dust particles coming into contact with the detectors clearly come from cosmic dust and not from impurities. In this regard, studies were carried out on the processing inside the large, honeycomb-shaped aluminium sensor housing and on the production of the grid system. These enable the dust particles to reach the dust collection container almost unhindered.
Everything functioning flawlessly even after 18 years
Another technological challenge was the curved shape of an impact sensor made from the extremely brittle and hard metal rhodium for the integrated mass spectrometer, with which the chemical composition of the dust particles is determined. In extensive sequences of tests consisting of heating and cooling cycles, a solution for the required shape of the impact surface was found. In addition, qualification tests were planned and carried out under the supervision of DLR and a structural and thermal model generated. The design of patterns for the thermal insulation was also part of DLR's work package.
The Jet Propulsion Laboratory's decision to make numerous cost savings on the Cassini mission in 1992-3 meant an unexpected increase in DLR's work package. To enable a large field of view for the CDA in its scientific work, it was originally planned that the CDA would be fitted on a rotating platform attached to the body of the space probe. However, doing away with rotating platforms in the new design of the Cassini probe meant attaching the CDA directly to the space probe. Consequently, the scientific measurement would have been potentially restricted, because the entire probe would have to be turned exclusively for CDA measurement, in constant conflict with other experiments. Furthermore, turning it would have been a very sluggish process, so a separate rotating platform was designed for the location at which the space probe was set to be attached, and this was successfully constructed and qualified at the DLR Institute of Planetary Research. Even so, this enables coverage of 270 degrees of space. Despite being used every day, the rotating mechanism maintains its original performance after a service life of 18 years.
CDA measurements up to the last minute
Another success factor for the scientific results of the CDA was the extremely tightly controlled cleanliness of the materials and processes used. Data that has formed the basis for astrobiology will be gained from the integrated flight time mass spectrometer literally up to the last minute. The significance that this simple spectrometer would achieve was not foreseen at the time. The only German instrument contribution to the 12 experiments on Cassini, the still globally unique CDA instrument has made invaluable contributions to science today. The measurements and expertise gained from its construction today form the basis for numerous new missions to the Jovian moons Europa and Ganymede and Saturn's moon Enceladus.