Cassini in the Shadow of Saturn
The Cassini spacecraft traversed the shadow of Saturn from 12:11 pm to 1:08 pm (UTC) on July 10th, 2011. Most remarkable about this event is that it coincided with a ring plane crossing. The radial distance from the center of Saturn was 4.7 Saturnian radii, close to the orbit of Enceladus. This moon is the source of a dusty ring, called E-Ring, for fountains of ice strew tiny particles along its way. The illumination conditions in the shadow lead to variable plasma states, which, in turn, induce different electrical charges. The Cosmic Dust Analyser (CDA) made specific measurements of negatively charged dust grains with high abundances. The data are currently analysed.
The field of view of the CDA
The general purpose of this instrument is to cover the whole hemisphere with its field-of-view. On Galileo, this was achieved with the wide aperture of +/- 70 degree and a mounting of the instrument by 55 degree with respect to the Galileo spin axis. Originally the Cassini design included a continuous rotating pointing platform for the fields and particles instruments which was canceled during a descoping process in order to lower the spacecraft costs. Although the CDA instrument was mounted nearly perpendicular to the Cassini spin axis, wide coverage cannot be obtained with a mainly 3-axis stabilized spacecraft. Furthermore, the rotation rate of Cassini is restricted to the maximum value of 0.26 degree/s and, during high activity periods, other instruments may determine the orientation of the spacecraft. All these constraints lead to a redesign of the instrument and a turntable was added at the interface to the spacecraft. The mounting vector of the turntable points 15 degrees below the spacecraft x-y plane. Furthermore, this vector points 30 degrees away from the +y-axis towards -x. The coordinates of the rotation axis with respect to the spacecraft x-y-z coordinate system are (-0.483; 0.837; 0.259). The Dust Analyzer detectors (IID, CAT and HRD) are mounted at 45 degrees with respect to the turntable rotation axis.
The boresight vector of the field of view has the coordinates (-0.250; 0.433; 0.866) in the launch position (0° position, downwards to +z). The turntable enables the instrument to rotate by 270 degrees. The cable wrap drum inside the turntable does not allow a full revolution. The "lower right" quarter of the full circle cannot be reached by the instrument.
The spacecraft coordinate system is such that the x-y plane is perpendicular to the spacecraft spin axis z. The +z direction points away from the high gain antenna (big dish), whereas the Huygens probe points towards -x. Besides the high gain antenna (which points towards -z) Cassini has two low gain antennas (LGA). LGA 1 points towards the -z direction whereas LGA 2 points towards the Huygens probe (-x-axis). During cruise, the 3-axis stabilized spacecraft has an orientation such that the high gain antenna points towards the sun and the selected low gain antenna points towards the earth as precisely as possible.
Technical Instrument Description
CDA consists of the sensor housing with its impact targets, the High Rate Detector, the electronics box and the turntable. The interior of the sensor housing was purged with dry nitrogen until launch in order to avoid any contamination of the sensitive multiplier and the rhodium target of the chemical analyzer.
A redundant pyro device moves a lever which unlatches the cover, and preforced springs jettison the cover to a normal direction. All major parts were made of milled aluminum while a honeycomb structure provided the required stiffness for the cover and the cylindrical sensor housing. The preamplifier box is located directly above the main electronics and occupies a separate housing to keep the input cables as short as possible and to eliminate any interference with the main electronics.
The turntable of the instrument allows a rotation of 270 degrees. The turn limit is given by the capability of the integrated cable wrap drum and the mechanical end stops. A design with 2 layers of plastic balls and a bearing diameter of 240 mm was selected and qualified. The torque necessary for the turn is provided by a Phytron ZSS32 stepper motor and a gear with a total gear ratio of approx. 1000:1. Special electronics were developed to achieve very low power consumption and a maximum torque. The motor has a compensating pole configuration and a Mu-metal shielding to keep the stray magnetic fields as low as possible. The motor can be operated with motor currents between 150 and 300mA and consumes between 2 and 5 Watts. The turn speed of the platform can be set and is normally in the range of 10 degrees per minute.
The grid system at the front end allows measurement of the dust charge. The grids are made of stainless steel and each of them has a transmission of 95%. The innermost and outermost of the four grids are grounded, the other two grids are connected to a charge sensitive amplifier.
A charged dust particle entering the sensor will induce a charge which corresponds directly to the charge of the particle. When the dust particle is far away from the sensor walls, all field lines are ending on the grids and the error in charge measurment is small. The output voltage of the amplifier will rise until the particle passes the second grid. As long as the particle is located between the second and third grid the output voltage remains more or less constant. As soon as the dust particle has passed the third grid, the voltage begins to fall until the fourth grid is passed. Due to the inclination of 9 degrees for the inner two grids, the path length between the grids depends on the angle of incidence, and allows a determination of the directonality of the incident particle in one plane. The choice of 9 degrees is a compromise between angular resolution and tube length of the detector. The larger the angle the better the angular resolution, but the bigger and heavier the instrument. The detection of particle charges as low as 10-15 C should be possible although the grid capacitance is approximately 200 pF.
A particle can impact either on the big gold plated impact ionization target or the small rhodium chemical analyzer target. In both cases the impact physics is the same: The impact produces particle and target fragments (ejecta), neutral atoms, ions and electrons (impact plasma). An electric field separates electrons (collected by the targets) and ions (collected by the ion grid). Charge sensitive amplifiers collect the charges at the various targets and grids. Amplifiers are connected at the chemical analyzer target (QC), the chemical analyzer grid (QA), the impact ionization target (QE), the ion grid (QI), the entrance grids (QP), the multiplier anode (QM) and the multiplier dynodes (DLA).
In order to increase the dynamic range, the amplifier for QC, QE and QI are working with two measurement ranges. The signals at the output of the electron multiplier must cover an exceptionally large dynamic range for two reasons. A wide dynamic range is required for measurement of a large range of ion abundances for any one impact, but, more importantly, a wide dynamic range is needed to make chemical analysis measurements over the desired six orders of magnitude in range of particle masses impacting the system. Because of the random nature of the impact events and the short ion time of flight, it is clearly impossible to make real-time gain changes for each event. Ordinary logarithmic amplifiers are not fast enough and do not have sufficient dynamic range for the time of flight measurements. An innovative solution to this problem has been created through the development of the Dynode Logarithmic Amplifier. This system sums the linear signals from six different dynodes of the Johnston MM-1 multiplier in such a way that for large impacts the amplifiers for highest gain dynodes produce fixed (saturated) outputs that sum with an unsaturated low gain dynode signal. Thus it is a fast, low-noise, piece-wise linear approximation to true logarithmic performance. This special electronics was developed by the Rutherford Appleton Laboratory, U.K.. All the outputs of the amplifiers are permanently compared with a channel specific reference value (threshold), and if it is exceeded an event trigger is released.
What happens now is that the sampling frequencies for the QC, QE, QI channels and the DLA are increased and the signals are digitised and stored in memory. The data processing by the 6 MHz MA31750 microprocessor system includes the calculation of signal risetimes, amplitudes and integrals. A wavelet algorithm allows signal smoothing and a lossy compression. A lossless RICE compression algorithm can reduce the raw data by a factor of three. It is expected that not more than 1500 bytes are necessary for the lossless storage of one data frame. The data processing limits the dead time of the instrument to one second. The calculated signal parameters are used for onboard data classification. Each event increases the value of one of twenty counter values. About half of the instrument memory is needed for the execution of the onboard software. The remaining memory is used to store event data. The classification and priorization of detected events is a very complex procedure and is still under development. The onboard program is written in ADA using a TARTAN development system.
The instrument is thermally isolated from the S/C by its mount and by multilayer thermal blankets covering the turntable, electronics boxes, the HRD and the cylindrical housing. The normal operating power of the instrument produces an acceptable overall temperature without supplementary heaters. When the instrument is off, the temperature is maintained by the S/C-controlled replacement heater attached on the top of the main electronics box. The instrument monitors five temperature locations internally when it is turned on, and the S/C provides monitoring of seven locations at all times. A special instrument-controlled heater is provided for periodic decontamination of the chemical analyzer target through heating to about 90° C. Because of the high depth to diameter ratio, the cylindrical housing aperture is a very effective radiator, and the interior required a specially prepared gold coating to reduce emissivity.
The detection of dust particle impacts is accomplished by two different methods:
- a High Rate Detection system (HRD), using two separate polyvinylidene fluoride (PVDF) sensors, for the determination of high impact rates during Saturnian ring plane crossings and
- a Dust Analyzer (DA) using impact ionisation. The DA measures the electric charge carried by dust particles, the impact direction, the impact speed, mass and chemical composition, whereas the High Rate Detector is capable of determining particle mass for particles with a known speed. The following section describes the DA.
An electrically charged particle flying through the two inclined entrance grids at the front of the DA will induce charge signals on the grids. This induced charge is directly proportional to the charge of the particle and allows therefore a direct determination of its electric charge. The inclined grid geometry leads to asymmetric signal shapes allowing the measurement of the particle direction in one plane. The particle can impact either on the outer big gold plated Impact Ionization Detector IID (QE signal) or on the small inner Chemical Analyzer Target, CAT (signal QC), which has a diameter of 16 cm. The gold target has a diameter of 41 cm. The impact generates charged and uncharged fractures (ejecta), atoms and ions. The electrons of this plasma are collected by the target, and the ions are accelerated towards the inner grids (ion grids, signal QI) by an applied field of -350 V. Some of the ions fly through the grids producing a multiplier signal (QM). The integrated chemical analyzer consists of the chemical analyzer target, the chemical analyzer grid (68% transmission) and the multiplier. The chemical analyzer grid is located 3 mm in front of the target and electrically grounded whereas the target is on a potential of +1000 Volts. The strong electric field between target and grid separates the impact charges very quickly and accelerates the ions towards the multiplier. The curved shape of target and grid provides a better focusing of the ions onto the multiplier. This time-of-flight mass spectrometer has a flight path length of 230 mm and gives information about the elemental composition of the micrometeoroids [Ratcliff 1996b]. The chemical analyzer is a development of the University of Kent, Canterbury, U.K..
The sensor concept for the High Rate Detector (HRD) is different from the impact ionization mechanism of the DA: Hypervelocity dust particles impacting a PVDF sensor produce rapid local destruction of dipoles (crater or penetration hole) which result in large and fast (ns range) current pulse to the input of the electronics. The output pulse amplitude depends on impacting particle mass and velocity. PVDF stands for polyvinylidene fluoride which is a type of permanently polarized polymer. More details about the function of PVDF dust sensors can be found in [Simpson and Tuzzolino, 1985 and 1989]. The HRD is an independent instrument with its own memory and processor. The interface to the Dust Analyzer is given by power and data lines only.
Scientific goals of the CDA