Cosmic Dust Analyzer
The Cosmic Dust Analyzer ( CDA ) is an independent and full-fledged instrument developed in Germany for the Cassini-Huygens probe, launched in 1997 , which explores the ring planet Saturn and its surroundings.
The Cassini-Huygens probe is a joint mission between the space agencies NASA and ESA . A total of twelve orbiter instruments and six instruments on the Huygens probe began a seven-year journey, which reached its destination on July 1, 2004 at a distance of almost ten Earth-Sun distances from the sun. Then the actual four year long exploration of the Saturn system with its numerous moons , its magnetosphere and its spectacular ring system began.
The Cosmic Dust Analyzer can detect both interplanetary dust (cometary and asteroidal) and interstellar dust (i.e. dust which penetrates our solar system and does not originate in our solar system) with great sensitivity and reliability. Particles can be detected with a speed of 5 kilometers per second and a mass of only kg (this corresponds to a size of two thousandths of a millimeter). In addition to the particle speed (1-100 kilometers per second) and the particle size (10 nanometers to 100 micrometers), the instrument also determines the electrical charge of the dust particles (1-100 femtocoulombs) and their elemental composition.
For five years, the scientists at the Max Planck Institute for Nuclear Physics (MPI-K) developed, built and tested the experiment under the direction of Eberhard Grün in cooperation with the German Aerospace Center (DLR) from Berlin-Adlershof . The mechanics were designed by the Pahl engineering office in Munich, and Phytron supplied a stepper motor with extremely low energy consumption for the instrument's own turntable. The University of Canterbury and the Rutherford Appleton Laboratory (RAL) in England participated in the CDA with the development of the Chemical Analyzer (CA). The restrictions imposed by NASA on mass (17 kilograms) and electrical power (12 watts) for the CDA could be observed despite the size of the experiment, which is 80 cm high and a detector diameter of 40 cm. Fig. 1 shows the 5.6 ton Cassini / Huygens probe and the attached CDA experiment.
A dedicated microcomputer allows for months of autonomous and reliable measurements. A special Ada software developed by KCS GmbH, the University of Mannheim and the Mannheim company Helfert-Informatik enables flexible processing, internal storage and compression of the data before it is sent to the computer system at a low data rate (one hundredth of an ISDN line ) Cassini to be passed. The data from the neighboring experiment high-rate detector (HRD) developed by the University of Chicago also flow into the data stream . Scientific planning, mission operations and data analysis are carried out at the MPI-K in cooperation with other Max Planck institutes and the universities of Münster , Potsdam and Munich . The MPI-K works closely with the Cassini project at the Jet Propulsion Laboratory in Pasadena / Los Angeles (USA) and the 25 directly involved scientists from seven countries. The CDA experiment has been successfully commanded in interplanetary space since 1999 and constantly provides fascinating data on Earth, which has already brought the dust scientists many new discoveries and results.
Although the CDA looks similar to an optical telescope (see Fig. 2), it does not provide direct images of the dust particles. For this purpose, the researchers receive a piece of the picture of the formation and evolution of the solar system and its planets from the data.
Measuring principle of the dust detector
The CDA sensors are based on three different measuring principles: electrical influence , impact ionization and impact depolarization . Before the particles hit one of the two sensor surfaces (targets), they pass through four grids in the entry area (Fig. 3). The two inner tilted grids are connected to a charge amplifier , which measures the influence charge of a charged particle flying through. If the particle is between the two tilted grids, the charge induced on the grids corresponds exactly to the primary charge of the particle. Fig. 3 shows the trapezoidal signal of a particle flying through the measuring channel QP.
In order to determine the trajectory of the particle more precisely, the two measuring grids are inclined by 9 ° in relation to the detector axis of symmetry. This inclination leads to an asymmetry of the rising and falling signal edge, from which the direction of incidence of the particle can then be determined. The charge of a four-micrometer-sized dust particle measured on channel QP is in the range of one femtocoulomb, provided the electrical surface potential is five volts (1 femtocoulomb corresponds to the charge of 6,000 electrons).
Once the particle has passed through the grids, it hits one of the two targets and produces particle fragments, neutral atoms and an impact plasma. The charge of this plasma is separated in an electric field between the target and the ion collector and flows off via connected charge amplifiers. The electron signals on the target and the ion signals on the ion lattice are digitized and recorded for data processing. The rise times of the signals allow the impact velocity v to be determined, while the amplitudes with the plasma charge Q are generally proportional to the mass of the particles m according to the relationship .
The primary charge, velocity and mass of each dust particle is determined by the combination of the entry grid and the impact target. The integrated time-of-flight mass spectrometer (or TOF-MS ) uses this to determine the elementary composition of the micrometer-sized dust particles. The plasma charge from impacts on the CA target is separated by a stronger electric field and the ions are accelerated towards the ion detector (multiplier). The light ions such as hydrogen and carbon arrive at the multiplier earlier than the heavy ions of the target material rhodium . The time-of-flight mass spectrum of the ions of the impact plasma is obtained by quickly recording the signals on the multiplier. The noble metal rhodium is suitable as a target material because its atomic mass of 103 is so far removed from the mass range of the elements expected in dust particles (hydrogen to iron) that the mass lines in the spectrum can be easily separated.
The detector described above is based on the measurement of plasma charge. For reasons of internal data processing, this detector has a dead time of one second. High impact rates, such as those occurring in the inner E-ring of Saturn, cannot be fully recorded as a result. For this reason the high-rate detector (HRD) is part of the CDA experiment. Its functional principle and its signal processing is much simpler, which means that up to 10,000 events per second can be registered. The HRD sensors are made of thin polyvinylidene fluoride films that are permanently polarized. A high-speed impact destroys the local dipoles along the particle trajectory by forming a crater or a puncture hole in the foil. The destruction of the dipoles causes a short current pulse, the amplitude of which is a function of particle mass and particle speed. The detection of particles smaller than one micrometer requires particularly thin foils. However, this makes the sensors fragile and sensitive to mechanical vibrations. The HRD therefore has a small 6 micrometer thick film and a large detector with an area of 50 cm² and a thickness of 28 micrometers. For comparison, normal household aluminum foil is 20 micrometers thick.
Scientists are often asked whether the numerous dust impacts do not pose a threat to the instrument and whether it is even damaged in the process. A simple consideration of the expected dust flows, the size distribution of their particles and the material properties of the target helps here . The target is 0.3 millimeters thick and high-speed impacts create small craters on the surface with depths of a few micrometers. At a tenth of a square meter, the target area is so large that a billion particle impacts of 10 micrometers in size would be necessary to erode the target area. Only particles with a speed of a few kilometers per second and a size of a tenth of a millimeter become dangerous for the CDA, the other experiments and the Cassini space probe. However, these large particles are so rare on the way from Cassini to Saturn and the chosen trajectory around Saturn that a loss of mission must not be expected for this reason. Most often, one expects impacts from micrometer-sized ice particles.